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Hello there, and welcome to Physics with William, where complicated ideas are given enough room to make themselves comfortable.
Exploring Cubism, a quantum mechanics interpretation where quantum states reflect personal expectations, not objective reality.
Key Takeaways
- Quantum states in Cubism are personal expectations, not objective properties.
- Different observers can validly assign different quantum states based on their information.
- Reality exists independently, but quantum theory describes an agent's interaction with it.
- Cubism offers a fresh perspective on the measurement problem and quantum probabilities.
- Objectivity does not require universal agreement on quantum states among observers.
What the video covers
- The video introduces Cubism, an interpretation of quantum mechanics emphasizing personal probability assignments rather than objective quantum states.
- Cubism suggests quantum states represent an agent's expectations about future experiences, not intrinsic properties of physical systems.
- The interpretation challenges the traditional view that quantum states depict physical reality, emphasizing the role of perspective and information.
- Different agents with different information may assign different quantum states to the same system, reflecting their unique expectations.
- Cubism maintains that reality exists independently and pushes back, but personal expectations shape how quantum theory is applied.
- The video uses everyday analogies like weather forecasts and train schedules to illustrate how personal probabilities work in ordinary life.
- Cubism does not deny physical interactions or objective reality but highlights the standpoint from which quantum theory is used.
- The interpretation addresses the measurement problem by focusing on the agent's experience and updating of expectations.
- Cubism distinguishes objectivity from the idea that all observers must share the same information or quantum state.
- The video encourages viewers to reflect on their own perspective and the role of personal experience in understanding quantum mechanics.
Chapters
- 00:00 Introduction to Cubism and Quantum Mechanics
- 08:21 Personal Probability and Everyday Analogies
- 16:54 Applying Personal Probability to Quantum States
- 25:50 Different Perspectives and Quantum State Assignments
- 34:11 Objectivity and Shared Reality in Cubism
- 42:29 Measurement Problem and Agent's Experience
- 50:31 Summary and Implications of Cubism Interpretation
- 56:01 Closing Thoughts on Quantum Mechanics and Reality
Full Transcript — Download SRT & Markdown
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Where physics is allowed to move at a sensible evening pace, and where we can explore some of the stranger questions about the universe without turning bedtime into an unexpected examination.
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Tonight, we're going to look at an interpretation of quantum mechanics that asks us to reconsider something so basic that we rarely think to question it.
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When you and I talk about reality, are we necessarily talking about exactly the same thing? Not whether we agree about politics, whether pineapple belongs on pizza, or whether the room is slightly too warm. Those disagreements are difficult enough.
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The question tonight goes deeper. When physics describes what may happen in the world, could part of that description belong to the person making it?
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The idea we'll be exploring is called Cubism, a particular way of understanding quantum mechanics.
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Its name comes from quantum Bayesianism, although the people who developed the interpretation eventually preferred the shorter name because Cubism grew into something more specific than simply applying one school of probability to quantum theory.
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At the center of it is a surprisingly simple suggestion. A quantum state does not have to be treated as an objective property carried around by a physical system.
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Instead, it can represent the expectations of an agent, meaning the person using quantum mechanics to decide what experiences may result from interacting with the world.
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In Cubism, quantum probabilities are personal probability assignments, expressions of an agent's expectations about future experience rather than physical labels hidden inside particles.
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That sentence may already sound suspicious. Physics is supposed to describe an objective world, so introducing words such as personal, agent, belief, and experience can make it seem as though we have wandered out of a laboratory and into a very serious conversation at the
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back of a coffee shop. Yet, Cubism is not saying that the universe exists only in your imagination.
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It is not suggesting that every person may choose whatever laws of nature they prefer.
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You cannot wake up tomorrow, decide that gravity is inconvenient, and float downstairs. You cannot assign yourself a personal speed of light because the official one feels unnecessarily strict.
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Reality still pushes back. Experiments still surprise us. Predictions can still fail. What becomes personal in Cubism is not the existence of the world, but the expectations an individual agent forms about what will happen when that agent interacts with it.
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Before we go any further, think about where you're listening from tonight. Perhaps you're in bed with the lights already low.
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Perhaps you're on a sofa telling yourself that you will move to bed after one more minute.
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A promise that may have been made several minutes ago. Maybe you're listening while traveling, studying, resting, or staring at a ceiling that has somehow become more interesting than it appeared during daylight.
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Wherever you are, what can you actually say about the world from where you are sitting or lying?
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You have what you can see, what you can hear, what you remember, what other people have told you, and what you expect to happen next.
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Someone else, where, has a different collection of experiences. They may know things you do not know, and you may know things they do not. So, where are you listening from? And what is the room around you like right now?
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It is an unusually appropriate question for this subject because Cubism places considerable importance on perspective.
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If two people possess different information, they may reasonably make different predictions about the same future event.
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We already accept this in ordinary life without finding it remotely mysterious. Imagine that you and a friend are waiting for a train.
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Your friend checked the departure board 30 seconds ago. You did not. They expect the train to arrive shortly because the platform number has just appeared.
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You still think it may be delayed because the last information you saw was 10 minutes old.
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There is only one railway station, thankfully. But there are two sets of expectations because there are two people with different experiences. If you enjoy these slow journeys through physics, and if this is the sort of thing you find
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relaxing rather than an unnecessarily elaborate way of becoming suspicious of railway timetables, you can give the video a like and subscribe to the channel.
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It helps other curious listeners find this little corner of science, and it lets us keep wandering into questions that become stranger the longer we look at them.
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You're also very welcome to leave a comment telling me where you are listening from tonight.
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It seems particularly fitting for a discussion in which perspective matters. Two people may be listening to exactly the same words at exactly the same point in the video while having completely different experiences of the evening around them.
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The easiest way to approach Cubism is not to begin with quantum mechanics at all. Start with an ordinary prediction.
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Imagine that tomorrow morning you're planning to leave the house without an umbrella. You look at the sky, check the weather forecast, notice the suspicious enthusiasm of the clouds, and decide how likely you think rain is.
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That probability represents your expectation. Another person may have seen a newer forecast and form a different judgment.
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Neither probability is a substance floating inside the clouds. If you say there is a strong chance of rain, nobody expects a tiny percentage sign to be hiding somewhere behind a cumulus cloud waiting to be measured.
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The probability belongs to your assessment of what may happen. Then the weather makes its own contribution. You leave the house, perhaps it rains, perhaps it does not.
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Whatever happens, you receive new information. If you repeatedly make terrible weather predictions and arrive home soaked while insisting that everything is proceeding exactly as expected, the problem is probably not with meteorology.
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Your expectations must eventually respond to experience. Personal probability therefore does not mean that all beliefs are equally useful.
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The world remains capable of correcting you. Sometimes by providing evidence, and sometimes by providing evidence directly onto your coat.
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Cubism takes this personal understanding of probability and applies it seriously to quantum mechanics. That is more radical than it may first appear because quantum theory is filled with probabilities.
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When physicists prepare a quantum system and consider a measurement they might perform, the theory allows them to assign probabilities to the different experiences that may result.
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The traditional temptation is to think of those probabilities as revealing something about the physical system itself.
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Perhaps the particle really possesses hidden tendencies corresponding to those numbers. Perhaps the quantum state is an objective description of what the particle is doing before the measurement.
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Cubism asks us not to make that extra step automatically. The quantum state, on the Cubist view, is an assignment made by an agent.
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It summarizes that agent's expectations for the consequences of actions the agent may take. To understand why that matters, imagine that a physicist named Anna prepares a quantum system in a laboratory.
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Anna knows exactly how she prepared it and assigns a quantum state based on that information.
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Across the building is another physicist named Ben who knows that Anna has prepared something but does not yet know exactly what procedure she used.
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Anna and Ben therefore possess different information. In Cubism, there is no requirement that they assign the same quantum state before Ben learns what Anna knows.
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Their state assignments reflect their own expectations. When Anna later tells Ben what she did, Ben can update his assignment. Nothing physically has to ju
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What changed was Ben's information and therefore the expectations he uses when deciding what may happen next.
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This sounds perfectly ordinary when we describe boxes, trains, weather forecasts, or forgotten passwords. Quantum mechanics makes it uncomfortable because physicists have often hoped that the quantum state might provide a direct picture of physical reality.
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If the state belongs to the agent, then we cannot simply point at the mathematics and say that this is what the world looks like when nobody's interacting with it.
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Cubism deliberately refuses that move. It does not claim that there is no external reality. Instead, it separates the external world from the mathematical expectations an agent uses when dealing with that world.
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The distinction is small enough to fit into one sentence and large enough to change how many familiar quantum puzzles are discussed.
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Consider a simple sealed box. Someone places an object inside while you're not looking. You know that it is either a blue ball or a red ball.
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Before opening the box, you're uncertain. The person who filled the box already knows the color.
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Your uncertainty obviously does not mean the ball itself is experiencing an identity crisis. The difference belongs to what you know.
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Classical examples like this are easy because we naturally assume that the ball has a definite color whether you know it or not.
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Quantum mechanics is more difficult because experiments have repeatedly shown that we cannot simply assume every measurable quantity possesses a predetermined value in the same straightforward classical way.
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The quantum theory does not behave like an ordinary list of unknown properties waiting patiently for someone to uncover them.
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This is where discussions of quantum mechanics often become tangled. We use the same word probability for several very different ideas.
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Sometimes probability means ignorance. If I shuffle a normal deck of cards and ask you which card is on top, you may assign probabilities because you do not know the answer.
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We still assume that one particular card is already there. Your uncertainty reflects missing information.
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But quantum probabilities have resisted attempts to interpret every case as nothing more than ignorance about ordinary pre-existing properties.
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The structure of quantum theory, together with the experiments supporting it, forces us to be more careful about the picture we attach to those probabilities.
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Cubism takes a strong position on that care. It says that all quantum probabilities belong to the agent, including assignments of complete certainty.
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Even when an agent is completely confident that a particular experience will occur, Cubism does not automatically convert that confidence into an objective property possessed by the system independently of the agent.
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Its proponents describe the rules of quantum theory as normative guidance for how an agent's probability assignments should fit together, rather than as a conventional catalog of physical properties that nature carries around before anyone interacts with it.
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That immediately changes the role of measurement. In many popular explanations of quantum mechanics, measurement sounds like a dramatic physical event in which a cloud of possibilities suddenly becomes one reality.
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Before measurement, several outcomes appear in the mathematics. After measurement, one result is recorded. This leads naturally to the famous question of collapse.
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What exactly collapsed? When did it happen? Did a conscious observer cause it? Did the measuring device cause it?
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Was there really a physical object corresponding to the quantum state that suddenly changed? Cubism approaches the situation differently.
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If the quantum state represents an agent's expectations, then changing the quantum state after an experience does not require us to imagine an objective physical wave suddenly collapsing throughout space.
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The agent took an action, encountered a result, and now possesses new experience. Naturally, the agent's expectations change.
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The mathematical update reflects that change. This does not make the external event unreal, quite the opposite. Something happened that the agent could not dictate beforehand, and the agent must now respond to it.
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Suppose you are waiting for an important message. Before checking your phone, you assign different possibilities in your mind.
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Perhaps the message has arrived. Perhaps it has not. Perhaps the only notification waiting for you is an application announcing that someone you barely remember from school has posted a photograph of breakfast.
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Then you look at the screen. Your uncertainty changes immediately because you have had a new experience.
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Nobody finds this mysterious because we never thought your uncertainty was a physical substance stored inside the phone.
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Cubism suggests that part of the mystery surrounding quantum state update may come from treating an agent's mathematical expectations as if they were physical objects that must literally transform when the agent learns something.
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There is an important limit to that analogy. A phone already contains whatever message it contains before you look, while quantum measurement cannot always be understood as merely revealing a value that existed in exactly the same form beforehand.
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Cubism does not reduce quantum mechanics to ordinary ignorance. The point of the comparison is narrower.
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It shows how naturally an agent changes expectations after receiving new experience. The difficult question is what kind of world can produce quantum experiences with exactly the pattern of probabilities that quantum theory requires.
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That question prevents Cubism from becoming a philosophy of wishful thinking. If probabilities belong to you, you might wonder whether you can simply choose whichever ones make you happiest.
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Perhaps every laboratory result will be the result you wanted. Perhaps every coin will land in your favor.
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Perhaps every supermarket queue will suddenly choose to move efficiently the moment you join it.
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Unfortunately, personal probability does not grant personal control. You choose your expectations and your actions, but you do not choose the experience the world gives you in return.
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The external world remains independent enough to surprise you. This element of surprise is essential.
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Imagine playing a game against someone whose moves you cannot control. You decide what move to make based on everything you have learned so far.
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You estimate what the other player may do, then they respond, perhaps exactly as expected, perhaps not.
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Your next decision must incorporate what happened. Cubism treats interaction with the physical world in a somewhat similar way.
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An agent chooses an action. Quantum theory helps organize expectations about possible consequences, and then an experience occurs.
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The agent has to live with the answer nature provides. Cubist writing goes further and treats measurement outcomes as personal experiences for the agent who acts.
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That wording can sound much stranger than it needs to. If Anna performs a measurement and sees a particular result, that result is part of Anna's experience.
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Ben, standing elsewhere and unaware of what Anna observed, has not yet had Anna's experience. Ben may later ask Anna what happened.
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Hearing her answer then becomes a new experience for Ben, allowing him to update his own expectations.
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Cubism therefore refuses to pretend that every observation must immediately be written into a single universal ledger available from some imaginary viewpoint outside the universe. At first, this can feel dangerously close to saying that Anna and Ben live in different
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realities. That is where the title of our discussion needs some care. What if reality is personal does not mean what if everyone occupies a private dream from which communication is impossible.
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Anna and Ben can meet. They can compare notes. They can repeat experiments. They can build instruments together.
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They can disagree and then gather more evidence. They inhabit a world in which their actions have consequences and in which those consequences can become experiences for other people through further interaction.
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The personal element concerns the standpoint from which predictions and experiences occur. There is no agent who possesses every possible viewpoint at once.
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Every actual scientist works from somewhere with some information, some history, and some set of expectations.
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This sounds almost too obvious to mention until quantum theory puts pressure on the assumption that physics must always be written as though those standpoints can be removed without leaving anything important behind.
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Classical physics makes that removal seem natural. Imagine calculating the path of a planet. The planet does not care whether the astronomer is hopeful, annoyed, hungry, or wearing particularly uncomfortable shoes.
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Different competent observers who know the same relevant information can agree on the same calculation.
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We therefore become accustomed to imagining a description of nature with the observer erased. There are positions, velocities, forces, fields, and objects doing what they do.
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The scientist merely discovers the facts. Quantum mechanics complicated that picture almost from the beginning.
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Its mathematical tools give probabilities for the outcomes of possible measurements, and the question of what those probabilities refer to has never produced one universally accepted answer.
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Some interpretations treat the quantum state as describing something objective. Others add hidden structures. Others describe branching worlds. Others emphasize relations between physical systems.
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Cubism takes another route by placing the agent and the agent's probability assignments directly into the meaning of the theory.
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The important thing tonight is not to decide immediately whether Cubism is correct. It is much too early for that, and physics is generally more interesting when we allow an unfamiliar idea to explain itself before throwing furniture at it.
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Instead, notice what the interpretation is asking us to give up. It asks us to stop assuming that every piece of the quantum mathematics must correspond directly to a physical feature existing independently in the world.
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It asks us to distinguish a description used by an agent from the world the agent is attempting to navigate.
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It asks us to take probability seriously as a statement of expectation, rather than quietly turning it into a hidden physical property.
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There is something almost practical about this. Every day you act without possessing a complete description of reality.
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You decide when to cross the road, whether to trust a forecast, whether the milk is still usable, whether your alarm really needs to be set that early, and whether the strange noise downstairs deserves investigation or can safely be
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blamed on the heating. You constantly combine past experience with present information to form expectations about what an action may produce.
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Usually, the stakes are ordinary. Quantum mechanics takes the general structure of acting under uncertainty and places it in a domain where the rules connecting expectations are unlike anything classical intuition prepared us for.
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The unusual part is that Cubism does not regard uncertainty as an embarrassment that must eventually disappear here when we discover the hidden true state of everything.
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It treats probability as a fundamental part of how an agent uses quantum theory. That changes the ambition of the theory.
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Instead of asking quantum mechanics to provide a complete view of the universe from nowhere, Cubism asks what guidance the theory provides to an agent located inside the universe who must decide what to expect from future interactions.
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You may already see why the idea creates resistance. Physics has achieved enormous success precisely by searching for features of nature that do not depend on personal opinion.
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We want the boiling point of water, the behavior of light, or the motion of a satellite to be more dependable than somebody's mood before breakfast.
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If we hear that quantum probabilities are personal, the immediate worry is that objectivity has been abandoned.
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Cubism responds that objectivity does not require pretending that probability assignments exist independently of the people assigning them.
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Agents can communicate, share evidence, update expectations, and learn from a common world even though each agent begins from a particular standpoint.
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Think again about the room around you. Someone standing outside it does not currently share your exact visual field.
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They do not hear every sound you hear. They cannot feel the pillow under your head your skin.
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Yet it would be strange to conclude that you therefore inhabit unrelated universes. If they enter the room, you can point to the lamp, move a chair, hand them a cup, or ask them whether that faint buzzing sound is genuinely present, or whether
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you have finally listened to too many physics videos. Through interaction, your experiences become connected.
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Cubism takes seriously the fact that physical knowledge always arises through interactions of this kind.
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There is an agent. There is something outside the agent. There is an action, and there is a consequence experienced by the agent.
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Quantum theory helps that agent manage expectations about those consequences. The interpretation is not trying to replace laboratories with introspection.
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It is trying to say more carefully what laboratory predictions actually mean. The strange consequences only begin to appear once we ask what happens when two agents describe one another.
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Suppose Anna performs a quantum experiment while Ben remains outside her laboratory. Anna obtains a result. Ben does not know which result she saw.
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Anna now has an experience that Ben lacks. If quantum states are personal assignments, Anna and Ben may legitimately use different quantum descriptions.
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Later, Ben can interact with Anna and learn what she observed. His description changes. Nothing about this sounds impossible when told as an ordinary story. Yet, when we place the same situation inside quantum theory, it leads directly towards some of the most difficult
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arguments about observers, measurement, and whether there can be one completely detached description containing everybody's facts at once.
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That is where the apparently harmless question about personal reality begins to earn its place.
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We're not asking whether the world is whatever you want it to be. We're asking whether quantum mechanics describes the world directly, or whether it describes how an individual participant should form expectations when acting within that world.
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Those are very different jobs for a physical theory, and the distinction affects nearly every famous quantum mystery that comes afterward.
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To see why anyone would propose such an unusual interpretation, we need to look at the problem that existed before Cubism tried to solve it.
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Long before physicists argued about personal probabilities, they were already with experiments in which quantum systems refused to behave like ordinary objects carrying neat sets of predetermined properties.
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They found a theory that predicted experimental statistics with extraordinary success while leaving basic questions about measurement, probability, and physical reality open to interpretation.
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The next step is therefore not to make reality even stranger. Quantum mechanics has already taken care of that for us.
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We need to look at the original carefully and see what forced physicists to reconsider the familiar idea that a good physical theory should simply tell us what everything is doing, whether anyone is looking or not.
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Once that problem is clear, the Cubist move from an objective quantum state to an agent's expectations will stop looking like an arbitrary philosophical choice, and start looking like one possible answer to a difficulty that has been sitting inside quantum mechanics
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from the beginning. That difficulty begins with a peculiar fact about quantum mechanics. The theory is extraordinarily good at telling us what we should expect from experiments, but it is much less cooperative when we ask for an ordinary story about what was
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happening before the experiment produced its result. In everyday physics, those two things normally sit together quite comfortably.
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If you throw a ball across a garden, you can describe where the ball is, how fast it is moving, and where it should land.
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If someone closes their eyes halfway through the flight, the ball does not become uncertain because nobody is watching it.
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We assume that it continues along its path with definite physical properties, whether or not anyone happens to know them.
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Quantum mechanics makes that familiar assumption much harder to maintain, and the trouble begins long before we arrive at anything as unusual as cubism.
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Imagine that we have a laboratory containing a very small physical system. It could be an atom, a photon, or another quantum object, but the particular object is not important yet.
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We prepare it carefully, perform the same kind of measurement again and again, and record what happens.
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If classical intuition were completely reliable, we might expect that enough careful preparation would eventually allow us to predict the individual result with certainty.
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Perhaps the system possesses a collection of definite properties, and our only difficulty is that we have not learned all of them yet.
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That would be a familiar kind of ignorance. You do not know which card is on top of a shuffled deck, but you assume that one particular card is already there.
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Your uncertainty belongs to you, while the answer belongs to the deck. Quantum mechanics does not generally work that way. Even when physicists prepare systems in the same controlled manner, the theory often provides probabilities for different possible measurement outcomes rather than one
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guaranteed answer. Repeat the experiment many times and a very stable statistical pattern appears. Individual outcomes may vary, yet the overall frequencies follow the predictions of quantum theory with remarkable success.
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This is one reason quantum mechanics is so impressive and so irritating. It can tell you extremely precisely what pattern to expect from thousands or millions of trials while refusing, in many situations, to tell you exactly what the next single trial will produce.
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It is rather like meeting someone who can predict your average tea consumption over the next 10 years, but will not tell you whether you're about to make another cup.
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The obvious reaction is to assume that something important must be missing. Perhaps quantum mechanics gives probabilities because it does not know the complete state of the system. If we could look underneath the theory and discover some deeper variables, maybe
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every apparently random outcome would become predictable. This idea has an enormous amount of intuitive appeal because it is how uncertainty usually works in ordinary life.
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A coin toss seems random because we do not know its exact starting position, speed, rotation, air resistance, and every tiny detail of the motion.
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The coin itself is not consulting probability theory during the flight. If we somehow knew the relevant physical conditions with impossible precision, classical mechanics would in principle allow us to predict the result.
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For a long time, it was tempting to hope that quantum probabilities might hide the same kind of ordinary ignorance.
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Perhaps an electron has definite properties all along and quantum mechanics merely gives us an incomplete summary because those hidden details are inaccessible.
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The history of quantum foundations made that simple picture increasingly difficult to preserve. Results associated with Bell and later experiments showed that a broad and natural class of hidden variable explanations could not reproduce quantum predictions while maintaining the familiar local structure people hoped
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for. Other results place different restrictions on the possibility of treating measurement outcomes as revealing a complete set of pre-existing values independent of how the measurements are arranged.
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Quantum mechanics was not behaving like a shuffled deck for which all the answers were quietly sitting underneath our ignorance.
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To get a feeling for the problem, forget particles for a moment and imagine a peculiar collection of sealed boxes.
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Each box allows you to ask one of several questions, but there is an inconvenient rule.
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You may choose which question to ask, yet opening the box in the required way gives you only the answer associated with that choice. You cannot simply open everything at once and inspect an each sheet containing all possible answers.
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After repeating the experiment with many boxes prepared in the same way, you discover dependable patterns connecting the questions and results.
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The temptation is to assume that every box carried all the answers internally from the beginning.
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Quantum experiments make that assumption much more difficult than the box story suggests. The context in which a quantity is measured matters in a way that does not fit naturally with the idea that every possible measurement merely uncovers a
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value that was sitting there in advance. This does not mean that human consciousness magically manufactures physical properties.
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That popular picture has caused more confusion than it has solved. A laboratory detector does not need someone staring intensely at it with philosophical determination before anything can happen.
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The deeper issue is that the mathematical structure of quantum mechanics does not let us casually treat every possible outcome as the revelation of an ordinary pre-existing property.
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Measurement is not always like lifting a cloth from the table to discover which object was underneath.
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Whatever quantum measurement is doing, the classical story becomes unreliable when we try to push it too far.
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The double-slit experiment gives us one of the most familiar examples. Send quantum objects toward a barrier containing two narrow openings and then detect where they arrive on a screen behind it.
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If the experiment is arranged so that information about which opening was used is unavailable, repeated detections can build an interference pattern, the kind of pattern we associate with waves.
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Yet each detection appears at a particular location. If the arrangement is changed so that information distinguishing the paths becomes available, the statistical pattern changes.
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The important point for us is not to turn this into the old slogan that something knows when it is being watched.
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The important point is that the experimental arrangement cannot always be treated as a passive window through which we inspect a completely independent set of microscopic properties.
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What we choose to measure is part of the physical situation to which quantum theory assigns probabilities.
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That raises an uncomfortable question. What exactly does the quantum state describe before the measurement occurs?
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The mathematics allows a physicist to assign a state and use it to calculate probabilities for possible outcomes.
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If we treat that state as a literal description of physical reality, then we have to explain what happens when measurement produces one particular result and the state used for future predictions changes.
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This is where the famous language of collapse enters the discussion. Before the measurement, the mathematical description may involve several possible results.
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Afterward, the observer has one recorded outcome and uses a different description. If the quantum state is physically real in the same straightforward sense as the position of a chair, then its sudden change demands a physical explanation.
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When does the change occur? Does it happen when the system interacts with the measuring device?
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When the device produces a visible record? When somebody reads that record? How large must an object become before we stop treating it quantum mechanically and begin treating it as a classical measuring instrument?
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If the entire laboratory is made from atoms and atoms obey quantum mechanics, why should the detector occupy a fundamentally different category from the particle it detects?
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This collection of difficulties is usually called the measurement problem, although that polite name makes it sound as though somebody simply forgot to calibrate a ruler.
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The real problem concerns the relationship between the quantum description containing possible outcomes and the definite experience we actually obtain when a measurement is performed.
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Different interpretations of quantum mechanics respond in very different ways. Some add additional physical structure.
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Some treat the quantum state as evolving universally and place the different outcomes into separate branches.
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Some emphasize relations between systems. Some question whether the demand for a single classical style story is appropriate in the first place.
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Cubism will eventually take another route by changing what we think the quantum state represents.
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Before making that move, however, it helps to understand why probability becomes so central. Suppose you prepare the same kind of quantum system a thousand times and perform the same measurement each time.
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Quantum theory gives you expectations for the distribution of results. Once you have enough trials, the observed pattern can be compared with those predictions. This is ordinary experimental science.
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The odd part appears when we focus on one individual trial. The theory may give several possible outcomes and assign different probabilities to them. But it does not generally provide an additional hidden sentence saying which one nature is secretly selected in advance.
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The probability therefore sits in a strange position. We know how to calculate it. We know how to test the statistical predictions it helps produce.
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What we do not automatically know is what kind of thing the probability is. One possibility is that it represents an objective chance built into nature.
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Perhaps the physical system itself possesses a certain tendency to produce one result rather than another.
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Another possibility is that the probability expresses incomplete knowledge about a deeper reality. Another is that all the possible outcomes are represented within a larger physical structure and our experience follows only one of them.
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Cubism will say something different again. It will place the probability with the agent using the theory.
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The distinction may seem philosophical until we notice what happens to certainty. Imagine quantum theory tells an observer to assign complete confidence to a particular measurement result.
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The natural classical reaction is to say that the corresponding property must therefore already be present.
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If you are completely certain that opening a cupboard will reveal a mug because you just placed the mug there, it seems harmless to say that the mug exists inside the cupboard before you open it. Cubism will later resist
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carrying that inference automatically into quantum mechanics. Even complete certainty from the QB's perspective remains an agent's probability assignment rather than a license to declare that a particular element of the mathematical description must be an objective property of the
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external world. This is easier to appreciate if we separate prediction from description. Imagine a weather service that becomes astonishingly accurate.
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It gives you excellent probabilities for rain, wind, temperature, and storms. Those predictions may be so useful that you trust the service when planning almost everything outdoors.
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But the success of the forecast does not automatically tell you what the numbers inside the forecast represent physically.
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A probability of rain may summarize enormous amounts of information and sophisticated modeling, yet the probability itself is not another cloud floating above the city.
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Predictive success and a literal picture of reality are related questions, but they are not identical questions.
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Quantum mechanics gives us predictive success on a scale that is difficult to overstate. Technologies built using quantum theory work.
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Experiments agree with quantum predictions to astonishing precision. None of the interpretational arguments change the fact that physicists know how to use the theory.
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A researcher does not need to settle the nature of reality before operating a laser.
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An engineer does not need to choose a favorite interpretation before working with a semiconductor. You can calculate, build, test, and measure while leaving the deepest philosophical questions politely waiting outside the laboratory door. Eventually though, someone opens the door. If quantum mechanics is only a
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calculation method, we may simply accept the rules and move on. But physicists are curious people and telling them not to ask what their most successful theory means is roughly equivalent to placing a large button on a table with a sign
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saying, "Please do not press." The questions return immediately. What exists between measurements? Does the quantum state belong to the physical system or to our knowledge of it?
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Are measurement outcomes created in the interaction or merely revealed? Is probability a feature of nature or a feature of our expectations?
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Can two observers legitimately assign different states to the same system? If they can, what happens when one observer treats the other observer as part of a quantum experiment?
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That last question becomes especially uncomfortable. Imagine Anna inside a sealed laboratory. She performs a quantum measurement and sees an outcome.
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From Anna's perspective, the experiment has produced a definite result. Outside the laboratory stands Ben, who has not yet learned what she saw.
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If Ben tries to describe the entire laboratory using quantum mechanics, should he assign a quantum state that still include several possibilities for Anna's result?
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If so, Anna and Ben appear to be using different descriptions of the same physical situation.
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This kind of scenario is related to the thought experiment known as Wigner's friend, and we will return to it later because it becomes particularly interesting when cubism enters the discussion.
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For now, notice the basic tension. Quantum mechanics seems to allow different observers to possess different information about an event.
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That by itself is perfectly ordinary. The problem becomes sharper if we insist that every observer's quantum state must simultaneously be a literal objective description of what physically exists.
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If Anna assigns one state and Ben assigns another, which state is the real one?
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One response is to search for a single deeper description that includes everything. Another is to claim that the different descriptions concern different branches or different relations.
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Cubism starts moving in a different direction by asking why we assume the quantum state had to be an objective physical object in the first place.
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If it is instead part of the agent's bookkeeping of expectations, then Anna and Ben can assign different states without creating two contradictory physical universes.
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They have different experiences and What requires explanation is no longer how one objectively real quantum state can somehow be two different things at once.
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This does not make quantum mechanics classical again. Anna cannot necessarily imagine that every possible outcome existed as an ordinary hidden value before she looked.
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The quantum rules connecting her expectations remain different from the rules of classical probability. Cubism does not remove the strange structure of quantum theory.
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It changes where that strangeness is located and what the mathematical machinery is being asked to describe.
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The Born rule becomes especially important here. In standard quantum mechanics, it is the rule that connects a quantum state and a contemplated measurement with probabilities for possible outcomes.
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Cubism keeps the practical rule but changes its interpretation. Rather than treating it as a law describing how an objective quantum state produces objective chances, Cubists understand it as a normative constraint on the probability assignments of an agent.
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In simpler language, quantum mechanics tells the agent that if they hold certain expectations about possible experiments, those expectations cannot be chosen independently in any way they please. They must fit together according to a specifically quantum pattern.
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The word normative can sound unnecessarily formal, so consider an ordinary betting example. Imagine that you confidently announce a set of probabilities for several related events.
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If those probabilities contradict one another badly enough, a clever person may arrange bets that guarantee you lose regardless of what happens.
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Basic probability theory gives rules for keeping your expectations internally coherent. You're still free to believe that it will rain tomorrow, but you cannot sensibly assign probabilities to related events in completely contradictory ways and expect your reasoning to remain consistent.
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Cubism says quantum theory adds another kind of guidance. The world has taught us through experiment that our expectations concerning different possible quantum actions must be related in a particular way.
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The quantum formalism constrains an agent's probability assignments. It does not tell nature which outcome to produce.
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The agent uses the rule before acting, then experiences whatever outcome actually occurs, and afterward revises expectations.
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This is where quantum uncertainty begins to look different from simple ignorance. If you merely lacked information about a hidden answer that was already there, then the ideal goal would be to discover the missing facts and eliminate the uncertainty. Cubism does not interpret
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quantum probabilities as temporary stains on an otherwise complete description. The probability is part of how the agent relates to an uncertain future interaction with the world.
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The actual outcome is something the agent has to encounter. Consider again the sealed box, but now make the situation less classical.
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You can perform one of several possible interventions on the box. Quantum theory tells you how to organize your expectations for each intervention, yet it does not require you to imagine that every possible answer for every possible intervention sits inside the
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box in advance. When you choose one action and receive one result, you have acquired a new experience. You can use that experience when deciding what to expect from later actions. The sequence becomes an ongoing exchange between the agent and the
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physical world, rather than a gradually unveiling of a completely written list. That is a subtle change, but it matters enormously.
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In the classical picture we instinctively carry around, reality possesses its properties and measurements simply discovers them.
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Uncertainty usually means that the observer has failed to learn something. In the Cubist picture we are approaching, the external world is still real, but quantum theory is not treated as a catalog containing all of its hidden properties.
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The theory is a tool used by an agent who must act without knowing exactly which experience will follow.
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Some people find this attractive because it dissolves certain traditional puzzles, rather than solving them with additional invisible machinery.
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Others think it avoids questions that physics ought to answer. If you ask what an electron really is doing between measurements and receive an explanation of how an agent should manage expectations, you may reasonably wonder whether somebody has changed the
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subject. Cubists would respond that the original question may have assumed the quantum state was designed to provide a detached microscopic picture, and that assumption is precisely what they reject.
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The disagreement therefore goes deeper than competing stories about particles. It concerns what we expect a physical theory to accomplish.
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Should fundamental physics describe the world exactly as it exists independently of any observer, ideally from a perspective belonging to nobody in particular, or can a fundamental theory instead give rules to physical agents who are themselves inside the world, helping
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each of them navigate the consequences of their own actions? Quantum mechanics makes this question unusually difficult because the observer cannot simply be imagined as an all-knowing spectator.
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A real experiment requires someone to choose what to do. Different possible measurements correspond to different questions we can physically ask of a system.
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The outcome then becomes part of what the experimenter has experienced. Afterward, future predictions change.
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Whether we interpret these facts as merely practical features of experimentation or as clues to the meaning of the theory is where the major interpretations begin to separate from one another.
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There is also a temptation to hear the word observer and immediately imagine consciousness having magical influence over matter.
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That is not what we need here. An agent in Cubism is not powerful because the agent possesses a mysterious mental force.
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The emphasis is on action and experience. The agent uses quantum theory to decide what to expect if a particular action is taken on the external world.
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Nature then supplies an outcome that was not under the agent's control. The interaction matters, but the agent does not get to write the answer in advance.
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That distinction is what keeps the world from becoming a private fantasy. If you were creating reality entirely by belief, surprise would be difficult to explain. Yet experiments surprise us constantly.
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Scientists propose predictions and discover that nature has other arrangements. They revise models, repeat experiments, compare results, and gradually build reliable expectations.
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Cubism takes this resistance from the external world seriously. The personal element sits in probability assignments and experiences, not in an ability to command what reality must do.
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Now, the original puzzle can be stated more clearly. Quantum mechanics gives us probabilities. Measurement gives us one actual experienced result.
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The theory works exceptionally well, but the mathematics by itself does not force every physicist to tell the same story about what the quantum state means. If we treat it as objective reality, we inherit questions about collapse, measurement, and competing descriptions.
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If we treat it as incomplete information about hidden classical properties, experiments place severe restrictions on what that hidden picture can look like.
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If we insist that probabilities must be objective features of nature, we still have to explain what those chances are and how they relate to actual outcomes.
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Cubism begins by changing one of those assumptions. Instead of asking which physical thing in the world corresponds to a quantum probability, it asks us to consider the person who assigned the probability.
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Instead of treating the quantum state as something owned by the particle, it treats the state as something assigned by an agent. Instead of asking why an objective state collapses when a result appears, it asks why we should be
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surprised that an agent changes expectations after receiving new experience. That does not answer every question. It opens a different set of them.
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If quantum states are personal, why do physicists so often agree? If measurement outcomes are experiences, how do different people build a shared scientific world? If an agent assigns probabilities based on personal belief, what prevents quantum theory from becoming arbitrary?
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Why does nature impose exactly the quantum constraints it does rather than some other set of rules.
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Most importantly, if the mathematical state is not the reality itself, what does Cubism think is actually out there?
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Those questions are where the interpretation becomes much more interesting than the simple phrase personal probability might suggest because Cubism is not merely replacing one definition of probability with another.
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It is changing the relationship between the user of quantum mechanics and the world the theory is used to navigate.
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To understand that relationship, we now need to meet Cubism on its own terms and see why it places an agent not outside the physical description looking in, but directly inside the situation, making choices, taking risks, receiving answers, and changing expectations each
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time reality responds. The puzzle becomes easier to approach once we stop asking quantum mechanics to speak from some imaginary position outside the universe, and instead ask a much simpler question.
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Who is actually using the theory? In every real laboratory, there is someone preparing equipment, choosing which experiment to perform, considering what might happen, observing a result, and deciding what to do next.
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Quantum mechanics is never floating by itself in empty space calculating probabilities for its own amusement.
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Someone uses it. Cubism begins by taking that ordinary fact much more seriously than most interpretations do.
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The person using quantum theory is called an agent, and from the Cubist point of view, the theory is a tool that helps that agent organize expectations about what may happen when the agent acts upon the world.
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This is not merely a convenient way of speaking. It changes what the mathematical objects of quantum mechanics are supposed to mean.
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A quantum state becomes an expression of an agent's expectations rather than a photograph of a microscopic object. A measurement becomes an action the agent performs on the external world, and the outcome becomes an experience that the world delivers in response.
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These ideas form some of the central commitments of cubism as presented by its developers.
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The word agent deserves a little attention because it can sound more mysterious than it is.
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You do not need to imagine a special kind of consciousness hovering over a laboratory.
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You do not need to suppose that the human mind sends invisible instructions into atoms.
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In the discussions that developed cubism, the important point is that an agent is a user of probability in quantum theory, someone who must make decisions about possible actions in a world whose responses are not fully under that agent's control.
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For our purposes, imagining an ordinary physicist is perfectly sufficient. A physicist decides which measurement to perform, assigns probabilities to the different experiences that might follow, performs the action, and then encounters whatever actually happens.
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The agent may have excellent expectations or poor ones, but the world is not required to produce the outcome the agent prefers.
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That last detail is essential because without it, cubism would quickly become the rather unhelpful idea that reality is whatever somebody wants it to be, and nature has spent several centuries demonstrating that it has very little interest in making scientific life that
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convenient. Suppose Anna is standing beside a piece of laboratory equipment. She's prepared a quantum system, and now has several possible measurements available to her.
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Before choosing one, she can use quantum mechanics to decide what probability she should assign to the possible experiences that may follow. The theory does not choose the measurement for her.
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Anna chooses the action. The theory helps her organize expectations concerning its consequences. Then she presses a button, adjusts an instrument, sends the system through an apparatus, or performs whatever physical operation the experiment requires.
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Something happens. A detector responds. A mark appears. A light flashes. Anna experiences an outcome. She now possesses something she did not possess a moment earlier, namely the experience produced by this particular interaction.
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She can update her expectations and consider what she might do next. That sequence sounds almost disappointingly ordinary.
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You act, something happens, you learn from it. We do essentially the same thing whenever we check a weather forecast, taste a soup before adding salt, or open an email whose subject line has already made us slightly nervous.
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Yet placing this structure at the center of quantum mechanics has surprisingly large consequences. Traditional discussions often begin by imagining that the quantum state belongs to the physical system. The electron has a state.
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The atom has a state. The photon has a state. The phrase is so natural that it is easy to forget there is an interpretation hidden inside it.
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Cubism asks us to replace that picture with a different one. Anna assigns a quantum state to the system.
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The distinction between the system having a state and Anna assigning a state may look like a small change in grammar, but it shifts the state from the world itself to Anna's expectations about her possible future experiences.
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Christopher Fuchs and Rüdiger Schack, two of the central developers of Cubism, have argued explicitly that a quantum state does not represent an element of physical reality in the usual sense.
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Instead, it represents an agent's personal probability assignments concerning future experience. Later presentations of Cubism make the same point even more directly by treating the quantum state as the personal judgment of the agent using the theory.
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This means that asking for the true quantum state of a system can already be misleading if we imagine that the answer must be something written into the system independently of any agent.
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Cubism would first ask whose state assignment we're talking about and what experiences that agent has had.
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Imagine Anna prepares a system while Ben waits outside the room. Anna knows exactly what preparation procedure she used. Ben only knows that Anna selected one procedure from several possibilities.
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Anna and Ben therefore do not possess the same information. If quantum states were objective physical properties in exactly the same sense as mass or electric charge, we might expect there to be one correct state that both scientists ought to
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discover. Cubism does not require that. Anna can assign one quantum state based on her experience, while Ben assigns another based on his.
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When Anna later tells Ben what she did, Ben acquires new information and changes his assignment.
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The physical system did not need to transform merely because two scientists had a conversation in the corridor.
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What changed was Ben's expectation concerning what he might experience in future interactions. We already tolerate this kind of difference in ordinary probability.
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Imagine you and a friend are waiting to hear whether a particular football match has finished.
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Your friend secretly checks the result while you continue talking. A moment later, you still assign probabilities to the possible winner because you do not know the result. Your friend does not share your uncertainty.
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There is no contradiction. You're standing in the same room and talking about the same match, but the expectations available to each of you differ because your experiences differ.
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Personalist Bayesian probability treats probabilities as expressions of an individual's degrees of belief rather than physical ingredients attached to events.
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Cubism brings that understanding of probability into quantum mechanics and follows the consequences much farther than the simple football example can take us.
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This is where the letter B in the historical name quantum Bayesianism came from. Bayesian approaches to probability emphasize how beliefs should be organized and how they should respond to new information.
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Cubism adopts a particularly personal form of Bayesian probability. The probability belongs to the agent. It expresses the strength of that agent's expectation about what may happen.
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If Anna assigns a high probability to one outcome, that is a statement about Anna's confidence.
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It is not a claim that nature contains a physical quantity called probability stored inside the apparatus.
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Another agent with different information can assign a different probability without either person thereby creating a second universe.
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There is still discipline. Personal does not mean careless. Suppose you announce that you're almost certain it will rain tomorrow, completely certain it will not rain tomorrow, and equally certain that both predictions are reasonable.
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A patient friend may eventually suggest that your probability assignments need work. Ordinary probability theory places consistency requirements on beliefs.
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If your judgments contradict one another badly enough, they can lead to decisions that are guaranteed to work against you.
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The point of Bayesian reasoning is not that every belief is automatically sensible because someone happens to hold it.
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The point is that beliefs belong to individuals and can still be judged according to standards of coherence and evidence.
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Cubism adds quantum structure to this picture. An agent's probabilities are personal, but quantum mechanics imposes additional relationships among them.
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The theory tells the agent that expectations concerning different possible experiments cannot be chosen independently without restriction.
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In Cubist language, the quantum formalism is normative. That word sounds as if quantum mechanics has suddenly become a stern school teacher, but the meaning is straightforward.
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The rules guide how an agent ought to organize expectations if the agent wants those expectations to fit together in the way demanded by quantum experience.
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Cubism therefore keeps the mathematical discipline of quantum mechanics while changing the story we tell about what the mathematics represents.
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Later Cubist accounts explicitly identify this normative character as one of the interpretation's central principles. You might compare this with a navigation system.
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A map does not control the streets. It does not force a bridge to exist because the mapmaker drew one.
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It helps a traveler organize expectations about what will happen after choosing a route. A good map is constrained by the structure of the world. If it repeatedly tells you to turn left into a lake, confidence in the map should decline
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fairly quickly. Quantum theory, in the Cubist picture, is obviously far more sophisticated than a road map, but there is a similar separation between the guide and the territory.
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The quantum state is part of the agent's guide for navigating possible experiences. It is not automatically identical to the underlying territory.
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This distinction also changes what happens during measurement. In the familiar textbook language, a quantum system is described by a state, a measurement occurs, and the state changes.
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If we imagine the state as a physical object, we naturally ask what physical process caused the change.
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Did the state collapse when the detector clicked? Did it collapse when someone looked at the detector?
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Was there a special border between the microscopic system and the large measuring device? These questions produced much of the traditional measurement problem.
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Cubism does not attempt to explain a physical collapse because it denies that the quantum state was the kind of physical object that needed to collapse in the first place.
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For Anna, the state changes because Anna's experience changes. Before the measurement, she has one set of expectations.
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After the measurement, she has encountered a result. Naturally, her future probabilities are different. Imagine that Anna is expecting one of several possible detector readings. She performs the experiment and sees one particular result.
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Her new state assignment reflects the fact that she has learned something through interaction with the world.
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The update is real as an update in Anna's beliefs, but Cubism does not require an invisible physical wave spread across space to undergo a sudden mysterious transformation simply because Anna now possesses new information.
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There is a danger here of making the whole process sound like ordinary ignorance. And Cubism does not intend that either.
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Remember the sealed box containing a colored ball. Before opening it, you're uncertain about the color, but we comfortably imagine that the ball already has one definite color.
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The uncertainty exists only because you have not looked. Quantum measurement is not treated by Cubism as merely revealing a complete collection of ordinary pre-existing answers.
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The agent performs an action on the world and receives an experience in response. The outcome is not simply assumed to have been sitting there as a hidden classical value all along.
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This is one reason Cubists emphasize action rather than passive observation. The word measurement can encourage the wrong picture.
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When you measure the length of a desk, it seems obvious that the desk possessed its length before you arrived with a ruler.
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The ruler merely reveals something already present. Because physicists use the same word measurement in quantum mechanics, we may instinctively import that classical story into every quantum experiment.
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Cubism asks us to resist doing so. A quantum measurement is better understood within the interpretation as an action an agent takes upon the world.
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The resulting outcome is the agent's experience of the world's response. Fuchs described Cubism as a form of participatory realism precisely because it retains an external reality while denying that reality can always be understood as a complete third-person description that excludes the
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participating agent. Participatory does not mean that Anna controls what happens. Suppose she chooses to perform a particular measurement and strongly expects one result.
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Nature produces another. Anna cannot complain that the universe has violated Cubism because she personally preferred the first outcome.
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Her probability assignment was her own commitment, not an instruction issued to reality. The difference between choosing an action and choosing an outcome is fundamental.
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You may choose to ask someone a question, but you cannot choose the answer they will give.
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You may choose to open a door, but you do not thereby decide what is standing behind it.
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You may choose to check your bank balance after a holiday. Although some actions are admittedly easier to postpone than others, the external world therefore has an active role.
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It is not a blank screen onto which the agent projects beliefs. Cubism assumes something outside the agent that can answer actions with experiences the agent did not determine in advance.
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That is why the interpretations defenders reject the accusation that Cubism is simply solipsism, the idea that only one's own mind can be known to exist.
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The whole framework depends on an agent interacting with an external world capable of providing genuine novelty.
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Fuchs uses the phrase participatory realism to emphasize precisely this combination. There is reality, but agents participate in their encounters with it, rather than merely uncovering a completely finished list of properties from a detached viewpoint.
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Consider an ordinary conversation. You decide what to ask another person. Your question affects what kind of response becomes possible, but it does not determine the response.
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The other person contributes something of their own. Afterwards, the conversation contains something that did not exist for you before you asked the question.
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Cubism sees a quantum measurement in a somewhat comparable way. The measurement is an intervention chosen by the agent, and the outcome is something created in the encounter between agent and world.
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We should not take the comparison too literally because atoms are not tiny people waiting to answer questionnaires, which would make experimental physics considerably more awkward, but the comparison captures the idea that measurement is an interaction rather than the passive reading of a
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hidden label. This also helps explain why Cubism speaks so strongly about experience. In ordinary physics language, an outcome sounds like an impersonal fact written into the universe.
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Cubism deliberately brings the agent back into the sentence. Anna experiences the detector reading. Ben may later experience Anna telling him about it.
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Those are not identical events from the Cubist standpoint. Anna's experience belongs to Anna. Ben's later interaction with Anna belongs to Ben.
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Each agent uses their own experiences to update their own expectations. The theory is therefore sometimes described by Cubists as a single-user theory.
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Each user applies quantum mechanics for themselves based on their own situation and their own possible actions.
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That phrase can sound alarming if we hear single-user and imagine everyone sealed into an isolated private reality.
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The idea is more practical. Your weather forecast is also used from your perspective. You decide whether to carry an umbrella.
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Your friend makes a separate decision based based on their own expectations. You can share forecasts, compare experiences, and learn from each other, but nobody can literally make your decision from inside your own point of view.
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Cubism says something similar about quantum theory. An agent cannot borrow a non-existent perspective outside all agents and pretend to possess every experience at once.
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Each application of the theory is made from the standpoint of a particular user. There is a useful humility in that picture.
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Physics often encourages us to imagine what philosophers sometimes call a view from nowhere. An ideal description that contains every fact while belonging to no particular observer.
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Classical mechanics makes such a picture feel natural because we can describe planets, projectiles, and pendulums without mentioning who is watching them.
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Cubism suggests that quantum mechanics may be telling us that this style of description has limits.
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Perhaps a fundamental theory used by agents who are themselves parts of the universe cannot always erase those agents from the story.
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This is not the same as saying that objective science becomes impossible. Scientists already begin from individual experiences and still produce shared knowledge.
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One researcher records a detector signal. Another repeats the experiment. Laboratories exchange data. Instruments are calibrated. Procedures are described so other people can perform similar actions and compare what happens.
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Personal experience does not prevent public science because agents can interact. They can communicate, influence one another's expectations, and construct stable agreements through repeated encounters with the same external world.
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Imagine Anna performs an experiment every morning for a year. Ben performs the same type of experiment in another laboratory.
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The individual outcomes may vary, yet their accumulated experiences reveal stable statistical patterns. They compare results and discover remarkable agreement about the expectations that work.
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Cubism can accommodate that without saying the probabilities existed as objective numerical properties before either scientist arrived.
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The world provides enough regularity that rational agents exposed to similar evidence can often move toward similar expectations.
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Their beliefs remain personal in ownership while becoming highly constrained by shared experience. Something similar happens outside physics.
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Two experienced doctors may independently become confident about the likely course of an illness because years of evidence have trained their expectations.
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Their confidence belongs to each doctor, but the agreement is not accidental. Both have been shaped by evidence from a common world.
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If new evidence appears, both may revise their expectations. Personal judgment and objective constraint can coexist.
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Cubism claims quantum theory should be understood through a comparable separation, although with uniquely quantum rules governing how expectations fit together.
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This becomes even clearer when we consider certainty. If Anna assigns complete confidence to one possible outcome, ordinary language tempts us to say that the corresponding result must already be an objective fact.
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Cubism refuses that inference. Complete certainty is still Anna's judgment. It tells us how strongly she's committed to an expectation, not that she has reached through the mathematics and uncovered an element of reality guaranteed to exist independently of her.
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Modern summaries of Cubism emphasize this explicitly. Even probability assignments of complete certainty are treated as judgments rather than direct pieces of ontology. That is a much stronger claim than simply saying people sometimes lack information.
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Suppose Anna has prepared a system in a way that makes her completely confident about what she would experience if she performed a certain measurement.
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She may be willing to bet everything on that expectation. Cubism still says the certainty belongs to Anna.
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If she never performs that measurement, the interpretation does not require us to imagine that the unperformed experiment nevertheless possessed a hidden outcome waiting in the background.
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This connects Cubism with the broader lesson that quantum mechanics does not comfortably support the idea of assigning ordinary pre-existing answers to every possible measurement.
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Now we can see why the agent is doing more work than an ordinary observer.
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An observer sounds passive, like someone sitting quietly in the back row watching nature perform.
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An agent chooses actions. The choice matters because different possible actions correspond to different possible experiences.
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Anna must decide which experiment to carry out. Quantum theory helps her connect the probabilities associated with those choices.
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The world then answers the action she actually performs. An action she never takes does not produce an experience for her.
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This makes Cubism feel less like a theory about objects viewed from far away, and more like a theory about encounters.
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The external world is not erased. The agent is not erased, either. Between them lies the event in which an action meets a response.
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For Cubists, this is not an unfortunate contamination of pure physics by human involvement. It may be a clue about what quantum mechanics has been trying to tell us all along. There is still a natural objection. Surely physics should
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tell us what exists, not merely how we should bet on our experiences. If Cubism tells Anna how to organize expectations, but refuses to identify the quantum state with something objective, has it actually explained the physical world, or has it retreated into a manual
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for prediction? This criticism has followed Cubism for years. The response from its advocates is that removing the quantum state from objective reality does not mean abandoning realism.
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It means searching for reality somewhere deeper than the mathematical objects we may have mistaken for reality.
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Fuchs has repeatedly argued that Cubism is intended as a realist project, but one in which reality cannot be exhausted by a detached third-person account.
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In that sense, Cubism performs an unusual act of restraint. Instead of taking every mathematical ingredient of quantum mechanics and declaring it physically real, it asks which parts may belong to the user and what remains to be learned about the
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world once those personal elements are removed. The quantum state is assigned by the agent.
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Probabilities belong to the agent. The choice of measurement belongs to the agent. The outcome is experienced by the agent.
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Yet, the agent does not control the outcome. Something external contributes the surprise. For cubism, that stubborn ability of the world to produce something new may be closer to the physical heart of quantum theory than the state we write in a
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calculation. We can return to Anna one more time. She prepares a system, assigns a state, considers several possible measurements, chooses one, forms expectations, acts, receives an outcome, and changes her expectations.
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Then, she acts again. Her scientific life becomes a continuing sequence of encounters. There is no moment when she possesses a complete view of everything from outside the universe.
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She's always embedded within the situation, working with what she's experienced and what she believes may happen next.
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Ben is doing the same thing. His experiences are not Anna's experiences. His probability assignments are not automatically Anna's assignments.
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When they communicate, each communication becomes another physical interaction from which new experiences arise. Over time, they can agree extraordinarily well.
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They can develop common methods, build common instruments, and discover common patterns. Their shared science grows not because either one escaped their personal perspective, but because reality repeatedly constrains what works.
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This picture is far removed from the idea that reality is merely a matter of opinion.
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If anything, it gives the external world a particularly direct role. Every experiment is a risk because the agent does not possess the future outcome in advance. Every interaction offers the possibility of surprise.
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A theory helps the agent make better expectations, but no theory allows the agent to order nature around.
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When the detector gives an unexpected result, the sensible response is not to remind the detector that probability is personal.
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The sensible response is to learn. That emphasis on learning is one reason Cubism fits naturally with Bayesian probability.
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The agent begins with expectations formed from previous experience. New experience arrives. Expectations change. Then another action becomes possible.
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Science proceeds through this constant cycle of judgment, intervention, surprise, and revision. Quantum mechanics provides a particularly strict framework for keeping those judgments coherent in situations where classical expectations fail.
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Yet one object has quietly appeared throughout this discussion without receiving the attention it deserves.
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Anna assigns it. Ben may assign a different one. It changes when experience changes. Physicists use it constantly to make predictions, and entire debates about quantum reality have been built around the question of what it represents.
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If Cubism is correct that this object belongs to the agent rather than the physical system itself, then one of the most familiar pieces of quantum mechanics has been wearing the wrong name tag for a very long time.
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That object is the wave function. And once we ask whether it describes the world or describes an agent's expectations about the world, the difference between conventional quantum language and the Cubist picture becomes impossible to ignore.
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The wave function sits at the the of this disagreement because it is one of the most useful objects in quantum mechanics, and also one of the easiest to misunderstand.
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Physicists use it to calculate what they should expect when they perform measurements on quantum systems.
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Give a physicist the appropriate wave function, tell them what measurement is being considered, and quantum mechanics provides a way to assign probabilities to the different outcomes that might be experienced.
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This procedure has worked with extraordinary reliability. The difficulty begins when we ask what the wave function itself represents.
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Is it describing something that physically exists in the world, something possessed by an electron or an atom whether anyone knows about it or not?
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Or is it describing what a particular agent believes about the possible consequences of interacting with that system?
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Cubism gives a clear answer. The wave function is not treated as an element of physical reality. It represents the personal probability judgments of the agent who assigns it.
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That claim becomes more significant when we remember how naturally we speak about quantum states.
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We say that an electron is in a particular state, that an atom has a particular wave function, or that a system evolves from one quantum state to another.
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The grammar encourages us to imagine that the state belongs to the object in roughly the same way that its mass or location might belong to it.
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If a suitcase has a mass, that mass does not depend on whether Anna or Ben knows it.
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If a planet has a certain diameter, the diameter does not change because one astronomer has better information than another.
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We are accustomed to physics being about properties that objects possess independently of what anyone believes.
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It is therefore very tempting to place the wave function into the same category, and imagine that quantum mechanics has discovered another unusual property carried by microscopic things.
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Cubism asks us to be careful about that step. The fact that a mathematical object is useful for making predictions does not automatically mean that the mathematical object must correspond directly to something physically sitting inside the world.
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A map is useful because it organizes information about a landscape, but the blue line representing a river is not another river.
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A weather forecast can be extremely accurate without the probability of rain existing as a separate object somewhere in the atmosphere.
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A medical risk estimate can guide an important decision without the percentage appearing physically inside the patient.
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In each case, there is a difference between the world and the tool used by someone trying to reason about the world.
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Cubism says that the wave function belongs on the tool side of that distinction. Return to Anna in the laboratory.
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She has prepared a quantum system and based on everything she's experienced, she assigns a wave function to it.
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That assignment allows her to calculate probabilities for the results of different actions she might perform.
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She might choose one measurement or she might choose another. Each possible action leads her to consider a different collection of possible experiences.
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The wave function helps her keep those expectations organized according to the rules of quantum mechanics.
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From the Cubist point of view, however, Anna should not imagine that the wave function is a hidden object attached to the system. It is Anna's state assignment. Another agent can assign another state if that agent has different information.
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Suppose Ben arrives after Anna has completed the preparation, but before she has performed the next measurement.
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Anna knows exactly what she did. Ben only knows that she selected one of several preparation procedures.
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Because their experiences differ, their expectations can differ. Anna may assign one wave function, while Ben assigns another.
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If there had to be one objectively correct wave function physically belonging to the system, this would seem troubling. The system cannot surely possess two incompatible physical conditions merely because two people standing nearby know different things.
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In Cubism, there is no such problem because the wave functions belong to Anna and Ben as expressions of their respective expectations.
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There is still one external system with which they may interact. What differs is the information each agent brings to the encounter.
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Ben can later ask Anna what preparation she used. She tells him. Ben now has a new experience, namely hearing Anna's answer, and he changes his state assignment accordingly.
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Nothing mysterious needs to happen to the quantum system because Ben learned something. His description changes because his expectations change.
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This is one of the reasons Cubists resist treating the wave function as an objective physical object.
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If a state assignment can depend on what a particular agent has experienced, then changes in the assignment need not correspond to sudden physical transformations in the distant system itself.
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The distinction may remind you of an ordinary deck of cards. Imagine that Anna draws a card, looks at it, and places it face down on a table.
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Ben has not seen the card. Anna knows exactly what she saw. Ben assigns probabilities to the different possibilities.
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When Anna tells him the card, his probability assignment changes immediately. Nobody asks what physical mechanism caused the card to transform when Ben learned the answer because everyone understands that the uncertainty belonged to Ben.
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The card had not been fluctuating among different identities while waiting for him to receive the news. Quantum mechanics cannot simply be reduced to that classical example, however.
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This is where it is important not to make QBism sound too easy. A hidden card already possesses a definite identity.
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Ben's uncertainty reflects ordinary ignorance about an answer that exists independently of him. Quantum mechanics does not generally permit us to imagine that every possible measurement has a complete set of ordinary hidden answers waiting to be revealed.
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The structure of quantum experiments is more restrictive than that. QBism, therefore, does not say that the wave function is merely ignorance about a perfectly classical microscopic reality.
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It says that the wave function represents the agent's expectations while remaining cautious about what underlying description of the world should replace it.
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This caution is sometimes frustrating because we naturally want to know what is actually there.
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If the wave function is not a physical object, perhaps we would like QBism to hand us another picture immediately.
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Maybe there are tiny particles carrying hidden instructions. Maybe there are invisible waves moving through space.
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Maybe there is some deeper mechanism that produces the quantum statistics. QBism does not provide that sort of complete mechanical picture.
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It's first move is more modest. It says that before deciding what reality must contain, we should stop mistaking an agent's predictive machinery for the reality being predicted.
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There is a useful lesson here from other parts of science. Temperature is a perfectly meaningful property at the level of ordinary objects, but our microscopic explanation of temperature involves the collective behavior of enormous numbers of particles.
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A line on a graph can represent the growth of a population without being a physical object living inside the population.
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Coordinates can describe a location without becoming ingredients of the landscape. Mathematics is powerful partly because it allows us to represent relationships, expectations, and structures without requiring every mathematical symbol to correspond to a separate physical thing.
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Cubism argues that the wave function should be understood with similar restraint. The difficulty is that many interpretations of quantum mechanics do take the wave function very seriously as part of physical reality.
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If the wave function is real in that stronger sense, then the strange behavior of the mathematical state becomes something the world itself must somehow be doing.
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This leads directly back to the problem of measurement. Before a measurement, the wave function may support several possible outcomes.
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After the measurement, the experimenter experiences one result and uses new state for future predictions.
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If the wave function is an objective physical entity, what caused that abrupt change? Did the wave physically collapse?
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If so, where did it collapse? When did it collapse? And what counts as the event that triggers the collapse?
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These questions become particularly uncomfortable if the quantum system is spread over a large region.
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A change in the state used to describe it can appear to occur immediately once a measurement result is obtained.
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If the state is merely an agent's expectations, there is less mystery in such an immediate update.
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Beliefs can change immediately when new information arrives without requiring a physical signal to race through space.
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You can learn on the telephone that a friend in another country has arrived safely and instantly, revise your expectations about what they are doing.
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Your change of belief does not cause anything to travel from your mind to the distant airport.
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Again, Cubism is not claiming that nothing physical happened at the airport or in the laboratory. Anna performs an action and receives an outcome.
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The outcome matters because it was not determined by Anna's prior belief. The world contributed something.
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What Cubism denies is that the mathematical update of Anna's wave function must itself be interpreted as a physical object undergoing a mysterious transformation.
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The update belongs to Anna's changing expectations after experience. This becomes even clearer when several agents are involved. Suppose Anna measures a system and records an outcome while Ben remains outside the laboratory.
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Anna now knows what happened to her. Ben does not. Anna changes the wave function she assigns when thinking about what she might experience next.
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Ben may continue using a different state because he lacks Anna's result. If the state is personal, both descriptions can be legitimate from the standpoint of the agents assigning them.
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Once Ben talks to Anna and learns the result, he updates his own assignment. The difference between them gradually disappears as their experiences become more similar.
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If the wave function were instead a single objective object that everyone had to describe identically at every moment, this situation would demand another explanation.
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We would need to decide exactly when the objectively real state changed and why Anna and Ben appear to have different descriptions.
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Some interpretations provide answers by changing the larger picture of reality. Cubism avoids the demand for one observer-independent wave function because it never gave the wave function that role to begin with.
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One consequence is particularly surprising. Even a wave function associated with complete certainty is still an agent's judgment in Cubism.
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Suppose Anna assigns complete confidence to a particular future outcome. It is tempting to conclude that the outcome must correspond to an objective property already possessed by the system.
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Cubism does not permit that automatic step. Complete confidence tells us something about how strongly Anna is willing to commit herself to an expectation.
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It does not by itself establish that the predicted result exists as a pre-written fact before the relevant action is performed.
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This point is explicitly included among the central principles of modern Cubist presentations. That may feel almost unreasonable.
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If Anna is completely certain, surely something must guarantee her prediction. Yet ordinary life already teaches us that certainty and reality are different categories.
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People can be absolutely certain and still be wrong. More importantly, even when certainty is justified, the psychological fact of being certain is not itself the physical cause of the event.
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You can be completely certain that the sun will rise tomorrow without imagining that your confidence is what pushes the planet through its rotation.
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Cubism makes a stronger technical point than this everyday example, but the distinction helps. A probability assignment belongs to an agent.
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Moving that probability all the way to complete certainty does not suddenly transform it into an objective physical property.
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The agent has reached the strongest possible commitment available within their expectations. The world will still be encountered through an actual interaction.
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This is also why Cubism refuses to treat the wave function as a hidden catalog of outcomes for experiments that were never performed.
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Imagine Anna could choose between several different measurements. She actually performs only one of them.
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Classical intuition encourages us to imagine that the answers to all the alternatives existed anyway.
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Had she chosen the first experiment, one answer was waiting. Had she chosen the second, another answer was waiting. Had she chosen the third, a third answer was already sitting somewhere in the system.
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Quantum mechanics gives us good reasons to be suspicious of that picture. And Cubism takes the suspicion seriously.
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An action that was not performed produced no outcome for the agent. Anna can assign probabilities to what she might experience if she chooses that action. But probabilities for hypothetical experiences automatically be converted into a secret list of facts.
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Think of a conversation again. Before asking a friend a question, you may have expectations about how they will answer. If you ask a different question, you have different expectations.
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It would be strange to imagine that all possible conversations already exist inside your friend as completed transcripts. And that speaking merely reveals the correct page.
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The answer emerges from a particular interaction. The analogy is imperfect because human conversation involves psychology, language, and many complexities absent from a simple quantum experiment.
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But it captures the Cubist resistance to imagining every possible measurement result as a stored property waiting for discovery.
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The wave function therefore becomes less like a portrait and more like a guide to possible encounters.
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It tells Anna how to organize expectations concerning the experiences that different actions may bring.
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It does not tell her that every possible experience already exists somewhere. This is one reason Cubism describes quantum mechanics as normative rather than directly descriptive.
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The theory gives rules connecting an agent's probability assignments. It tells the agent how expectations concerning different possible actions should fit together if they are to respect the empirical structure revealed by quantum physics.
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The important point is that these constraints are not invented by Anna. She cannot choose whatever relationship among probabilities happens to appeal to her.
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The quantum formalism has been shaped by experiments. Generations of physicists have interacted with the world and discovered patterns that classical probability alone does not capture.
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Cubism treats those patterns as guidance that any agent should respect when reasoning about quantum experiences.
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The probabilities themselves remain personal, but the structure connecting them reflects something learned from the world.
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This gives us a useful answer to the objection that a personal wave function would make quantum mechanics arbitrary.
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Imagine several navigators using maps of the same unfamiliar coastline. Each navigator may begin with different information and therefore draw a somewhat different map.
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Yet repeated journeys constrain them. Rocks damage boats regardless of the cartographer's opinions. Harbors repeatedly appear in the same places.
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Over time, agents who take experience seriously are pushed toward increasingly effective expectations. Their representations are personal in the sense that each agent uses their own map, but the world imposes strong limits on which maps remain useful. In Cubism,
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the world plays a similar role without being identified with the wave function. Agents can disagree initially. They can exchange information. They can perform experiments. They can discover that some expectations work and others fail.
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Science remains possible because personal judgments are continuously tested against a world that does not belong to any one person.
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The personal character of the quantum state therefore does not imply that experimental evidence is personal in the casual sense of being optional.
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There is a more subtle question hiding underneath this. If the wave function belongs to the agent, then what features of quantum theory belong to the world?
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Qubists have spent considerable effort thinking about that problem, because the interpretation is not satisfied with saying that everything is subjective.
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Its advocates want to separate the parts of the formalism that arise from an agent's judgments from whatever structure may reveal something objective about nature.
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The hope is that once the personal elements are identified correctly, the remaining features of quantum theory may provide clues about the character of the external world.
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This is one reason the interpretation continues to develop, rather than stopping with the slogan that the wave function is belief.
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Calling it belief is only the beginning. We still need to understand why rational agents living in this particular universe should obey specifically quantum rules.
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We still need to understand why the dimension associated with a quantum system matters. We still need to understand why interactions produce the patterns observed in laboratories.
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Removing the wave function from the list of objective physical things does not eliminate the scientific problem.
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It changes which problem we think needs to be solved. A helpful comparison is money.
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The number written in your banking application represents something important about your relationship with a financial system.
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But the number is not a pile of physical coins hidden inside your phone. If the number changes after a purchase, nobody imagines that tiny currency symbols physically collapsed inside the screen.
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The number is part of a representation governed by rules connecting your actions with a larger external structure.
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This does not make your bank balance unreal, particularly when rent is due. It simply means that confusing the representation with a physical object would produce the wrong kind of explanation.
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The wave function is obviously far more fundamental and mathematically sophisticated than a bank balance.
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But Cubism asks for a comparable act of conceptual discipline. Do not assume that because the wave function changes, some matching physical substance must have changed in exactly the same way.
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First ask what role the wave function plays for the agent using it. If it represents expectations, then it's update after new experience is exactly what we should expect from a tool whose job is to organize expectations.
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The strongest resistance to this idea usually comes from the desire for an observer-independent description of nature.
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Surely, we might think the electron is doing something whether Anna is there or not.
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Cubism does not need to deny that. The external world exists independently of Anna. What it denies is that Anna's wave function should automatically be identified with the complete description of what that external reality is.
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There can be a real electron without the wave function being the electron's private instruction manual.
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This difference between reality and description is easy to say, but difficult to maintain because our scientific language constantly blends the two.
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We speak of the state of a system when, in Cubism, we should more carefully speak of the state an agent assigns.
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We speak of the state collapsing when Cubism would describe the agent updating expectations. We say that the system is in a superposition, while the Cubist wants us to remember that the mathematical expression summarizes the agent's probability judgments about possible experiences.
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The interpretation therefore asks for a substantial change in habit before it asks for any change in calculation.
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The calculations remain the familiar ones. A physicist working as a Cubist and a physicist using another interpretation can often perform exactly the same laboratory calculation and predict exactly the same statistical results.
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The disagreement appears when they explain what the mathematical object mean. One may say that the wave function is physically real.
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Another may say it represents an ensemble or a relation. The cubist says it is the agent's personal judgment.
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The experiment may not distinguish these interpretations directly because they can share the same operational predictions. The argument concerns what kind of picture we should place behind those successful predictions.
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That can make foundational debates seem strangely detached from ordinary physics. If everyone predicts the same detector readings, why argue about the meaning of the wave function at all?
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The answer is that interpretation shapes the questions we think are worth asking. If the wave function is physically real, we may search for the mechanism behind its collapse or its branching.
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If it is personal, those particular problems change form and different questions become urgent. Why does the external world force agents to organize expectations according to quantum rules?
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What exactly is created in a measurement interaction? How should two agents relate their separate experiences?
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What features of a quantum system can be regarded as genuinely objective if its wave function is not one of them?
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This brings us back to the basic image of Anna standing beside her apparatus. Before acting, she assigns a wave function.
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That wave function summarizes how she expects different possible actions to turn out. She chooses one action.
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The external world responds with an experience that she could not simply decree in advance.
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Afterward, she changes her wave function because her situation has changed. The mathematics follows her expectations through the encounter, but cubism refuses to identify the mathematics with a hidden object moving behind the scenes.
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Ben can tell a different but compatible story from his own position. He assigns his own state, makes his own choices, and updates after his own experiences.
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When Anna and Ben interact, each gains new information and can revise accordingly. No universal wave function needs to descend from above and announce which observer possessed the official perspective all along.
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Each agent uses quantum mechanics from within the world. This point, the wave function has lost one familiar job. It is no longer being asked to tell us directly what the system is in itself.
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That leaves the actual measurement outcome with much more weight. The outcome is not merely the unveiling of a value contained in the wave function, because the wave function was an expectation rather than a container of hidden facts.
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Something occurs when the agent acts on the world. Something new enters the agent's experience. The interaction matters in a way that cannot be reduced to opening a drawer and reading a label.
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Once that is taken seriously, measurement begins to look less like observation and more like participation.
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The agent chooses what kind of encounter to initiate. The world supplies a consequence that cannot be fully controlled, and the experience changes what the agent can reasonably expect afterward.
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The wave function guides the agent toward that encounter, but it does not stand behind the outcome as a complete script already written in advance.
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Understanding exactly what cubism means by that interaction is where the next difficulty begins. Because if measuring the world is not simply discovering a property that was already there, then asking a question of nature may be doing something more interesting than merely
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reading the answer. Once the wave function is moved away from the system and placed with the agent who assigns it, measurement begins to look very different.
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The familiar word encourages us to picture something simple. We imagine a property already sitting in the world while an instrument comes along and reveals it.
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A ruler measures the length of a table. A thermometer measures the temperature of water. A bathroom scale delivers information that some of us would occasionally prefer it kept to itself.
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In each case, the ordinary assumption is that the measurement uncovers something that was already there.
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The table had a length before the ruler arrived. The water had a temperature before anyone looked at the thermometer.
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Nothing about the act of checking seems necessary for the property itself to exist. Quantum mechanics has always made this picture difficult to apply without qualification, and Cubism responds by changing what the word measurement means in the first place.
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For a QBist, a quantum measurement is not fundamentally the passive inspection of a property that has been waiting for discovery.
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It is an action performed by an agent upon the external world. The agent chooses what to do. Quantum mechanics helps the agent organize expectations about the possible consequences, and then the world responds with an outcome that becomes part of that
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agent's experience. The outcome is not treated as merely reading a value that quantum theory had secretly stored somewhere beforehand.
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In the QBist picture, something new occurs in the encounter. Fuchs and Schack have described quantum measurement precisely in this way, as an action whose result is a new experience for the agent, rather than a comparison with something simply assumed to
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preexist the act of the measurement. Imagine Anna returning to her laboratory. On On table in front of her is a quantum system she has prepared carefully.
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Beside it is an apparatus that can be arranged in several different ways. Anna has to decide what she wants to do.
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She could perform one measurement, or she could change the apparatus and perform another. Before touching anything, she uses quantum mechanics to form expectations about what might happen under each choice.
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Those expectations belong to Anna. They depend on the wave function she assigns, the measurement she's considering, and everything she has learned from her previous experience.
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Then Anna chooses one action and performs it. A detector responds. She sees a result.
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At that moment, she has something genuinely new to work with. The result becomes part of our experience, and her expectations about future actions can change.
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Notice how different this is from imagining Anna as a spectator standing outside nature. She's not simply waiting for the system to announce a fact it possessed all along. She has to do something.
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Her choice of action determines which kind of encounter takes place. Had she chosen a different experimental arrangement, she would have created a different situation and faced a different collection of possible experiences.
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This does not mean that Anna gets to choose the result. She chooses the action, not the answer.
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That distinction is essential. You can choose to ask someone whether they remembered your birthday, but you cannot choose the expression that appears on their face before they answer.
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The question is yours. The response is not. This is the sense in which asking the universe becomes part of the answer.
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The phrase should not be taken to mean that nature understands human questions or waits for a physicist to speak before deciding what to do.
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Quantum systems are not tiny contestants on a quiz show. The point is more practical.
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The physical action selected by the experimenter helps to find the situation in which an outcome occurs.
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Different experimental arrangements correspond to different possible interactions. Quantum mechanics does not give Anna a single enormous list containing answers to every measurement she might possibly perform.
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It gives her a way of organizing expectations for the consequences of the actions she could choose.
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Classical intuition makes this difficult because we're used to assuming that every sensible question has an answer whether we ask it or not.
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Imagine a wooden box containing several objects. You could ask how many objects are inside, what color they are, how much they weigh, or which one is closest to the left wall.
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Even if you never open the box, it feels natural to assume that all those answers already exist simultaneously. The box does not need you. Measurement merely reveals part of a completed description.
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We carry that intuition into quantum physics and imagine that every possible experiment must also reveal something the system already possessed.
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Quantum theory has repeatedly warned us that this classical picture cannot simply be assumed. Cubism takes the warning seriously.
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If Anna considers three different measurements, but performs only one of them, she may assign probabilities to the results she would experience under all three possible choices.
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Those probabilities help her decide what to expect. Yet, the interpretation does not require her to imagine that the outcomes of the two unperformed measurements exist as hidden facts somewhere inside the system.
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They remain possibilities associated with actions she could have taken. Only the action she actually performs produces an experience for her.
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This idea can sound as though the human choice creates reality from nothing, which is not what Cubism claims.
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There is an external world confronting Anna. Her apparatus is physical. The system is physical. The interaction is physical.
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Anna cannot force the detector to show whatever result she likes. She can prepare carefully, calculate carefully, and have extremely strong expectations, only to receive an outcome she considered unlikely.
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The existence of surprise is one of the clearest signs that the world is contributing something independent of Anna.
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Cubism places the agent inside the story without placing the agent in command of it.
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The difference can be compared with opening a conversation. Suppose you meet someone you have never spoken to before.
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You can decide what question to ask. Your choice matters because different questions invite different responses.
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Asking where someone grew up creates one kind of conversation. Asking whether they have ever tried to assemble flat-pack furniture without reading the instructions creates another, probably accompanied by a longer pause.
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Your question influences the encounter, but the other person contributes something you did not control.
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Their answer is not simply your question reflected back at you. Cubists use a similar general idea when they talk about measurement as an interaction between agent and world. The agent contributes the action. The world contributes something through the
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outcome. The resulting experience is new for that agent. This is one reason Fuchs has described Cubism as a form of participatory realism.
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Reality is not discarded, but neither is the agent treated as an irrelevant spectator who can be removed from every description without consequence.
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The word experience can make people uncomfortable because it sounds psychological, and physics usually tries to avoid psychological language.
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If a detector clicks, surely the click is a physical event. Why bring experience into it?
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Cubism is not denying the physical character of laboratory interactions. It is emphasizing the standpoint from which quantum theory is used.
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Anna assigns probabilities to what she may experience as a consequence of an action. When the detector responds, Anna receives a result that enters her future reasoning.
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The theory is being applied by Anna for Anna's expectations. Another agent applies the theory from another standpoint.
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This also helps with the old question of wave function collapse. Before Anna performs the measurement, she has a wave function representing her expectations.
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After she sees the outcome, she assigns a new one. If the wave function is an objective physical object, that abrupt change looks like a strange event requiring a mechanism.
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Something in nature apparently jumped from one mathematical condition to another at the moment measurement occurred.
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Physicists then have to ask where that jump happened, how quickly it happened, and what exactly counts as a measurement strong enough to cause it.
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Cubism removes much of that problem by refusing the starting assumption. The wave function was Anna's assignment. It summarized her expectations before the experiment.
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Once Anna experiences a result, of course, her expectations change. The update in the wave function represents that change.
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There is no need to imagine an invisible physical substance collapsing because Anna learned something.
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The physical event was the interaction and the experience it produced. The mathematical update belongs to Anna.
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Think about waiting for a train again. You believe the train is probably several minutes away. Then you hear it entering the station behind you. Your expectations change immediately.
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Nobody asks whether the train's physical condition collapsed when you heard the sound. The train was doing whatever it was doing while your information changed.
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Quantum mechanics is not merely classical ignorance about a train, but the example shows why an instantaneous change in expectation does not require an instantaneous physical transformation of the thing being considered.
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The quantum case goes further because the outcome is not treated by Cubism as simply revealing a hidden value that had already been fixed in advance.
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The measurement experience is something arising from the particular interaction. Before Anna acts, she has expectations about possible consequences.
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Afterward, she has one actual experience. This makes measurement creative in a limited and specific sense.
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Something occurs in the world that was not merely a line Anna was entitled to regard as already written in a secret ledger.
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That is a stronger statement than saying Anna did not know the answer. If you place a coin underneath your hand and I do not know whether it shows heads or tails, my uncertainty says nothing profound about the coin.
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One side is already facing upward. I simply lack the information. If you lift your hand, I discover the answer.
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Cubism does not treat every quantum experiment as that kind of discovery. The outcome of a measurement is associated with the actual encounter between agent and world.
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Fuchs and Schack emphasize this creation of something new as an important part of their account of quantum measurement.
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Naturally, this raises the question of what exactly is created. If Anna sees a particular detector result, has she created a property of the system? Has the apparatus created it? Has the interaction created an event?
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Cubism is cautious about translating the outcome into an observer-independent property that supposedly existed before the measurement.
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The safest statement within the interpretation is that Anna has acquired a new experience resulting from her action on the world.
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That experience is real for Anna and becomes part of the information she uses in future decisions.
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There is nothing casual about the phrase real for Anna. Suppose Anna's detector gives a result she did not expect. She cannot simply decide that she dislikes it and substitute another one.
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Her experience constrains her. If the result repeatedly conflicts with her expectations, she must reconsider those expectations.
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Perhaps her preparation was poor. Perhaps her apparatus behaves differently than she thought. Perhaps her model is wrong. The world is capable of teaching Anna precisely because the outcome is not hers to select.
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This makes scientific experimentation look almost like a conversation in which nature is the participant who never explains its reasoning.
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We prepare a situation, decide what interaction to initiate, predict what might happen, and receive a response.
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Then we adjust our expectations and ask again. Sometimes the responses form patterns quickly. Sometimes they force generations of physicists to argue for a century.
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Quantum mechanics falls rather firmly into the second category. The agent's choice also explains why Cubism treats the measurement itself as part of the agent's probability assignments.
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Anna has to describe what action she intends to perform and what possible experiences she associates with it.
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These descriptions are part of the framework she uses to reason. Cubism does not imagine an entirely objective measurement procedure floating independently of every agent while one universal quantum state waits to be processed by it.
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The theory is used from a particular perspective for a particular contemplated action. This does not imply that laboratory procedures become arbitrary.
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If Anna says she's measuring one thing while Ben looks at the apparatus and says she has connected the equipment backwards, the interpretation does not magically make both descriptions equally effective. Agents live in a world that constrains them. Experience,
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communication, calibration, and repeated testing allow physicists to develop reliable shared practices. A laboratory instruction can become extremely stable and reproducible, even though individual agents use it from their own perspectives. Suppose Anna and Ben build identical experimental arrangements in neighboring rooms. They
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repeat the same preparation and measurement thousands of times. Each receives individual experiences as the experiments proceed.
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When they compare their records, stable statistical regularities emerge. They can agree that quantum theory provides excellent guidance for the expectations associated with this class of experiments.
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Personal outcomes do not prevent shared science because Anna and Ben can interact, communicate, and expose their expectations to the same stubborn external world.
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The phrase personal outcome therefore does not mean private fantasy. Your experience of hearing a thundercloud is personal in the simple sense that you are the one who hears it.
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Another person nearby can hear it, too. But their hearing is their experience. Afterwards, you can both compare what happened.
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You may agree closely because both experiences arose from interactions with the same world. The fact that neither of you possesses the other's first-person experience does not stop you from developing a common description of the storm.
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QBism pushes this distinction into quantum physics. Anna experiences her measurement outcome. Ben does not automatically acquire Anna's experience merely because he is another physicist somewhere in the universe.
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If he wants to know what happened, he must interact with Anna or with some record of the experiment.
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He can walk into the laboratory, read her notebook, examine the detector, receive a message, or ask her directly.
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Whatever method he chooses, something has to happen to Ben before Anna's result becomes part of Ben's experience.
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That sounds almost trivial until we ask how Ben should describe Anna before speaking to her.
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Anna has already performed the experiment. From her standpoint, she saw one definite result. Ben, however, has not learned which result she obtained.
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He must form expectations based on his own situation. Cubism says he cannot simply borrow Anna's experience as though all information belonged to a single universal observer.
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His quantum state assignments are his own, just as Anna's are hers. Here, we begin approaching a situation where the personal nature of quantum measurement becomes much harder to ignore.
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Imagine Anna is not merely standing beside a detector while Ben waits down the corridor. Put Anna, the detector, and the quantum system together inside a completely isolated laboratory.
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Anna performs her measurement and sees a definite outcome. Ben remains outside with no information about what happened inside.
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For Anna, there is no ambiguity about her experience. She looked at the apparatus and obtained a result. She may update her expectations immediately.
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Ben has not had that experience. From his perspective, the entire sealed laboratory is something external to him, something on which he may later choose to act.
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If Ben is an extremely ambitious experimental physicist with equipment far beyond anything currently practical, he might even attempt to treat the whole laboratory, including Anna, as part of a larger quantum experiment.
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Now, the comfortable idea of measurement begins to strain. Anna has an outcome. Ben has not yet encountered that outcome.
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Anna assigns probabilities based on what she experienced. Ben assigns probabilities based on what he experienced.
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Neither agent can simply claim access to the other's first-person perspective. If quantum theory is genuinely a tool used by each agent for their own future experiences, their descriptions do not have to coincide at every stage.
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This is exactly the kind of situation that makes people worry that Cubism has gone too far.
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If Anna says a measurement produced one result while Ben still uses a description containing several possibilities, are there now two realities?
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Does Anna's fact fail to be a fact for Ben? What happens when they eventually meet and compare notes?
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Could they disagree about what occurred inside the laboratory, or does the structure of their later interaction force their accounts into agreement?
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Cubism's answer requires more care than simply saying everyone has their own truth. That phrase would badly misrepresent the interpretation.
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Anna and Ben share an external world and can interact with one another. Their experiences are personal, but they are not disconnected.
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Ben can ask Anna what she saw. Anna can answer. The answer becomes part of Ben's experience.
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Their expectations change through communication, just as they change through any other physical interaction. The difficult part is accepting that physics may not provide a single perspective from which Anna's experience and Ben's expectations are simultaneously collected into one privileged account belonging to nobody.
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Each agent has to speak from somewhere. Each application of quantum mechanics belongs to an agent who has some experiences and lacks others.
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The hope that we can step completely outside all these perspectives and write down one quantum state representing reality exactly as it is for everyone may be precisely the assumption Cubism wants us to reconsider.
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Measurement therefore becomes more than a technical procedure inserted into the theory after the interesting physics has already happened.
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It becomes the point where an agent meets a world that is not fully under that agent's control.
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The agent chooses the action. The world contributes the consequence. The outcome enters the agent's experience. The agent updates expectations and continues.
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There is no magical consciousness forcing matter into existence, but there is also no detached spectator simply reading values from a finished cosmic spreadsheet.
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Once two agents become involved, however, the situation becomes much more demanding. Anna can treat a detector as something external to herself.
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Ben can treat Anna and her detector as things external to himself. Anna's measurement outcome belongs to her experience, while Ben may still be uncertain about it.
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When Ben finally opens the laboratory door and asks what happened, his own interaction creates the next experience in his story.
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If quantum mechanics is supposed to guide each agent separately, we must understand how those separate stories can meet without producing contradiction.
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And that brings us directly to one of the most famous observer puzzles in quantum theory.
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Where a physicist inside a laboratory sees one result, while another physicist outside tries to describe the entire situation from a different point of view.
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The sealed laboratory gives us exactly the kind of situation that makes quantum mechanics difficult to talk about in ordinary language.
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Anna is inside. She performs a measurement on a quantum system and sees one definite result.
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Ben remains outside and has not yet spoken to her. From Anna's point of view, there is no mystery about what she experienced. The detector gave one outcome rather than another.
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She has already updated her expectations and can decide what she would expect from whatever experiment she performs next.
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Ben, however, has not seen the detector, has not heard Anna's report, and has not entered the room.
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If he wants to use quantum mechanics to describe what he may experience when he eventually interacts with the laboratory, he has to reason from his own information rather than quietly borrowing Anna's.
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This simple separation between what Anna has experienced and what Ben has experienced leads us into one of the most famous thought experiments in quantum foundations, Wigner's friend.
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The idea was introduced by the physicist Eugene Wigner as a way of pushing the problem of quantum measurement beyond the usual particle and detector.
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Instead of asking what happens when a tiny quantum object meets a measuring device, imagine that the measuring device is being watched by a person inside a laboratory.
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That person sees an outcome. Then imagine another observer outside the laboratory who treats the entire laboratory, including the person inside it, as one physical system.
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The thought experiment asks whether quantum mechanics gives both observers a consistent way to describe what is happening.
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It sounds like the sort of problem physicists might invent after deciding that ordinary measurement was not confusing enough. But the reason it matters is very simple. If quantum mechanics is supposed to apply universally, then a human observer is made of
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physical matter, too. There should not be a magical line where quantum rules stop simply because a scientist happens to be wearing a lab coat.
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Let us keep Anna inside the room and Ben outside. Anna measures a quantum system with two possible outcomes. She sees one of them. Perhaps a light on her detector turns green rather than red.
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From Anna's perspective, the measurement has happened. She does not experience a vague mixture of green and red. She experiences green.
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She can write the result in her notebook, make herself a cup of tea, and begin wondering why Ben is taking so long to open the door.
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For her, the event is definite because it is part of her experience. Ben does not yet know any of this. He only knows how the experiment was arranged before the laboratory was sealed. If he uses the ordinary machinery of quantum theory from
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outside, he may describe the whole laboratory in a way that still includes different possible correlations between Anna and her detector.
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One possibility corresponds to Anna seeing green. Another corresponds to Anna seeing red. From Ben's perspective, before he interacts with the laboratory, both possibilities remain relevant to the probabilities he assigns to his own future experiences.
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This is where the discomfort begins. Anna says that she has already seen one definite result.
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Ben uses a quantum description that still includes several possibilities. If the quantum state is supposed to be one objective description of reality, then Anna and Ben seem to be offering incompatible accounts of the same laboratory.
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Anna says there is a definite outcome. Ben's description has not yet singled out that outcome. Which one is correct?
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Different interpretations of quantum mechanics answer that question in different ways. Some say the larger quantum state never collapses and both possible outcomes continue in different branches.
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Some introduce a physical collapse at some stage. Some argue that the state describes relations rather than absolute properties.
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Cubism makes a move that should now sound familiar. It says we should not begin by assuming that Anna's state assignment and Ben's state assignment are competing photographs of one underlying physical state.
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The quantum state belongs to the agent who assigns it. Anna assigns a state based on what she has experienced.
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Ben assigns another based on what he has experienced. Their descriptions differ because their situations differ.
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That does not mean Anna and Ben live in two disconnected realities. It means that quantum mechanics is being used separately by two agents.
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Anna uses it to organize her expectations about the consequences of actions she may take.
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Ben does the same from outside the laboratory. Anna's measurement outcome is part of Anna's experience.
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It does not automatically become part of Ben's experience merely because it happened to Anna.
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Ben has to interact with the laboratory before he acquires something new. Imagine this in a less exotic setting.
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Anna is inside a restaurant and discovers that the last slice of cake has already been sold.
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Ben is outside and has not entered yet. For Anna, the situation is settled. There is no cake.
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Ben may still think there is a reasonable chance that cake remains because he has not received the information.
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Their expectations differ. We do not accuse the restaurant of existing in two realities. Ben simply lacks an experience Anna already has.
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When he enters and sees the empty display, his expectations change. The quantum version is more subtle because Ben is not merely uncertain about an ordinary hidden fact in the same classical way.
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If he treats the entire sealed laboratory quantum mechanically, the state he assigns may contain a structure representing different possible outcomes for his future interaction with it.
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Cubism tells us that this state is still Ben's tool for managing his own expectations.
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It should not be converted into a declaration that Anna herself literally exists in some strange mixture of experiences from her own point of view.
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Anna never reports feeling partly certain that she saw green and partly certain that she saw red. She saw what she saw.
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From her perspective, the measurement outcome is an actual experience. Ben's uncertainty does not reach backward and erase that.
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It tells us something about Ben. This is one of the clearest places where Cubism insists on discipline about pronouns. Who assigned the state? Who performed the action? Who experienced the result?
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Who is uncertain? These questions matter. Saying the system is in a state can hide the fact that some agent is making the assignment.
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Saying the outcome is uncertain can hide the fact that it is uncertain for someone.
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Wigner's friend becomes especially confusing when we allow all these perspectives to blend into a single sentence and then wonder why the story seems contradictory.
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Suppose Ben now opens the laboratory door and asks Anna what she saw. Anna says green.
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For Ben, hearing that answer is a new experience. His expectations change. The state he assigns for future actions changes as well.
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Anna's earlier experience has now become connected with Ben's later experience through a physical interaction between them.
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They can compare notebooks. They can inspect the detector. They can repeat the experiment. They can argue over whether the tea inside the laboratory has gone cold. Nothing in Cubism prevents them from agreeing.
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The important point is that Ben's agreement comes from interaction. There is no need for a universal observer standing above both of them and declaring the result before either perspective is considered.
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Anna had an experience. Ben later had an experience involving Anna. Their accounts become linked because the world allows agents to interact.
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This sounds innocent until we make Ben's experiment more ambitious. Instead of simply opening the door and asking Anna what she saw, imagine Ben has complete quantum control over the entire laboratory.
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This is far beyond anything realistic, but thought experiments are allowed to be unreasonable as long as they are unreasonable in a useful way.
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Ben may decide to perform a measurement on the whole laboratory that is sensitive not to Anna's recorded green or red result individually, but to a broader quantum property of the combined system.
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Now Anna's earlier certainty and Ben's later possible experiments seem to clash more sharply. Anna experienced a definite outcome.
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Ben may use quantum theory to assign probabilities for an experiment that treats the entire laboratory as one coherent quantum system.
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If we insist that Anna's measurement caused an objective collapse for the whole universe, Ben's larger experiment might be described one way. If we insist that no collapse occurred and the complete quantum state remained intact, it might be described another way.
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Wigner's friend exposes the difficulty of trying to combine an observer's definite experience with a single observer independent quantum state.
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Cubism tries to avoid the clash by refusing to create the observer independent state in the first place.
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Anna's quantum state is Anna's. Ben's quantum state is Ben's. Anna's measurement outcome is an experience for Anna.
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Ben's later measurement outcome is an experience for Ben. Neither agent is permitted to use their own quantum state as if it were an absolute physical description that everybody else must regard as the world itself.
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There is a subtle consequence here. Ben may use quantum mechanics to assign a state to Anna, the detector, and everything else in the laboratory because all of those things are external systems from Ben's point of view.
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Anna, however, does not assign a quantum state to her own experience in the same way.
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She does not need a mathematical prediction to discover whether she just saw green. She has already seen it.
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Quantum theory is used for expectations about possible future experiences, not to replace the experience currently being had.
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This is where Cubism becomes strongly first-person in its structure. The theory tells each agent how to organize expectations from that agent's own standpoint.
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It is not a cosmic diary written from nowhere. Anna cannot jump into Ben's perspective and use his uncertainty as a description of what she herself experienced.
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Ben cannot claim Anna's result as part of his own experience before he interacts with her.
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Each person starts with what they have encountered. To some physicists, this feels uncomfortable because science aims for a view of the world that does not depend on particular people.
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If Anna and Ben are allowed different descriptions, have we abandoned objectivity? Cubism says no. It distinguishes objectivity from the idea that every observer must possess the same information at every moment.
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Science does not require that everyone begin with identical knowledge. It requires that agents can communicate, compare evidence, repeat actions, and adjust expectations when experience demands it.
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Suppose Anna tells Ben that she saw green. Ben believes her. Later, he performs another experiment and receives a result consistent with what she reported. They repeat the procedure many times.
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Over days and weeks, they build a network of shared expectations. Other scientists join them. They perform similar experiments in different laboratories and discover stable patterns.
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Eventually, the entire community becomes extraordinarily confident about what kinds of outcomes should be expected under particular conditions.
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Nothing about that process requires a single universal wave function to exist as the official description behind every agent's experience.
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Shared science can emerge through communication and repeated interaction with a common world. There is a useful difference between shared and identical.
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Anna and Ben do not need identical experiences to share knowledge. If Anna looks through a telescope and tells Ben that a particular star has brightened, Ben has not had Anna's visual experience. He can still learn from her report.
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Later, he may look through another telescope. His own observation gives him another experience connected with hers.
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Scientific communities are built from enormous networks of interactions like this. No person possesses every observation personally, yet agreement becomes possible because information travels through communication, instruments, records, and repeated tests.
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QBism treats quantum experiments in this same general spirit, although with the added complication that quantum states and probabilities remain tied to the agents assigning them.
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Personal does not mean isolated. It means located. The difference is important because people sometimes hear cubism and imagine millions of private universes, each existing only for one observer.
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That picture goes much farther than the interpretation requires. Anna is real for Ben as part of the world he encounters.
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Ben is real for Anna. They can affect one another. They can surprise one another.
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If Ben knocks on the laboratory door while Anna is concentrating, Anna may discover the reality of Ben's presence rather abruptly.
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The external world is not dissolved into private imagination. What cubism rejects is the assumption that there must be a single quantum state representing both agents from a viewpoint that belongs to neither of them. The state always belongs to some user of the theory. If
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Ben assigns a state to Anna, that is Ben's state assignment for a system external to him.
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If Anna assigns a state to Ben, that is Anna's assignment. Neither description automatically outranks the other simply because we would prefer one universal perspective.
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Wigner's friend therefore becomes less of a contradiction and more of a warning about mixing viewpoints. Anna says, "I saw green." Ben says, "Before I interact with the laboratory, I assign these probabilities to what I might experience." Those statements do not have to conflict
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because they refer to different agents and different experiences. Trouble appears when we quietly transform Ben's state assignment into an absolute description of what Anna must be experiencing or when we transform Anna's experience into information that Ben must somehow
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already possess. This does not mean every possible statement by every observer is valid. If Ben opens the door, speaks to Anna, inspects the apparatus, and repeatedly receives evidence that she saw green, he cannot sensibly continue behaving as if her result were completely unknown.
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His own experiences have changed. A rational agent updates. QBism does not protect beliefs from evidence. It makes evidence central because the agent's expectations are supposed to respond to experience. Likewise, Anna cannot decide afterward that she saw red simply
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because red would have been a more interesting result. Her experience constrains her. Personal does not mean optional.
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The world keeps contributing consequences that the agent did not control. There is also a deeper point about memory.
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Anna remembers seeing green. That memory is part of her present experience and influences her expectations.
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Ben does not possess Anna's memory. He may trust her report, but his trust is still part of Ben's own web of beliefs.
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The distinction between their perspectives persists even after they communicate. Complete merging of viewpoints never actually occurs.
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They can agree very closely without literally becoming one observer. In ordinary life, we accept this without concern.
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Two people watch the same sunset from slightly different places. They describe nearly the same colors, the same clouds, the same horizon.
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Their experiences are not numerically identical because each sees from a different position, yet nobody concludes that sunsets are subjective inventions.
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Agreement does not require one person to possess the other's eyes. QBism asks us to bring that humility into quantum theory.
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Perhaps the demand for one observer-independent quantum state is not a necessary condition for realism.
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Perhaps reality can be common while experiences and probability assignments remain personal. The Wigner's friend setup becomes even more interesting when we consider what Ben expects Anna to say.
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Before opening the laboratory, Ben can assign probabilities to possible conversations. He may expect Anna to report green or red.
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Once he opens the door, he hears one answer. That answer is now part of his own experience.
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He updates. From Ben's viewpoint, the encounter with Anna functions much like any other measurement.
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Anna is part of the external world with which he interacts, and her reply is one possible consequence of that action.
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For Anna, of course, being asked the question is a different event. She already knows what she remembers seeing.
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Ben's arrival produces a new experience for her, too. Both agents participate in the interaction, but each participates from their own standpoint.
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This is a useful place to notice how far QBism moves away from the picture of observers as passive cameras.
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Anna and Ben are not merely recording a finished universe. They act upon one another and upon other physical systems.
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Their actions lead to experiences that shape what happens next in their own expectations and decisions.
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The world is not described as a collection of facts waiting to be copied into notebooks.
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It is encountered through events. That does not make the past disappear. Once Anna has experienced green, she has a memory and a record.
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Once Ben has heard her report, that becomes part of his own history. QBism is not suggesting that reality resets every time someone looks away.
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It is saying that quantum theory itself should be understood as a tool each agent uses from the standpoint of their own past experiences when facing uncertain future interactions.
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Seen this way, Wigner's friend exposes a tension less about consciousness and more about perspective.
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Traditional language tempts us to ask for the quantum state of the laboratory as though such a state must exist independently of anyone assigning it.
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Cubism instead asks, whose expectations are being represented? Once that question is answered, the apparent contradiction between Anna and Ben becomes less severe.
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Their state assignments do not need to be identical cuz they do not have identical experiences.
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There is still something unresolved. If every agent starts from a personal perspective, why do different agents agree so often?
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Why does physics not fragment into billions of incompatible private descriptions? Anna and Ben may begin with different information, yet after communication, they usually expect the same equipment to behave in similar ways.
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Scientists in different countries can repeat experiments and obtain compatible statistics. Engineers can build machines based on quantum theory and trust that they will work for people who had nothing to do with the original calculations.
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If quantum probabilities are personal and measurement outcomes belong to individual experience, the remarkable stability of shared science needs an explanation.
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It is not enough to say that agents communicate. We need to understand why communication works so well. Why common patterns emerge and what kind of external world allows separate observers to build a reliable public reality from private streams of experience?
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This is where the question becomes more interesting than whether Anna and Ben are each entitled to their own quantum state.
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They clearly can begin differently. What matters next is how those separate viewpoints become coordinated.
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Somewhere between Anna's detector, Ben's later question, their shared notebook, and the repeated experiments of thousands of other physicists, something remarkably stable appears.
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Personal expectations begin to converge. Private experiences become public evidence. Different agents discover that they can rely on the same regularities, even though none of them possesses a view from outside the universe. And understanding how cubism makes room for
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that shared world is the next problem waiting just beyond the laboratory door. That shared world is where the most obvious objection to cubism finally has to be faced directly.
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If quantum states are personal, if probabilities express the expectations of individual agents, and if measurement outcomes enter the experience of the person performing the action, why does science not dissolve into millions of separate stories?
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Anna sees one result. Ben forms his own expectations. Another physicist on the other side of the world begins with different information again.
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Yet when scientists build the same equipment, prepare systems in similar ways, and repeat experiments, they find remarkably stable patterns.
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Laboratories separated by oceans can compare results. Engineers can use quantum mechanics to build devices that work for people they have never met.
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A laser does not behave differently because the person switching it on has a different childhood, favorite breakfast, or preferred interpretation of quantum theory.
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If cubism makes the standpoint of the agent so important, it has to explain how all these agents manage to live in what appears to be one dependable world.
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The first thing to notice is that personal does not mean unconstrained. This distinction has been following us throughout the discussion, but here it becomes essential.
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Anna may assign probabilities based on her own experiences, but she does not get to choose what happens simply because the probabilities belong to her.
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She can expect an outcome with great confidence and still receive another. She can be surprised.
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She can discover that her model was poor, that her apparatus was misaligned, or that something she had believed for years does not survive contact with experiment.
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The ability of the world to resist an agent's expectations is exactly what prevents cubism from becoming a theory in which everyone simply invents whatever reality suits them.
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Imagine several people trying to learn the behavior of an unfamiliar coffee machine in a hotel lobby.
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Nobody has the instruction manual, which is perhaps not very different from the situation humanity faces with the universe.
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Anna presses one button and receives a small cup of extremely strong coffee. Ben presses another and receives hot water.
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Clara makes what appears to be the same choice as Anna and somehow receives nothing except an alarming mechanical noise.
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Each person begins with different experiences and therefore different expectations about what the machine will do next. Their beliefs are personal, but the machine is not.
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It keeps responding according to whatever internal structure it actually has, whether or not the users understand it.
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After enough attempts, the group begins comparing notes. Anna explains what happened when she pressed the first button.
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Ben reports his experience. Clara tries again and discovers that the machine needed more water.
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Gradually, their expectations become better coordinated. They may never possess a perfect internal description of the machine, but they learn reliable rules for dealing with it.
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Pressing certain buttons tends to produce certain results. Some expectations fail and are abandoned. Others survive repeated testing.
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Cubism treats interaction with the physical world in a much more sophisticated version of this general pattern.
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Agents begin from their own histories and form their own probability judgments, but those judgments are repeatedly exposed to a world they do not control.
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Similar experiences can therefore push different agents towards similar expectations. A shared world does not require probabilities themselves to float objectively in nature.
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It requires the world to possess enough regularity that agents who interact with it can learn from what happens.
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Cubism's advocates describe the interpretation as realist precisely because the external world is not reduced to belief.
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The agent participates in reality, but reality remains something that can answer back. This is also why agreement between scientists should not be imagined as a mysterious synchronization of minds.
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People become coordinated through ordinary physical interactions. Anna performs an experiment and records a result.
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Ben reads her report. Clara repeats the experiment. Another laboratory checks the equipment. Someone notices an error. Someone else improves the preparation method.
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The community gradually develops procedures that other agents can use and test for themselves. At no point does anyone need direct access to another person's private experience.
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Communication provides new experiences that connect one agent's history with another. Suppose Anna tells Ben that her detector produced a particular result.
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Anna's original detector reading was Anna's experience. Ben does not somehow receive that earlier experience directly.
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Instead, he hears Anna speak, reads her message, or examines her notebook. That interaction produces a new experience for Ben.
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He may trust Anna completely, doubt her, check the apparatus, or repeat the experiment. Every one of those choices creates further opportunities for the world to constrain his expectations.
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This is not unusual. Most of what you know about the world was not personally observed by you.
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You probably did not watch the Earth form, measure the distance to every planet, verify the structure of every molecule, or personally inspect the manufacturing process of the device on which you are listening.
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Fortunately, science does not require one extremely busy individual to perform every experiment. Knowledge moves through communities because people can communicate, preserve records, reproduce procedures, and test claims.
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The personal origin of an experience therefore does not stop it from contributing to public knowledge. A scientist sees a result. The results are recorded. Another scientist interacts with the record. A third repeats the procedure. Agreement develops through a
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chain of physical encounters. Cubism simply refuses to pretend that all those encounters must be combined into a single imagined perspective belonging to nobody.
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This is where the Wigner's friend situation becomes useful again. Anna sees green inside the laboratory.
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Ben has not yet entered. The descriptions differ because their experiences differ. Once Ben interacts with Anna and hears that she saw green, his situation changes.
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Cubism does not insist that Ben must have possessed Anna's fact before that interaction, nor does it insist that Anna's personal experience somehow failed to be real until Ben confirmed it.
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Each agent continues from their own standpoint. Work on Cubism's treatment of Wigner's friend emphasizes this equal status between agents.
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Neither observer occupies a privileged position above the other, and each can treat the other as part of the external world when considering possible actions.
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That equality matters because an older habit in quantum discussions is to imagine one observer somehow more official than another.
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Perhaps Anna is merely part of Ben's experiment. So, Ben's description is the important one.
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Then, we can move outward again and imagine Clara observing Ben, making Clara's perspective even more official.
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Continue long enough, and we begin searching for a final observer standing outside everything, which becomes difficult once we remember that everything includes the universe.
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QBism refuses to build such a hierarchy. Anna is an agent. Ben is an agent.
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When Anna acts on something outside herself, quantum theory helps her organize her expectations. When Ben acts on something outside himself, he uses the theory in the same way.
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If Ben happens to treat Anna as part of the system he's investigating, that does not turn Anna into a less legitimate user of quantum mechanics.
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From Anna's perspective, Ben is equally capable of appearing as part of her external world.
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QBist analysis describe this reciprocity as a kind of quantum Copernican principle. No agent occupies a uniquely privileged center.
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There is something reassuringly ordinary about that idea. Imagine two people negotiating the use of a narrow doorway while carrying large boxes.
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Each person sees the other as an obstacle requiring prediction. Neither person is the official center of the doorway. Each adjusts based on what the other does.
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After several awkward movements and at least one unnecessary apology, they usually coordinate. The shared environment allows their separate perspectives to become compatible enough for both people to get through.
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Scientific agreement works on a far larger and more careful scale. Agents do not need identical beliefs before they interact. They need a world in which interaction has repeatable enough consequences that poor expectations can be exposed.
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If Anna repeatedly predicts one outcome while Ben repeatedly predicts another, experiments can provide experiences that push one or both of them to reconsider.
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This is what gives empirical evidence its authority. The authority does not come from probabilities being objective objects.
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It comes from the stubbornness of what happens. The difference becomes clearer if we imagine the opposite kind of universe.
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Suppose reality changed completely according to whatever each agent happened to believe. Anna expects her detector to show green, so it does.
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Ben expects red, so when he enters the laboratory it becomes red for him. Clara expects the apparatus to turn into a bowl of fruit, and the universe politely obliges.
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In such a world, disagreement would be almost impossible to correct because there would be no stable external resistance. Scientific testing would lose its purpose. Every prediction could protect itself by producing its own confirmation.
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That is not the world cubism describes. Agents have personal expectations precisely because they're uncertain about what an external world will give them. The result of an action is not chosen by the agent. Nature gets a vote, and it is generally the deciding one.
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This allows different agents to converge without requiring them to begin from identical probability assignments.
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Imagine Anna believes an outcome is very likely while Ben regards it as only moderately likely.
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They perform the experiment repeatedly, exchange their results, and gather more experience. Their expectations may move closer together because they are being shaped by similar evidence.
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Nothing guarantees that every person will always agree. Scientists are quite talented at proving otherwise.
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Yet disagreement can become smaller when agents take account of common evidence and use compatible methods of reasoning.
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Cubism therefore separates agreement about quantum states from agreement about how to function successfully in the world.
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Two agents are not required to assign exactly the same quantum state simply because they are discussing the same system.
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Their assignments can reflect different histories and information. Recent discussion of intersubjective agreement in Cubism has emphasized that identical quantum state assignments are not required for objectivity.
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What matters more fundamentally is the reciprocal ability of agents to interact, test expectations, and treat one another as physical participants rather than imagining a privileged observer whose account automatically overrides everyone else's.
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That may sound like a weaker form of agreement than physics normally wants. But consider what scientists actually do.
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They rarely begin by demanding that every researcher possess exactly the same internal confidence about a theory.
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One physicist may consider a model almost certainly correct. Another remain cautious. A third may suspect the experiment contains an unnoticed problem.
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What matters is that they can describe procedures, make predictions, perform tests, and compare what happens.
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Public science does not require private certainty to be identical. Think about weather forecasting again.
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Two meteorologists can study the same storm while assigning somewhat different probabilities to tomorrow's rain.
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They still agree on large amounts of shared structure. They agree that there is an atmosphere, that certain measurements were recorded, that particular forecasting methods exist, and that tomorrow will provide new evidence.
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After the weather arrives, each meteorologist updates. Over many forecasts, methods that repeatedly perform badly lose credibility.
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Quantum physics is different from meteorology in important ways, but the example helps separate two ideas that are often blended together.
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Agreement about reality does not necessarily require identical probabilities. Probabilities can remain judgments made by agents, while the experiences those judgments confront arise from a shared world.
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Cubism goes further by claiming that the rules of quantum theory constrain those personal judgments.
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An agent cannot assign quantum probabilities in any arbitrary pattern, and still claim to be using quantum mechanics coherently.
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The formalism places relationships among the expectations associated with different possible actions. This is one of the reasons Cubists call the theory normative.
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It tells an agent how certain probability judgments should relate to others if the agent wants to reason in accordance with quantum theory.
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The personal character of the probabilities therefore exists inside a common framework discovered through our long interaction with nature.
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Consider a game whose rules everyone gradually learns through playing. Each player still has personal expectations about what another player will do next.
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Anna thinks Ben is likely to make one move. Clara expects another. Their predictions belong to them.
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Yet all of them are playing the same game, and their possible actions are constrained by its structure.
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If Anna repeatedly reasons as if chess pieces can move according to the rules of football, her personal perspective does not make the strategy successful.
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Quantum theory plays a much deeper role than a game rule, but the comparison is useful.
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Agents may enter with different beliefs, while the world teaches them that successful quantum reasoning has a particular structure.
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That structure is not erased because the probabilities themselves are personal. There may also be features associated with physical systems that Cubists hope can be treated more objectively than the quantum state itself.
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Christopher Fuchs has argued that the dimension associated with a quantum system may be understood as a kind of capacity belonging to the system, rather than to the agent's personal beliefs.
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In that line of thought, the wave function and probabilities remain agent-dependent, while some structural aspects of the physical system point toward properties of the external world itself.
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This is part of a broader Cubist effort to ask what remains genuinely objective once the personal elements of the usual quantum formalism have been separated out.
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That is an important detail because Cubism is sometimes described as though it takes the entire content of physics and places it inside someone's head.
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The actual ambition is almost the reverse. By identifying which parts of quantum theory belong to the agent's expectations, Cubists hope to become clearer about what features might truly belong to the world.
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If the wave function is not an objective property, then perhaps we should stop spending our effort asking what physical substance it represents, and instead investigate the deeper structure that makes quantum probability necessary in the first place.
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The shared world would then be responsible for the constraints that make quantum mechanics useful to many different agents.
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Anna, Ben, and Clara may assign different states, but all of them discover that certain patterns of reasoning work better than others.
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They may disagree about the probability of one particular outcome while agreeing that quantum theory gives the right framework for connecting their expectations.
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Experience repeatedly reinforces that framework. There is another useful analogy in language. Every speaker has a personal history with words.
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You may associate a particular word with a childhood memory that means nothing to someone else. Your voice is yours. Your exact thoughts are yours.
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Yet language works because communities share enough structure to communicate. There are rules, conventions, repeated patterns, and a world people often talk about together.
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Personal meaning does not prevent public language. Physics is far stricter than ordinary conversation because experiments allow extremely precise comparison, but something similar happens.
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Individual experience enters through particular agents. Shared structures emerge because agents communicate and because the external world keeps imposing regularities.
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A scientific community becomes possible without requiring the members to merge into one enormous observer, which would make conferences efficient, but probably uncomfortable.
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This also changes the way we should think about objectivity. We often imagine objectivity as the removal of every personal standpoint.
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A statement becomes objective when we can write it as though nobody in particular discovered it.
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Cubism suggests another possibility. Perhaps objectivity can arise through relationships among many perspectives, rather than through the invention of a perspective belonging to no one.
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Anna makes an observation. Ben checks it. Clara performs another experiment. Their individual experiences remain individual, but the network of interactions becomes increasingly stable.
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If a result can be reproduced by many agents under similar conditions, confidence grows. If an expectation consistently fails, it is revised.
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Objectivity then becomes something built through resistance, communication, repetition, and agreement, rather than something that requires us to pretend the observers were never there.
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This is not a uniquely QBist insight about science. Experimental practice has always depended on reproducibility and communication.
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What QBism does is refuse to hide that process when interpreting quantum mechanics. The agent is not an embarrassing temporary feature that will disappear once the theory becomes complete.
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The agent may be part of how quantum theory is fundamentally used. At first, that can feel like a retreat from the old dream of physics.
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Newtonian mechanics encouraged the hope that we might describe the universe as a complete machine.
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If we somehow knew every relevant property and every law, reality could be written as one enormous objective story.
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The observer would have no special role. The universe would simply proceed. QBism suspects that quantum mechanics is telling us something different.
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An agent inside the universe never possesses the entire story. The agent faces a future that contains genuine uncertainty from their own standpoint.
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Actions matter cuz different actions lead to different possible experiences. The world is not merely something watched from a distance. It is something encountered. Still, the encounters are not random chaos.
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That is the part that makes a shared world possible. Nature displays regularities. Similar actions performed under similar conditions lead to dependable statistical structures.
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Quantum theory captures those structures with extraordinary success. Agents learn them, communicate them, and build technology around them.
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Imagine Anna designing a quantum device. She uses her own probability assignments while working. Eventually, the design is manufactured and sent to Ben, who has never met her.
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Ben switches it on. It behaves according to the same quantum regularities Anna relied upon.
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Clara buys another one a year later. It works for her, too. Their personal experiences differ, but the device reveals the same underlying stability in the world.
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No QBist needs to say that the device works only because Anna believed in it.
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Her beliefs helped her decide what actions to take during the design process. The world determined whether those actions produced a functioning machine.
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Bad physics does not become good engineering through confidence alone. This external correction is what keeps QBism connected to ordinary scientific realism, even while it changes what counts as a description of reality.
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Reality is not the quantum state. Reality is not the agent's probability assignment. Those are tools the agent uses.
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Something beyond the agent is responsible for the fact that some expectations survive repeated encounters, while others fail.
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QBism's participatory language therefore contains two sides that have to remain together. Remove the agent, and the interpretation loses the personal probabilities and experiences that define its use of quantum theory.
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Remove the external world, and the interpretation loses the source of surprise and constraint that makes learning possible.
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Agent and world meet in interaction. Neither can simply be replaced by the other. This balance is central to Fuchs's account of participatory realism, where reality is treated as more than a complete third-person description can capture, without thereby being reduced
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to private thought. This may help with the question that sounded so threatening at the beginning.
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Can Anna and Ben have different realities? In the loose everyday sense, they can certainly possess different experiences, information, and expectations.
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Cubism takes those differences seriously. But that does not mean there is no common world connecting them. Anna can surprise Ben.
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Ben can surprise Anna. Their actions affect what happens to each other. Their expectations can be corrected by shared encounters. They can build instruments together and discover regularities that no other person invented.
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Perhaps the word reality was doing too many jobs for us. We wanted it to mean the external world, the mathematical description of that world, every individual's experience of it, and the collection of facts everyone ought to agree upon all at once.
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Cubism separates those roles. The world exists. Agents experience it from particular standpoints. Quantum states represent their expectations.
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Communication creates new experiences through which those expectations can become coordinated. Once those pieces are separated, agreement stops being mysterious.
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We agree not because we secretly share one universal wave function, and not because every observer has direct access to everyone else's experience.
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We agree because we keep encountering the same world because that world contains enough dependable structure to teach us, and because human beings have become remarkably good at comparing what happened after we asked it similar questions.
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There remains one larger issue, however. If this is genuinely what quantum mechanics is telling us, then Cubism is making a claim that goes beyond how physicists should interpret laboratory probabilities.
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It is suggesting a different picture of our relationship with nature. The universe is not being presented as a completed object that could in principle be described from a single perfect viewpoint. While every observer disappears from the account, agents are themselves physical
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participants. Their actions matter to the experiences that follow. Reality constrains them, but it is encountered through interaction rather than revealed as a finished catalog.
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That raises a final set of questions that cannot be answered merely by saying that scientists eventually agree.
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If the wave function is not the world itself, what sort of world lies underneath our probability assignments?
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If measurement outcomes are genuinely new experiences rather than simple revelations of pre-written values, what does that imply about whether the future is fully settled?
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If every agent is a participant rather than a spectator, does quantum mechanics suggest that participation is not an inconvenience of experimentation, but something basic about how reality works?
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These are the questions that push Cubism from an interpretation of probability toward its more ambitious idea of participatory realism, where the strange lesson is no longer simply that different people can assign different quantum states, but that a universe containing agents
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who act, experience, and learn may be fundamentally different from the silent mechanical universe physics once hoped to describe from the outside.
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Once the shared world is taken seriously, the question changes again. We're no longer asking only why Anna and Ben can begin with different quantum states or why their probability assignments eventually become more closely aligned.
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We can now ask what kind of world makes this entire situation possible. There are agents inside it who can choose actions. There are physical systems outside each agent that respond to those actions. The responses are not completely determined by what the agent
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expects. New experiences occur, expectations change, and different agents can communicate well enough to build a common science.
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Cubism takes this structure seriously enough that its advocates describe their position as participatory realism.
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The word realism matters because the world is not being reduced to someone's imagination. The word participatory matters because the agent is not treated as a spectator standing outside a completed universe and merely reading properties from it.
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Christopher Fuchs has used participatory realism to describe the idea that reality exists independently of us while also being more than a complete third-person description can capture.
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This is probably the point where the title of our discussion can finally be understood without giving the wrong impression.
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Asking whether reality is personal does not mean asking whether your bedroom exists only for you or whether another person can have a different gravitational constant because they feel strongly about it.
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The personal part of Cubism concerns the standpoint of the agent, the probabilities the agent assigns, the actions the agent chooses, and the experiences the agent receives.
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The external world is precisely what prevents those personal judgments from becoming arbitrary. Anna can believe that a detector will show one result. She can prepare carefully and assign very high confidence to that expectation.
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She can still be surprised. Whatever reality is, it contains something that is not reducible to Anna's beliefs because it can give Anna an experience she did not choose.
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That simple fact gives the Cubist picture much of its character. Imagine walking into a dark room that you've never visited. You have expectations about what might be inside.
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You expect a floor beneath your feet. You expect a wall somewhere around you. You might expect a light switch near the door because experience has taught you that people often put them there. You reach out.
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Perhaps your hand finds the switch immediately. Perhaps it finds an empty wall. Perhaps it finds something hanging there that makes you withdraw your hand rather quickly and reconsider how urgently you need the light.
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Your expectations belong to you, but the room does not. The room contributes consequences to your actions.
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Cubism treats quantum interaction in a more fundamental way, but the separation remains useful. The agent contributes an action. The world contributes an outcome that the agent did not determine in advance. The resulting experience changes the agent's future expectations.
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Measurement is therefore not merely a passive check on a finished collection of properties. Fuchs and Rüdiger Schack describe a quantum measurement as an action upon the world in which the outcome represents something new for the agent, rather than simply the comparison
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of a pre-existing value with a measuring standard. The phrase something new needs to be handled carefully because it is one of the strongest claims in the Cubist picture.
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It does not simply mean that Anna learns a fact that she happened not to know.
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If she opens a cupboard and discovers a mug, the mug did not come into existence because she opened the door.
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Ordinary ignorance explains why the discovery was new to Anna. Cubism does not assume that quantum measurement always works that way.
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An outcome is associated with the actual interaction between agent and world, and the interpretation resists treating every possible result as a value that had already been written into the system before the interaction occurred.
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This is where the idea of participation begins to mean something more than the obvious fact that experiments require equipment.
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The action selected by the agent helps determine which encounter takes place. Anna chooses one measurement rather than another.
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She does not control the result, but her choice helps define which set of possible experiences is relevant.
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If she chooses a different action, she creates a different experimental situation. The world responds to the action that was actually performed, not to every experiment Anna might have imagined while drinking coffee earlier that morning.
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It would be easy to exaggerate this and say that consciousness creates reality. QBism does not need that claim. Nothing in the basic QBist structure requires a human mind to stare at a detector until matter decides what to do.
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The important distinction is between an agent who uses the theory and the external systems on which that agent may act.
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The role of the agent is connected with probability decision action and experience, not with a mysterious mental force capable of rearranging atoms by attention alone.
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Modern QBist summaries make the agent-centered structure explicit while treating quantum theory as a single-user theory for each agent, rather than a universal state description written from nowhere.
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Suppose Anna places a detector in front of a quantum system and decides what interaction she wants to perform.
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Before the experiment, she assigns probabilities. During the experiment, something happens. Afterward, she has a result.
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From the QBist standpoint, the outcome is not simply a number that belonged to the system independently of every possible interaction.
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It is part of an event involving Anna and the external world. Ben can later interact with Anna and learn what she experienced, but Ben's learning is another event.
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Clara can read Ben's report producing another. The world becomes a network of actual encounters rather than a single book of facts that every observer is assumed to possess simultaneously.
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That picture sounds unusual because physics has trained us to imagine the opposite. Classical mechanics encouraged an extremely successful model in which the world could be described from outside.
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A planet has a position and motion. A stone falls. A pendulum swings. Whether anyone watches seems irrelevant.
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The ideal description contains the physical variables and the laws connecting them, while the observer disappears from the final account.
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If enough information were available, the hope was that the entire future could in principle be read from the present.
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Cubism does not simply deny classical physics where classical physics works. Nobody needs a personal interpretation of a falling sandwich to calculate approximately when it reaches the floor.
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The argument is about what quantum mechanics may be telling us at a deeper level.
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If quantum states are judgments assigned by agents, if measurements are actions, and if outcomes are experiences arising in those interactions, then perhaps the old ideal of a complete description from outside the universe was asking for something quantum theory
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was never designed to provide. There is no actual place outside the universe from which a physicist can inspect everything at once.
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Every real scientist is part of the physical situation. Every instrument is built from physical matter. Every record is stored somewhere in the world. Every piece of information reaches an agent through interaction.
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The view from nowhere is therefore an intellectual construction. It can be extremely useful, but cubism asks whether usefulness should be confused with the final structure of reality.
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Participatory realism answers that reality is not exhausted by such a detached account. Fuchs connects this view with a line of thinking influenced strongly by John Wheeler, who emphasized observer participation and the possibility that the laws and structure we observe should
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not automatically be pictured as features of a completely finished universe independent of every act of investigation.
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QBism does not simply copy Wheeler's ideas, but Fuchs has explicitly identified Wheeler's influence on its development.
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We should be cautious here because it's very tempting to turn participation into a dramatic claim that the universe needs human beings in order to exist. That is not required.
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The Earth existed long before physicists began arguing about quantum foundations. Stars formed before anyone built a detector.
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Dinosaurs managed an impressive amount of existence without assigning wave functions to one another. QBism is not claiming that human observation brought these things into being.
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The more limited and more interesting claim concerns events and interactions. When an agent acts on an external system, the result is not treated as the uncovering of a fully predetermined response that the quantum state had already encoded as an objective fact.
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Something happens in the encounter. The future experience was uncertain for the agent, and the agent could not force the world to provide a chosen answer.
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This gives the future a different character from the simple mechanical picture. In a perfectly deterministic classical universe, uncertainty can be imagined as merely practical.
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The future is already fixed by the present, even if nobody knows enough to calculate it.
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Our ignorance does not change what will happen. Cubism does not interpret quantum probabilities as merely reflecting ignorance of a hidden classical future.
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An agent genuinely faces several possible experiences, and the particular outcome becomes actual through the interaction.
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That does not automatically settle the enormous philosophical question of whether the universe as a whole has one objectively open future.
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Cubism is careful about making claims from an imaginary universal viewpoint because its entire framework begins with actual agents.
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What it does say is that an agent should not convert personal certainty or quantum probability into a declaration that an unperformed measurement already possesses an outcome.
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Even complete confidence remains a judgment, and an experiment that was never carried out produced no result for that agent.
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Fuchs includes these ideas among the explicit principles of Cubism. Imagine Anna has two possible experiments available.
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She performs the first and sees one result. She never performs the second. In ordinary classical thinking, we may want to say that the second experiment also had a definite answer, and Anna simply failed to discover it.
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Cubism does not grant that assumption automatically. Anna could assign probabilities to what she might experience if she chose the second action, but the possible outcomes remain expectations concerning a hypothetical encounter.
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No actual encounter means no actual outcome for Anna. This is one reason the QBist universe can be described as participatory without becoming mind-dependent.
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What happens depends partly on which interactions actually occur. An agent does not choose the response, but the choice of action matters because it determines which encounter takes place.
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Reality is not a film that agents merely watch from different seats. Their physical actions are among the events occurring in the world.
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That sounds less strange when we consider ordinary causation. The future course of your evening depends partly on what you do.
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If you switch off the light, the room changes. If you leave the window open, the temperature may change.
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If you decide at midnight to begin reorganizing every cupboard in the kitchen, tomorrow morning may contain consequences that were avoidable.
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We do not find it mysterious that actions participate in producing later events. Quantum mechanics adds a deeper kind of uncertainty to this familiar fact.
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The agent can choose the action while remaining unable to determine the exact experience that follows.
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Quantum theory helps manage that uncertainty. In cubism, this is not a temporary weakness caused by an incomplete wave function.
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It is central to what the theory is for. This also changes how we think about physical law.
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The usual instinct is to imagine a law as a command issued by nature. Matter must behave according to the equation, and physics discovers the instruction.
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Cubism interprets at least the quantum formalism differently. The rules of quantum theory guide the agent's probability assignments.
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They tell an agent how different expectations should fit together. In that sense, the formalism is normative rather than a direct description of invisible machinery.
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That does not mean quantum theory is invented freely by human beings. The reason agents use these particular rules is that centuries of interaction with the world have shown that classical expectations are not enough.
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Nature has taught us that successful reasoning about certain experiments must obey quantum structure. The normative rule is therefore constrained by experience.
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We did not sit in a room and decide that quantum mechanics would make an entertaining addition to physics.
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The world forced increasingly reluctant physicists toward it. This creates an interesting division between what belongs to the agent and what may belong more directly to the physical system. Cubism identifies the quantum state as personal, along with the probabilities derived from it.
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That naturally raises the question of whether anything in the mathematical structure can still represent an objective feature of a system.
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Fuchs has explored the possibility that quantum dimension may be treated as a capacity associated with the system itself rather than as another personal belief.
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This remains part of Cubism's continuing attempt to identify what in quantum theory reflects the world once the agent-dependent elements have been separated out.
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The importance of this point is easy to miss. Cubism is not satisfied with saying that physics is merely about beliefs.
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If that were the whole story, there would be little reason to continue asking what quantum mechanics teaches us about nature.
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The more ambitious project is to use the distinction between personal and objective elements to expose something about reality that may have been hidden by the usual interpretation of the formalism.
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Think of an astronomer studying a distant star through an imperfect telescope. Some features in the image come from the star. Others come from the telescope, the atmosphere, and the processing of the data.
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If the astronomer wants to learn what the star is really like, it helps to identify which features belong to the observing system rather than mistakenly attributing everything in the image to the star.
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Cubism performs a similar conceptual separation. The quantum state, probabilities, and state updates may contain contributions belonging to the agent's expectations.
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If we remove the temptation to treat those as direct properties of the world, perhaps the remaining structure tells us something more interesting about what physical systems can do and how they can interact.
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This is why the question about the wave function was not merely philosophical decoration. If the wave function is objective, physicists may spend their time trying to understand how that physical object collapses, splits, or evolves.
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If the wave function is personal, those problems change. The deeper task becomes understanding why agents in our universe are rationally required to use quantum probability rather than classical probability when dealing with certain physical systems.
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That question points outward toward the world rather than inward toward the mind. Why does reality confront agents with exactly these constraints?
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Why do different possible actions and experiences fit together according to quantum rules? Why does the structure remain stable enough that physicists can discover it, teach it, and build technologies that work across laboratories and generations?
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Cubism does not yet provide a complete answer to all of those questions. Its advocates openly treat the search for an ontology, an account of what kind of world lies behind the normative structure of quantum theory, as unfinished work.
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Fuchs has described the continuing problem as discovering an ontology appropriate to the agent-centered structure, rather than simply declaring the existing formalism to be the final inventory of reality.
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That incompleteness is worth acknowledging because an interpretation can sound much more powerful if every open question is quietly turned into an answer.
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Cubism gives a clear account of what quantum states and probabilities mean for an agent.
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It offers a clear way of thinking about measurement as action and outcome as experience.
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It provides a way to approach Wigner's friend without requiring one agent state assignment to become the universal description.
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It argues that personal probabilities can coexist with an external world and with shared science.
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What the deeper structure of that external world ultimately is remains a research question rather than a solved mystery.
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One provocative direction appears in the idea that experience itself may be more fundamental than we usually assume.
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Fuchs and Schack have explored whether the creation of new experience in measurement could provide a clue to an ontology suitable for Cubism, drawing on Wheeler's idea of observer participation.
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They present this as a possibility to investigate rather than as a finished physical theory of what everything is made from.
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That distinction matters because saying experience may be fundamental can easily sound as though the universe is made from human thoughts.
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That is not what is being proposed. Human experience is one familiar example of the broader structure Cubism is trying to understand, but the interpretation does not reduce physics to psychology.
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The question is whether events arising in interactions should be given a more basic role than a picture in which all properties are completely fixed before the interaction takes place.
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The result is a universe that looks less like a warehouse filled with finished facts and more like a world in which physical encounters matter.
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We do not need to turn that into a metaphor to see the difference. In the warehouse picture, every possible measurement result would ideally correspond to something already stored somewhere.
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Measurement would simply locate the correct item. In the Cubist picture, the agent chooses an action, the world responds, and the resulting experience becomes part of what actually happened.
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The probability calculation guided the expectation, but did not contain the outcome as an already existing object.
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Anna's laboratory can now be seen in this wider context. She prepares the system, assigns a quantum state, and considers what she might do.
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Her state is not a physical property carried by the system. It is her tool.
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She selects a measurement. That action is real. The world responds with an outcome she did not control. That experience is real for Anna.
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She updates her expectations. Ben later interacts with Anna and obtains his own experience. Their beliefs become coordinated because the same external world constrains both of them.
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Nothing about this requires one universal observer. Nothing requires the universe to consult a human mind before behaving.
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Nothing permits Anna to invent outcomes. The participatory element comes from the fact that Anna is physically inside the situation, and that the encounter she chooses becomes part of what happens next.
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This may also explain why cubism resists the comforting idea that physics should eventually eliminate every first-person statement.
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If every agent is inside the universe, perhaps phrases such as what I expect, what I do, and what I experience are not merely temporary inconveniences caused by imperfect knowledge.
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They may mark a feature of how physical theories are actually used by participants who cannot stand outside reality.
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The great challenge is preventing this first-person structure from sliding into relativism. Cubism tries to do that by keeping the external world firmly in place.
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Anna's expectations are personal, but the answer is not hers to choose. Ben's expectations are personal, but his later experiences force updates.
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Shared scientific practice works because agents can communicate, and because reality repeatedly displays stable enough behavior to coordinate their judgments.
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The strange result is that objectivity and participation no longer have to be opposites. A world can be independent of you without being completely describable from a perspective that belongs to no one.
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Personal experience can matter without making physical law a matter of personal taste. Different agents can assign different states while still learning about the same external systems.
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Agreement can emerge through interaction rather than being assumed from the beginning. This brings us back to the question with which we started.
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What if reality is personal? Cubism would probably tell us that the question needs one final adjustment.
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Reality itself is not merely personal. Our access to it is. Our expectations are. Our actions begin from particular standpoints. Our measurement outcomes enter particular experiences.
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We never possess the universe from every viewpoint at once. Yet those viewpoints do not float separately. They meet through the external world.
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You're having one experience of these words right now. Someone else may hear the same explanation tomorrow in another country, in another room, while carrying an entirely different collection of memories and expectations.
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The two experiences will not be identical. There is no need for them to be.
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What makes communication possible is that both listeners inhabit a world where actions can connect one person to another, where information can be recorded, where experiments can be repeated, and where nature does not change its behavior merely to protect
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whatever someone happened to believe beforehand. Quantum mechanics, read through Cubism, may therefore be telling us less about a strange microscopic substance called the wave function, and more about the relationship between any agent and a world that cannot be completely
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possessed in advance. We act without knowing exactly what will happen. We use experience to form expectations.
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The world answers. We revise. Then we act again. For a working physicist, that cycle becomes preparation, prediction, measurement, result, and update.
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For Cubism, however, the cycle is not simply a temporary method used while we wait for a more complete description.
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It may be the proper form of quantum theory itself. The uncertainty belongs to the agent, but the surprise comes from reality.
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The probability belongs to the agent, but the constraint comes from reality. The action belongs to the agent, but the outcome cannot be commanded.
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The universe in this picture is neither a private dream nor a completely finished machine whose future can be read from an all-knowing position outside it.
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It is a physical world containing agents who are themselves part of what happens. Those agents encounter systems, make choices, receive consequences, communicate with one another, and gradually discover dependable structures in the responses they receive.
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Whether that is ultimately the right way to understand quantum mechanics remains open. Cubism is one interpretation among several serious attempts to understand the theory whose calculations are far less controversial than the story we should tell about them.
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Its value is not that it makes every mystery disappear. It is that it forces us to notice assumptions we normally carry without examining them.
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We assume a quantum state must belong to the object. Cubism asks why. We assume certainty must correspond to an existing property. Cubism asks why.
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We assume measurement merely uncovers what was already there. Cubism asks why. We assume physics must ultimately speak from a viewpoint belonging to nobody.
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Cubism asks whether an agent inside the universe could ever genuinely occupy such a viewpoint.
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After following those questions from uncertainty to probability, from the wave function to measurement, and from Wigner's laboratory to the problem of a shared world, the original title looks rather different.
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Reality being personal does not mean that each of us receives a separate universe. It means that no actual agent encounters the universe except from somewhere with some previous experience, some expectations, and some action they are considering next.
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The external world remains there, refusing to become whatever we would find convenient. It surprises us often enough to keep physics employed.
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We respond by updating what we believe, changing what we do, and asking better questions.
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Cubism takes that ordinary scientific process and suggests that quantum mechanics may have been built around it more deeply than physicists initially realized.
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Perhaps the most important difference is therefore not between your reality and mine. It is between a universe imagined from the outside and a universe encountered from within.
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Cubism chooses the second picture. We do not watch reality from a safe distance. We are physical participants in it, and every experiment is another encounter in which our expectations meet something that does not belong to us.
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Then the detector clicks. The result is there for the agent who receives it. Expectations change.
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Another action becomes possible. Another agent may eventually enter the story and compare what happened.
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From those separate encounters, a shared science continues to grow. Not because anybody has escaped their perspective, but because the world keeps giving all of us something real to respond to.
Topics:
Quantum Mechanics
Cubism
Quantum Bayesianism
Quantum State
Personal Probability
Measurement Problem
Physics Interpretation
Quantum Theory
Agent Perspective
Quantum Probabilities
