Speaker A
At about six o'clock in the morning on September 14, 2015, scientists witnessed something no human had ever seen: two black holes colliding.
Learn how LIGO detected gravitational waves from two colliding black holes, and how these ripples in spacetime reveal the universe's hidden events.
Key Takeaways
- Gravitational waves are ripples in spacetime produced by accelerating massive objects.
- LIGO detected gravitational waves from a black hole merger in 2015, opening a new way to observe the universe.
- Gravitational wave data reveals properties of their sources, such as mass and orbital shape.
- Each new astronomical tool tends to produce unexpected discoveries that can reshape our understanding.
What the video covers
- On September 14, 2015, LIGO detected gravitational waves from two black holes, each about 30 times the Sun's mass, colliding and merging.
- The signal lasted only a fifth of a second and was LIGO's first observation of gravitational waves.
- Gravity is the attractive force between any two masses; more mass means a stronger pull, and pull weakens with distance.
- Fluctuations in gravity travel outward as gravitational waves, like ripples on a pond, weakening as they spread.
- Einstein's general relativity describes gravity as curvature in spacetime, with massive objects creating depressions in it.
- Moving masses send ripples through spacetime; a passing gravitational wave would stretch and compress a body in alternating directions.
- LIGO uses L-shaped detectors with laser-measured arms; tiny length changes can indicate a passing gravitational wave.
- The video compares gravitational wave detectors to radios, since they extract signals that would otherwise go unnoticed.
- Gravitational wave signals can be converted to sound, producing a 'chirp' that signals two objects orbiting into each other.
- Other events, such as star collapses before supernovae and neutron star collisions, may also produce detectable gravitational waves.
Full Transcript — Download SRT & Markdown
Speaker A
Both about 30 times as massive as our Sun, they had been orbiting each other for millions of years.
Speaker A
As they got closer together, they circled each other faster and faster. Finally, they collided and merged into a single, even bigger, black hole.
Speaker A
A fraction of a second before their crash, they sent a vibration across the universe at the speed of light.
Speaker A
And on Earth, billions of years later, a detector called the Laser Interferometer Gravitational Wave Observatory, or LIGO for short, picked it up.
Speaker A
The signal only lasted a fifth of a second and was the detector's first observation of gravitational waves.
Speaker A
What are these ripples in space? The answer starts with gravity, the force that pulls any two objects together.
Speaker A
That's the case for everything in the observable universe. You're pulling on the Earth, the Moon, the Sun, and every single star, and they're pulling on you.
Speaker A
The more mass something has, the stronger its gravitational pull. The farther away the object, the lower its pull.
Speaker A
If every mass has an effect on every other mass in the universe, no matter how small, then changes in gravity can tell us about what those objects are doing.
Speaker A
Fluctuations in the gravity coming from the universe are called gravitational waves. Gravitational waves move out from what caused them, like ripples on a pond, getting smaller as they travel farther from their center.
Speaker A
But what are they ripples on? When Einstein devised his Theory of Relativity, he imagined gravity as a curve in a surface called space-time.
Speaker A
A mass in space creates a depression in space-time, and a ball rolling across a depression will curve like it's being attracted to the other mass.
Speaker A
The bigger the mass, the deeper the depression and the stronger the gravity. When the mass making the depression moves, that sends out ripples in space-time.
Speaker A
These are gravitational waves. What would a gravitational wave feel like? If our bodies were sensitive enough to detect them, we'd feel like we were being stretched sideways while being compressed vertically.
Speaker A
And in the next instant, stretched up and down while being compressed horizontally, sideways, then up and down.
Speaker A
This back and forth would happen over and over as the gravitational wave passed right through you.
Speaker A
But this happens on such a minute scale that we can't feel any of it.
Speaker A
So we've built detectors that can feel it for us. That's what the LIGO detectors do.
Speaker A
And they're not the only ones. There are gravitational wave detectors spread across the world.
Speaker A
These L-shaped instruments have long arms, whose exact length is measured with lasers. If the length changes, it could be because gravitational waves are stretching and compressing the arms.
Speaker A
Once the detectors feel a gravitational wave, scientists can extract information about the wave's source.
Speaker A
In a way, detectors like LIGO are big gravitational wave radios. Radio waves are traveling all around you, but you can't feel them or hear the music they carry.
Speaker A
It takes the right kind of detector to extract the music. LIGO detects a gravitational wave signal, which scientists then study for data about the object that generated it.
Speaker A
They can derive information, like its mass and the shape of its orbit. We can also hear gravitational waves by playing their signals through speakers, just like the music a radio extracts from radio waves.
Speaker A
So those two black holes colliding sounds like this. Scientists call this slide whistle-like noise a chirp, and it's the signature of any two objects orbiting into each other.
Speaker A
The black hole collision was just one example of what gravitational waves can tell us.
Speaker A
Other high-energy astronomical events will leave gravitational echoes, too. The collapse of a star before it explodes in a supernova, or a very dense neutron star colliding.
Speaker A
Every time we create a new tool to look at space, we discover something we didn't expect, something that might revolutionize our understanding of the universe.
Speaker A
LIGO's no different. In the short time it's been on, LIGO's already revealed surprises, like that black holes collide more often than we ever expected.
Speaker A
It's impossible to say, but exciting to imagine, what revelations may now be propagating across space towards our tiny blue planet and its new way of perceiving the universe.
Topics:gravitational wavesLIGOblack holesgeneral relativityspacetimeastronomygravityneutron starsTED-Edastrophysics
