I recently posted a track on SoundCloud that included the sonification of LIGO’s gravitational wave data. A student asked me what that meant. Since today is Albert Einstein’s birthday, what better time to try to formulate an answer?
https://soundcloud.com/ethanhein/ligo-chirp
First, some context: 1.3 billion years ago, two black holes collided. Each was about thirty times as heavy as the sun. The collision took a tenth of a second and released fifty times more energy than all the stars in the observable universe. Here’s how it looked:
Of course, black holes being black, you can’t see them; the graphic shows the way that they would warp the appearance of stars behind them.
Here’s how the collision “sounded”:
In the first two runs of the animation, the sound-wave frequencies exactly match the frequencies of the gravitational waves. The second two runs of the animation play the sounds again at higher frequencies that better fit the human hearing range. The animation ends by playing the original frequencies again twice.
Sound is a vibration–of air, water, or any other material. The gravitational waves from the colliding black holes were vibrations of spacetime itself.
Wait, what?
To understand what the heck this all means, first you have to understand Albert Einstein’s theory of what gravity is.
In Einstein’s conception, gravity is the warping of spacetime by mass and energy. Think of the Earth as stretching space and time around it like a bowling ball on a trampoline. If you throw a baseball up in the air, it’s like rolling it up the slope of the trampoline–eventually it’ll roll right back down. If you could throw the baseball hard enough, you could roll it all the way up and out of the indentation in the trampoline, where it would keep rolling forever. And if you could throw with exactly the right amount of force, the baseball would roll around the sides of the indentation, never escaping or falling back in. That’s how moons and satellites stay in orbit.
Gravity isn’t just the warping of space. It’s also the warping of time. To understand what that means, you have to wrap your head around an extremely weird idea: everything is moving through spacetime at the speed of light. Some of that motion is through space, and some of it is through time. If you move faster through space, you necessarily move slower through time, and vice versa. You and I don’t notice this, because we move through space so slowly, but gravitational time dilation has a measurable impact on GPS satellites. Their speed relative to the Earth is about seven thousand miles per hour, which is fast enough for their movement through time to slow very slightly. The GPS system has to account for that, or it won’t work. Smaller and lighter things, like subatomic particles, move fast enough through space so that their movement through time can be dramatically slowed. Light itself moves through space at the speed of, well, light, so it doesn’t move through time at all. When gravity warps space, it also warps time. Dude.
When black holes collide, they produce a hell of a lot of spacetime warping. The distortion ripples outward, exactly like ripples in a pond after you throw a rock in. By the time the ripples have traveled 1.3 billion light-years, they’re pretty small. But if you can detect very small deviations in space (or time), you can detect gravitational waves.
So, how do you detect gravitational waves?
Lasers!
Laser light, like all electromagnetic radiation is a wave. (And also particles. Quantum mechanics is hard.) If you shoot a laser through a half-silvered mirror, you get two lasers. If these two lasers travel the same distance and then shine onto precisely the same spot, they neatly cancel each other out through the magic of wave interference. If the distance traveled by one laser is slightly different from the distance traveled by the other, they won’t exactly cancel out. So lasers are a great way to measure distances precisely. This is good, because the size of the spacetime warp that LIGO detected is orders of magnitude smaller than the width of a proton.
The real technological achievement here is not so much detecting the gravitational wave itself. The really big deal is isolating the lasers from all other sources of vibration and electromagnetic interference so they don’t totally bury the signal you’re looking for. The LIGO detectors are so sensitive that they pick up lightning strikes on the other side of the world. The engineering required to keep miles-long laser beams free of even the slightest outside interference was utterly heroic and is well worth reading about if you like this kind of thing.
That’s cool and all, but why should I care?
A substantial majority of the stuff in the observable universe doesn’t interact with light (or other kinds of electromagnetic radiation) at all. We know that dark matter and energy are there, because they have an effect on the visible stuff like stars and galaxies. But we’ve never been able to actually look at it. Being able to detect gravity directly could radically expand our understanding of the universe. A good analogy is the amount of the universe we can see using only visible light, versus with infrared light and X-rays and gamma rays and microwaves and so on. We’re about to widen our perspective even further.
So what is that sound we’re hearing?
Turning the LIGO data into sound is pretty easy because it’s already sound. All you have to do is take the contraction and expansion of spacetime and translate it into contraction and expansion of your speaker cones, and thereby the air. It’s a vague thump at its original frequency, but if you transpose it up to human hearing range, you can hear the rising pitch as the black holes accelerate into their death spiral. Science!