Gravitational Waves: The Universe's Subtle Soundtrack, with Janna Levin | Big Think

Gravitational waves are such a difficult concept. When Einstein first wrote down his theory of curved space-time, he said the most important question that he needed to look at next was the question of whether or not there were waves in the shape of space-time.

If space-time can curve, so let's say you imagine around the earth or around a black hole there are these deep curves, and those are the paths that you naturally fall along if you're falling around a black hole or the earth or the sun or anything else. That traces out the shape of space-time. But if I take that black hole and I start to move it around, those curves in the shape of space have to follow the moving black hole. They have to somehow readjust, straighten out in some places and curve in other places. And those are the gravitational waves.

And they actually move like a wave in space and time. If you were floating nearby, you would kind of bob on the wave. You would be slightly squeezed and stretched as it passed. And in Einstein's approach to gravity, we think that the gravitational waves travel at the speed of light so they emanate out through the universe kind of as though fish were swirling in a pond and if the pond started to create waterways, those would emanate out through the ocean.

When black holes collide, they're like mallets on a drum and space-time itself rings. And if we could hear that sound— which you might be able to actually, technically if you were an astronaut floating near enough to colliding black holes—you might actually have your eardrum resonate in response and hear something. And it would sound—the word that we use is a chirp—it sounds like a chirp.

And what that really means is the ringing of space sweeps up in frequency as the black holes get faster and faster and finally merge. So they're going to near the speed of light by the time it's loud enough to be heard. A gravitational wave is much closer to a sound than just some kind of scientific analogy. You can take a lot of different wave forms and translate them into sound, it doesn't really mean that they're actually sounds.

But a gravitational wave you could liken it to how an electric guitar, when you pluck the string of an electric guitar, it doesn't technically make a sound. It vibrates and the body of the guitar translates that vibration into a sound. In some sense, LIGO, the instrument that measured the gravitational wave, it's doing something like that. The gravitational wave is a ringing of space-time, a vibration of space-time and the machine records the vibration and plays it back as sound.

You can actually sit in the control room at the two sites of the LIGO observatories and listen to the detector. It's surprising to people I think to know that gravity is actually very weak. We think that gravity is this incredibly strong force. We feel heavy in our chairs, we feel heavy when we lie in bed at night and we think that gravity is this thing we resist, but it's actually incredibly weak. The whole earth is pulling on me and I can still jump.

And so by the time the gravitational waves get to the earth, the phenomenon is so small that the experiment has to be incredibly sensitive to detect this ringing of space-time. Presumably right now the room is flooded with the gravitational waves coming from incredibly far away, but they're just imperceptibly weak by the time that they get here.

So what the instrument has to do is it has to record over four kilometers variations in the shape of space-time of less than 1/10000th the width of a proton. Another way to try to have a sense of that scale is to make the comparison that it's equivalent to trying to measure the width of a human hair compared to 100 billion times the circumference of the world.

Even though LIGO had to record an incredibly quiet ringing of space-time, incredibly subtle and quiet, and it was this incredibly difficult measurement. It was from an event that was the most powerful event that we've detected since the Big Bang. More power came out of the merger of those two black holes, 1.3 billion years ago, than all...