Ladies and Gentlemen, we have detected gravitational waves! We did it!” And with that, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the discovery of a strange feature of General Relativity that’s been hiding since the 1960s.
Gravitational waves are ripples in the fabric of space, caused by the acceleration of mass. The bigger the mass and the faster it’s accelerated, the better, and there’s nothing that better fits that description than two black holes orbiting each other. These waves expand and compress space, which slightly changes distances between objects in their path. Two objects floating freely in deep space far from any forces, would find themselves carried a little towards each other and then a little away from each other and back and forth, as the wave passed over them. If you could measure the distance between them accurately, like with a laser range-finder, you’d detect that oscillation. But this is tough to do on the Earth, because you can’t have the two objects floating freely any more. The best you can do is to hang them from fine threads, like a swing. They’re not free to move in every direction, but they can swing back and forth.
So, how did we find them? LIGO shines laser beams down two 4-kilometer (2.5-mile) vacuum pipes, arranged in an “L” shape. At the end of each pipe is a mirror that reflects the beam back and measures its length, with an accuracy of 10^-18 m, a thousandth the width of a proton. This always amazes me, because the mirror itself is made out of atoms, which are bumpy and much larger than protons. And yet, it works because the bumps average themselves out. When a gravitational wave comes by, it shrinks the distance along one pipe while expanding the distance along the other (as they’re at right angles) and then it switches.
What they’ve announced today is the detection of gravitational waves coming from the merger of two black holes, spiraling into each other a billion light-years away from us. At 29 and 36 times the mass of the Sun, these aren’t very big for black holes: the ones in the centers of galaxies have millions or billions of solar masses. But the smaller ones are more plentiful, which means they’re more likely to collide. As the two in question orbited each other, the gravitational waves carried away more and more of their energy, making them spiral inward, until they finally collided and released massive amounts of energy (the equivalent of the mass of three Suns converted directly into energy).
Take a look at the wave patterns that LIGO felt:
The vibration is vertical, and time goes to the right. Early on, the waves are slow (i.e., broad) and weak (i.e., short). As the black holes spiral inward, the waves strengthen and increase in frequency until they make a loud “chirp!“ when they collide. Then, there’s a faint wiggling at the end, as the new, combined black hole rings like a bell until it settles down.
This pattern is so clear, you can see it without needing the computer to sort it through for you. What are the odds of detecting this? My wife — who works in numerical relativity (the computer simulation of the waves) — tells me that it was entirely possible they’d run this for decades before they heard anything. Instead, they found this within days of turning the machine on. That suggests that these black holes are colliding all over the place, and we’re going to find plenty more.
So what good is this? The press is mostly writing headlines shouting, “Einstein is proven right!” This is true in a simplified sense, but not too many of us were expecting otherwise and that’s not really why all of this was built. With more detections and better signals, we’ll get to the point where we can test theories that go beyond General Relativity, which is interesting. But I think the real value of this discovery will be observational.
Until now, astronomy has almost exclusively relied on what we can see with light: i.e, electromagnetic radiation, ranging from radio waves on one end to gamma rays on the other. We can see the gas falling into black holes — glowing x-ray hot before it disappears — and we can see supernovas from the light they emit. But there’s a lot going on down in the compact centers of these things that we can’t see with light. Gravitational waves will give us an entirely different way to probe them.
Already, the gravitational wave people and the visible light astronomers have teamed up to do joint searches for flashes of light that might accompany a gravitational wave burst. My wife, for example, is working on the computer simulations for these. This photo shows her working out the algorithm for her models.
The discovery of gravitational waves has been a long time coming. Searches started in the 1960s, and there’s even a detector on the Moon, placed there by Apollo 17. But the evidence up to this point had always been indirect. Now, we’ve finally got something both direct and obvious.
That’s big news.