# About those gravitational waves they just discovered

First, what is a gravitational wave?

I find it interesting that some people are expressing difficulty in understanding what a gravitational wave is, as though everybody (who is not a physicist) has a perfectly good understanding of what any kind of wave is. We don’t need to go too deeply beneath the surface, as it were, to understand this well enough to be amazed at the discovery, but not well enough to get a job being a Gravitational Waveologist.

Imagine a perfectly flat pond. Imagine throwing a stone out into the middle of the pond. Now imagine ripples, tiny waves, spreading out from the point where the rock hit the pond.

Now imagine the smallest possible stone that could produce a visible wave on the water. It would be pretty small. The ripples would be pretty small.

Gravitational waves are just like that, but even smaller, so you can’t see them. Not that you would “see” them with your eyes, but rather, you can’t detect them, in any normal way.

They won’t make the moon wobble, or a bird fall out of the sky. You know about super sensitive satellites that orbit the earth detecting variations in gravity such as the decrease in the pull of a glacier on the nearby ocean when the glacier melts by way less than one percent, and that sort of thing. Gravitational waves are presumably passing through that satellite all the time, but it can’t detect them.

Since they had not been detected for a long time, it had not been 100% certain that they exist. However, evidence for their existence has been mounting, and they have, in a sense, been indirectly observed.

Gravitational waves were first postulated by Einstein as early as 100 years ago as part of his application of his General Theory of Relativity, though he later temporarily retracted the idea (but then put it back).

Russian scientists advanced ways of detecting them in the 1960s, and one scientist thought he had found them a bit later in time. Further methods were developed to try to see them. In 1974 one group detected the slowing down of a pulsar orbiting a neutron star in such a way that implicated “gravitational radiation,” and this work stood (and earned a Nobel Prize). That was probably the first actual “detection” of the waves, in a sense.

In the late 1970s, the LIGO project was started to build a gravitational wave detecting antenna. In 1996, the VIRGO project was started in Italy. The Germans had just built GTO600, and partnered with LIGO soon after.

Over the last five years, the LIGO wave detector was upgraded, and after a period of time being turned off, was started up again in September. And bingo, there was a gravitational wave. Detected. Probably.

From Science Magazine:

Long ago, deep in space, two massive black holes—the ultrastrong gravitational fields left behind by gigantic stars that collapsed to infinitesimal points—slowly drew together. The stellar ghosts spiraled ever closer, until, about 1.3 billion years ago, they whirled about each other at half the speed of light and finally merged. The collision sent a shudder through the universe: ripples in the fabric of space and time called gravitational waves. Five months ago, they washed past Earth. And, for the first time, physicists detected the waves, fulfilling a 4-decade quest and opening new eyes on the heavens.

The discovery marks a triumph for the 1000 physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of gigantic instruments in Hanford, Washington, and Livingston, Louisiana. Rumors of the detection had circulated for months. Today, at a press conference in Washington, D.C., the LIGO team made it official. “We did it!” says David Reitze, a physicist and LIGO executive director at the California Institute of Technology (Caltech) in Pasadena. “All the rumors swirling around out there got most of it right.”

The way you detect a gravitational wave is to place two objects some distance from a laser source. You hit both objects with the laser and measure the return time, which should be exactlyvthe same for each object. If either object moves, that might have been because of a gravitational wave.

Obviously objects are going to move. Temperature changes, vibrations in the earth, all those things, can cause the objects to vibrate or move in some way. So you figure all that out and make all those things impossible. Also, the objects have to be really far out in order to get a signal. In the case of LIGO, the objects are four kilometers out from the laser source.

Yes, there is a reason that LITO was first funded in 1979 and only now has a result!

Now, here’s the part I don’t like. I don’t like the fact that the device was turned off and upgraded, then, very soon after being turned on, found a gravitational wave, and apparently hasn’t found another since then. I’m worried that this is similar to when CERN detected faster than light neutrinos. Everybody knew there were no faster than light neutrinos, but the instruments detected them anyway. Eventually, it was discovered that something was going on with the way the instruments were wired up that made the detector wrong.

I asked Jeffrey Bennett, author of What Is Relativity?: An Intuitive Introduction to Einstein’s Ideas, and Why They Matter, how convincing the results are. He told me,

“I think it’s very convincing. The reason they’ve waited months from the actual detection to report the event was so that they could check every possible source of artifact that might be something other than a real signal. So at this point, it’s well over 99% likely that the detection is real. Remember that the detection was made by both LIGO sites — Louisiana and Washington — and the delay between the two signals agrees with the light travel time. As to the “coincidence” — it depends on how rare or common these events are. Currently, no one has a good prediction for how often 2 black holes should collide and merge somewhere in the universe. If such events are very rare — e.g., one every 10 years or one every century — then it would indeed be a surprising coincidence. But if these events happen, say, once a year or more, then we’d expect to get a signal within a few months of starting a machine like this. The real test, then, will be whether other similar signals are detected over the next few years. I suspect they will be, and so it will turn out that these events are relatively common…”

Is there something wrong with the LIGO detector?

See the link to Science Magazine above for discussion of how hard the team worked to eliminate alternative explanations. Quite a bit, and quite convincing. But the gravity of this situation, this new discover, is so great that it may be good to retain a small amount of healthy skepticism.

By the way, don’t confuse “gravitational waves” with “gravity waves.” Going back to the pond, the ripples or waves on the water. Those are technically called gravity waves, as are some of the patterns we see in clouds. This is simply wave energy within a gravitational field. Gravitational waves are a totally different thing.

ADDED: Neil dGrasse Tyson on this discovery: