on Gravitational Waves

Light waves are a fundamental way we interact with and understand the universe. It’s what our eyes see, and it was the first thing we saw through a telescope. In fact, when it comes to astronomy, light waves (whether gamma rays, X-rays, UV, optical, infrared or radio) are basically the ONLY way we can detect anything about the rest of the universe. Barring planetary science where we can actually set down instruments on objects, and the odd cosmic ray, that’s basically it.

…at least it was, until Einstein proposed the General Theory of Relativity a hundred years ago. General Relativity implies an ADDITIONAL kind of detectable waves completely unlike anything we’ve known before: Gravitational waves, the motion of spacetime itself. There was absolutely no evidence of such a thing, and no reason to expect it actually exists, except that every other prediction of General Relativity has been proven true.

Still from the Caltech LIGO movie

What was rumored was that Advanced LIGO (the Laser Interferometer Gravity-wave Observatory) had detected a 65 solar mass event made up of 32 and 27 solar mass black holes colliding, and two papers would be released during the webcast.

What actually happened? On September 14, 2015, LIGO detected a signal that looked exactly (exactly! The figures in the press conference were impressive) like the prediction for two thirty solar mass black holes colliding. The LIGO team has spent the last few months confirming this. The resulting paper is here (or here if the journal’s server goes down again).

This is a huge deal. LIGO has proven that gravitational waves exist. There’s a whole new way to look at the universe we weren’t aware of before. This is like the fantasy-book staple of suddenly discovering a hidden world that normal people simply can’t see – except this is real, and has been opened up by physics. This will probably win the LIGO team a Nobel Prize (and a bunch of others).

So where are these gravitational waves supposed to come from? Einstein’s general theory of relativity famously considers gravity to cause a bending (or warping) of space-time. So black holes are often represented like this:

Actually, this is a still from the opening credits of Disney’s 1979 film “The Black Hole”, but that kind of proves my point about “commonly”…

When objects move, the warping of space-time should change, too… but at the speed of light, so you get a spiralling effect like a lawn sprinkler head.  So if you have two black holes orbiting each other, spacetime should look like this:

From Wikipedia

I’ve been using black holes here because they’re expected to deliver the biggest gravitational wave signals, because of how “deep” their gravity wells are. Despite the fact that neutron stars (and pairs of neutron stars) are more common, they are theoretically not supposed to produce signals as large as black holes. It’s also predicted that neutron star pairs will usually be so far apart that it will take billions of years for them to actually merge, rather than a few million. So, black holes are what LIGO expected to see, and that’s what they did see.

Specifically, they saw black holes merging. As black holes merge, the gravitational waves get larger and larger (seriously, click that), until the actual moment of collision. The waves from such a system should be detectable (only light waves are visible). Now, gravity isn’t actually dropping “down” into anything; that flat 2d plane is supposed to be our 3d universe. The warping just represents the way spacetime is bent in a way that makes more intuitive sense. You could also think about space as squeezing like an accordion, or jello (as the press conference used): It squishes space along the direction the waves are expanding in, and stretches them out along the other two. Yes, spacetime is wibbly-wobbly. In a sense. The problem is, the waves will be tiny, because (for reasons still unknown) gravity is actually incredibly weak compared to the other three fundamental forces of nature.

So, the detection is very difficult, and has taken multiple iteratively improving attempts over the past 40-50 years to achieve. It’s been a risky process, because it requires incredibly expensive equipment and isn’t guaranteed to return anything: General Relativity didn’t necessarily need to be an accurate explanation of everything. Nevertheless, because it’s so important, it was still deemed very worthwhile. The Astronomy Decadal survey from 2010 considered the LISA space-based laser interferometer a super-high priority (although it was cancelled anyway, due to the expense and is now limping along with only European support). As it is, LIGO is the most expensive project NSF has ever funded.

The current state of the art is LIGO, now in its Advanced LIGO upgrade, a joint project of Caltech, MIT, and numerous other institutions around the world. This is how LIGO is designed: it’s designed to measure the distance between the laser target and a mirror. When gravity waves pass by, they will compress space, and the slight deviations will be detectable by a sensitive enough instrument. That’s where the whole laser interferometer thing comes in: they are incredibly accurate at measuring small deviations in position. Light gets split into two beams, sent down two different tunnels, bounced off mirrors, and when they come back they should interfere with each other and cancel each other out exactly. If they don’t, it’s because something changed the length of one or both of the tunnels.

An interference pattern. Not like LIGO’s, but the point is that there’s a large difference in the appearance of the interference pattern with each tiny change in the distance between S1 and S2.

In LIGO’s case, this is light interfering with itself after travelling down two long L-shaped tunnels. Advanced LIGO uses an L-shaped tube 4 kilometers long, flatter than the curvature of the Earth, in a complete vacuum, to measure vibrations smaller than the width of an atom. That’s really small.

Of course, LIGO is, in principle, also very good at detecting earthquakes, trucks rumbling by on the highway… that’s why the experimental equipment has been so carefully designed and redesigned, and why there are two (soon to be three) installations; one in Louisiana, one in Washington, and soon one in India.

The press conference was impressive: The very first thing out of the mouth of the first speaker was “We have detected gravitational waves.” The black holes had 29 and 36 solar masses, and the resulting black hole is 63 solar masses – three solar masses of energy was given off in gravitational waves, over 20 milliseconds. That rate of energy emission is 50x higher than every star in the universe put together (thank goodness it only lasted 20 milliseconds.) And that was detected 1.3 billion light years away, 1.3 billion years later (but just barely! It’s hard to warp spacetime.) The signal came vaguely from the location of the Magellanic Clouds, but it’ll be difficult to tell exactly where until more detectors come online and improve the ability to triangulate sources.

The signal LIGO detected was almost strong enough to have been found by the original LIGO setup; that means there are probably many more weaker signals in LIGO we’ll see over the coming months as people play with the data with increased confidence. LIGO India and Italy’s Virgo detector are all supposed to come online later this year and next year, which will allow more precise triangulation of the source of the signals. Japan is also making their own device to be ready in 2019; it’s only going to have a 3 kilometer baseline, but it’s got significant technological advances over what LIGO has now. LIGO itself is, apparently, only operating at 1/3 of its planned capacity, so even signals like this will improve over time. As Kip Thorne pointed out in the press conference, every new region of the electromagnetic spectrum facilitated new discoveries. With radio, we found pulsars and distant quasars. With X-rays, we found black holes and gamma-ray bursters. Now, with gravity waves, we can see the otherwise-invisible merging of two black holes billions of light years away… and who knows what else we’ll find. This is incredibly exciting for science.

Some other notes from the press conference:

  • Apparently Joe Weber was a pioneer in the early work and was later pushed out of the field contentiously. The presenters mentioned his work frequently and seem to be trying to make up for whatever past bad blood existed between them.
  • Surprisingly, the frequencies of gravitational waves are within the frequency of human hearing and literally sound like a dripping noise. (or, in the slowed-down version they played in the press conference, a “THWUP”).
  • The existence of 30-solar-mass stellar black holes was contentious before this. Stars shed a lot of mass as they die, so either the stars were truly gargantuan to start with, or (as seems to be the case) there are ways to retain most of the mass. There are some stellar astrophysicists feeling pretty well vindicated right now.
  • This discovery might even help constrain quantum mechanics (and the elusive grand unified theory that would unite the two). In quantum mechanics, gravity is conveyed by (potentially massless) particles called gravitons, and this new result (according to Kip Thorne) puts the best upper limit on the mass of the hypothetical graviton – the waves observed by LIGO would have been distorted if the mass of the graviton was significant – at something like 10^-55 grams. Having numbers like that will help theorists.
  • EDITED TO ADD: A lot of people all over the internet are asking if this means any advances in time travel or warp drive. This question was even asked by a reporter during the press conference, and the answer is… no, not really. This discovery is more about suddenly being able to see in a new way. We might (heck, probably will) learn new things about physics from things we detect in gravity waves, but there’s no reason to think that will necessarily lead to warp drives or time travel.

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