Gravitational waves

February 11, 2016

There’s a very exciting announcement today from the LIGO experiment: they are reporting the first ever direct observation of gravitational waves. Read all about it.

The existence of gravitational waves — which are “ripples” in spacetime produced by catastrophic astrophysical events like black hole collisions or supernovae — are one of the most important predictions of general relativity. Today’s discovery will go into every future textbook on the subject, and the scientists involved go straight to the Nobel shortlist.

The LIGO experiment (LIGO = Laser Interferometer Gravitational-Wave Observatory), if you haven’t heard of it, is one of the most amazing physics experiments ever conceived. Gravitational waves travelling through the detector change its size by a small amount, and so the experiment consists of making continual, very precise measurements of distance. The sensitivity is exquisite: they can detect a change in length of a fraction of the radius of a proton.

The particular observation reported today is of a collision of two black holes at an estimated distance of 1.3 billion light years. Here is the technical paper describing the discovery.

What a great day!

8 Responses to “Gravitational waves”

  1. Mac Horton Says:

    So how does this affect me personally? 🙂

    Of course I can’t understand any explanation much more detailed than the one you gave and the Science article you linked to, but I have two questions, aware that they may not be answerable in terms I can grasp: 1) how do they know the waves originate in the black hole collisions? 2) does this reveal something about the nature of gravity that was not previously known? I.e. does it go further than confirming relativity?

  2. cburrell Says:

    Good questions.

    The signal observed by the experiment is the only evidence for the black hole collision; we have no independent way of confirming it. The reason they interpreted the data as originating from a black hole collision is because over the years people have studied what GR predicts a black hole collision would look like in this sort of detector. The experiment observed steadily mounting oscillations as the black holes circled one another at steadily decreasing distances, then a merger, and then a ‘ring down’ as the new black hole settled down. A supernova, for instance, would look quite different.

    This particular observation appears to be fully consistent with General Relativity, so, no, it doesn’t teach us anything new about gravity. However, it is the most complicated test of General Relativity to date, and so the fact that GR got it right is exciting.

  3. Mac Horton Says:

    But do they have *some* other reason to believe a black hole collision happened in a particular region, right? If not, this wouldn’t seem to quite rise to the level of *proof*.

    Does the duration of the “chirp” actually match the amount of time it took the black holes to collide and merge? If so that’s mind-boggling.

    Which reminds me: I’ve said for years that when I retire I’m going to read some or at least one of those books explaining contemporary physics to the lay reader. Do you have any recommendations? It can’t really have any math to speak of, as I never went beyond basic calculus and have long forgotten that.

  4. cburrell Says:

    No, there’s no other reason to think a black hole collision happened. By its nature, a black hole collision can’t be observed with telescopes. The only way to see it is by its gravitational effects.

    So the conclusion that a black hole collision occurred is indeed the result of an inference from the data, and it is true that inferences are not air-tight, but the number of possible explanations in this case is small. Catastrophic astrophysical events like that which produced the spacetime fluctuations observed at LIGO can only be made by a small number of objects, and, according to those involved with the experiment, the signal they observed fits what one would observe from black holes and not from the other candidates. So, as you say, not a “proof”, but proofs are not really what the empirical sciences deliver.

    By the way, have you ever stopped to think how odd it is that so much of the scientific enterprise is built on a logical fallacy? If A, then B. We observe B, and so conclude A. (Example: If two black holes of such-and-such masses collide, then we would observe signal X. We observe X, therefore two black holes collided.) It’s rather surprising that it works as well as it does.

    I am sometimes asked to recommend a good book for laymen, and I’m afraid I don’t have a good answer. I would avoid books about string theory, as they are generally more empty promises than substance. Perhaps one of the other physicists whom I know read this blog from time to time will chime in with a suggestion. If I think of one, I’ll let you know.

  5. Mac Horton Says:

    Thanks. I do sometimes think about that fallacy, although I hadn’t actually identified it as a fallacy. It strikes me when I think about the theories that construct descriptions of the remote past, both earthly and cosmic. No doubt they’re correct, or mostly correct, but it amuses me to imagine the shock that would be felt if they turned out to be wrong in some fundamental way–e.g. if the stars were actually much much closer and much much smaller than we believe them to be. I’m sure the evidence against that is so overwhelming as to make our measurements effectively certain, but, like I say, it amuses me.

  6. cburrell Says:

    The fallacy has a name: affirming the consequent.

    Classical physics found itself overturned in something like that way a century ago. The premises on which much of the structure rested were changed. Of course most of what classical physics predicted is also predicted by the new physics, but conceptually there was a major shake-up.

    The stars are indeed far away.

  7. Mac Horton Says:

    Oh, and another question about black holes: I know they can’t be seen, but I had the impression that their presence could be (and has been) detected with a telescope by virtue of their effects on other bodies in the neighborhood. Is that the case? You don’t mean that their existence was only theorized until this experiment, do you?

    • Vince Says:

      Yes, you’re right. By observing how other objects close to the suspected black hole behave, one can infer the mass and size of the prospective black hole and deduce (using some astrophysics) that the object is much too massive to be anything but a black hole.

      By the way, a couple of good books (I’ve heard) are the two “Theoretical Minimum” books by Susskind. The first is on classical dynamics, the second on quantum mechanics. Some math (calculus) is used.


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