Gravitational Waves Detected!

This is an artist's conception of two merging black holes and the gravity waves they generate.
This is an artist’s conception of two merging black holes and the gravity waves they generate.

Einstein’s General Theory of Relativity makes some outlandish claims. For example, it says that the rate at which time passes depends on the strength of the gravitational field to which you are being exposed. It also says that gravity isn’t really a force. Instead, it is a consequence of how massive bodies warp spacetime, a four-dimensional mesh in which the three dimensions of space are merged with time. When I first read about this wild theory, the scientist in me was very skeptical. However, its predictions have been verified time and time again, so the scientist in me is forced to accept it as a reasonable description of the natural world.

For example, the global positioning system (GPS) must take relativity into account in order to work properly. Because they are farther from the center of the earth, the satellites that make up the GPS experience a lower force of gravity than we do on the surface of the earth. As a result, time passes more quickly for them than it does for us. If this were not taken into account, the GPS couldn’t accurately determine your absolute position on the surface of the earth.1 (There are many other factors that must be taken into account, including the effect of relative motion on time, but that is a part of Einstein’s Special Theory of Relativity and is not related to this post.)

Of course, there are many other confirmations of Einstein’s General Theory of Relativity. Mercury’s closest approach to the sun is best explained by general relativity. General relativity gives the only correct description of how a massive object bends the path of light. An experiment first done in 1959 showed that gravity causes a shift in the wavelength of light, which was predicted by general relativity. More recently, satellites confirmed a process called frame dragging, which is also a prediction of general relativity.

Just a few days ago, Physical Review Letters published a paper that provides yet another confirmation of general relativity, but this one is more important than many of the others.

In Einstein’s view of the universe, we are embedded in a four-dimensional mesh called spacetime. Anything with mass warps this mesh, and that produces the effect we call gravity. Two massive objects accelerate towards one another not because there is a force acting between them, but because the way they warp spacetime requires that they accelerate towards one another. Well, because massive objects warp spacetime, their motion should send ripples (gravitational waves) through that mesh.

Unfortunately, these waves are predicted to be very small, so they are incredibly hard to detect. Nevertheless, physicists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) set out to find them. They built two L-shaped detectors, each of which is 4 kilometers long. Laser light is bounced back and forth in the detectors. If everything is stable, the laser light overlaps. If something “wiggles” the detector (including a gravitational wave) the laser light won’t overlap perfectly, and a signal is generated.

Of course, there are a lot of things that can wiggle the detectors, such as earthquakes, the movement of heavy traffic outside, and even the crash of waves on the shoreline. Lots of things are done to reduce the importance of such effects, but one of the most crucial is that the two detectors are placed about 3,000 kilometers apart from one another. If they both see the same signal, the time between those two signals can tell the physicists the speed at which whatever caused the signal is moving. To me, that’s the most important part of this experiment.

On September 14, 2015 at 09:50:45 UTC, both detectors recorded the same signal. This isn’t unusual. Even when things are working perfectly, the detectors get signals from all sorts of unexpected “wiggles.” In addition, the system is set up to generate false signals from time-to-time in order to see if the scientists analyzing the experiment are being careful enough to weed out false positives.

In this case, the signal wasn’t a false positive, and the two detectors saw the same signal about 0.007 seconds apart from one another.2 There is some error associated with the time measurement, but it is consistent with something that is traveling at the speed of light. Waves from earthquakes, traffic, and surf cannot travel at the speed of light. However, gravity waves are supposed to. It is hard to imagine any other explanation for essentially identical signals produced by something traveling at the speed of light.

Based on the characteristics of the signals, the scientists estimate that the gravity waves were produced by two black holes that are 29 and 36 times as massive as our sun spiraling towards one another and merging, as depicted in the drawing above. That’s two really massive objects experiencing a violent “collision,” but they produced such a weak signal that it took two 4-kilometer-long detectors to observe it. That should tell you something about how difficult it is to detect gravity waves!

As an experimental feat alone, then, this result is incredible. However, it is also a very important confirmation of general relativity. First, I don’t know of a nonrelativistic theory of gravity that predicts gravitational waves. More importantly, however, this directly confirms Einstein’s view of the universe. Based on this measurement, we really do live embedded in a mesh of spacetime, and massive objects really do warp it. Gravitational waves set Einstein’s theory of gravity apart from any other gravitational theory of which I am aware, and that makes this measurement incredibly important.

Now, of course, this is just one measurement. It is always possible that it is the result of some strange error that no one has considered. However, if this result is real, there should be more violent collisions of massive objects in the universe, so LIGO should see more signals as time goes on. If so, this will probably be known as one of the most important confirmations of Einstein’s General Theory of Relativity.


1. Neil Ashby, “Relativity and the Global Positioning System,” Physics Today, May 2002, pp. 41-47
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2. LIGO Scientific Collaboration and Virgo Collaboration, Neil Ashby, “Observation of Gravitational Waves from a Binary Black Hole Merger,” Physical Review Letters, 116:061102, 2016
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13 thoughts on “Gravitational Waves Detected!”

  1. Thank you, that was a very clear summary and explanation of the relevance of this event.

    To me this does look like a confirmation of something VERY BIG moving and causing gravitational waves. That seems more likely than an earthquake positioned just right for a nearly identical wave pattern to reach both sensors just 7 milliseconds apart. However, even though the waves fit their model of black holes colliding, I suspect other models could be constructed that would also fit that data, so I remain skeptical about their exact description of what caused it. It will be interesting to see how often we see these kind of signals, and whether we will see repeating patterns in them. Data from more than 2 sensors would be helpful also. My understanding is are a handful of other sensors in the world of this (huge, expensive) nature, but they were all down at the time of this event.

    1. I agree that their conclusion about what caused the gravity waves is incredibly model-dependent, so I take it with a grain of salt.

      I didn’t know there were other interferometers that big out there. It will be interesting to see if these other detectors are online when the next event happens.

    2. The wave form was not at all like an earthquake which starts with a sudden release of energy and then trails off. The gravity signal was oscillatory and increased in frequency and suddenly ended.

  2. Having never actually carefully learned GR, I don’t know about all of its previous successfully-observed predictions, but the sense I’m getting from people in my department is that it would’ve been very strange if we hadn’t seen gravitational waves eventually – i.e., that we’ve already seen enough confirmations that no one thinks GR could possibly be wrong anymore (meaning classically, of course). So while everyone here is excited about the new observational window and the precision of the detectors (as am I), I haven’t heard much talk about the confirmational aspect of the detection. I guess I’ll ask my astro friends about it. I personally was interested in the possibility of detecting anomalous gravitational couplings in the collision signatures, but unsurprisingly such terms are suppressed by the Planck mass, making them ridiculously small. And apparently even black hole mergers aren’t radical enough events to see that.

    Finally, if we’re to believe what I overheard in conversations after my department’s press conference viewing, more detections have already occurred. If I recall correctly, the initial one was within a month of bringing the more advanced detectors online.

  3. I remember back in September of 2014 (or 15?) the BICEP 2 experiment was attempting to detect GW as confirmation of the inflation period after the Big Bang. After some initial excitement it turned out space dust caused a false positive.

    What are your thoughts on the inflation theory and the newly detected GW confirming it? Can they differentiate between GW caused by black holes and those caused by inflation?

    1. I actually predicted that the BICEP 2 reading was the result of dust, and that’s what it turned out to be. Of course, it wasn’t a hard prediction to make. In their rush to confirm the physically-unrealistic process of inflation, astronomers have claimed to make that observation before, and it was also the result of dust. Also, another group had published a paper showing that dust was a more likely explanation for the observation than inflation.

      In any event, these gravitational waves are unrelated to the physically-unrealistic hypothesis of inflation. These waves relate simply to the violent motion of two black holes, and that is independent of any model of cosmology. The hoped-for signature of inflation that the BICEP2 group desperately wanted to see was in the cosmic microwave background radiation. That’s not what the LIGO group observed in this study. They observed ripples in the fabric of spacetime itself.

      I think inflation is just another “epicycle,” tacked on to the Big Bang in an ad-hoc way so as to force the model into compliance with astronomical observations. I seriously doubt that confirmation of it will ever be found, because I seriously doubt that it happened.

    1. I would have to disagree with your premise, Tom. These results indicate that, just like space, time is actually a part of creation. It is wrapped up in the fabric of spacetime. It changes depending on relative speed as well as the strength of the local gravitational field, and, along with space, it is warped by mass. Thus, from a scientific view, it is just as much a created thing as is space. This, of course, isn’t contrary to Scripture in any way.

  4. The best part of this was the timing, Dr Jay. We have just started using your book, ‘General Science’. Module 1 mentions Einstein, and his theories of relativity. We were just finishing up Module 2, when this was announced last week. What fun to discuss it over the breakfast table, and then quiz my boy thus: “After all these years, I now know why it’s called the Theory of Relativity. Now we have the sound of those black holes exploding, is it a Law, now?” “No, Mama.” “Why not?” “Cos we need more information over A LONG PERIOD OF TIME.” “Oh right…” How wonderful to be able to apply the ideas in the modules so soon after reading about them!

    Another good part was when the lightbulb went off in my head “Perhaps that’s why scientists still call it the Theory of Evolution, despite all the social pressure to believe it?”

    PS Yesterday, I went next door to show the Egg Drop Inertia experiment to our neighbours, who are on half term holiday. The cheers and giggles were something to behold! That’s from your new book, ‘The Scientific Revolution’, which I am using with our daughter. You are making me very popular as an edifying child entertainer!!

  5. Thanks for the reply.
    You mentioned the BICEP experiment was looking for waves in the CMB. What about those waves would be different than the waves detected by LIGO. The signature?
    Any good web sites you could recommend for further reading?

    1. The BICEP experiment wasn’t looking at gravity waves. It was looking at the CMB. The thought is that if inflation occurred, the gravity waves that resulted from inflation would have left a tell-tale signature in the CMB. They thought they had found that signature. They were wrong.

      The gravity waves detected by LIGO are ones that were produced by black holes and are currently rippling through the universe. The signature that the BICEP team was looking for in the CMB would have been produced by gravity waves that have long since dissipated. This link discusses the BICEP failure, and there are several associated links at the bottom that might help.

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