Monday, February 22, 2016

The Truth About Gravitational Waves

For the first time ever, gravitational waves have been detected directly, and the news — broken by the LIGO team on February 11, 2016 — has made some waves of its own. To sort out some difficult-to-explain points that have made their way into the media this week, we turn to the experts: Lynn Cominsky, Professor and Chair of the Physics Department at Sonoma State University, who joined Physics Central recently for a live Q&A following the announcement, and Rana Adhikari, Professor of Physics at Caltech.

First, we take a close look at a couple of common analogies for gravitational waves and the instruments that search for them. Human senses, especially hearing, are often invoked to help explain these ripples in spacetime, and there are many good reasons for that. Just like having two ears, one on either side of your head, helps you to localize the source of sounds around you, multiple detectors can help us pinpoint the source of gravitational waves. Moreover, the frequencies of gravitational waves that can be detected by LIGO — the Laser Interferometer Gravitational-wave Observatory — happen to fall in the same range of frequencies that our ears are tuned to pick up, which makes it easy to convert the observed signature into an audio clip. In that sense, we can “hear” gravitational waves, but the analogy can also introduce confusion. The now-famous “chirp” played during the announcement is an aural representation of the waveform recorded by the two LIGO detectors, but many headlines and newscasts incorrectly reported that scientists had “heard the sound” of gravitational waves. To Dr. Adhikari, detecting these elusive signals is as much like touch as sound — imagine letting your hands trail in the water on each side of a rubber raft and feeling the waves on the surface of the water brush your fingers — but neither really does it justice. “You have to sort of try to grasp new phenomena by analogy,” he explains, “but it's just a completely different way of viewing the universe...There's no sense that you can appeal to, because humans don't have a spacetime sensing sense.”

Another common trope in the excitement following the announcement has to do with the implications of the discovery. “Einstein was right!” was on everyone’s lips, but what exactly was he right about? His theory of general relativity suggested that gravitational waves had to exist, but he himself remained skeptical that we would ever be able to observe them. September’s detection thus fulfilled the last outstanding prediction of his theory, but it didn’t — as many headlines would have you believe — prove general relativity or vindicate Einstein, because neither were in any danger of being rejected. General relativity has been subjected to a plethora of tests over the last century, beginning with the anomalous perihelion shift of Mercury up to the success of GPS satellites today. Einstein was indeed right that gravitational waves are real — and that’s exciting! — but that’s just the beginning. “When you find something in astronomy,” Dr. Cominsky explains, “there’s a lot more of them out there: different sizes, different shapes, different orientations, different spin rates...This really is the birth of a new field.”

Part of that new field will involve testing Einstein’s theories in the regime of strong gravitational fields, and perhaps finding that it’s not entirely complete. “The interesting thing happens when the theory breaks down,” says Dr. Adhikari. “What I hope, expect, guess — one of those three — is that eventually we'll get our detector good enough, and there'll be a black hole merger loud enough that we'll start to see the first hints at the new theory beyond Einstein's relativity.” Einstein was right, but he may prove to be wrong (or, at any rate, incomplete) in the very near future.

Finally, we’ve been hearing a lot about the “dance of death” that these two massive black holes executed, but death isn’t what comes to mind for a physicist. On the one hand, these extreme events — supernovae and perhaps neutron star mergers — are so energetic that they create and seed the galaxy with heavy elements that are necessary for life to take root and evolve. On the other, it’s hard not to think of how lucky we are to have witnessed this grand collision in September, a billion years after it took place. It’s an exciting time to be alive, and it’s just the beginning of a new era. “Any time you open a new branch of astronomy,” says Dr. Cominsky, “you detect things people never even imagined.”

Podcast and post by Meg Rosenburg
(Full disclosure: The interview with Dr. Adhikari was made possible with travel support provided by Caltech.)


  1. The recent detection of gravitational waves by LIGO (Laser Interferometer Gravitational-Wave Observatory) has captured the imagination of the public. It will stand as one of the great feats of experimental physics, alongside the famous Michelson-Morley experiment of 1887, which it resembles. In fact by comparing these two experiments, you will see that understanding gravitational waves is not as hard as you think.

    Michaelson and Morley measured the speed of light at different times as the earth moved around its orbit. To their - and everyone’s - surprise, the speed was constant, independent of the earth’s motion. This discovery caused great consternation until FitzGerald and Lorentz came up with an explanation: objects in motion contract. Einstein then showed that this contraction is a consequence of his Principles of Relativity - one of which was that the speed of light is constant!. However there was no explanation for why they contract (other than a desire to conform to Einstein’s Principle?). It wasn’t until Quantum Field Theory came along that the explanation was found. In QFT, at least in Julian Schwinger’s version, everything is made of fields, even “particles”, and contraction occurs because motion affects the way fields interact.

    Now if gravity is a field, then it’s easy to see why there are gravitational waves. Waves are a natural property of fields. Just as an oscillating electron in an antenna sends out electromagnetic radio waves, so a large mass moving back and forth sends out gravitational waves. However it doesn’t take QFT to show this. Einstein also believed that gravity is a field that obeys the equations he derived, just as the electromagnetic field obeys the equations of James Maxwell. In fact, gravitational waves are accepted by all physicists who, like Einstein, see gravity as a field.

    The first experimental evidence for gravitational waves came in 1978 when Joseph Taylor and Russell Hulse observed energy loss from a rotating pulsar that was consistent with Einstein’s theory. This confirmed not only Einstein’s theory, but also the existence of gravitational waves.

    But what about “curvature of space-time”, which many people today say is the cause of gravity? You may be surprised to learn that’s not how Einstein saw it. He believed that gravity is a force field that causes changes in dimensions, just as motion causes contraction - by affecting the way fields hold everything together. In fact Einstein used this analogy to show the similarity between the two effects.

    The LIGO apparatus is similar to Michelson’s and Morley’s. In both experiments the time for light to travel along two perpendicular paths was compared, but because the gravitational field is much weaker than the EM field, the distances in the LIGO apparatus are much greater (miles instead of inches). Another difference is that while Michelson, not knowing about motion-induced contraction, expected to see a null result, the LIGO staff used the known gravity-induced contraction to see a change when a gravitational wave passed through.

    QFT not only provides a simple explanation for gravitational waves, it also resolves the many paradoxes of Relativity and Quantum Mechanics. Fields of Color: The theory that escaped Einstein explains QFT to a lay audience, without any math. It makes QFT understandable and describes its many accomplishments, including a simple derivation and understanding of Einstein’s e=mc2 (the only equation in the book).

    To buy Fields of Color, or just look inside, visit

  2. gravitational waves have been detected for the first time. LIGO scientists work are commendable.
    prof premraj pushpakaran

  3. gravitational waves have been detected for the first time. LIGO scientists work are commendable.
    prof premraj pushpakaran

  4. I have a couple of questions. I don't understand how there is spatial and temporal resolution in this experiment. 1. How do these two inteferometers "focus" on the supposed part of the universe in which these two black holes combined? 2. Since they were believed to have been 1.3 billion light years from earth, any photons would have take 1.3 billion years to arrive. Would gravitational waves take the same time? Wouldn't the collapse of the two black holes have required years or perhaps centuries to occur? If so, how can the interference signal be only half a second in length?

  5. Why does the detector need two black holes colliding in an awesome collision for a detection? Shouldn't you have seen waves from many different sources?

  6. Also, over time there must have been many other gravitational waves in less than a billion years or over a billion. In the observance of the extremely long time function you would think LIGO would be awash and buzzing with all the different sources of gravitational waves. Do the gravitational waves really have to be that powerful to be detected by what seems to be an extremely sensitive apparatus?
    Thanks for all your work, Ken Dean