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.)

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