When Einstein first published his general theory of relativity, it was mostly a beautiful bit of mathematical invention. One of the riddles that inspired his thinking on the subject, small changes in Mercury’s orbit, immediately lined up in quantitative perfection with what the theory predicted. But at first, the theory was long on prognostication and short on experimental support. Perhaps the most well known “proof” of the theory involved some astronomical observations during an eclipse which showed that light is indeed bent around a massive object by its gravity. Einstein made lots of other predictions and many of them, too, have turned out to be correct. But there was one phenomenon he said we should observe if he was right about mass, gravity, and the curvature of spacetime, gravitational waves, that had stubbornly evaded detection until very recently.
General relativity stipulates that certain events, such as the collision of two black holes, will send waves through spacetime, curving it as they pass through. There were multitudinous challenges in actually seeing these waves, though. Most importantly, the math told us that they would be only tiny events by the time they got here from most likely sources. Hence, building an effective detection device became an engineering problem more than anything else.
LIGO, The Laser Interferometer Gravitational-Wave Observatory, was eventually successful in seeing some gravitational waves, and if you have a strong desire to know exactly how this technological marvel works, you will be edified by Govert Shilling’s new book, Ripples in Spacetime: Einstein, Gravitational Waves, and the Future of Astronomy. Although the book does eventually get to a thorough explanation of the principles and practicalities of LIGO, there’s a lot of material before and after that fantastic section that will be old hat for veteran popular science readers. If you’ve been through Kip Thorne’s Black Holes and Time Warps, for example, you’ll find yourself skipping a lot of introductory astrophysics in Ripples in Spacetime. That shouldn’t dissuade you from picking the book up, though, because every question you had about how LIGO works is answered within. I thought I understood LIGO’s operation before I started reading Ripples, but I wasn’t even close. What LIGO actually measures is the degree to which two harmonized laser beams are put out of sync by a passing gravitational wave. It’s fascinating stuff, and reveals LIGO to be a supremely elegant solution to a vexingly difficult problem.
Another thing Ripples does well is explain all the implications of LIGO’s success. It has opened up a whole new form of astronomy, and now that we have achieved simultaneity of observation with the optical and radio spectrum, we stand to learn a tremendous amount about the universe. In my admittedly amateur opinion, it is hard to overstate the possibilities for learning about the cosmos opened up by LIGO.
General relativity is a mind-bending concept and studying it is thoroughly enjoyable when you have good materials at hand. Ripples in Spacetime is a great example of such materials. You don’t have to be a genius to read or understand the book, but you can at least feel like one having done so. It’s close enough for me!