The Perfect Theory by Paolo G Ferreira is a delightful history of General Relativity (GR), Einstein's explanation of gravity. It focuses on people (a wild menagerie of characters) and problems (including the most important questions in the cosmos). The Prologue puts it neatly:
It sounds like the ultimate overstatement, but I can’t resist it: the reward for harnessing Albert Einstein’s general theory of relativity is nothing less than the key to understanding the history of the universe, the origin of time, and the evolution of all the stars and galaxies in the cosmos. General relativity can tell us about what lies at the farthest reaches of the universe and explain how that knowledge affects our existence here and now. Einstein’s theory also sheds light on the smallest scales of existence, where the highest-energy particles can come into being out of nothing. It can explain how the fabric of reality, space, and time emerges to become the backbone of nature.
What I learned during those months of intense study is that general relativity brings space and time alive. Space is no longer just a place where things exist, nor is time a ticking clock keeping tabs on things. According to Einstein, space and time are intertwined in a cosmic dance as they respond to every single speck of stuff imaginable, from particles to galaxies, weaving themselves into elaborate patterns that can lead to the most bizarre effects. And from the moment he first proposed his theory, it has been used to explore the natural world, revealing the universe as a dynamic place, expanding at breakneck speed, filled with black holes, devastating punctures of space and time, and grand waves of energy, each carrying almost as much energy as a whole galaxy. General relativity has let us reach further than we ever imagined.
Much of the feel of the GR research in The Perfect Theory is that of exploring a cave or a labyrinth – as mathematician Andrew Wiles said of his work (in Amir Aczel's book Fermat's Last Theorem – see "Good Mistakes"):
"Perhaps I could best describe my experience of doing mathematics in terms of entering a dark mansion. You go into the first room and it's dark, completely dark. You stumble around, bumping into the furniture. Gradually, you learn where each piece of furniture is. And finally, after six months or so, you find the light switch and turn it on. Suddenly, it's all illuminated and you can see exactly where you were. Then you enter the next dark room ..."
That's precisely what the physicists, astronomers, and astrophysicists are doing throughout The Perfect Theory – usually on timescales of decades. In Chapter 11 ("Seeing Gravity"), for instance:
Weber died in 2000, before LIGO started operations. It had taken decades of devotion to get the most perfectly tuned instrument to work. Along the way, there had been delay after delay. Kip Thorne had made a number of bets with colleagues in the 1980s and 1990s that gravitational waves would be discovered before the turn of the millennium, and he lost them all. Even in the beginning of the twenty-first century, LIGO faced setbacks, from the loggers with their circular saws in the Louisiana forest who set off the detectors at Livingston, to mysterious whirrings in the nuclear reactors around the Hanford site in Washington. But when it was finally turned on in 2002 and run for a few years, LIGO was able to achieve the sensitivity everyone had been gunning for. It was the first stage in the experimental journey laid out in the proposal in the early 1990s. Its detectors could pick up vibrations of less than a proton’s width, as had been envisioned decades before. In fact, the LIGO team announced, the instrument was even more sensitive than they had predicted. LIGO was, by all means, a resounding success, even though it didn’t see anything. As expected in its first incarnation, LIGO was not yet sensitive enough to actually detect gravitational waves, but it did show the way forward. The LIGO team can now improve the existing instrument so that at some point it will see the ripples in spacetime that Einstein had first predicted.
It is a long game. Unlike Weber’s results, which came fast and steady the moment he turned on his instrument, LIGO will have used up thousands of technicians over many decades before it can actually detect gravitational waves. The founding trio, Ron Drever, Kip Thorne, and Rainer Weiss, now in their seventies and eighties, might not all be around when that moment comes, and they may have devoted their lives to something they will never see. But there is unwavering confidence that waves are out there; Einstein’s theory predicts them, and they have been seen, albeit indirectly, through the gentle but steady orbital decay of the millisecond pulsars. It is just a matter of time before gravitational waves are seen, and then a field of research that started with Weber’s bang will end with a whimper: the whimper of spacetime shimmering as it passes through Earth.
The Perfect Theory was published in 2014, and LIGO made two detections of gravitational waves that reached the Earth in 2015. The 2017 Nobel Prize in physics was shared Barry Barish, Kip Thorne, and Ray Weiss. (Ronald Drever died shortly before that; R.I.P. Ron!)
Near the end of the book in Chapter 13 ("A Spectacular Extrapolation") Ferreira discusses next steps for GR:
Now, in the early twenty-first century, we seem to be in a similar situation, with a wonderful theory of gravity that, to explain cosmology, requires that more than 96 percent of the universe be made up of something we can’t see or detect. Could this be yet another crack in the edifice that Einstein had constructed almost one hundred years before? That general relativity might have to be corrected due to quantum physics had been accepted without too much fuss. But questioning general relativity’s efficacy on large scales was something different. If the dark matter and dark energy of the universe were eliminated from the picture, Einstein’s beautiful theory would have to be modified. The prospect was as unappealing to many astrophysicists as taking a sledgehammer to a classic car just so it would fit in the garage.
The Perfect Theory is a loving, lovely history of a central field of physics.
(cf Vulnerable Theories (1999-05-17), Relativity Plus Astrophysics (2000-03-29), Cherished Beliefs (2000-04-19), No Concepts At All (2001-02-22), Universal Knowns (2002-06-13), Gravitational Waves - Thirty Years Later (2011-07-15), Little Book of Cosmology (2023-03-31), ...) - ^z - 2023-08-23