In 1921, Albert Einstein delivered a series of lectures at Princeton that marked his final attempt at a comprehensive synopsis of the special and general theories of relativity. He had introduced those theories in 1905 and 1915, respectively, and by 1921 they included refinements resulting from his and his colleagues’ sustained attention to his original work. In THE FORMATIVE YEARS OF RELATIVITY: The History and Meaning of Einstein’s Princeton Lectures (Princeton University, $35), the physicist Hanoch Gutfreund and the historian Jürgen Renn declare those talks to be “the paradigmatic text” of this pivotal period in relativity’s development.
They note, however, a conspicuous absence. There is “no trace” in Einstein’s lectures of what is today considered a key topic in relativity: gravitational waves. These are tiny vibrations in space-time that arise from the universe’s most cataclysmic events, such as the collision of two black holes. The first direct detection of gravitational waves, which occurred two years ago, on Sept. 14, 2015, was celebrated as a landmark scientific event. Einstein’s reluctance to even mention gravitational waves in 1921 — despite having contemplated their existence as early as 1915 — suggests how thorny a problem they presented.
According to the theory of relativity, gravity is not a ghostly tug between objects but a warping of the realm between them. A massive body, like a planet, indents the space-time around it, like a bowling ball placed on a trampoline. The “attraction” to the planet of, say, a moon is actually the result of the moon’s following the curved path of space-time, much as a marble rolling across the trampoline would veer toward the indentation created by the bowling ball. As the physicist John Archibald Wheeler put it: “Matter tells space-time how to curve; space-time tells matter how to move.”
The equations Einstein used to formulate this theory resembled the equations of electrodynamics. If you take an electric charge and accelerate it, you create an electromagnetic wave of some sort: a radio wave, a microwave, a wave of visible light. If you were to take a massive object and accelerate it, would you create a gravitational wave, a ripple in space-time? It was conceivable. These would be peculiar waves: not waves moving through space-time, like a wave moving through the ocean, but waves in space-time — the world itself (and everything in it) would be alternately lengthened and squashed, as patches of greater and lesser density propagated through it, like a bowl of gelatin that has been jostled.
Einstein went back and forth on whether his theory entailed gravitational waves. Gutfreund and Renn give a blow-by-blow history of his correspondence with various peers on this topic. Early in 1916, Einstein wrote that “there are no gravitational waves analogous to light waves.” Later that year, he changed his mind and published the first paper about them. In 1917, a colleague helped him see a miscalculation in that paper, and things looked uncertain again. In 1918, Einstein published an improved article, “On Gravitational Waves,” after which he went quiet on the issue (including his silence during the Princeton lectures) until 1936, at which point he started waffling again. He would die in 1955 never entirely convinced of their existence.
As the journalist Govert Schilling explains in RIPPLES IN SPACETIME: Einstein, Gravitational Waves, and the Future of Astronomy (Belknap Press/Harvard University, $29.95), one reason for the neglect of the topic in the 1920s and ’30s was that even if gravitational waves did exist, “they would be far too small to detect.” Space-time, he notes, is “incredibly stiff”; enormous amounts of energy are required to produce the slightest tremors in it. Even with today’s state-of-the-art equipment, two ordinary stars orbiting each other don’t emit gravitational waves at a measurable level. You need a far more powerful event, involving something like a neutron star (the dense remnant of a supernova) or a black hole. What point was there in arguing about whether relativity implied the existence of these waves, if they couldn’t be observed?
Schilling’s book, a detailed account of the quest to detect gravitational waves, is joined by a similar book by the science writer Marcia Bartusiak: EINSTEIN’S UNFINISHED SYMPHONY: The Story of a Gamble, Two Black Holes, and a New Age of Astronomy (Yale University, paper, $18). Where Schilling is excitable and geeky, Bartusiak is dreamier in mood and more stately in tone. Both have an important, multifaceted scientific story to tell — part theoretical physics, part astronomy, part experimental physics, part engineering. Schilling notes that he expected to finish his book before the first detection of gravitational waves, but like most people was taken by surprise by the discovery in 2015, and ended up doing the vast majority of his research and writing after the fact. Bartusiak’s book was originally published in 2000 and has been substantially updated.
In both books, the hunt gets underway in earnest in 1957. That year, theoretical physicists became confident that gravitational waves must be a real physical phenomenon, not a quirk of relativity’s mathematics. Suddenly, building a gravitational-wave detector no longer seemed like a waste of time. By the early 1960s the first versions of detectors were built and deployed, though with no success. In 1978, physicists indirectly confirmed the existence of gravitational waves: They discovered a binary pulsar (two rotating neutron stars revolving around each other at tremendous speed) whose shrinking orbit was losing energy precisely in the amounts relativity would predict if it were emitting gravitational waves. But even such a fierce disturbance in space-time wasn’t something we were yet able to directly detect on Earth.
By 2001, scientists had constructed and begun to operate the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which comprises a pair of exquisitely sensitive detectors, one in Washington State, the other in Louisiana. Why two detectors, nearly 2,000 miles apart? To rule out false positives: LIGO searches for such small vibrations in space-time that external jolts, like a distant thunderclap, can pose a problem. But if precisely the same vibration occurs in both detectors at the same time (or rather about the same time, as a gravitational wave takes an instant to cover that distance), that’s a better indication a gravitational wave has passed through Earth.
LIGO ran for about a decade, detecting no gravitational waves, and then was shut down to undergo improvements to increase its sensitivity tenfold. By September 2015 it was ready to start looking again.
On Sept. 14, 2015, soon after being turned back on, LIGO detected a portentous “chirp,” first in Louisiana, then seven milliseconds later in Washington. Both detectors measured the exact same stretching and squeezing of space-time. Scientists consulted a database of hundreds of thousands of pre-calculated wave forms, to see what sort of cosmic event matched these vibrations. The answer: two black holes, 36 and 29 times the mass of our sun, that had spiraled together and then collided (the “chirp” was what Schilling calls their “death cry”) 1.3 billion light-years away.
How small were the vibrations that reached us? Quite small. For one-fifth of a second, LIGO measured waves in space-time with an amplitude of 0.0000000000001 centimeters, which is millions of times smaller than an atom. By the standards of gravitational waves, though, these were strong.
That black-hole collision, Schilling writes, was one of the most powerful events ever observed in the universe.
The detection of gravitational waves has ushered in a new era in astronomy. After hundreds of years of relying only on electromagnetic radiation (such as light seen through a telescope), scientists can now use gravitational waves to directly observe a black hole (otherwise an impossibility, as it does not emit light in any form) or, as Bartusiak marvels, “to peer into the very heart of an exploding star” (gravitational waves, unlike electromagnetic radiation, are not dispersed or absorbed by matter as they pass through the universe).
In addition, the detection of gravitational waves has been another vindication for the theory of relativity — not only because the theory predicted them, but because, as Bartusiak notes, “their very existence demonstrates, in a firm and vivid way, that space-time is indeed a physical entity in its own right.” Maybe Einstein shouldn’t have been so reticent in 1921, after all.
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