This year marks the 100th anniversary of a British astronomer and his team confirming Einstein's theory of general relativity by observing a solar eclipse and finding that gravity bends light. We spoke with Ron Cowen, the author of Gravity’s Century, about the importance of this event and what we've learned about gravity since then.
On May 29, 1919, astronomer Arthur Eddington and his team observed a solar eclipse and found something extraordinary: gravity bends light. Why is this observation still relevant one hundred years later?
Both the human drama and the observations make this finding stand out a century later.
First the drama: With the ravages of World War I still fresh, two teams of British astronomers in 1919 dared to test the strange new theory of gravity developed behind enemy lines by the German-born Albert Einstein. Hatred of the “evil Hun” was still rampant and the leader of the British teams, Quaker astronomer Arthur Eddington, had been threatened with jail time for his refusal to serve in the British army.
Then, of course, the observation itself: the sun’s heft had bent the path of starlight, meaning that gravity bends space and time. The finding erased the idea of gravity as a force and refuted long-held notions of space and time as featureless, silent spectators to the comings and goings in the universe. Space-time was as malleable as putty, shaped by the presence of mass and energy. A body did not fall because Earth tugged on it; rather, Earth’s mass and energy curved the surrounding space-time in such a way that a passing body would inevitably have its path bent toward the planet. The same mutual influence applied to any two objects in the universe. Even light was subject to this law of nature: if it passed near a massive enough body—the Sun, for instance—its path, too, would bend.
When the results were announced on Nov. 6, 1919, it created a sensation among the public, unlike any other scientific finding. “This is the most important result obtained in connection with the theory of gravitation since Newton’s day,” declared British physicist J. J. Thomson, head of Britain’s Royal Society. In a triple-decker headline, the normally staid Times of London wrote: “Revolution in Science / New Theory of the Universe / Newtonian Ideas Overthrown.” The New York Times followed suit with a front-page story on November 10: “Lights All Askew in the Heavens…Einstein Theory Triumphs.”
Einstein became an overnight celebrity. He figured his fame would soon die down. He was wrong. The public’s fascination with his theory of gravity endured for the next century and continues to revolutionize our understanding about the universe.
How has Albert Einstein’s theory of relativity influenced the fields of astrophysics and astronomy since it was proven?
One hundred years ago, when Einstein formulated his general theory of relativity, the universe seemingly consisted of a single galaxy; today we know not only that the universe has at least 100 billion galaxies but that it is expanding, ballooning at a faster rate every second.
Over the past century, astronomy has grown from the study of the narrow optical band of the electromagnetic spectrum, visible through individual telescopes, to the whole range of the electromagnetic spectrum, from microwaves through gamma rays. By the start of the twenty-first century, astronomy had extended even beyond the electromagnetic spectrum: mysterious, invisible entities known as dark matter and dark energy are now known to make up 96 percent of the composition of the cosmos. All this is a consequence of Einstein’s theory of gravity. Anyone trying to make sense of these discoveries owes a debt to his theory of general relativity.
What are the most important findings we’ve learned about gravity thanks to Einstein’s theory?
Two findings stand out.
1) As strange as the notion that space-time can be bent and sculpted by massive objects, no phenomenon is quite as counter-intuitive as a black hole. The idea was a natural outcome of general relativity, as some theorists realized as soon as Einstein had devised his theory. If an object was massive and dense enough, wouldn’t space-time become so distorted that it would close in on itself? Not only would light passing close to such an object be bent, but if it passed too close, it would fall into the gravitational trap and never escape.
Einstein never liked the idea of black holes; it made his elegant equations blow up and lose their meaning. For decades, he and other physicists could afford to ignore the concept.
But then another kind of revolution arrived—this one in telescope technology. Observations beginning in the early 1960s revealed that compact beacons of radiation from the distant reaches of the universe were outshining entire galaxies and that stars were whipping around galactic centers at staggeringly high speeds. The enormous energies and furious velocities betrayed the presence of unseen gravitational hulks at the cores of galaxies. Black holes, light-guzzling gravitational maws in space-time, had become real.
Soon theorists began to take a strong interest. They realized that black holes were a crucible for marrying the world of the very tiny—the realm of quantum theory—with the realm of extreme gravity, where general relativity rules supreme. That was a marriage Einstein had spent decades trying to forge but never accomplished.
2) Soon after Einstein developed his theory, he predicted the existence of gravitational waves, ripples or undulations in the fabric of space-time. Einstein never thought the ripples could be detected—he knew they were incredibly feeble. In fact, after more than five decades of hunting for these space-time oscillations, it took a pair of detectors that could record changes in length smaller than one ten-thousandth the diameter of a proton to find the waves. And those first waves were generated by the collision of two stellar-mass black holes—those weird objects Einstein never quite believed in. The discovery was honored with the 2017 Nobel Prize in Physics.
What is your favorite anecdote about the scientists researching gravity?
One of my favorite stories is the reaction physicist Scott Hughes had when he first saw the signal of the first gravitational wave. As I related in my book:
“When MIT physicist Scott Hughes first saw the image of that signal on a colleague’s cellphone, he felt a rush of emotion he had experienced only twice before—when he saw his newborn daughter’s face for the first time and his dying father’s face for the last time. His colleague kept talking, but Hughes couldn’t hear him. All he could think about was the image on the cellphone. It showed a pattern of wiggles that first grew in amplitude and increased in frequency, and then rapidly diminished in amplitude—a picture that Hughes had been seeing in his mind’s eye since the dawn of his career more than twenty years earlier.
“In 1995, Hughes was a graduate student at the California Institute of Technology, studying how black holes, the powerful gravitational traps from which not even light can escape, affect their surroundings. Hughes knew that a black hole at rest would dent space-time the way a bowling ball would sag a rubber sheet. But what would happen to space-time if heavy objects were shaken or accelerated like two black holes about to smash into each other?
“Just as a bouncing bowling ball would jiggle the rubber sheet, shaking a chunk of matter would generate undulations in the fabric of space-time. These undulations, known as gravitational waves, would spread out across the universe like ripples in a cosmic pond….
“At the time Hughes and his collaborator did their calculation in the mid 1990s, researchers lacked the computational tools to determine what the gravitational waves would actually look like—their size, duration, and change in frequency. That breakthrough came nearly a decade later, when other researchers developed the techniques to solve Einstein’s equations of gravity on computers.
“The match between those computer calculations and the trace from LIGO that Hughes was now seeing on his colleague’s cellphone was more than uncanny. It was exact. A detailed analysis would later show that the masses of the two coalescing black holes were smack in the middle of the range that he and his collaborator had calculated nearly twenty years earlier.” Hughes was spellbound.
What is the Event Horizon Telescope project and what is the most exciting thing we may learn from it? How will the project provide a crucial new test of Einstein’s theory of gravity?
The Event Horizon Telescope is actually an array of radio telescopes around the world that are electronically correlated so that they have the resolving power of a single telescope with a lens as large as Earth’s diameter. It needed such acuity in order to take the first images of black holes—or more precisely the region just outside them, known as the event horizon. The collaboration chose to target two black holes in particular. One of those gravitational monsters is Sagittarius A*, which churns at the center of our Milky Way galaxy and has a mass four million times that of the Sun. The second, possessing a mass some 1,000 times greater, occupies the core of M87, a galaxy 54 million light years distant. Together they may provide science with a crucial test of Einstein’s general theory of relativity: determining how well its predictions match observations in the most extreme gravitational environment known in the universe. Einstein’s theory has passed every other test with flying colors. But no one has tested the theory in such detail so close to a black hole. The shape of the event horizon is predicted by the general theory of relativity. The first results, announced in April, provide a capstone for a century of work on Einstein’s masterpiece, his theory of gravity.
