In the summer of 1895, while the rest of the world was locked in the Victorian age, Albert Einstein was pondering the phenomenon of light. Though he was just sixteen, Einstein had renounced his German citizenship, rejected Judaism, and begun “a positively fanatic orgy of free thinking.” Bored in his prep school near Zurich, he took long walks in the Swiss Alps, experimenting in his own private chamber — his mind.
All his life, Einstein would perform Gedankenexperimente — thought experiments. That fall, he did his first. What would the world look like, he wondered, if he could ride on a beam of light? “If a person could run after a light wave with the same speed of light,” he thought, “you would have a wave arrangement which could be completely independent of time.” He knew that “such a thing is impossible,” yet he could not stop thinking about the ride. Viewed from a beam of light, clocks would seem frozen, the light from their moving hands never reaching the retreating passenger. And yet, like someone in a slow elevator, light’s rider would barely know he was moving. All the staid laws of Newton’s universe would be upset.
For the next ten years, Einstein struggled with this thought experiment. “If I pursue a beam of light with the velocity c,” he imagined, “I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating…” The problem, he later wrote, caused him a great “psychic tension.” The tension followed him for a decade as earned his Ph.D., married, and toiled as a patent clerk in Bern, Switzerland. Then in 1905, Einstein dared to believe something few physicists were willing to accept — that light behaved in ways contrary to ordinary matter. “The difficulty to be overcome,” Einstein later wrote, “then lay in the constancy of the velocity of light in a vacuum, which I first thought would have to be abandoned.”
Recent experiments had disproved the existence of “luminiferous ether,” an invisible substance through which, since the days of Aristotle, everyone assumed that light traveled. The American physicist Albert Michelson had shown that no matter which direction a beam of light was aimed, no matter the velocity of its source, light’s speed was a constant. There was no “ether drag” to slow or speed light depending on its direction. There was no ether at all. Many continued to believe in — and test for — ether, but Einstein, having renounced his heritage, his religion, his nationality, rejected the concept with ease. If what he called “the light medium” did not exist, he reasoned, then the metal ride on his beam was still more perplexing. If light’s speed was constant, then two observers, one traveling toward a beam, another standing still, would have to perceive the same speed of light. But this defied the laws of classical physics, not to mention common experience.
When I walk on an airport’s moving sidewalk, my speed is added to its speed. My legs carry me three miles per hour, the sidewalk moves at five, and my combined speed is eight m.p.h. So I get to wait for my luggage a minute sooner. But as the Michelson-Morley experiment showed, light defies this simple summation. Shoot a beam of light from a speeding jet, aim it forward, backward, sideways, and it always travels at the same speed — 186,282 miles per second. Light simply refuses to behave like anything else in the universe. Its constancy seemed impossible, unless…
Late one morning in May 1905, Einstein was walking through Bern with his fellow patent clerk Michele Besso. Einstein often shared his latest thought experiment with Besso. Now he confessed despair over light’s constancy. “I’m going to give it up,” Einstein said. Then, right there on the Kramgasse, as trams passed and pedestrians jostled, the answer came to him. If light traveled at a constant speed, as seen by all observers, in all tests, then time must be mutable. The next day, Einstein rushed up to his friend. “Thank you,” Einstein told Besso. “I’ve completely solved the problem.”
Einstein spent five weeks writing “On the Electrodynamics of Moving Bodies.” He would spend the rest of his life explaining it, sometimes with equations, sometimes with a joke: “Put your hand on a hot stove for a minute and it seems like an hour. Sit with a pretty girl for an hour and it seems like a minute. That’s relativity.” Nowhere in his paper did Einstein suggest riding a beam of light, yet he later called the idea the “germ of the theory of special relativity.” The constant speed of light, Einstein saw, made time “relative.”
Picture two observers, one riding a train, the other sitting on an embankment watching the train pass. The passenger puts a flashlight on the ceiling of the coach and a mirror on the floor. Turning the flashlight on and off, he sees a beam travel straight down and bounce straight up, its path forming a lower case ‘l’. Now imagine what the observer on the embankment sees. From this point of view, the light beam is stretched by the moving train. It hits the floor at an angle and bounces back at a similar angle. The shape is not an ‘l’ but a ‘V’. Clearly light’s V-shaped path is longer than the ricocheted ‘l’. Yet if light’s speed is constant, how could it travel different distances in exactly the same time? Time is “the culprit,” Einstein realized. If light is constant, time cannot be. The only way to explain the paradox was to believe — begin to believe — that time slows on moving bodies.
It sounded — still sounds — incredible, ridiculous, but Einstein used an equation – the Lorentz Transformation – to calculate how time slows negligibly at jet speed but nearly stops as an object approaches the speed of light. (If the train were approaching light speed, the ‘V’ seen from the embankment would be enormously long, while the ‘l’ observed from the train would be the same. Light would trace each letter at its constant speed, forcing time, as experienced by the passenger, to nearly stop.) And time has since proven Einstein correct.
Seven decades after Einstein conceived relativity, atomic clocks had become sufficiently precise to test time dilation. A pair of clocks were flown around the world, then compared to stationary clocks. The moving timepieces returned a fraction of a second slower, precisely as calculated by Einstein. And when today’s particle accelerators blast sub-atomic muons from an atom, each survives for micro-fractions of a second. But when accelerated toward the speed of light, their clock slows and the particles survive slightly longer. Time is the culprit, light its accomplice.
For more than a dozen years after Einstein conceived special relativity, the idea met with more bewilderment than acceptance. The public heard nothing of it, and only a few physicists found it intriguing. After four more years in the patent office, Einstein was given a professorship and more time for thought experiments. Soon he would rewrite the laws of gravity, predict light’s bending around a star, and become world famous. Light, with its iconic bulb the very symbol of a new idea, had given the world a new genius. And light’s constant speed — 186,282 miles per second — coming at you, going away, in a lab, beamed from a train or plane, across galaxies, or in the mind of Albert Einstein — had disturbed the universe.
Bruce Watson is the author of Light: A Radiant History from Creation to the Quantum Age (Bloomsbury, February 2016). The book traces humanity’s evolving understanding and control of light, starting with creation myths, then moving into scripture, philosophy, architecture, Islamic science, art history, poetry, physics, and quantum physics.
Watson’s previous books include Freedom Summer, Sacco and Vanzetti, and Bread and Roses. Watson’s work has appeared in the Boston Globe, the Los Angeles Times, American Heritage, the Wall Street Journal, the Washington Post, Yankee, Reader’s Digest, and Best American Science and Nature Writing 2003.