You have to change the time.
“Thank you!” Einstein greeted Besso the morning after their momentous discussion. “I have completely solved the problem.”
According to Einstein’s calculations, time itself wasn’t constant, an absolute, an immutable part of the universe. Now it was a variable that depended on how you and whatever you’re observing are moving in relation to each other. “Every other physicist assumed that there was a universal world clock that kept time,” says Schwartz. “Einstein completely removed that idea.” From the point of view of the person on the dock, the time it took the light to reach the ship’s deck was longer than a second. That means the time on board the ship appeared to be passing more slowly than on the dock. The reverse, Einstein knew, would also have to be true. From the sailor’s point of view, the dock would be moving, and therefore a beam of light sent down from a tall post on land would appear to him to travel a bit farther than it would to you on the dock. To the sailor, the time onshore would appear to be passing more slowly. And there we have it: a new principle of relativity.
“Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows,” the German mathematician Hermann Minkowski declared in 1908. Other physicists had done calculations that showed a similar difference in measurement of time between two observers, but they always added some version of “but not really.” For them, a difference in time might be in the math, but it wasn’t in the world. Einstein, however, said there is no “really.” There is only what you on the dock can measure about time on board the moving ship and what the sailor can measure about time on board the moving ship. The difference between the two is in the math, and the math is the world. Einstein’s insight was that because these perceptions are all that we can ever know, they are also, in terms of taking the measure of the universe, all that matter.
This was pretty heady stuff for a 26-year-old clerk who only a couple of weeks earlier had submitted his doctoral thesis to the University of Zurich. Einstein would keep his day job at the patent office until 1909, but his obscurity was over, at least among physicists. Within a year of completing his relativity paper, his ideas were being debated by some of the most prominent scientists in Germany. In 1908 physicist Johann Jakob Laub traveled from Würzburg to Bern to study with Einstein, exclaiming that to find the great man still laboring in a patent office was one of history’s “bad jokes.” But Einstein wasn’t complaining. His “handsome” pay, as he wrote a friend, was sufficient to support a wife and 4-year-old son, Hans Albert, and his schedule left him “eight hours of fun in the day, and then there is also Sunday.” Even on the job, he found plenty of time to daydream.
During one such daydream, Einstein experienced what he would later call “the most fortunate thought of my life.”
He knew that his 1905 special relativity theory applied only to the relationship between a body at rest and a body moving at a constant velocity. What about bodies moving at changing velocities? In the fall of 1907, he saw a vision in his mind’s eye not unlike a beam of light descending from a mast: a man falling off a roof.
What’s the difference? Unlike the beam of light, which moves at a constant velocity, the falling man would be accelerating. But in another sense, he would also be at rest. Throughout the universe, every scrap of matter would be exerting its exquisitely predictable influence on the man, through gravity. This was Einstein’s key insight—that acceleration and gravitation are two ways of describing the same force. Just as someone on board Galileo’s ship would have as much right to think of the dock leaving the ship as the ship leaving the dock, so the man in free fall from the roof would have as much right to think of himself being at rest while the earth hurtles toward him. And there we have it: another principle of relativity, called general relativity.
“Einstein always took what everyone else thought to be two completely different scenarios of nature and saw them as equivalent,” says Gerald Holton of Harvard, a leading Einstein scholar. Space and time, energy and mass, and acceleration and gravitation: as Holton says, “Einstein was always confronting the question, Why should there be two different phenomena with two different theories to explain them when they look to me like one phenomenon?”
After his 1907 vision, however, another eight years would pass before Einstein worked out the equations to support it. Einstein told friends that when he finally figured out the math to demonstrate general relativity in 1915, something burst inside him. He could feel his heart beating erratically, and the palpitations didn’t stop for days. He later wrote a friend, “I was beyond myself with excitement.”