{ EINSTEIN’S CENTURY } Albert Einstein’s key breakthrough — relativity — came when he looked at a few ordinary things from a different perspective. /// BY RICHARD PANEK Relativity turns 1001 How’d he do it? This question has shadowed Albert Einstein for a century. Sometimes it’s rhetorical — an expression of amazement that one mind could so thoroughly and fundamentally reimagine the universe. And sometimes the question is literal — an inquiry into how Einstein arrived at his special and general theories of relativity. Einstein often echoed the first, awestruck form of the question when he referred to the mind’s workings in general. “What, precisely, is ‘thinking’?” he asked in his “Autobiographical Notes,” an essay from 1946. In somebody else’s autobiographical notes, even another scientist’s, this question might have been unusual. For Einstein, though, this type of question was typical. In numerous lectures and essays after he became famous as the father of relativity, Einstein began often with a meditation on how anyone could arrive at any subject, let alone an insight into the workings of the universe. An answer to the literal question has often been equally obscure. Since Einstein emerged as a public figure, a mythology has enshrouded him: the lone- ly genius sitting in the patent office in Bern, Switzerland, thinking his little thought experiments until one day, suddenly, he has a “Eureka!” moment. “Eureka!” moments young Einstein had, but they didn’t come from nowhere. He understood what scientific questions he was trying to answer, where they fit within philosophical traditions, and who else was asking them. He knew which giants’ shoulders he was standing on. WHEN ALBERT EINSTEIN developed his theories of relativity, no one knew what galaxies were. Today, astronomers use Einstein’s theories to decipher a universe filled with 100 billion galaxies and other objects not known during his lifetime. ESO © 2015 Kalmbach Publishing Co. This material may not be reproduced in any /// 32 astronomy februaryform without permission 05 from the publisher. www.Astronomy.com For Einstein, the answer to how anyone could do what he did “Thank you!” Einstein greeted Besso the following morning. “I might have been, “Who knows?” But the answer to how Einstein have completely solved the problem.” The trouble with the current 1 did what he did is simple: Here’s how. conception of the universe, he explained, wasn’t space. It was time. 1 Space and time Time changes The problem was space. When Einstein finally solved it in the Go back to the dock and look at Galileo’s ship, still in harbor after spring of 1905, he had already been thinking about it for 10 years. all these centuries. While it’s at rest in the water, you on the dock But the problem had been around for nearly 3 centuries, since the and an observer on the ship measure the mast and agree it is modern scientific era dawned. 186,282 miles high. (It’s a very tall ship.) In 1632, in Dialogue Concerning the Two Chief World Systems, Now suppose that, as in the earlier example, the ship is moving Galileo Galilei tried to explain how a Copernican view of the uni- at a constant speed across your line of sight. If the person at the verse alters physics. He invited you to imagine yourself on a dock, mast’s top sends a light signal straight down, where will it land? observing a ship moving at a steady rate across your line of vision. The Aristotelian answer is some distance behind the mast’s If someone at the top of the ship’s mast dropped a rock, where base. The Galilean answer is the base of the mast — which is the would it land? At the base of the mast? Or would it land some Einsteinian answer as well. small distance behind the mast — a distance corresponding to the From your point of view on the dock, the base of the mast will THE PERSON DROPPING THE ROCK from the top of the ship’s mast distance the ship covers between the release of the rock at the top have moved out from under the top of the mast during the light AN ELEVATOR FLOATING in space — or falling in a gravitational field observes a perpendicular drop — the rock appears to fall straight of the mast and its arrival on the deck? beam’s descent, just as it did during the rock’s descent. This — feels the same to a passenger inside it. You can’t see outside, so down to the base of the mast. The intuitive, Aristotelian answer is a small distance back. The means the distance the light has traveled, from your point of you have no frame of reference, and you feel weightless. correct, counterintuitive — and, Galileo argued, Copernican — view, has lengthened. It’s not 186,282 miles. It’s more. answer is the base of the mast, because the movement of the ship You can easily find how much more by measuring the light’s 2 and the movement of the rock together comprise a single motion. journey time — and this is where Einstein’s interpretation begins 2 From an observer’s point of view at the top of the mast, the to depart from Galileo’s. rock’s motion seems to be a perpendicular drop — the kind Any velocity is distance divided by time — for instance, miles Aristotle argued a rock would make in seeking to return to its divided by seconds. In the case of light, though, the velocity isn’t natural place in the universe, the ground. A person at the ship just 186,282 miles per second; according to Maxwell, and now mast’s top would take into account only the rock’s motion. Einstein, it’s always 186,282 miles per second. It’s a constant. It’s But for you, observing from the dock, both the rock and the on one side of the equal sign, moving at its imperturbable rate. ship would be moving, and together, those movements constitute On the other side of the equal sign are the parts of the equa- a single system in motion. To you, therefore, the motion of the tion that can vary: distance and time. They can undergo infinite rock falling toward the ship would not seem perpendicular but changes in value, so long as they continue to divide in such a way at an angle. that the result is 186,282 miles per second. Change the distance And vice versa. If, instead, you the observer standing on the the light beam travels, as happened from your point of view on dock dropped a rock, then, to you, the motion of that rock rela- the dock when the light beam on the moving ship “fell” from the tive to Earth would appear perpendicular, while to an observer on top of the mast, and you have to change the time. AN OBSERVER ON A DOCK sees the ship and rock moving together. the ship, the trajectory of the rock would make an angle. You have to change the time. IN AN ACCELERATING ELEVATOR, you feel a force on your feet. But Instead of a straight fall to the base of the mast, the dockside Either way, to trace the rock’s trajectory would require just “The step,” Einstein called this insight later, as if it had been because you can’t see outside, you can’t tell if the elevator is mov- observer sees the rock fall at an angle, taking a longer path. basic geometry, and you on the dock or the observer on the ship merely a matter of putting one foot in front of another. ing upward or if you’re at rest on a planetary surface. would have equal claim to being right. And there you have it — And one person, in fact, already had arrived at the idea that a 3-century-old Galilean principle of relativity. time might differ for different observers: Henri Poincaré, proba- motion of ship Einstein, however, introduced a complication into this sce- bly the most eminent mathematician of the day — and one of the 3 nario: What if the object descending from the top of the mast two sets of shoulders on which Einstein rested. As a member of 3 wasn’t a rock but a beam of light? the French Bureau des Longitudes and a professor at the École His choice of falling object wasn’t arbitrary. According to the Professionelle Supérieure des Postes et Télégraphes, Poincaré electromagnetic theory that Scottish-born physicist James Clerk presided over one of the most pressing practical matters at the force of Maxwell devised 40 years earlier, the speed of light is constant. It’s turn of the 20th century: the coordination of clocks, specifically, resultant gravity the same no matter what. What changes isn’t the speed of the the coordination of electrical clocks. path of rock on rock light waves but their frequency — the number of waves that Unlike mechanical clocks, electrical clocks allowed the trans- reaches you in a certain length of time. mission of information from city to city, capital to capital, even Part of Einstein’s larger ambition was to reconcile electromag- shore to ship once radio signals came into use, and all at the netism with Galileo’s version of relativity. And one night in May speed of light. Poincaré understood that this speed is a natural 1905, after discussing the problem with his longtime friend and limit in the transmission and reception of information. In 1898, patent office sounding-board, Michele Besso, Einstein understood he published an essay on the subject, “The Measure of Time.” how to do it.