ALBERT EINSTEIN, THE DISCOVERER OF THE SPECIAL AND general theories of relativity, was born in Ulm, Germany, in 1879, but the following year the family moved to Munich, where his father, Hermann, and uncle, Jakob, set up a small and not very successful electrical business. Albert was no child prodigy, but claims that he did poorly at school seem to be an exaggeration. In 1894 his father's business failed and the family moved to Milan. His parents decided he should stay behind to finish school, but he did not like its authoritarianism, and within months he left to join his family in Italy. He later completed his education in Zurich, graduating from the prestigious Federal Polytechnical School, known as the ETH, in 1900. His argumentative nature and dislike of authority did not endear him to the professors at the ETH and none of them offered him the position of assistant, which was the normal route to an academic career. Two years later, he finally managed to get a junior post at the Swiss patent office in Bern. It was while he held this job that in 1905 he wrote three papers that both established him as one of the world's leading scientists and started two conceptual revolutions—revolutions that changed our understanding of time,
space, and reality itself. Toward the end of the nineteenth century, scientists believed they were close to a complete description of the universe. They imagined that space was filled by a continuous medium called the "ether."
Light rays and radio signals were waves in this ether, just as sound is pressure waves in air. All that was needed for a complete theory were careful measurements of the elastic properties of the ether. In fact, anticipating such measurements, the Jefferson Lab at Harvard University was built entirely without iron nails so as not to interfere with delicate magnetic measurements. However, the planners forgot that the reddish brown bricks of which the lab and most of Harvard are built contain large amounts of iron. The building is still in use today, although Harvard is still not sure how much weight a library floor without iron nails will support.
(FIG. I.I, left) THE FIXED ETHER THEORY
If light were a wave in an elastic material called ether, the speed of light should appear higher to someone on a spaceship (a) moving toward it, and lower on a spaceship (b) traveling in the same direction as the light.
By the century's end, discrepancies in the idea of an all-pervading ether began to appear. It was expected that light would travel at a fixed speed through the ether but that if you were traveling through the ether in the same direction as the light, its speed would appear lower, and if you were traveling in the opposite direction of the light, its speed would appear higher (Fig. 1.1). Yet a series of experiments failed to support this idea. The most careful and accurate of these experiments was carried out by Albert Michelson and Edward Morley at the Case School of Applied Science in Cleveland, Ohio, in 1887. They compared the speed of light in two beams at right angles to each other. As the Earth rotates on its axis and orbits the Sun, the apparatus moves through the ether with varying speed and direction (Fig. 1.2). But Michelson and Morley found no daily or yearly differences between the two beams of light. It was as if light always traveled at the same speed relative to where one was, no matter how fast and in which direction one was moving (Fig. 1.3, page 8). Based on the Michelson-Morley experiment, the Irish physicist George FitzGerald and the Dutch physicist Hendrik Lorentz suggested that bodies moving through the ether would contract and that clocks would slow down. This contraction and the slowingdown of clocks would be such that people would all measure the same speed for light, no matter how they were moving with respect to the ether. (FitzGerald and Lorentz still regarded ether as a real substance.) However, in a paper written in June 1905, Einstein pointed out that if one could not detect whether or not one was moving through space, the notion of an ether was redundant. Instead, he started from the postulate that the laws of science should appear the same to all freely moving observers. In particular, they should all measure the same speed for light, no matter how fast they were moving. The speed of light is independent of their motion and is the same in all directions. This required abandoning the idea that there is a universal quantity called time that all clocks would measure. Instead, everyone would have his or her own personal time. The times of two people would agree if the people were at rest with respect to each other, but not if they were moving. This has been confirmed by a number of experiments, including one in which two accurate clocks were flown in opposite directions around the world and returned showing very slightly different times. This might suggest that if one wanted to live longer, one should keep flying to the east so that the plane's speed is added to the earth's rotation. However, the tiny fraction of a second one would gain would be more than canceled by eating airline meals.
(Fig. 1.5.above) THE TWINS PARADOX - In the theory of relativity each observer has his own measure of time. This can lead to the socalled twins paradox. One of a pair of twins (a) leaves on a space jorney during wich he travels close to the speed of light (c) while his brother (b) remanins on Earth. Because of (a)'s motion, time runs more slowly in the spacecraft as seen by the earthbound twin. So on his return the space traveler (a2) will find that his brother (b2) has agged more than himself. Although it seems against common sense, a number of experiments have implied that in this scenario the traveling twin would indeed be younger.
A very important consequence of relativity is the relation between mass and energy. Einstein's postulate that the speed of light should appear the same to everyone implied that nothing could be moving faster than light. What happens is that as one uses energy to accelerate anything, whether a particle or a spaceship, its mass increases, making it harder to accelerate it further. To accelerate a particle to the speed of light would be impossible because it would take an infinite amount of energy. Mass and energy are equivalent, as is summed up in Einstein's famous equation E = mc2 This is probably the only equation in physics to have recognition on the street. Among its consequences was the realization that if the nucleus of a uranium atom fissions into two nuclei with slightly less total mass, this will release a tremendous amount of energy. In 1939, as the prospect of another world war loomed, a group of scientists who realized these implications persuaded Einstein to overcome his pacifist scruples and add his authority to a letter to President Roosevelt urging the United States to start a program of nuclear research. This led to the Manhattan Project and ultimately to the bombs that exploded over Hiroshima and Nagasaki in 1945. Some people have blamed the atom bomb on Einstein because he discovered the relationship between mass and energy; but that is like blaming Newton for causing airplanes to crash because he discovered gravity. Einstein himself took no part in the Manhattan Project and was horrified by the dropping of the bomb. After his groundbreaking papers in 1905, Einstein's scientific reputation was established. But it was not until 1909 that he was offered a position at the University of Zurich that enabled him to leave the Swiss patent office. Two years later, he moved to the German University in Prague, but he came back to Zurich in 1912, this time to the ETH. Despite the anti-Semitism that was common in much of Europe, even in the universities, he was now an academic hot property. Offers came in from Vienna and Utrecht, but he chose to accept a research position with the Prussian Academy of Sciences in Berlin because it freed him from teaching duties. He moved to Berlin in April 1914 and was joined shortly after by his wife and two sons. The marriage had been in a bad way for some time, however, and his family soon returned to Zurich. Although he visited them occasionally, he and his wife were eventually divorced. Einstein later married his cousin Elsa, who lived in Berlin. The fact that he spent the war years as a bachelor, without domestic commitments, may be one reason why this period was so productive for him scientifically. Although the theory of relativity fit well with the laws that governed electricity and magnetism, it was not compatible with Newton's law of gravity. This law said that if one changed the distribution of matter in one region of space, the change in the gravitational field would be felt instantaneously everywhere else in theuniverse. Not only would this mean one could send signals faster than light (something that was forbidden by relativity); in order to know what instantaneous meant, it also required the existence of absolute or universal time, which relativity had abolished in favorof personal time.
