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By the end of the nineteenth century, most physicists were feeling quite smug. They seemed to have theories in place that would explain all physical phenomena. There was clearly a lot of cleaning up to do, but it looked like a fairly mechanical job: turn the crank on the calculator until the results come out. Apart from a few niggling problems like those lines in the light emitted by gas discharges, and the apparent dependence of the mass of high-speed electrons on their velocity .... Twenty-five years later, this complacency had been completely destroyed by the invention of three entirely new theories: special relativity, general relativity, and quantum mechanics. The outstanding figure of this period was Albert Einstein. His name became a household word for his development, virtually single-handedly, of the theory of relativity, and he made a major contribution to the development of quantum mechanics in his explanation of the photoelectric effect. Einstein was a clerk in a Swiss patent office when he published his special theory of relativity in 1905. He claimed in later life that the need for this theory emerged out of Maxwell's equations. Those equations changed their form when one rewrote them from the conventional perspective of a person moving at constant velocity. On the other hand, our experience tells us that we cannot tell if we are moving as long as our velocity is constant: you can throw a ball back and forth in a rapidly moving train car just as you can when the train is still. It is only when it accelerates -- slows down or speeds up -- that one experiences a change. Moreover, Maxwell's equations indicated that the speed of light did not depend on the speed of the person measuring this speed, whereas if one throws a stone while running, the speed of the runner contributes to the speed of the stone. To overcome these apparent difficulties with Maxwell's theory, which Einstein believed to describe reality correctly, he considered the effect of two postulates. The first was that all physical phenomena must obey the same equations for people moving at different constant velocities (the principle of relativity), and the second was that the speed, c, measured for light does not depend on the speed of the "observer" (the person carrying out the measurement). These two postulates led directly to almost unbelievable results. They showed that the measurement of space and time depended on each other (that the time you measured for an occurrence depended on your position), and also depended on the speed of the observer. One immediate result is that "simultaneity " is relative to the observer. Two "events" that occur at the same time for one observer occur at different times as seen by an observer in motion relative to the first, provided that the events occur at different spatial locations; the concept of absolute time and space which had underpinned mechanics for two centuries lay in shatters. Einstein's theory also showed that the measured mass of an object depended on its velocity, and that mass (m) could be converted to energy (E) according to E=mc2, the principle behind the atomic bomb and nuclear power plants. One of the beauties of Einstein's theory was that, as you let a body's speed become small compared to the speed of light, the equations would reduce to those of Newtonian mechanics. This requirement of physics, that a more general theory must reduce in some limit to more restrictive theories, is called the "correspondence principle". Thus we see that the development of the special theory of relativity in no way diminishes the stature of Newton. Although his concept of absolute space and time were incorrect, his genius remains: Newton's mechanics is still correct except for bodies whose speeds approach that of light. It is important to discuss the fact that the results of the special theory contradict "common sense": we know that we do not have to correct our watches after we have been in a car, and that people who are running do not appear thinner than when at rest. The problem here is that our common sense is, by definition, the sense of how the common world works. However, the effects predicted by the special theory are significant only at a speed approaching that of light, and none of us has ever moved at such a speed relative to another object with which we can interact. Therefore, we must not assume that our low-speed common sense also applies at very high speeds. Similarly, we will see that the mechanics governing sub-microscopic bodies such as atoms is quite different to the mechanics describing 60-kg human beings. In 1887 the Americans Albert Michelson and Edward Morley had attempted to measure the speed of the Earth through the ether by measuring the difference in the speed of light travelling in two perpendicular directions. A difference was expected, for the same reason that the speed of a water wave relative to you depends on whether you are travelling in the same direction as the wave or otherwise. They found no dependence on the direction of motion of the light, and interpreted this null result by claiming that the Earth dragged the ether with it. But if the ether interacted with matter in this way, why could it not be detected directly? Moreover, the observation by James Bradley in 1725 of stellar aberation rules out the hypothesis of ether drag. (Stellar aberation is the apparent movement of the stars in a small ellipse over the course of a year, because the Earth is moving and it takes some time for the light of the stars to reach Earth.) In 1892, Hendrik Lorentz and G.F. Fitzgerald independently hypothesized that the size of Michelson and Morley's measuring device must depend on its velocity so as to contract in the direction of motion exactly enough to give the null result. Einstein's second postulate presented yet another possibility: the measured speed of light was intrinsically independent of the speed of the observer. However, it went much beyond interpreting the Michelson -Morley result and explained, for example, the experimental observation that an electron's mass depended on its velocity. In fact, Henri Poincaré, a renowned physicist, had suggested a year before Einstein's publication that a whole new mechanics might be required, in which mass depended on velocity. Einstein's theory cleared up so many outstanding problems that it was quite quickly accepted by most physicists. Before leaving special relativity it is important to discuss briefly Einstein's role in the development of nuclear weapons. Nuclear fission had been discovered in Germany in 1938, just after the invasion of Austria by Hitler's forces. In 1939, faced with the threat that Germany would develop a nuclear bomb, Einstein was convinced by physicist Leo Szilard to write to President Roosevelt, pointing out the possibility and encouraging American research in this direction. In spite of this, Einstein actively opposed further development of nuclear weapons following the Second World War. In fact, he and British philosopher/mathematician Bertrand Russell founded the Pugwash organization, named after its first meeting in Pugwash, Nova Scotia, in 1954. This organization of leading scientists throughout the world, and its student wing, still meet regularly to discuss issues concerning the impact of science on society, and to prepare position papers for presentation to governments and the United Nations. The General Theory of Relativity extended Einstein's ideas to bodies which are accelerating, rather than moving at constant velocity. Einstein showed that spacetime near masses could not be described by Euclidean geometry, but rather that a geometry invented by Riemann must be used. In this way, gravitation was shown to be a result of the curvature of spacetime in the vicinity of mass. The general theory allowed Einstein to predict the amount of the deflection of light in the eclipses of 1919 and 1921, a value which agreed with that measured. However, Einstein's theory of general relativity was not the last word on the subject. General relativity is still an active area of research today, partly because it provides us with much evidence on the evolution of the universe including such questions as, "Will the universe someday begin to collapse back upon itself under its gravitational attraction?" |
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