A Briefer History of Time Read Online Free PDF

of light is so great compared to the speeds at which we might move toward a light source, this difference in speed was a very difficult effect to measure.
    In 1887, Albert Michelson (who later became the first American to receive the Nobel Prize for physics) and Edward Morley carried out a very careful and difficult experiment at the Case School of Applied Science (now Case Western Reserve University) in Cleveland. They realized that because the earth orbits the sun at a speed of nearly twenty miles per second, their lab itself must be moving at a relatively high rate of speed through the ether. Of course, no one knew in which direction or how fast the ether might be moving with respect to the sun, or whether it was moving at all. But by repeating an experiment at different times of the year, when the earth was in different positions along its orbit, they could hope to account for this unknown factor. So Michelson and Morley set up an experiment to compare the speed of light measured in the direction of the earth’s motion through the ether (when we were moving toward the source of the light) to the speed of light at right angles to that motion (when we were not moving toward the source). To their great surprise, they found the speed in both directions was exactly the same!
    Between 1887 and 1905, there were several attempts to save the ether theory. The most notable was by the Dutch physicist Hendrik Lorentz, who attempted to explain the result of the Michelson-Morley experiment in terms of objects contracting and clocks slowing down when they moved through the ether. However, in a famous paper in 1905, a hitherto unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the w hole idea of an ether was unnecessary, provided one was willing to abandon the idea of absolute time (we’ll see why shortly). A leading French mathematician, Henri Poincaré, made a similar point a few weeks later. Einstein’s arguments were closer to physics than those of Poincaré, who regarded this problem as purely mathematical and to his dying day did not accept Einstein’s interpretation of the theory.
    Einstein’s fundamental postulate of the theory of relativity, as it was called, stated that the laws of science should be the same for all freely moving observers, no matter what their speed. This was true for Newton’s laws of motion, but now Einstein extended the idea to include Maxwell’s theory. In other words, since Maxwell’s theory dictates that the speed of light has a given value, all freely moving observers must measure that same value, no matter how fast they are moving toward or away from its source. This simple idea certainly explained—without the use of the ether or any other preferred frame of reference—the meaning of the speed of light in Maxwell’s equations, yet it also had some remarkable and often counterintuitive consequences.
    For example, the requirement that all observers must agree on how fast light travels forces us to change our concept of time. Picture again the speeding train. In Chapter 4, we saw that although someone on the train bouncing a Ping-Pong ball up and down may say that the ball traveled only a few inches, someone standing on the platform would perceive the ball as traveling about forty meters. Similarly, if the observer on the train shone a flashlight, the two observers would disagree on the distance the light traveled. Since speed is distance divided by time, if they disagree on the distance the light has traveled, the only way for them to agree on the speed of light is for them to also disagree about the time the trip has taken. In other words, the theory of relativity requires us to put an end to the idea of absolute time! Instead, each observer must have his own measure of time, as recorded by a clock carried with him, and identical clocks carried by different observers need not agree.
    In relativity there is no need to introduce the idea of an ether, whose presence, as