Special relativity: Difference between revisions
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==Einstein's Assumptions== | ==Einstein's Assumptions== | ||
Einstein rested his theory on two uncontroversial postulates. He presumed physical experiments performed in any room moving at any constant speed in any constant direction, i.e. in any [[inertial frame]], must always produce the same results. Along with assuming this [[Principle of Galilean Relativity]], Einstein assumed a second fact based on recent work published by experimental physicists [[Albert Michaelson]] and [[Edward Morley]]. The Michaelson-Morley experiment aimed to determine the speed of light relative to the background ether, which required detecting differences in light's speed depending on how it moved through that ether. Surprisingly, the experiment found that light moves with exactly the same speed all the time, regardless of the motion of the object from which the light emanates or is measured. Einstein took this result at face value and postulated that the speed of light is always exactly the same in any inertial frame. | Einstein rested his theory on two uncontroversial postulates. He presumed physical experiments performed in any room moving at any constant speed in any constant direction, i.e. in any [[inertial frame]], must always produce the same results. Along with assuming this [[Principle of Galilean Relativity]], Einstein assumed a second fact based on recent work published by experimental physicists [[Albert Michaelson]] and [[Edward Morley]]. The Michaelson-Morley experiment aimed to determine the speed of light relative to the background ether, which required detecting differences in light's speed depending on how it moved through that ether. Surprisingly, the experiment found that light moves with exactly the same speed all the time, regardless of the motion of the object from which the light emanates or is measured. Einstein took this result at face value and postulated that the speed of light is always exactly the same in any inertial frame. | ||
Aside from its basis in physicists' experimental results, assuming the constancy of light's speed also does not contradict human perception in any obvious way. In everyday life we experience light's speed as invariably infinite: turn on a light switch and a room is illuminated instantaneously. A simple [[thought experiment]], however, reveals the strangeness of light's speed: | |||
Imagine driving a car straight down a highway at 60mph. An observer on the side of the road measures our speed at 60mph. If another car comes toward us at 50mph as measured by the observer on the side of the road, we inside our car would perceive it coming at us at 60 + 50 = 110mph. Both cars and the outside observer are in inertial frames. From experience, we know that speeds simply add together. Now imagine that we turn on our headlights. Designating the speed of light in the traditional manner by the symbol ''c'', we see the light beam travel away from us at light's constant speed ''c''. We might also presume that the oncoming car's driver sees our lightbeam traveling at ''c'' + 110mph because experience tells us we must add the speed of our car and the oncoming car to our light beam. Our assumption that observers always measure light's speed the same, however, means that the other car sees the light beam moving at speed ''c'' and that the extra 110mph makes no difference. The observer on the side of the road must also see our lightbeam traveling at speed ''c'' even though it emanates from a moving car. The cars' speeds make no difference. If both cars were traveling at half the speed of light, the oncoming car would still measure our lightbeam as traveling at speed ''c'', not (''c'' + 1/2''c'' + 1/2''c'') regardless of the great speed of his and our car. | |||
==References== | ==References== | ||
Revision as of 17:58, 23 June 2007
Albert Einstein developed his theory of special relativity by 1905, when he was a twenty-six year old clerk in the Swiss patent office. The theory accounted for the paradoxical results of certain 19th century physical experiments attempting to detect the universe's background ether, which was supposed to be the ultimate neutral background or reference point against which the entire physical universe moved. Physicists had always assumed the ether's existence, but experiments--most notably the Michaelson-Morley experiment of the 1880s--always failed to detect it. By boldly refusing to assume the possibility of an ether and theorizing laws of motion without referring to an absolute background, Einstein's simple presumption of objects' "relativity" revolutionized the fundamental view of the physical universe in that his results utterly countered humans' intuitive view of the everyday world. In particular, humans' perception of time and distance, while quite correct for everyday life, inadequately understand these intuitive ideas when high speeds are involved, and so ultimately misunderstand them fundamentally. Einstein's theory says that when speed is an appreciable fraction of light's speed time passes more slowly and length shortens in the direction of motion, and so human perception fails in a fundamental way to grasp what are thought to be the intuitive ideas of time and distance.
Einstein's Assumptions
Einstein rested his theory on two uncontroversial postulates. He presumed physical experiments performed in any room moving at any constant speed in any constant direction, i.e. in any inertial frame, must always produce the same results. Along with assuming this Principle of Galilean Relativity, Einstein assumed a second fact based on recent work published by experimental physicists Albert Michaelson and Edward Morley. The Michaelson-Morley experiment aimed to determine the speed of light relative to the background ether, which required detecting differences in light's speed depending on how it moved through that ether. Surprisingly, the experiment found that light moves with exactly the same speed all the time, regardless of the motion of the object from which the light emanates or is measured. Einstein took this result at face value and postulated that the speed of light is always exactly the same in any inertial frame.
Aside from its basis in physicists' experimental results, assuming the constancy of light's speed also does not contradict human perception in any obvious way. In everyday life we experience light's speed as invariably infinite: turn on a light switch and a room is illuminated instantaneously. A simple thought experiment, however, reveals the strangeness of light's speed:
Imagine driving a car straight down a highway at 60mph. An observer on the side of the road measures our speed at 60mph. If another car comes toward us at 50mph as measured by the observer on the side of the road, we inside our car would perceive it coming at us at 60 + 50 = 110mph. Both cars and the outside observer are in inertial frames. From experience, we know that speeds simply add together. Now imagine that we turn on our headlights. Designating the speed of light in the traditional manner by the symbol c, we see the light beam travel away from us at light's constant speed c. We might also presume that the oncoming car's driver sees our lightbeam traveling at c + 110mph because experience tells us we must add the speed of our car and the oncoming car to our light beam. Our assumption that observers always measure light's speed the same, however, means that the other car sees the light beam moving at speed c and that the extra 110mph makes no difference. The observer on the side of the road must also see our lightbeam traveling at speed c even though it emanates from a moving car. The cars' speeds make no difference. If both cars were traveling at half the speed of light, the oncoming car would still measure our lightbeam as traveling at speed c, not (c + 1/2c + 1/2c) regardless of the great speed of his and our car.