# STR5 - STR The notion of the Present also involves the...

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STR The notion of the Present also involves the notion of simultaneity . All of the events simultaneous with this event are part of the Present. But, as Einstein showed, the notion of simultaneity is a relative one. Let us look at Einstein’s Special Theory of Relativity (STR). First, some background. Newton founded his dynamics upon the twin notions of absolute space and time. As he says in the Scholium, “Absolute, true, and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent, and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time; such as an hour, a day, a month a year.” He says similar things about absolute space. The postulation of absolute space and time bothered many of his contemporaries, insofar as both are invisible and undetectable. But Newton needed them in order to found his principles of motion, and his theory of universal gravitation. A century and a half later, Maxwell, building upon the physical insights of Michael Faraday, found the laws that unified electricity and magnetism. Among other things, these laws indicated that light was an electro-magnetic field propagating as a wave through space. On Maxwell’s view, a propagating wave required a medium, as a sound wave requires air, so the luminiferous aether was proposed to serve that role. The aether was thought to fill all of space, and to be at rest relative to absolute space. After a time, the aether came to be identified with absolute space. But if the aether is at rest, and the earth moves through it on its journey around the sun, then effect of this motion on light should be observable. In a famous set of experiments in 1887 in Cleveland, Ohio, Michaelson and Morley attempted to measure the velocity of the earth through the aether. They found no evidence of any such motion. Their “null result” caused quite a stir. Einstein begins, in his 1905 paper “On the Electrodynamics of Moving Bodies”, with two assumptions, or postulates. 1. The Principle of Galilean Relativity, which states that the laws of nature are the same in any inertial reference frame. 2. The velocityof light is the same in everyinertial reference frame, independent of the velocity of the light source. The justification of the first comes from, among other things, “. ..the unsuccessful attempts to discover any motion of the earth relatively to the ‘light medium’, suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.” The second comes from the results of manymeasurements over manyyears. He then addresses one of Poincaré’s questions, viz., how do we establish the simultaneityof distantlyseparated events.

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STR5 - STR The notion of the Present also involves the...

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