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# Lec27 - The shift in the interference pattern is therefore...

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The shift in the interference pattern is therefore by nearly half a fringe, which is certainly observable (and was above detectability level already in 1887). The shocking news of the Michelson–Morley experiment were that no shift at all was mea- sured! As a note in passing, notice that if 1 1 = 1 1 - v 2 /c 2 the predicted e ff ect disappears. If that were to happen in practice, it would explain the null result of the Michelson–Morley experiment. Of course, there are no physical reasons to argue that such contraction actually occurs. Nevertheless, Fitzgerald and Lorentz tried to explain the null result of the Michelson– Morley experiment by just such a contraction, that they believed to be an actual physical e ff ect. The Principle of Relativity The Principle of Relativity was first formulated by Galileo, who suggested that one cannot do any mechanical experiment that will reveal that your reference frame is in a state of inertial motion. Of course, in Galileo’s time the only laws of physics known were the laws of mechanics. Einstein expanded galilean relativity to all physical interactions: The Einstein Principle of relativity states that one cannot do any experiment that will find a system to be in a state of inertial motion. The exact equivalency of all inertial frames, regardless of their relative motion, means that the laws of physics must be the same in all inertial frames. If they were not, then by finding how the laws of physics in our laboratory are di ff erent from the laws of physics in some preferred reference frame, we could find that our laboratory is in a state of inertial motion. Notice that it is the laws of physics that are the same in all inertial frames, not the description of phenomena. Say you juggle a ball. In your frame the ball is moving up and down along a straight line. But if you are a better juggler and toss the ball in exactly the same way (i.e., straight up in your frame) while walking, an observer who is static observes di ff erent kind of motion, specifically parabolic motion. So how can the Principle of Relativity be correct if one observer sees motion along a straight line and another sees parabolic motion? In fact, both observers do agree that the law of physics governing the motion of the ball is F = m g .

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Lec27 - The shift in the interference pattern is therefore...

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