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Simple Nature - Light and Matter

Simple Nature - Light and Matter

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ExercisesExercise 7A: The Michelson-Morley ExperimentIn this exercise you will analyze the Michelson-Morley experiment, <strong>and</strong> find what the resultsshould have been according to Galilean relativity <strong>and</strong> Einstein’s theory of relativity. A beam oflight coming from the west (not shown) comes to the half-silvered mirror A. Half the light goesthrough to the east, is reflected by mirror C, <strong>and</strong> comes back to A. The other half is reflectednorth by A, is reflected by B, <strong>and</strong> also comes back to A. When the beams reunite at A, partof each ends up going south, <strong>and</strong> these parts interfere with one another. If the time takenfor a round trip differs by, for example, half the period of the wave, there will be destructiveinterference.The point of the experiment was to search for a difference in the experimental results betweenthe daytime, when the laboratory was moving west relative to the sun, <strong>and</strong> the nighttime, whenthe laboratory was moving east relative to the sun. Galilean relativity <strong>and</strong> Einstein’s theoryof relativity make different predictions about the results. According to Galilean relativity, thespeed of light cannot be the same in all reference frames, so it is assumed that there is one specialreference frame, perhaps the sun’s, in which light travels at the same speed in all directions;in other frames, Galilean relativity predicts that the speed of light will be different in differentdirections, e.g., slower if the observer is chasing a beam of light. There are four different waysto analyze the experiment:• Laboratory’s frame of reference, Galilean relativity. This is not a useful way to analyzethe experiment, since one does not know how fast light will travel in various directions.• Sun’s frame of reference, Galilean relativity. We assume that in this special frame ofreference, the speed of light is the same in all directions: we call this speed c. In thisframe, the laboratory moves with velocity v, <strong>and</strong> mirrors A, B, <strong>and</strong> C move while the lightbeam is in flight.• Laboratory’s frame of reference, Einstein’s theory of relativity. The analysis is extremelysimple. Let the length of each arm be L. Then the time required to get from A to eithermirror is L/c, so each beam’s round-trip time is 2L/c.• Sun’s frame of reference, Einstein’s theory of relativity. We analyze this case by startingwith the laboratory’s frame of reference <strong>and</strong> then transforming to the sun’s frame.444 Chapter 7 Relativity

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