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The Quest for the Ether
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12545 |
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NATURAL SCIENCE
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7 / 1987 |
3,867 Words |
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Arthur I. Miller
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In 1880, Albert A. Michelson conceived of an optical instrument striking in its simplicity and breathtaking in its precision. Michelson's interferometer opened up new fields of research and led to his being the first American Nobel laureate for any scientific achievement (1907). Yet, ironically, he is best known to many scientists for an experiment that failed.
In July 1887, Michelson and Edward W. Morley attempted to measure the velocity at which the earth moves relative to the medium that was assumed to transmit light, the ether. Their failure was considered to be nothing short of extraordinary. It became a puzzle that would preoccupy physicists for almost two decades, and that is still discussed today. Why? To some extent the reply resides in how the puzzle was resolved. On the one hand, the eminent Dutch physicist H.A. Lorentz proposed a detailed explanation for the dilemma, while on the other hand, Albert Einstein declared that there was no puzzle at all because this was the way the experiment should have turned out. There is also speculation about the place that this experiment has taken in the history of ideas - namely, as a crucial experiment that was the catalyst for Einstein's invention of the special theory of relativity. In the centennial year of what is considered to be one of the most significant experiments over conducted, it is appropriate to recall its origins, intent, reception, and current interpretation.
Background
By the middle of the nineteenth century, physicists had agreed that light travels in waves, just as sound does. Sound waves require a medium, like air or water, for their transmission; waves are a form of energy that propagate, for example, by progressively compacting and expanding the medium without permanently moving or altering it. Physicists argued that light waves must require a similar medium, and they called this hypothetical substance the ether.
Many experiments were devised in an attempt to detect the presence of the ether. These experiments were collectively called "ether-drift" experiments, for they attempted to show the influence of the ether on the velocity of light. The intent of these experiments can be demonstrated with the following "thought experiment."
Suppose that a light wave is emitted from a source that is at rest on the ground. The velocity of a marked point on the wave relative to the source is c. After the wave has been emitted, an observer originally at rest begins to move in order to match the speed of the marked point. The velocity of the observer relative to the source of light is v. Then the velocity of the point on the light wave relative to the observer, V, is
V = c - v (1)
Clearly, the observer could increase his velocity until v = c, at which point V would equal zero. In this case the marked point on the light wave and the observer are at relative rest. Equation 1 is the addition law for velocities from Newton's mechanics, and it expresses how we expect intuitively to observe relative motion. For example, we can imagine the point on the light wave to be a car that is traveling ahead of you on a highway and that you are the observer. You can accelerate in order to match the speed of the other car, and when you do, V =
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