Einstein's Mistakes by Hans C. Ohanian

Author:Hans C. Ohanian
Language: eng
Format: epub
Publisher: W. W. Norton & Company
Published: 2008-05-05T16:00:00+00:00

Again, there was nothing very original about this idea. In Einstein’s days, the generation of artificial gravity by acceleration was well understood and widely applied in the technology of centrifuges. The ultra-modern Swiss dairies, like all other modern dairies, used centrifuges to separate cream from milk. These centrifuges subject the milk to a high-speed rotational motion, with a large centripetal acceleration, which generates strong artificial gravity of the kind envisioned by Einstein. The cream separates from the milk much faster than if you merely leave a pail of milk sitting on a table, where ordinary gravity slowly pulls the water away from the cream.

What was new was that instead of using artificial gravity to extract cream, Einstein used it to gain insights into the relationship between acceleration and gravity. He argued that the indistinguishability of the effects of acceleration and gravity meant that acceleration, like velocity, is relative. The physicist in the closed box cannot tell whether he is experiencing an acceleration or a gravitational force, unless he looks outside the box and checks its motion relative to some reference point. Thus, Einstein thought there must be a generalized form of relativity, in which both velocity and acceleration are relative. He called the theory of relative velocity he had formulated in 1905 “old” relativity, or special relativity; and he called the new theory of relative acceleration general relativity.

As in special relativity, the speed of light played a central role in Einstein’s attempts to achieve a general theory of relativity. He used the Principle of Equivalence to deduce predictions about the behavior of light in a gravitational field. He reasoned that if the effects of acceleration and of gravity are identical, then it must be possible to deduce what happens in a reference frame at rest in a gravitational field by examining what happens in an accelerated reference frame in the absence of gravity. The latter reference frame is a surrogate for the former, and if we can figure out what light does in the latter, we will know what it does in the former.

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