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Special relativity.

The Michelson-Morley experiment (see 1887) was still troublesome. The work of FitzGerald (see 1892) and Lorentz (see 1895) got rid of the difficulty in a way, but the notion of decrease of distance and increase of mass seemed to hang in the air without an overall physical theory for support.

The German-born physicist Albert Einstein (1879-1955) supplied that in 1905.

He began with the assumption that the speed of light in a vacuum would always be the same, regardless of the motion of the light source relative to the observer. This was what Michelson and Morley had observed, but Einstein maintained that he was unaware of the Michelson-Morley results when he worked out his theory.

From this assumption it was possible to deduce length contraction and mass increase with velocity. It was also possible to deduce that the speed of light in a vacuum was an absolute speed limit and that the rate of time-flow would decrease with velocity.

This is Einstein's theory of special relativity. It is relativity, because velocity has meaning only as relative to an observer, there being no such thing as "absolute rest" against which an "absolute motion" can be measured.

There is also no such thing as "absolute space" or "absolute time," since both depend on velocity and therefore have meaning only relative to the viewer.

Nevertheless, despite this absence of absolutes, the law of physics still held for all "frames of reference." In particular, Maxwell's equations (see 1865) still held, though the much older and more revered laws of motion as worked out by Newton had to be modified.

The theory is special, because it confines itself to the special case of objects that are moving at constant velocity. Under those conditions, the theory does not take into account the effect of gravitational interactions, which are everywhere present and which force accelerations on motion.

Einstein's view of the Universe seemed to go against common sense, but that was only because the average person deals with a Universe of small distances and small velocities. Under those conditions, Newton's theories hold almost perfectly. In fact, Einstein's equations reduce to Newton's under such conditions. However, where large distances and large velocities are involved, Einstein's equations hold and Newton's do not.

In the eight decades since special relativity was advanced, endless tests and observations have upheld it completely. No divergences between reality and the Einsteinian view have been found.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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