By David Terr
Einstein's theory of relativity is one of the greatest scientific theories of all time. It has radically altered our views of space, time, matter, energy, and gravity.
The theory of relativity consists of two parts, special relativity, which does not concern gravity, and general relativity, which does. Special relativity is by far the simpler of the two. Einstein devised the special theory of relativity in 1905 and the general theory in 1915.
Special relativity relies on two key assumptions. The first is that there is no experimental way to distinguish between two nonaccelerating observers, so that the notion of being "at rest" in an absolute sense is meaningless. The second assumption is that light always appears to travel at the same speed no matter how fast one is moving. This assumption, which was experimentally verified by Michaelson and Morley in 1887, is the more counterintuitive assumption.
Special relativity makes many bizarre predictions which defy common sense but have all been experimentally verified. Here are a few:
Time Dilation: Time slows down for a moving observer, i.e. if an observer at rest and an observer in motion are given two identical clocks, then the moving observer's clock will tick more slowly than the clock of the observer at rest.
Length Contraction: An object such as a meter stick shrinks in the direction in which it is moving, shrinking to zero length in the limit in which its speed approaches the speed of light.
Mass Increase: As an object approaches the speed of light, its mass increases, approaching infinity.
Relative Simultaneity: Two events which appear to occur at the same time for one observer do not necessarily appear to occur at the same time for a second observer moving relative to the first.
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