Einstein's theory of Special Relativity was one of the magical four papers in 1905. In it he starts from the supposition that all non-accelerating observers get the same result when they measure the speed of light, and deduces such counter-intuitive results as length contraction, time dilation and relativity of simultaneity. These can also be deduced starting from the following observation:

Suppose we are moving past each other - neither accelerating - and you are holding an electron. I see the electron moving, so I should see a magnetic field. You don't see the electron moving, so you should not see a magnetic field. This apparent inconsistency can lead to the development of the same theory, and the same predictions.

These predictions have been independently verified many times over, providing strong evidence that these rather bizarre predictions are, in truth, correct.

The theory is termed "special" because it applies the principle of relativity only to inertial frames. Einstein later developed general relativity to apply the principle more generally, handling coordinate transformations in accelerating systems, and hence General Relativity includes the effects of gravity.

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