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special relativity requires objects to be in uniform motion in order for the laws of physics to apply. Special relativity also states that the speed of light is constant in empty space. The speed of light is constant in empty space due to time dilation and length contraction. As an object approaches the speed of light time slows down (time dilation) and length contacts. The amount of time dilation  is equal to the amount of length contraction thereby maintaining the constant speed of light in a vacuum. In special relativity, Einstein defined space and time as a single infinity he called space-time.

General relativity states that acceleration is equivalent to gravity in the  principle of equivalence. According to the correspondence principle, all new theories must agree with the proven parts of existing theories. Therefore, Einstein general theory of relativity had to agree with Newtonian mechanics at slow speeds; which it did. finally, Einstein developed his famous equation, E=mc2 which states that energy and mass are equal. Mass is simply potential energy.