Einstein's theory of relativity consists of two major portions: the special theory of relativity and the general theory of relativity. Special relativity deals with phenomena that become noticeable when traveling near the speed of light and reference frames that are moving at a constant velocity, inertial reference frames. General relativity deals with reference frames that are accelerating, noninertial reference frames, and with phenomena that occur in strong gravitational fields. General relativity also uses the curvature of space to explain gravity.

In the seventeenth century, Isaac Newton completed a grand synthesis of physics that used three laws of motion and the law of gravity to explain motions we observe both on Earth and in the heavens. These laws worked very well, but by the end of the nineteenth century, physicists began to notice experiments that did not work quite the way they should according to Newton's classical...