Albert Einstein developed the theory of relativity in the early twentieth century. He published the theory of special relativity in 1905 and the theory of general relativity in 1916. Special relativity deals only with nonaccelerating (inertial) reference frames; general relativity deals with accelerating (noninertial) reference frames. Simply stated, according to the theory of special relativity, the laws of nature are the same for all observers whose frames of reference are moving with constant velocity with respect to each other. Published as an addenda to the special theory of relativity was the famous equation, E = mc², representing that mass and energy can be transformed into each other. In contrast, general relativity states that the laws of nature are the same for all observers even if they are accelerating with respect to each other.