Einstein's theory of relativity states that matter and energy are interchangeable, and the amount of energy in a portion of matter equals its mass times the speed of light squared, which Einstein expressed in his equation: E=mc². Relativity also states that space and time are aspects of a single fabric called the space-time continuum and that gravity is not a pulling force, but a change in the motion of objects due to the curvature of space.
Albert Einstein published two papers on the subject of relativity. He published his first paper on the special theory of relativity in 1905 and his second paper on the general theory of relativity in 1915. In his special theory of relativity, Einstein explained how the speed of objects is relative to the speed of the observer, and there is no frame of reference in the universe to determine an absolute speed of an object; therefore, speed is always relative. The exception to this rule, according to Einstein, is the speed of light, which always remains constant, regardless of the speed of the observer. These findings led Einstein to deduce that the mass of an object is relative to its speed. The faster an object travels, the greater its mass. Also, the passage of time is relative to an object's speed. The faster an object travels, the slower time passes for that object.