Theory of relativity from whatis.com
Albert Einstein's theory of relativity is actually two separate theories: his special theory of relativity , postulated in the 1905 paper, The Electrodynamics of Moving Bodies and his theory of general relativity , an expansion of the earlier theory, published as The Foundation of the General Theory of Relativity in 1916. Einstein sought to explain situations in which Newtonian physics might fail to deal successfully with phenomena, and in so doing proposed revolutionary changes in human concepts of time, space and gravity.
The special theory of relativity was based on two main postulates: first, that the speed of light is constant for all observers; and second, that observers moving at constant speeds should be subject to the same physical laws. Following this logic, Einstein theorized that time must change according to the speed of a moving object relative to the frame of reference of an observer. Scientists have tested this theory through experimentation - proving, for example, that an atomic clock ticks more slowly when traveling at a high speed than it does when it is not moving. The essence of Einstein's paper was that both space and time are relative (rather than absolute), which was said to hold true in a special case, the absence of a gravitational field. Relativity was a stunning concept at the time; scientists all over the world debated the veracity of Einstein's famous equation, E=mc2, which implied that matter and energy were equivalent and, more specifically, that a single particle of matter could be converted into a huge quantity of energy. However, since the special theory of relativity only held true in the absence of a gravitational field, Einstein strove for 11 more years to work gravity into his equations and discover how relativity might work generally as well.
According to the theory of general relativity, matter causes space to curve. It is posited that gravitation is not a force, as understood by Newtonian physics, but a curved field (an area of space under the influence of a force) in the space-time continuum that is actually created by the presence of mass. According to Einstein, that theory could be tested by measuring the deflection of starlight traveling near the sun; he correctly asserted that light deflection would be twice that expected by Newton's laws. This theory also explained why the light from stars in a strong gravitational field was closer to the red end of the spectrum than those in a weaker one.