Special relativity
Albert Einstein developed his theory of special relativity by 1905, when he was a twenty-six year old clerk in the Swiss patent office. The theory accounted for the paradoxical results of certain 19th century physical experiments attempting to detect the universe's background ether, which was supposed to be the ultimate neutral background or reference point against which the entire physical universe moved. Physicists had always assumed the ether's existence, but experiments--most notably the Michaelson-Morley experiment of the 1880s--always failed to detect it. By boldly refusing to assume the possibility of an ether and theorizing laws of motion without referring to an absolute background, Einstein's simple presumption of objects' "relativity" revolutionized the fundamental view of the physical universe in that his results utterly countered humans' intuitive view of the everyday world. In particular, humans' perception of time and distance, while quite correct for everyday life, inadequately understand these intuitive ideas when high speeds are involved, and so ultimately misunderstand them fundamentally. Einstein's theory says that when speed is an appreciable fraction of light's speed time passes more slowly and length shortens in the direction of motion, and so human perception fails in a fundamental way to grasp what are thought to be the intuitive ideas of time and distance.
Einstein's Assumptions
Einstein rested his theory on two uncontroversial postulates. He presumed physical experiments performed in any room moving at any constant speed in any constant direction, i.e. in any inertial frame, must always produce the same results. Along with assuming this Principle of Galilean Relativity, Einstein assumed a second fact based on recent work published by experimental physicists Albert Michaelson and Edward Morley. The Michaelson-Morley experiment aimed to determine the speed of light relative to the background ether, which required detecting differences in light's speed depending on how it moved through that ether. Surprisingly, the experiment found that light moves with exactly the same speed all the time, regardless of the motion of the object from which the light emanates or is measured. Einstein took this result at face value and postulated that the speed of light is always exactly the same in any inertial frame.