Albert Einstein’s first major theory, dating from 1905, special relativity builds on Galileo's more simplistic principle of relativity and relates what one person sees when looking at another person moving at constant speed relative to them. “Special” indicates that the theory restricts itself to observers in uniform or constant relative motion, a restriction Einstein addressed later in his General Theory of Relativity. The theory incorporates the principle that the speed of light is the same for all inertial observers, regardless of the state of motion of the source. Among other things, it reveals that the moving person appears to shrink in the direction of their motion (length contraction)and their time slows down (time dilation), effects which are ever more marked as speeds approach the speed of light. The theory also leads to some famous paradoxes like the so-called Time Travel Paradox and the Twin Paradox.