Albert Einstein published his special theory of relativity in 1905, which established two postulates - the laws of physics are the same in all inertial reference frames, and the speed of light has the same value in all reference frames. This challenged the prevailing Newtonian mechanics and the concept of the luminiferous ether. Experiments like the Michelson-Morley experiment found no evidence of the ether or different light speeds. Relativity led to phenomena like time dilation, where clocks in motion run slower, and length contraction, where objects appear shorter along the direction of motion.