Universality of the speed of light

One of the cornerstones of special relativity, it states that the speed of light is the same whether the observer measuring it is stationary or in constant motion. This was first proved experimentally by Michelson and Morley from 1881 onwards, in their failure to detect the ether. The ether was the hypothetical medium which pervaded the whole of space and through which electromagnetic radiation was thought to have propagated. In an attempt to measure the difference in light's velocity through this ether, it was shown that all experiments return the same value for the speed of light, regardless of the relative velocity of the experiment which researchers expected to be superimposed upon that of light. At a stroke, this disproved that the ether existed and paved the way for Einstein to develop the theory of special relativity.