Published in Annalen der Physik in 1905, this paper set out what became known as the special theory of relativity. It begins from two assumptions: that the laws of physics take the same form for all observers moving at constant velocity, and that light travels at the same speed regardless of the motion of its source.

Einstein showed that these two premises can only hold together if time and distance are not absolute. Clocks in relative motion do not agree on whether two events are simultaneous, moving clocks run slow, and moving objects are measured as shorter along their direction of travel.

The argument removed the need for the ether, the medium that light waves had been assumed to travel through, and it resolved long-standing inconsistencies between mechanics and the electrodynamics of Maxwell.