Zur Elektrodynamik bewegter Körper
In this foundational paper ("On the Electrodynamics of Moving Bodies") Einstein introduced the special theory of relativity, building physics on two postulates: that the laws of physics are identical in all inertial frames and that the speed of light in vacuum is the same for all observers regardless of the motion of the source. From these he derived the Lorentz transformation kinematically, abolishing the need for a luminiferous ether and showing that simultaneity, lengths, and time intervals are relative to the observer's frame of reference. The work reconciled mechanics with Maxwell's electrodynamics and established time dilation and length contraction as physical consequences of the structure of spacetime.