special relativity: a standalone construct

Albert Einstein introduced special relativity in his 1905 paper Zur Elektrodynamik bewegter Körper (On the electrodynamics of moving bodies), building on the earlier electrodynamics of Hendrik Lorentz and Henri Poincaré. The theory rests on two postulates: that the laws of physics take the same form in all inertial frames (observers moving at constant velocity relative to one another), and that light travels at the same speed for every such observer regardless of the motion of its source. From these follow time dilation (moving clocks run slow), length contraction (moving objects shorten along their direction of motion), the relativity of simultaneity (events judged simultaneous by one observer need not be by another), and, in Einstein's companion 1905 paper, the equivalence of mass and energy expressed as E=mc². Special relativity is restricted to inertial frames and flat spacetime without gravity, and its predictions are confirmed to high precision in particle accelerators, atomic clocks, and the operation of satellite navigation systems, which sets it apart as the framework that reconciles mechanics with electromagnetism at speeds approaching that of light.[1][2]

Unit

A physical theory: a set of postulates and their consequences for space, time, and energy in inertial frames

Recovery means

Not applicable

Measured by

Tested through time dilation of fast-moving particles and precise clocks, mass-energy conversion in accelerators, and the invariance of the speed of light in interferometry