In his annus mirabilis 1905 Einstein presents his special theory of relativity in the article On the Electrodynamics of Moving Bodies, starting out with a discussion of the concept of simultaneity:
- We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events.
- Thus with the help of certain imaginary physical experiments (clock synchronization) we have settled what is to be understood by synchronous stationary clocks located at different places, and have evidently obtained a definition of “simultaneous” or “synchronous,” and of “time.”
- In physics, the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame.
- According to the special theory of relativity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events are separated in space, such as a car crash in London and another in New York.
- The question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in other frames (in a different state of motion relative to the events) the crash in London may occur first, and in still other frames the New York crash may occur first.
- A mathematical form of the relativity of simultaneity ("local time") was introduced by Hendrik Lorentz in 1892, and physically interpreted (to first order in v/c) as the result of a synchronization using light signals by Henri Poincaré in 1900.
- However, both Lorentz and Poincaré based their conceptions on the aether as a preferred but undetectable frame of reference, and continued to distinguish between "true time" (in the aether) and "apparent" times for moving observers.
- It was Albert Einstein in 1905 who abandoned the (classical) aether and emphasized the significance of relativity of simultaneity to our understanding of space and time.