special relativity
The equation explained
In wordsWhen two events happen at the same time in the stationary system, the time between them in the moving system is minus gamma times the speed times their distance apart, over c squared.Full explanationTwo events at the same time in the stationary system but at different places are, in the moving system, separated in time by this amount. It is the Lorentz map applied to a pair of events with no time between them.