Special relativity

Developed by Albert Einstein, special relativity explains how measurements of space and time relate between observers moving at constant velocity relative to one another. It rests on the same laws of physics applying in inertial reference frames and the speed of light in vacuum being the same for all such observers. Its effects become especially noticeable when relative speeds approach the speed of light. Time dilation and length contraction follow from this framework, as does the finding that distant events judged simultaneous by one observer need not be simultaneous for another.

These are connected consequences of the structure of spacetime, rather than separate optical illusions. The theory is essential in particle physics and the analysis of high-speed motion. Students and engineers use it where ordinary Newtonian approximations become inadequate; gravity itself requires the broader framework of general relativity.