What is relativity?

Relativity names two connected theories describing space, time, motion and gravity. Special relativity relates inertial reference frames, while general relativity treats gravity through spacetime geometry. They do not claim that everything is subjective: they define precise relationships between different observers’ measurements.

Key consequences of special relativity

1. Basic principles Physical laws have the same form in inertial frames, and vacuum light speed is the same for these observers. Together these principles change classical expectations about relationships between time intervals and lengths measured in relative motion. 2. Time and simultaneity Observers moving relative to one another can measure different durations between events. Distant events simultaneous in one frame need not be simultaneous in another. This concerns spacetime relationships rather than defective clocks. Comparisons must specify which events are being measured. 3. Mass and energy E = mc² gives the energy associated with rest mass. A moving particle’s total energy also relates to momentum. Photons having zero rest mass does not prevent them carrying energy and momentum.

General relativity and observational evidence

4. Gravity and geometry General relativity relates matter and energy to spacetime geometry. Freely falling objects follow natural paths in this geometry. A stretched-sheet analogy illustrates some ideas but is not a complete model of four-dimensional spacetime. 5. Light and clocks Gravitational fields affect light paths and the compared rates of clocks. Gravitational lensing can magnify or multiply images of distant objects. Satellite timing systems need to account for effects of motion and gravity. 6. Validity and limits Many observations test relativity. At low speeds and in weak fields, Newtonian approximations are sufficiently accurate for many tasks. More comprehensive descriptions incorporating quantum effects under extreme conditions remain research topics. Investigating a theory’s limits does not invalidate the measurements it explains successfully.