Theory of relativity

Relativity is not a single equation but a framework for describing space, time, motion, matter and gravity. Its two main parts address different conditions. Special relativity describes physical laws for observers moving at constant velocity relative to one another and does not include gravity. General relativity extends the picture to gravity and the structure of spacetime. Einstein's work transformed earlier assumptions about absolute space and universal time, while retaining precise, testable relationships between measurements made by different observers.

Einstein presented special relativity in 1905. Its central principles are that the laws of physics take the same form in all inertial reference frames and that light in a vacuum has the same speed for all such observers, regardless of the motion of the source. These principles require a revision of everyday ideas about time and distance. Observers in relative motion can disagree about whether separated events happened simultaneously; they can also measure different elapsed times and lengths. Time dilation and length contraction are normally tiny at everyday speeds but become important near the speed of light. The relation between mass and energy is expressed by E = mc². The framework applies to inertial motion; by itself, it does not describe gravitational effects.

Einstein completed the general theory of relativity in 1915 to bring gravity into the relativistic account of space and time. Instead of treating gravity only as an invisible force acting across space, the theory describes matter and energy as shaping spacetime, with that geometry influencing how matter and light move. The equivalence principle—closely related to the experience of free fall—helped guide the theory's development. General relativity predicts effects including the bending of light by gravity and gravitational waves. It provides a framework for studying planetary motion, compact objects such as black holes, and large-scale cosmology.

Relativity is supported by a broad range of observations and experiments. One modern example is satellite navigation: GPS timing and positioning require corrections for relativistic effects on satellite clocks, which experience both motion-related and gravitational frequency shifts. In astrophysics, gravitational waves—predicted by general relativity—were directly detected by the LIGO observatory in 2015, providing a new way to test gravity and observe distant cosmic events. These examples show that relativity is not confined to theoretical physics; it is also part of the account needed for precise measurement and modern astronomical research.

Special and general relativity have passed demanding tests in the situations where they are designed to apply. A major unresolved challenge is how to reconcile general relativity, a theory of gravity and spacetime on large scales, with quantum theory, which describes matter and interactions at small scales. Relativistic quantum field theories successfully combine special relativity with quantum concepts, but physicists have not yet established a complete, experimentally confirmed theory of quantum gravity. This remains an active research area rather than a reason to treat the established tests of relativity as unsettled.

Einstein's Nobel Prize in Physics was designated for 1921 but awarded in 1922. The Nobel citation recognized his services to theoretical physics, especially his discovery of the law of the photoelectric effect; it did not name relativity as the basis of the award. The distinction is useful when separating the history of the prize from the later experimental tests of Einstein's theories.