Relativity

Relativity refers to theories that describe the structure of spacetime and the behavior of objects in motion and in gravitation. Special relativity addresses observers in uniform motion and introduces the invariance of the speed of light and the relativity of simultaneity. It leads to time dilation, length contraction, and the equivalence of all inertial frames. Mass-energy equivalence is expressed by E = mc^2, linking energy and mass.

General relativity extends these ideas to non-inertial frames and gravity, describing gravitation as the curvature of spacetime produced by mass-energy, formalized in the Einstein field equations. The equivalence principle underpins the theory, positing that local experiments cannot distinguish uniform acceleration from a gravitational field. Predictions include gravitational redshift, light deflection by massive bodies, time dilation in gravitational fields, gravitational waves, and the expansion of the universe. The theory reduces to Newtonian gravity in the weak-field, low-velocity limit.

Experiments and observations have confirmed many predictions: the 1919 solar eclipse light deflection, the perihelion precession of Mercury, gravitational redshift measurements, gravitational lensing, and, more recently, direct detection of gravitational waves and imaging of black holes. General relativity also provides the framework for modern cosmology, including models of the expanding universe and black holes.

Special relativity is essential for high-speed physics and particle physics, while general relativity governs large-scale structure and strong gravitational fields. Practical applications include GPS, which must account for relativistic time dilation to maintain accuracy.