## TL;DR

Einstein's relativity (1905 Special, 1915 General) redefined space, time, and gravity. Special relativity: laws of physics same for all non-accelerating observers; speed of light is constant → time dilation, length contraction, E=mc². General relativity: gravity is curvature of spacetime by mass/energy.

## Core Explanation

Special relativity: time slows for moving objects (experimentally confirmed — GPS satellites must correct for relativistic effects). E=mc²: mass can convert to energy. General relativity: predicted black holes, gravitational waves (detected 2015 by LIGO), expansion of universe. Einstein's happiest thought (1907): free-falling person doesn't feel own weight — equivalence principle.

## Further Reading

- [Relativity: The Special and General Theory (Einstein, 1916)](https://www.gutenberg.org/ebooks/5001)