Einstein's Special and General Relativity (1905 and 1915)

In his "miracle year" of 1905, Albert Einstein published special relativity, showing that the speed of light is constant for all observers and that mass and energy are equivalent (E=mc²). In 1915 he completed general relativity, reframing gravity not as a force but as the curvature of spacetime caused by mass. General relativity predicted the bending of starlight around the Sun (confirmed during the 1919 solar eclipse), the existence of black holes, gravitational waves (detected by LIGO in 2016, a century later), and the expansion of the universe. GPS satellites require relativistic corrections to remain accurate — Einstein's equations are baked into the technology in every smartphone on Earth.