Albert Einstein is widely considered the most influential physicist of the 20th century. In 1905, often called his 'Annus Mirabilis' (Miracle Year), he published four groundbreaking papers that changed the course of modern physics. These included his work on the photoelectric effect (for which he won the Nobel Prize), Brownian motion, and Special Relativity. The latter introduced the world's most famous equation, $E=mc^2$, which expressed the equivalence of mass and energy. In 1915, he published his General Theory of Relativity, which explained that gravity is not a force but a curvature of space-time caused by mass. Einstein’s theories provided the framework for cosmology, nuclear energy, and modern technology like GPS. Beyond science, he was a staunch advocate for civil rights and pacifism, famously warning against the dangers of nuclear proliferation.