Albert Einstein publishes the third of his Annus Mirabilis papers, introducing the special theory of relativity.

In 1905, often referred to as Albert Einstein’s Annus Mirabilis (miracle year), he published four groundbreaking papers that revolutionized physics. These papers were written while he was still working as a patent clerk in Bern, Switzerland, and they touched on fundamental aspects of space, time, matter, and energy. Each paper tackled a distinct problem that scientists of the era were struggling with, and together they laid the foundation for modern physics.

The first paper explained the photoelectric effect, introducing the revolutionary idea that light could be thought of as discrete packets of energy, or “quanta.” This was a key step toward the development of quantum theory. The second paper dealt with Brownian motion, providing strong evidence for the existence of atoms and molecules by showing how their random movements explained the erratic paths of particles suspended in fluids. Both of these contributions reshaped the way scientists understood the microscopic world.

The third and fourth papers were equally transformative. In the third, Einstein presented the special theory of relativity, which redefined concepts of space and time by proposing that the laws of physics are the same for all observers moving at constant speeds, and that the speed of light is a universal constant. The final paper, a follow-up, introduced the famous equation $E = mc^2$, revealing the profound equivalence of mass and energy. Collectively, these four works marked a turning point in physics, propelling Einstein to global fame and permanently altering the trajectory of science.