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

Albert Einstein was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence formula E = mc², which arises from special relativity, has been called "the world's most famous equation". He received the 1921 Nobel Prize in Physics for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.

The annus mirabilis papers are four papers that Albert Einstein published in the scientific journal Annalen der Physik in 1905. As major contributions to the foundation of modern physics, these scientific publications were the ones for which he gained fame among physicists. They revolutionized science's understanding of the fundamental concepts of space, time, mass, and energy.

1. The first paper explained the photoelectric effect, which established the energy of the light quanta , and was the only specific discovery mentioned in the citation awarding Einstein the 1921 Nobel Prize in Physics.
2. The second paper explained Brownian motion, which established the Einstein relation and compelled physicists to accept the existence of atoms.
3. The third paper introduced Einstein's special theory of relativity, which proclaims the constancy of the speed of light and derives the Lorentz transformations. Einstein also examined relativistic aberration and the transverse Doppler effect.
4. The fourth, a consequence of special relativity, developed the principle of mass–energy equivalence, expressed in the equation and which led to the discovery and use of nuclear power decades later.