Albert Einstein (14 March 1879 – 18 April 1955) 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.

Born in the German Empire, Einstein moved to Switzerland in 1895, forsaking his German citizenship the following year. He enrolled in the mathematics and physics teaching diploma program at the Swiss Federal Polytechnic School in Zurich and graduated in 1900. He later acquired Swiss citizenship and secured a position at the Swiss Patent Office in Bern. In 1905, he submitted a successful PhD dissertation to the University of Zurich.

In 1914, Einstein moved to Berlin to join the Prussian Academy of Sciences and the Humboldt University of Berlin, becoming director of the Kaiser Wilhelm Institute for Physics. In 1933, while Einstein was visiting the United States, Adolf Hitler came to power in Germany. Horrified by the Nazi persecution of his fellow Jews, he decided to remain in the US and was granted American citizenship in 1940.

In 1905, he published four groundbreaking papers, which outlined a theory of the photoelectric effect, explained Brownian motion, introduced his special theory of relativity, and demonstrated that if the special theory is correct, mass and energy are equivalent to each other. In 1915, he proposed a general theory of relativity that extended his system of mechanics to incorporate gravitation. A cosmological paper in 1916 laid out the implications of general relativity for the modeling of the structure and evolution of the universe as a whole.

Throughout his career, Einstein made significant contributions to statistical mechanics and quantum theory. Notably, he worked on the quantum physics of radiation, where light consists of particles called photons, and alongside physicist Satyendra Nath Bose, laid the groundwork for Bose-Einstein statistics. However, his attempts to devise a unified field theory by generalizing his geometric theory of gravitation to include electromagnetism did not succeed, leading him to become increasingly isolated from mainstream modern physics.