Albert Einstein (14 March 1879–18 April 1955) was a German-born theoretical physicist whose work transformed the understanding of space, time, matter, and radiation. He developed the theory of relativity and made foundational contributions to quantum mechanics. His explanation of the photoelectric effect was specifically recognized in the citation for the 1921 Nobel Prize in Physics, awarded to him in 1922. His scientific career extended from employment in the Swiss patent office to professorships in Europe and the United States. (nobelprize.org)
Early life and academic career
Einstein was born in Ulm, Germany, and grew up principally in Munich. After further schooling in Aarau, Switzerland, he entered the Swiss Federal Polytechnic in Zurich, now ETH Zurich, in 1896 to train as a teacher of physics and mathematics. He completed his studies in 1900 and acquired Swiss citizenship in 1901. Unable initially to secure a regular academic position, he worked at the Swiss patent office in Bern while conducting research independently. He earned his doctorate at the University of Zurich in 1905. (nobelprize.org)
His academic appointments included positions in Zurich, Prague, and, from 1914, Berlin. In 1933, following the establishment of Nazi rule in Germany, he settled in the United States and joined the Institute for Advanced Study in Princeton, New Jersey. The institute was an independent research institution, not a department of Princeton University. Einstein became an American citizen in 1940 while retaining Swiss citizenship. (nobelprize.org)
The papers of 1905
Einstein’s publications of 1905 are often described collectively as his annus mirabilis, or “miracle year.” Four papers addressed light quanta, Brownian motion, special relativity, and the relationship between mass and energy. Their subjects crossed several established areas of physics rather than constituting a single research programme. (ias.edu)
Building on Max Planck’s work on thermal radiation, Einstein proposed that light could behave as discrete packets of energy, later called photons. The energy of each packet depended on its frequency. This hypothesis explained why light could eject electrons from a material only above a characteristic frequency: individual light quanta transferred energy to individual electrons. It extended quantum reasoning beyond the exchange of energy by matter to the properties of radiation itself. (ias.edu)
His Brownian-motion paper connected the irregular movement of suspended particles with collisions involving surrounding molecules. By deriving measurable relationships between particle displacement, temperature, and the properties of the fluid, he provided a way to test the molecular account of matter. The work helped establish the physical reality of atoms through observable consequences of their motion. (ias.edu)
Special relativity rested on the equivalence of inertial reference frames for physical laws and the invariant speed of light in a vacuum. It replaced universal simultaneity with a description in which measurements of time and distance depend on relative motion. In a subsequent paper, Einstein showed that changes in a body’s energy entail corresponding changes in its mass, establishing the relationship commonly expressed as . (ias.edu)
General relativity
Einstein subsequently sought a relativistic account of gravity. A central starting point was the local equivalence between freely falling motion in a gravitational field and motion without gravity. He used thought experiments involving falling observers and accelerating laboratories to investigate the implications of this principle. (einstein-online.info)
On 25 November 1915, he presented the completed field equations of general relativity. The theory described gravity through the geometry of spacetime, rather than as a force acting within an otherwise fixed spatial framework. It accounted for the unexplained portion of Mercury’s orbital precession and predicted the bending of light by gravitational fields. (einstein.caltech.edu)
Observations during the solar eclipse of 29 May 1919 supported the predicted deflection of starlight near the Sun. Their announcement brought Einstein international public recognition. The significance of these observations was that they tested a distinctive quantitative prediction, rather than establishing every aspect of the theory through a single experiment. (ias.edu)
Quantum theory and later research
Einstein continued to contribute to quantum physics after developing relativity. His 1917 work on radiation introduced stimulated emission, in which incoming radiation induces an excited atom to emit additional radiation. This process later became the operating principle of the laser and maser. (aps.org)
In 1924–1925, he extended Satyendra Nath Bose’s treatment of light quanta to material particles. The resulting Bose–Einstein statistics led him to predict Bose–Einstein condensation, a collective state in which particles occupy the same lowest-energy quantum state. (einstein.caltech.edu)
Although instrumental in the development of quantum theory, Einstein questioned whether its standard formulation completely described physical reality. His disagreements with Niels Bohr concerned its interpretation and completeness, not simply its predictive success. In 1935, with Boris Podolsky and Nathan Rosen, he published the argument known as the Einstein–Podolsky–Rosen paradox, drawing attention to correlations associated with quantum entanglement. At Princeton, he also pursued a unified mathematical description of gravitation and electromagnetism, without producing an accepted unified field theory. (ias.edu)
Public activities and final years
Einstein publicly supported peace initiatives and assisted people escaping persecution. On 2 August 1939, he signed a letter prepared with physicist Leó Szilárd warning President Franklin D. Roosevelt about the possibility of powerful uranium-based bombs. The letter helped prompt American uranium research, one stage in the developments that eventually produced the Manhattan Project. Einstein did not work on that project or design its weapons. (ias.edu)
After World War II, he advocated international cooperation and controls on nuclear weapons. He remained associated with the Institute for Advanced Study until his death in Princeton on 18 April 1955, continuing his theoretical investigations during his final years. (ias.edu)