Albert Einstein (14 March 1879–18 April 1955) was a theoretical physicist who is widely regarded as one of the most influential scientists of all time. His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first post-Newtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of light by gravity and gravitational lensing, the first fluctuation dissipation theorem which explained the Brownian movement of molecules, the photon theory and wave-particle duality, the quantum theory of atomic motion in solids, the zero-point energy concept, the semiclassical version of the Schrödinger equation, and the quantum theory of a monatomic gas which predicted Bose–Einstein condensation. Einstein is best known for his theories of special relativity and general relativity. 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.” Einstein published more than 300 scientific and over 150 non-scientific works. He is often regarded as the father of modern physics.

Albert Einstein was born in Ulm, in the Kingdom of Württemberg in the German Empire on 14 March 1879. His father was Hermann Einstein, a salesman and engineer. His mother was Pauline Einstein (née Koch). In 1880, the family moved to Munich, where his father and his uncle founded Elektrotechnische Fabrik J. Einstein & Cie, a company that manufactured electrical equipment based on direct current. The Einsteins were non-observant Jews. Their son attended a Catholic elementary school from the age of five until ten. Although Einstein had early speech difficulties, he was a top student in elementary school. As he grew, Einstein built models and mechanical devices for fun and began to show a talent for mathematics. In 1889 Max Talmud (later changed to Max Talmey) introduced the ten-year old Einstein to key texts in science, mathematics and philosophy, including Kant’s Critique of Pure Reason and Euclid’s Elements (which Einstein called the "holy little geometry book"). Talmud was a poor Jewish medical student from Poland. The Jewish community arranged for Talmud to take meals with the Einsteins each week on Thursdays for six years. During this time Talmud wholeheartedly guided Einstein through many secular educational interests.

In 1894, his father’s company failed: Direct current (DC) lost the War of Currents to alternating current (AC). In search of business, the Einstein family moved to Italy, first to Milan and then, a few months later, to Pavia. When the family moved to Pavia, Einstein stayed in Munich to finish his studies at the Luitpold Gymnasium. His father intended for him to pursue electrical engineering, but Einstein clashed with authorities and resented the school’s regimen and teaching method. He later wrote that the spirit of learning and creative thought were lost in strict rote learning. In the spring of 1895, he withdrew to join his family in Pavia, convincing the school to let him go by using a doctor’s note. During this time, Einstein wrote his first scientific work, "The Investigation of the State of Aether in Magnetic Fields".

Einstein applied directly to the Eidgenössische Polytechnische Schule (later Eidgenössische Technische Hochschule (ETH)) in Zürich, Switzerland. Lacking the requisite Matura certificate, he took an entrance examination, which he failed, although he got exceptional marks in mathematics and physics. The Einsteins sent Albert to Aarau, in northern Switzerland to finish secondary school. While lodging with the family of Professor Jost Winteler, he fell in love with the family’s daughter, Marie. (His sister Maja later married the Winteler son, Paul.) In Aarau, Einstein studied Maxwell’s electromagnetic theory. At age 17, he graduated, and, with his father’s approval, renounced his citizenship in the German Kingdom of Württemberg to avoid military service, and enrolled in 1896 in the mathematics and physics program at the Polytechnic in Zurich. Marie Winteler moved to Olsberg, Switzerland for a teaching post.

In the same year, Einstein’s future wife, Mileva Marić, also entered the Polytechnic to study mathematics and physics, the only woman in the academic cohort. Over the next few years, Einstein and Marić’s friendship developed into romance. In a letter to her, Einstein called Marić “a creature who is my equal and who is as strong and independent as I am.” Einstein graduated in 1900 from the Polytechnic with a diploma in mathematics and physics; Although historians have debated whether Marić influenced Einstein’s work, the majority of academic historians of science agree that she did not.

In early 1902, Einstein and Mileva Marić had a daughter they called Lieserl in their correspondence, who was born in Novi Sad where the parents of Mileva lived. Her full name is not known, and her fate is uncertain after 1903. Einstein and Marić married in January 1903, and in May 1904 the couple’s first son, Hans Albert Einstein, was born in Bern, Switzerland. Their second son, Eduard, was born in Zurich in July 1910. In 1914, Einstein moved to Berlin, while his wife remained in Zurich with their sons. Marić and Einstein divorced on 14 February 1919, having lived apart for five years. Einstein married Elsa Löwenthal (née Einstein) on 2 June 1919, after having had a relationship with her since 1912. She was his first cousin maternally and his second cousin paternally. In 1933, they emigrated permanently to the United States. In 1935, Elsa Einstein was diagnosed with heart and kidney problems and died in December, 1936.

After graduating, Einstein spent almost two frustrating years searching for a teaching post, but a former classmate’s father helped him secure a job in Bern, at the Federal Office for Intellectual Property, the patent office, as an assistant examiner. He evaluated patent applications for electromagnetic devices. In 1903, Einstein’s position at the Swiss Patent Office became permanent, although he was passed over for promotion until he "fully mastered machine technology". Much of his work at the patent office related to questions about transmission of electric signals and electrical-mechanical synchronization of time, two technical problems that show up conspicuously in the thought experiments that eventually led Einstein to his radical conclusions about the nature of light and the fundamental connection between space and time.

With friends he met in Bern, Einstein formed a weekly discussion club on science and philosophy, which he jokingly named "The Olympia Academy." Their readings included Henri Poincaré, Ernst Mach, and David Hume, who influenced Einstein’s scientific and philosophical outlook. The next year, Einstein published a paper in the prestigious Annalen der Physik on the capillary forces of a straw. Throughout his life, Einstein published hundreds of books and articles. Most were about physics, but a few expressed leftist political opinions about pacifism, socialism, and zionism. In addition to the work he did by himself he also collaborated with other scientists on additional projects including the Bose–Einstein statistics, the Einstein refrigerator and others.

Einstein’s early papers all come from attempts to demonstrate that atoms exist and have a finite nonzero size. At the time of his first paper in 1902, it was not yet completely accepted by physicists that atoms were real, even though chemists had good evidence ever since Antoine Lavoisier’s work a century earlier. The reason physicists were skeptical was because no 19th century theory could fully explain the properties of matter from the properties of atoms.

Ludwig Boltzmann was a leading 19th century atomist physicist, who had struggled for years to gain acceptance for atoms. Boltzmann had given an interpretation of the laws of thermodynamics, suggesting that the law of entropy increase is statistical. While Boltzmann’s statistical interpretation of entropy is universally accepted today, and Einstein believed it, at the turn of the 20th century it was a minority position. The statistical idea was most successful in explaining the properties of gases. James Clerk Maxwell, another leading atomist, had found the distribution of velocities of atoms in a gas, and derived the surprising result that the viscosity of a gas should be independent of density. m, using a rotating slitted drum, showed that atoms in a gas had velocities distributed according to Maxwell’s distribution law. In addition to these successes, there were also inconsistencies. Maxwell noted that at cold temperatures, atomic theory predicted specific heats that are too large. The most glaring inconsistency was in the theory of light waves. These inconsistencies led some people to say that atoms were not physical, but mathematical. Notable among the skeptics was Ernst Mach, whose positivist philosophy led him to demand that if atoms are real, it should be possible to see them directly. Mach believed that atoms were a useful fiction, that in reality they could be assumed to be infinitesimally small, that Avogadro’s number was infinite, or so large that it might as well be infinite, and k_B was infinitesimally small. Certain experiments could then be explained by atomic theory, but other experiments could not, and this is the way it will always be.

Einstein opposed this position. Throughout his career, he was a realist. He believed that a single consistent theory should explain all observation, and that this theory would be a description of what was really going on, underneath it all. So he set out to show that the atomic point of view was correct. This led him first to thermodynamics, then to statistical physics, and to the theory of specific heats of solids.

In 1905, while he was working in the patent office, the leading German language physics journal Annalen der Physik published four of Einstein’s papers. The four papers eventually were recognized as revolutionary, and 1905 became known as Einstein’s "Miracle Year", and the papers, as the Annus Mirabilis Papers.

Einstein’s earliest papers were concerned with thermodynamics. He wrote a paper establishing a thermodynamic identity in 1902, and a few other papers which attempted to interpret phenomena from a statistical atomic point of view. His research in 1903 and 1904 was mainly concerned with the effect of finite atomic size on diffusion phenomena. As in Maxwell’s work, the finite nonzero size of atoms leads to effects which can be observed. This research, and the thermodynamic identity, were well within the mainstream of physics in his time. They would eventually form the content of his PhD thesis. His first major result in this field was the theory of thermodynamic fluctuations. His great breakthrough came in 1905. The theory of fluctuations, he realized, would have a visible effect for an object which could move around freely. Einstein’s theory of Brownian motion was the first paper in the field of statistical physics. It established that thermodynamic fluctuations were related to dissipation. The theory of Brownian motion was the least revolutionary of Einstein’s Annus mirabilis papers, but it had an important role in securing the acceptance of the atomic theory by physicists.

Einstein’s thinking underwent a transformation in 1905. He had come to understand that quantum properties of light mean that Maxwell’s equations were only an approximation. He knew that new laws would have to replace these, but he did not know how to go about finding those laws. He felt that guessing formal relations would not go anywhere. So he decided to focus on a-priori principles instead, which are statements about physical laws which can be understood to hold in a very broad sense even in domains where they have not yet been shown to apply. Einstein sought new principles of this sort, to guide the production of physical ideas. Once enough principles are found, then the new physics will be the simplest theory consistent with the principles and with previously known laws. The first general a-priori principle he found was the principle of relativity, that uniform motion is indistinguishable from rest. Other principles postulated by Einstein are the principle of equivalence and the principle of adiabatic invariance of the quantum number. Another of Einstein’s general principles, Mach’s principle, is fiercely debated, and whether it holds in our world or not is still not definitively established. The use of a-priori principles is a distinctive unique signature of Einstein’s early work, which has become a standard tool in modern theoretical physics.

His 1905 paper on the electrodynamics of moving bodies introduced his theory of special relativity, which showed that the observed independence of the speed of light on the observer’s state of motion required fundamental changes to the notion of simultaneity. In his paper on mass–energy equivalence, which had previously considered to be distinct concepts, Einstein deduced from his equations of special relativity what has been called the twentieth century’s best-known equation: E = mc². This equation suggests that tiny amounts of mass could be converted into huge amounts of energy and presaged the development of nuclear power. Einstein’s 1905 work on relativity remained controversial for many years, but was accepted by leading physicists, starting with Max Planck. In a 1905 paper, Einstein postulated that light itself consists of localized particles (quanta). Einstein’s light quanta were nearly universally rejected by all physicists, including Max Planck and Niels Bohr. This idea only became universally accepted in 1919, with Robert Millikan’s detailed experiments on the photoelectric effect, and with the measurement of Compton scattering. Einstein continued his work on quantum mechanics in 1906, by explaining the specific heat anomaly in solids. This was the first application of quantum theory to a mechanical system. Einstein showed in a simple model that the hypothesis that solid motion is quantized explains why the specific heat of a solid goes to zero at zero temperature. This work was the foundation of condensed matter physics.

Throughout the 1910s, quantum mechanics expanded in scope to cover many different systems. After Ernest Rutherford discovered the nucleus and proposed that electrons orbit like planets, Niels Bohr was able to show that the same quantum mechanical postulates introduced by Planck and developed by Einstein would explain the discrete motion of electrons in atoms, and the periodic table of the elements.

Although the patent office promoted Einstein to Technical Examiner Second Class in 1906, he had not given up on academia. In 1908, he became a privatdozent at the University of Bern. In "über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung" ("The Development of Our Views on the Composition and Essence of Radiation"), on the quantization of light, and in an earlier 1909 paper, Einstein showed that Max Planck’s energy quanta must have well-defined momenta and act in some respects as independent, point-like particles. This paper introduced the photon concept (although the name photon was introduced later by Gilbert N. Lewis in 1926) and inspired the notion of wave-particle duality in quantum mechanics. Einstein returned to the problem of thermodynamic fluctuations, giving a treatment of the density variations in a fluid at its critical point.

In 1907, while still working at the patent office, Einstein had what he would call his "happiest thought". He realized that the principle of relativity could be extended to gravitational fields. He thought about the case of a uniformly accelerated box not in a gravitational field, and noted that it would be indistinguishable from a box sitting still in an unchanging gravitational field. He used special relativity to see that the rate of clocks at the top of a box accelerating upward would be faster than the rate of clocks at the bottom. He concludes that the rates of clocks depend on their position in a gravitational field, and that the difference in rate is proportional to the gravitational potential to first approximation. Although this approximation is crude, it allowed him to calculate the deflection of light by gravity, and show that it is nonzero. From Prague, Einstein published a paper about the effects of gravity on light, specifically the gravitational redshift and the gravitational deflection of light. 

While developing general relativity, Einstein became confused about the gauge invariance in the theory. He formulated an argument that led him to conclude that a general relativistic field theory is impossible. He gave up looking for fully generally covariant tensor equations, and searched for equations that would be invariant under general linear transformations only. The Entwurf ("draft") theory was the result of these investigations. As its name suggests, it was a sketch of a theory, with the equations of motion supplemented by additional gauge fixing conditions. Simultaneously less elegant and more difficult than general relativity, Einstein abandoned the theory after realizing that the hole argument was mistaken.

In 1912, Einstein returned to Switzerland to accept a professorship at his alma mater, the ETH. Once back in Zurich, he immediately visited his old ETH classmate Marcel Grossmann, now a professor of mathematics, who introduced him to Riemannian geometry and, more generally, to differential geometry. On the recommendation of Italian mathematician Tullio Levi-Civita, Einstein began exploring the usefulness of general covariance (essentially the use of tensors) for his gravitational theory. For a while Einstein thought that there were problems with the approach, but he later returned to it and, by late 1915, had published his general theory of relativity in the form in which it is used today. This theory explains gravitation as distortion of the structure of spacetime by matter, affecting the inertial motion of other matter.

During World War I, the work of Central Powers scientists was available only to Central Powers academics, for national security reasons. Some of Einstein’s work did reach the United Kingdom and the United States through the efforts of the Austrian Paul Ehrenfest and physicists in the Netherlands, especially 1902 Nobel Prize-winner Hendrik Lorentz and Willem de Sitter of Leiden University. After the war ended, Einstein maintained his relationship with Leiden University, accepting a contract as an Extraordinary Professor; for ten years, from 1920 to 1930, he travelled to Holland regularly to lecture.

In 1917, Einstein applied the General theory of relativity to model the structure of the universe as a whole. He wanted the universe to be eternal and unchanging, but this type of universe is not consistent with relativity. To fix this, Einstein modified the general theory by introducing a new notion, the cosmological constant. With a positive cosmological constant, the universe could be an eternal static sphere. Einstein believed a spherical static universe is philosophically preferred, because it would obey Mach’s principle. He had shown that general relativity incorporates Mach’s principle to a certain extent in frame dragging by gravitomagnetic fields, but he knew that Mach’s idea would not work if space goes on forever. In a closed universe, he believed that Mach’s principle would hold.

In 1917, at the height of his work on relativity, Einstein published an article in Physikalische Zeitschrift that proposed the possibility of stimulated emission, the physical process that makes possible the maser and the laser. This paper was enormously influential in the later development of quantum mechanics, because it was the first paper to show that the statistics of atomic transitions had simple laws. In another major paper from this era, Einstein gave a wave equation for de Broglie waves, which Einstein suggested was the Hamilton–Jacobi equation of mechanics. This paper would inspire Schrödinger’s work of 1926. In 1924, Einstein received a description of a statistical model from Indian physicist Satyendra Nath Bose, based on a counting method that assumed that light could be understood as a gas of indistinguishable particles. Einstein noted that Bose’s statistics applied to some atoms as well as to the proposed light particles, and submitted his translation of Bose’s paper to the Zeitschrift für Physik. Einstein also published his own articles describing the model and its implications, among them the Bose–Einstein condensate phenomenon that some particulates should appear at very low temperatures. Bose–Einstein statistics are now used to describe the behaviors of any assembly of bosons.

Following his research on general relativity, Einstein entered into a series of attempts to generalize his geometric theory of gravitation, which would allow the explanation of electromagnetism. In 1950, he described his "unified field theory" in a Scientific American article entitled "On the Generalized Theory of Gravitation."  Although he continued to be lauded for his work, Einstein became increasingly isolated in his research, and his efforts were ultimately unsuccessful. In his pursuit of a unification of the fundamental forces, Einstein ignored some mainstream developments in physics, most notably the strong and weak nuclear forces, which were not well understood until many years after his death. Mainstream physics, in turn, largely ignored Einstein’s approaches to unification. Einstein’s dream of unifying other laws of physics with gravity motivates modern quests for a theory of everything and in particular string theory, where geometrical fields emerge in a unified quantum-mechanical setting.

The question of scientific determinism gave rise to questions about Einstein’s position on theological determinism, and whether or not he believed in God, or in a god. In 1929, Einstein told Rabbi Herbert S. Goldstein "I believe in Spinoza’s God, who reveals Himself in the lawful harmony of the world, not in a God Who concerns Himself with the fate and the doings of mankind." In a 1954 letter, he wrote, "I do not believe in a personal God and I have never denied this but have expressed it clearly.” In a letter to philosopher Erik Gutkind, Einstein remarked, "The word God is for me nothing more than the expression and product of human weakness, the Bible a collection of honorable, but still purely primitive, legends which are nevertheless pretty childish."

Throughout the November Revolution in Germany Einstein signed an appeal for the foundation of a nationwide liberal and democratic party, which was published in the Berliner Tageblatton 16 November 1918, and became a member of the German Democratic Party.
Einstein flouted the ascendant Nazi movement, tried to be a voice of moderation in the tumultuous formation of the State of Israel and braved anti-communist politics and resistance to the civil rights movement in the United States. He participated in the 1927 congress of the League against Imperialism in Brussels. He was a socialist Zionist who supported the creation of a Jewish national homeland in the British mandate of Palestine. After World War II, as enmity between the former allies became a serious issue, Einstein wrote, “I do not know how the third World War will be fought, but I can tell you what they will use in the Fourth – rocks!” In a 1949 Monthly Review article entitled “Why Socialism?” Albert Einstein described a chaotic capitalist society, a source of evil to be overcome, as the “predatory phase of human development” (Einstein 1949). With Albert Schweitzer and Bertrand Russell, Einstein lobbied to stop nuclear testing and future bombs. Days before his death, Einstein signed the Russell–Einstein Manifesto, which led to the Pugwash Conferences on Science and World Affairs.  Einstein was a member of several civil rights groups, including the Princeton chapter of the NAACP. When the aged W. E. B. Du Bois was accused of being a Communist spy, Einstein volunteered as a character witness, and the case was dismissed shortly afterward. Einstein’s friendship with activist Paul Robeson, with whom he served as co-chair of the American Crusade to End Lynching, lasted twenty years.

On 17 April 1955, Albert Einstein experienced internal bleeding caused by the rupture of an abdominal aortic aneurysm, which had previously been reinforced surgically by Dr. Rudolph Nissen in 1948. He took the draft of a speech he was preparing for a television appearance commemorating the State of Israel’s seventh anniversary with him to the hospital, but he did not live long enough to complete it. Einstein refused surgery, saying: "I want to go when I want. It is tasteless to prolong life artificially. I have done my share, it is time to go. I will do it elegantly." He died in Princeton Hospital early the next morning at the age of 76, having continued to work until near the end. Einstein’s remains were cremated and his ashes were scattered around the grounds of the Institute for Advanced Study, Princeton, New Jersey. During the autopsy, the pathologist of Princeton Hospital, Thomas Stoltz Harvey removed Einstein’s brain for preservation, without the permission of his family, in hope that the neuroscience of the future would be able to discover what made Einstein so intelligent.