If you can’t explain it simply, you don’t understand it well enough.

Albert Einstein was a renowned German-born theoretical physicist and one of the most influential scientists of all time. He is best known for developing the theory of relativity, which revolutionized the understanding of space and time, and his mass-energy equivalence formula E=mc2, which has been dubbed “the world’s most famous equation”. He also made significant contributions to the development of quantum mechanics and his work had a profound impact on the philosophy of science. In 1905, Einstein published four groundbreaking papers that outlined the theory of the photoelectric effect, explained Brownian motion, introduced special relativity and demonstrated mass-energy equivalence. Throughout his career, Einstein attempted to unify the laws of physics and opposed the probabilistic nature of quantum mechanics. He left Germany in 1933 due to the rise of the Nazi regime and later became an American citizen. He also played a key role in warning the US government of the potential dangers of nuclear weapons during World War II.

Albert Einstein was born on March 14, 1879 in Ulm, Germany, into a family of Ashkenazi Jews. His parents, Hermann Einstein and Pauline Koch, later moved the family to Munich where they owned an electrical equipment manufacturing company. As a child, Einstein attended a Catholic elementary school and later a gymnasium where he received advanced education. However, he clashed with the authorities and resented the strict rote learning method. In 1894, his family’s company faced financial difficulties and they moved to Italy. Einstein stayed behind in Munich to finish his studies, but later joined his family in Italy. From a young age, Einstein excelled in math and physics, teaching himself advanced concepts and even independently discovering a proof of the Pythagorean theorem. He also had a strong interest in philosophy, particularly the works of Kant. Despite failing the entrance exam for a polytechnic school in Switzerland, he received exceptional grades in physics and mathematics and was eventually admitted.

In 1900, Einstein published a paper on capillarity phenomena and in 1905, at the age of 26, he published four groundbreaking papers on the photoelectric effect, Brownian motion, special relativity, and the equivalence of mass and energy, which brought him to the attention of the academic world. This year is often referred to as his “annus mirabilis”. He received his PhD from the University of Zurich in 1906. By 1908, Einstein was recognized as a leading scientist and was appointed lecturer at the University of Bern. He was later appointed associate professor in 1909, and full professor at the German Charles-Ferdinand University in Prague in 1911. He returned to his alma mater in Zurich in 1912, where he was a professor of theoretical physics until 1914. In 1913, he was offered a position at the Prussian Academy of Sciences and the University of Berlin, which he accepted and moved to Berlin. He became director of the Kaiser Wilhelm Institute for Physics and was elected president of the German Physical Society from 1916 to 1918. In 1919, observations of a solar eclipse confirmed his theory of the bending of light by gravitation.

In the spring of 1925, Einstein traveled to South America, spending a month in Argentina, a week in Uruguay, and a week in Rio de Janeiro, Brazil. His trip was organized by Jorge Duclout and Mauricio Nirenstein, with support from Argentine scholars and funding from the Council of the University of Buenos Aires, the Asociación Hebraica Argentina and the Argentine-Germanic Cultural Institution.

In December of 1930, Einstein made a second trip to America, originally planning to spend two months as a research fellow at the California Institute of Technology. Despite the high level of public attention he had received during his previous visit, he and his team sought to protect his privacy by declining all public speaking engagements and awards. Upon arriving in New York City, Einstein was taken on a tour of various places and events, including Chinatown and a performance at the Metropolitan Opera, where he was cheered by the audience. He also met with the mayor and president of Columbia University, and visited a church that had erected a statue of him at the entrance. Einstein then traveled to California, where he met with the president of Caltech, Robert A. Millikan, though their friendship was reportedly “awkward” due to their differing views on militarism. During a speech at Caltech, Einstein emphasized the potential for science to cause harm. Einstein also formed friendships with author Upton Sinclair and film star Charlie Chaplin, both known for their pacifist views. He was given a tour of Universal Studios by the head of the studio, Carl Laemmle, and was introduced to Chaplin, with whom he had an instant connection. Chaplin invited Einstein and his wife to his home for dinner and to the premiere of his film City Lights in Hollywood, where they were greeted as special guests.

In 1933, as the Nazis rose to power in Germany, Einstein knew he could not return to his home country. He and his wife Elsa returned to Europe, but upon learning of the Enabling Act, which gave Hitler’s government dictatorial powers, and the persecution of Jews in Germany, they decided not to return to Berlin. Einstein renounced his German citizenship and his works were targeted in the Nazi book burnings. He was also included in a list of enemies of the regime with a bounty on his head. Fearing for his safety and that of other Jewish scientists, Einstein sought help from the Academic Assistance Council and was able to leave Germany. He spent some time in Belgium and England, where he met with political leaders and advocated for the rescue of Jewish scientists still in Germany. He eventually moved to the United States, where he became a citizen and continued his work in physics.

Einstein was a political and social activist throughout his life, holding views that leaned towards socialism and critical of capitalism. He advocated for a democratic global government and was a strong supporter of world peace. He was a founding member of the German Democratic Party and greatly admired Mahatma Gandhi, with whom he exchanged letters. Einstein’s views on Zionism were complex, he helped establish the Hebrew University of Jerusalem and supported the idea of Jews and Arabs living together in Palestine, but was against the creation of an independent Jewish state. He had a deep understanding of the need for human rights and was very critical of the Nazi regime’s treatment of Jews and other groups. Einstein also had deep philosophical and religious views and was an advocate of the idea that science and religion should coexist. He believed that religion and science have different realms of understanding, but both are necessary for a well-rounded understanding of the world. He also expressed that science alone cannot provide a sense of meaning or purpose in life.

Albert Einstein was a physicist who made significant contributions to the field of thermodynamics, statistical physics, and the theory of relativity. In 1900, he submitted his first paper on capillary attraction, which was published the following year. In 1902-1903, he published papers on thermodynamics that attempted to interpret atomic phenomena from a statistical point of view, laying the foundation for his 1905 paper on Brownian motion. He also studied the effect of finite atomic size on diffusion phenomena. In 1905, Einstein published “Zur Elektrodynamik bewegter Körper” (“On the Electrodynamics of Moving Bodies”), which became known as his special theory of relativity, reconciling conflicts between Maxwell’s equations and the laws of Newtonian mechanics. He also developed the theory of general relativity between 1907 and 1915, which explained the observed gravitational attraction between masses as the warping of space and time by those masses.

Energy momentum pseudotensor Main article: Stress–energy–momentum pseudotensor General relativity includes a dynamical spacetime, so it is difficult to see how to identify the conserved energy and momentum. Noether’s theorem allows these quantities to be determined from a Lagrangian with translation invariance, but general covariance makes translation invariance into something of a gauge symmetry. The energy and momentum derived within general relativity by Noether’s prescriptions do not make a real tensor for this reason. Einstein argued that this is true for a fundamental reason: the gravitational field could be made to vanish by a choice of coordinates. He maintained that the non-covariant energy momentum pseudotensor was, in fact, the best description of the energy momentum distribution in a gravitational field. This approach has been echoed by Lev Landau and Evgeny Lifshitz, and others, and has become standard. The use of non-covariant objects like pseudotensors was heavily criticized in 1917 by Erwin Schrödinger and others. Wormholes In 1935, Einstein collaborated with Nathan Rosen to produce a model of a wormhole, often called Einstein–Rosen bridges.[257][258] His motivation was to model elementary particles with charge as a solution of gravitational field equations, in line with the program outlined in the paper “Do Gravitational Fields play an Important Role in the Constitution of the Elementary Particles?”. These solutions cut and pasted Schwarzschild black holes to make a bridge between two patches.[259] If one end of a wormhole was positively charged, the other end would be negatively charged. These properties led Einstein to believe that pairs of particles and antiparticles could be described in this way. Einstein–Cartan theory Main article: Einstein–Cartan theory Einstein, sitting at a table, looks up from the papers he is reading and into the camera. Einstein at his office, University of Berlin, 1920 In order to incorporate spinning point particles into general relativity, the affine connection needed to be generalized to include an antisymmetric part, called the torsion. This modification was made by Einstein and Cartan in the 1920s. Equations of motion Main article: Einstein–Infeld–Hoffmann equations The theory of general relativity has a fundamental law—the Einstein field equations, which describe how space curves. The geodesic equation, which describes how particles move, may be derived from the Einstein field equations. Since the equations of general relativity are non-linear, a lump of energy made out of pure gravitational fields, like a black hole, would move on a trajectory which is determined by the Einstein field equations themselves, not by a new law. So Einstein proposed that the path of a singular solution, like a black hole, would be determined to be a geodesic from general relativity itself. This was established by Einstein, Infeld, and Hoffmann for pointlike objects without angular momentum, and by Roy Kerr for spinning objects. Old quantum theory Main article: Old quantum theory Photons and energy quanta The photoelectric effect. Incoming photons on the left strike a metal plate (bottom), and eject electrons, depicted as flying off to the right. In a 1905 paper,[213] 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 concluded that each wave of frequency f is associated with a collection of photons with energy hf each, where h is Planck’s constant. He does not say much more, because he is not sure how the particles are related to the wave. But he does suggest that this idea would explain certain experimental results, notably the photoelectric effect.[213] Quantized atomic vibrations Main article: Einstein solid In 1907, Einstein proposed a model of matter where each atom in a lattice structure is an independent harmonic oscillator. In the Einstein model, each atom oscillates independently—a series of equally spaced quantized states for each oscillator. Einstein was aware that getting the frequency of the actual oscillations would be difficult, but he nevertheless proposed this theory because it was a particularly clear demonstration that quantum mechanics could solve the specific heat problem in classical mechanics. Peter Debye refined this model.[260] Adiabatic principle and action-angle variables Main article: Adiabatic invariant 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. Einstein contributed to these developments by linking them with the 1898 arguments Wilhelm Wien had made. Wien had shown that the hypothesis of adiabatic invariance of a thermal equilibrium state allows all the blackbody curves at different temperature to be derived from one another by a simple shifting process. Einstein noted in 1911 that the same adiabatic principle shows that the quantity which is quantized in any mechanical motion must be an adiabatic invariant. Arnold Sommerfeld identified this adiabatic invariant as the action variable of classical mechanics. Bose–Einstein statistics Main article: Bose–Einstein statistics 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.[261] It was not until 1995 that the first such condensate was produced experimentally by Eric Allin Cornell and Carl Wieman using ultra-cooling equipment built at the NIST–JILA laboratory at the University of Colorado at Boulder.[262] Bose–Einstein statistics are now used to describe the behaviors of any assembly of bosons. Einstein’s sketches for this project may be seen in the Einstein Archive in the library of the Leiden University.[211] Wave–particle duality Einstein during his visit to the United States 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.[263] 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 saw this wave–particle duality in radiation as concrete evidence for his conviction that physics needed a new, unified foundation. Zero-point energy In a series of works completed from 1911 to 1913, Planck reformulated his 1900 quantum theory and introduced the idea of zero-point energy in his “second quantum theory”. Soon, this idea attracted the attention of Einstein and his assistant Otto Stern. Assuming the energy of rotating diatomic molecules contains zero-point energy, they then compared the theoretical specific heat of hydrogen gas with the experimental data. The numbers matched nicely. However, after publishing the findings, they promptly withdrew their support, because they no longer had confidence in the correctness of the idea of zero-point energy.[264] Stimulated emission 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.[265] This article showed that the statistics of absorption and emission of light would only be consistent with Planck’s distribution law if the emission of light into a mode with n photons would be enhanced statistically compared to the emission of light into an empty mode. 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. Matter waves Einstein discovered Louis de Broglie’s work and supported his ideas, which were received skeptically at first. 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. Quantum mechanics Einstein’s objections to quantum mechanics Newspaper headline on 4 May 1935 Einstein played a major role in developing quantum theory, beginning with his 1905 paper on the photoelectric effect. However, he became displeased with modern quantum mechanics as it had evolved after 1925, despite its acceptance by other physicists. He was skeptical that the randomness of quantum mechanics was fundamental rather than the result of determinism, stating that God “is not playing at dice”.[266] Until the end of his life, he continued to maintain that quantum mechanics was incomplete.[267] Bohr versus Einstein Main article: Bohr–Einstein debates Two men sitting, looking relaxed. A dark-haired Bohr is talking while Einstein looks skeptical. Einstein and Niels Bohr, 1925 The Bohr–Einstein debates were a series of public disputes about quantum mechanics between Einstein and Niels Bohr, who were two of its founders. Their debates are remembered because of their importance to the philosophy of science.[268][269][270] Their debates would influence later interpretations of quantum mechanics. Einstein–Podolsky–Rosen paradox Main article: EPR paradox In 1935, Einstein returned to quantum mechanics, in particular to the question of its completeness, in a collaboration with Boris Podolsky and Nathan Rosen that laid out what would become known as the EPR paradox.[270] In a thought experiment, they considered two particles, which had interacted such that their properties were strongly correlated. No matter how far the two particles were separated, a precise position measurement on one particle would result in equally precise knowledge of the position of the other particle; likewise, a precise momentum measurement of one particle would result in equally precise knowledge of the momentum of the other particle, without needing to disturb the other particle in any way.[271] Given Einstein’s concept of local realism, there were two possibilities: (1) either the other particle had these properties already determined, or (2) the process of measuring the first particle instantaneously affected the reality of the position and momentum of the second particle. Einstein rejected this second possibility (popularly called “spooky action at a distance”).[271] Einstein’s belief in local realism led him to assert that, while the correctness of quantum mechanics was not in question, it must be incomplete. But as a physical principle, local realism was shown to be incorrect when the Aspect experiment of 1982 confirmed Bell’s theorem, which J. S. Bell had delineated in 1964. The results of these and subsequent experiments demonstrate that quantum physics cannot be represented by any version of the picture of physics in which “particles are regarded as unconnected independent classical-like entities, each one being unable to communicate with the other after they have separated.”[272] Although Einstein was wrong about local realism, his clear prediction of the unusual properties of its opposite, entangled quantum states, has resulted in the EPR paper becoming among the most influential papers published in Physical Review. It is considered a centerpiece of the development of quantum information theory.[273] Unified field theory Main article: Classical unified field theories Following his research on general relativity, Einstein attempted to generalize his theory of gravitation to include electromagnetism as aspects of a single entity. In 1950, he described his “unified field theory” in a Scientific American article titled “On the Generalized Theory of Gravitation”.[274] Although he was lauded for this work, his efforts were ultimately unsuccessful. Notably, Einstein’s unification project did not accommodate the strong and weak nuclear forces, neither of which was well understood until many years after his death. Although mainstream physics long ignored Einstein’s approaches to unification, Einstein’s work has motivated modern quests for a theory of everything, in particular string theory, where geometrical fields emerge in a unified quantum-mechanical setting. Other investigations Main article: Einstein’s unsuccessful investigations Einstein conducted other investigations that were unsuccessful and abandoned. These pertain to force, superconductivity, and other research. Collaboration with other scientists The 1927 Solvay Conference in Brussels, a gathering of the world’s top physicists. Einstein is in the center. In addition to longtime collaborators Leopold Infeld, Nathan Rosen, Peter Bergmann and others, Einstein also had some one-shot collaborations with various scientists. Einstein–de Haas experiment Main article: Einstein–de Haas effect Einstein and De Haas demonstrated that magnetization is due to the motion of electrons, nowadays known to be the spin. In order to show this, they reversed the magnetization in an iron bar suspended on a torsion pendulum. They confirmed that this leads the bar to rotate, because the electron’s angular momentum changes as the magnetization changes. This experiment needed to be sensitive because the angular momentum associated with electrons is small, but it definitively established that electron motion of some kind is responsible for magnetization. Schrödinger gas model Einstein suggested to Erwin Schrödinger that he might be able to reproduce the statistics of a Bose–Einstein gas by considering a box. Then to each possible quantum motion of a particle in a box associate an independent harmonic oscillator. Quantizing these oscillators, each level will have an integer occupation number, which will be the number of particles in it.[citation needed] This formulation is a form of second quantization, but it predates modern quantum mechanics. Erwin Schrödinger applied this to derive the thermodynamic properties of a semiclassical ideal gas. Schrödinger urged Einstein to add his name as co-author, although Einstein declined the invitation.[275] Einstein refrigerator In 1926, Einstein and his former student Leó Szilárd co-invented (and in 1930, patented) the Einstein refrigerator. This absorption refrigerator was then revolutionary for having no moving parts and using only heat as an input.[276] On 11 November 1930, U.S. Patent 1,781,541 was awarded to Einstein and Leó Szilárd for the refrigerator. Their invention was not immediately put into commercial production, and the most promising of their patents were acquired by the Swedish company Electrolux.[note 6] Non-scientific legacy Einstein (second from left) at a picnic in Oslo in 1920. Heinrich Goldschmidt is at the left, Ole Colbjørnsen in the center and Jørgen Vogt sits behind Ilse Einstein. While traveling, Einstein wrote daily to his wife Elsa and adopted stepdaughters Margot and Ilse. The letters were included in the papers bequeathed to the Hebrew University of Jerusalem. Margot Einstein permitted the personal letters to be made available to the public, but requested that it not be done until twenty years after her death (she died in 1986[278]). Barbara Wolff, of the Hebrew University’s Albert Einstein Archives, told the BBC that there are about 3,500 pages of private correspondence written between 1912 and 1955.[279] Einstein’s right of publicity was litigated in 2015 in a federal district court in California. Although the court initially held that the right had expired,[280] that ruling was immediately appealed, and the decision was later vacated in its entirety. The underlying claims between the parties in that lawsuit were ultimately settled. The right is enforceable, and the Hebrew University of Jerusalem is the exclusive representative of that right.[281] Corbis, successor to The Roger Richman Agency, licenses the use of his name and associated imagery, as agent for the university.[282] Mount Einstein in New Zealand’s Paparoa Range was named after him in 1970 by the Department of Scientific and Industrial Research.[283]