Albert Einstein?(14 March 1879?– 18 April 1955) was a German-born?theoretical physicist,widely acknowledged to be one of the greatest physicists of all time. Einstein is best known for developing the?theory of relativity, but he also made important contributions to the development of the theory of?quantum mechanics. Relativity and quantum mechanics are together the two pillars of?modern physics. His?mass–energy equivalence?formula?E?=?mc2, which arises from relativity theory, has been dubbed "the world's most famous equation".?His work is also known for its influence on the?philosophy of science.?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",?a pivotal step in the development of quantum theory. His intellectual achievements and originality resulted in "Einstein" becoming synonymous with "genius".

In 1905, a year sometimes described as his?annus mirabilis?('miracle year'), Einstein published?four groundbreaking papers.?These outlined the theory of the photoelectric effect, explained?Brownian motion, introduced?special relativity, and demonstrated mass-energy equivalence. Einstein thought that the laws of?classical mechanics?could no longer be reconciled with those of the?electromagnetic field, which led him to develop his special theory of relativity. He then extended the theory to gravitational fields; he published a paper on?general relativity?in 1916, introducing his theory of gravitation. In 1917, he applied the general theory of relativity to model the structure of the universe.?He continued to deal with problems of?statistical mechanics?and quantum theory, which led to his explanations of particle theory and the?motion of molecules. He also investigated the thermal properties of light and the quantum theory of radiation, which laid the foundation of the?photon?theory of light.

However, for much of the later part of his career, he worked on two ultimately unsuccessful endeavors. First, despite his great contributions to quantum mechanics, he opposed what it evolved into, objecting that nature "does not play dice".?Second, he attempted to devise a?unified field theory?by generalizing his geometric theory of gravitation to include electromagnetism. As a result, he became increasingly isolated from the mainstream of modern physics.

Einstein was born in the?German Empire, but moved to Switzerland in 1895, forsaking his German citizenship (as a subject of the?Kingdom of Württemberg)?the following year. In 1897, at the age of 17, he enrolled in the mathematics and physics teaching diploma program at the Swiss?Federal polytechnic school?in?Zürich, graduating in 1900. In 1901, he acquired Swiss citizenship, which he kept for the rest of his life, and in 1903 he secured a permanent position at the?Swiss Patent Office?in Bern. In 1905, he was awarded a PhD by the?University of Zurich. In 1914, Einstein moved to?Berlin?in order to join the?Prussian Academy of Sciences?and the?Humboldt University of Berlin. In 1917, Einstein became director of the?Kaiser Wilhelm Institute for Physics; he also became a German citizen again, this time Prussian.

In 1933, while Einstein was visiting the United States,?Adolf Hitler?came to power in Germany. Einstein, of Jewish origin, objected to the policies of the newly elected?nazi government;?he settled in the United States and became an American citizen in 1940.?On the eve of?World War II, he endorsed?a letter?to President?Franklin D. Roosevelt?alerting him to the potential?German nuclear weapons program?and recommending that the US begin?similar research. Einstein supported the?Allies?but generally denounced the idea of?nuclear weapons.

Life and career

Early life and education

Albert Einstein was born in?Ulm,?in the?Kingdom of Württemberg?in the?German Empire, on 14 March 1879 into a family of secular?Ashkenazi Jews.?His parents were?Hermann Einstein, a salesman and engineer, and?Pauline Koch. In 1880, the family moved to?Munich, where Einstein's father and his uncle Jakob founded?Elektrotechnische Fabrik J. Einstein & Cie, a company that manufactured electrical equipment based on?direct current.

Albert attended a?Catholic elementary school?in Munich, from the age of five, for three years. At the age of eight, he was transferred to the Luitpold Gymnasium (now known as the Albert Einstein Gymnasium), where he received advanced primary and secondary school education until he left the German Empire seven years later.

In 1894, Hermann and Jakob's company lost a bid to supply the city of Munich with electrical lighting because they lacked the capital to convert their equipment from the?direct current?(DC) standard to the more efficient?alternating current?(AC) standard.?The loss forced the sale of the Munich factory. In search of business, the Einstein family moved to Italy, first to?Milan?and a few months later to?Pavia. When the family moved to Pavia, Einstein, then 15, stayed in Munich to finish his studies at the Luitpold Gymnasium. His father intended for him to pursue?electrical engineering, but Einstein clashed with the authorities and resented the school's regimen and teaching method. He later wrote that the spirit of learning and creative thought was lost in strict?rote learning. At the end of December 1894, he traveled to Italy to join his family in Pavia, convincing the school to let him go by using a doctor's note.?During his time in Italy he wrote a short essay with the title "On the Investigation of the State of the?Ether?in a Magnetic Field".

Einstein excelled at math and physics from a young age, reaching a mathematical level years ahead of his peers. The 12-year-old Einstein taught himself algebra and Euclidean geometry over a single summer.?Einstein also independently discovered his own original proof of the?Pythagorean theorem?at age 12.?A family tutor?Max Talmud?says that after he had given the 12-year-old Einstein a geometry textbook, after a short time "[Einstein] had worked through the whole book. He thereupon devoted himself to higher mathematics... Soon the flight of his mathematical genius was so high I could not follow."?His passion for geometry and algebra led the 12-year-old to become convinced that nature could be understood as a "mathematical structure". Einstein started teaching himself calculus at 12, and as a 14-year-old he says he had "mastered?integral?and?differential calculus".

At age 13, when he had become more seriously interested in philosophy (and music),?Einstein was introduced to?Kant's?Critique of Pure Reason. Kant became his favorite philosopher, his tutor stating: "At the time he was still a child, only thirteen years old, yet Kant's works, incomprehensible to ordinary mortals, seemed to be clear to him."

In 1895, at the age of 16, Einstein took the entrance examinations for the Swiss?Federal polytechnic school?in?Zürich?(later the Eidgen?ssische Technische Hochschule, ETH). He failed to reach the required standard in the general part of the examination,?but obtained exceptional grades in physics and mathematics.?On the advice of the principal of the polytechnic school, he attended the?Argovian cantonal school?(gymnasium) in?Aarau, Switzerland, in 1895 and 1896 to complete his secondary schooling. While lodging with the family of Professor?Jost Winteler, he fell in love with Winteler's daughter, Marie. Albert's sister?Maja?later married Winteler's son Paul.?In January 1896, with his father's approval, Einstein renounced his?citizenship in the German Kingdom of Württemberg?to avoid?military service.?In September 1896 he passed the Swiss?Matura?with mostly good grades, including a top grade of 6 in physics and mathematical subjects, on?a scale of 1–6.?At 17, he enrolled in the four-year mathematics and physics teaching diploma program at the Federal polytechnic school. Marie Winteler, who was a year older, moved to?Olsberg, Switzerland, for a teaching post.

Einstein's future wife, a 20-year-old?Serbian?named?Mileva Mari?, also enrolled at the polytechnic school that year. She was the only woman among the six students in the mathematics and physics section of the teaching diploma course. Over the next few years, Einstein's and Mari?'s friendship developed into a romance, and they spent countless hours debating and reading books together on extra-curricular physics in which they were both interested. Einstein wrote in his letters to Mari? that he preferred studying alongside her.?In 1900, Einstein passed the exams in Maths and Physics and was awarded a Federal teaching diploma.?There is eyewitness evidence and several letters over many years that indicate Mari? might have collaborated with Einstein prior to his landmark 1905 papers,?known as the?Annus Mirabilis?papers, and that they developed some of the concepts together during their studies, although some historians of physics who have studied the issue disagree that she made any substantive contributions.

Patent office

After graduating in 1900, Einstein spent almost two frustrating years searching for a teaching post. He acquired?Swiss?citizenship in February 1901,?but was not?conscripted?for medical reasons. With the help of?Marcel Grossmann's father, he secured a job in?Bern?at the?Swiss Patent Office,?as an?assistant examiner – level III.

Einstein evaluated?patent applications?for a variety of devices including a gravel sorter and an electromechanical typewriter.?In 1903, his 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 a few friends he had met in Bern, Einstein started a small discussion group in 1902, self-mockingly named "The Olympia Academy", which met regularly to discuss science and philosophy. Sometimes they were joined by Mileva who attentively listened but did not participate.?Their readings included the works of?Henri Poincaré,?Ernst Mach, and?David Hume, which influenced his scientific and philosophical outlook.

First scientific papers

In 1900, Einstein's paper?"Folgerungen aus den Capillarit?tserscheinungen"?("Conclusions from the Capillarity Phenomena") was published in the journal?Annalen der Physik.?On 30 April 1905, Einstein completed his dissertation,?A New Determination of Molecular Dimensions?with?Alfred Kleiner, Professor of Experimental Physics at the?University of Zürich, serving as?pro-forma?advisor.?His work was accepted in July, and Einstein was awarded a Ph.D.

Also in 1905, which has been called Einstein's?annus mirabilis?(amazing year), he published?four groundbreaking papers, on the?photoelectric effect,?Brownian motion,?special relativity, and the?equivalence of mass and energy, which were to bring him to the notice of the academic world, at the age of 26.

Academic career

By 1908, he was recognized as a leading scientist and was appointed lecturer at the?University of Bern. The following year, after he gave a lecture on?electrodynamics?and the relativity principle at the University of Zurich,?Alfred Kleiner?recommended him to the faculty for a newly created professorship in theoretical physics. Einstein was appointed associate professor in 1909.

Einstein became a full professor at the German?Charles-Ferdinand University?in?Prague?in April 1911, accepting?Austrian?citizenship in the?Austro-Hungarian Empire?to do so.?During his Prague stay, he wrote 11 scientific works, five of them on radiation mathematics and on the quantum theory of solids.

In July 1912, he returned to his alma mater in Zürich. From 1912 until 1914, he was a professor of theoretical physics at the?ETH Zurich, where he taught analytical mechanics and?thermodynamics. He also studied?continuum mechanics, the molecular theory of heat, and the problem of gravitation, on which he worked with mathematician and friend?Marcel Grossmann.

When the "Manifesto of the Ninety-Three" was published in October 1914—a document signed by a host of prominent German intellectuals that justified Germany's militarism and position during the First World War—Einstein was one of the few German intellectuals to rebut its contents and sign the pacifistic "Manifesto to the Europeans".

The New York Times?reported confirmation of "the Einstein theory" (specifically, the bending of light by gravitation) based on 29 May 1919 eclipse observations in Principe (Africa) and Sobral (Brazil), after the findings were presented on 6 November 1919 to a joint meeting in London of the?Royal Society?and the?Royal Astronomical Society.?(Full text)

In the spring of 1913, Einstein was enticed to move to Berlin with an offer that included membership in the Prussian Academy of Sciences, and a linked University of Berlin professorship, enabling him to concentrate exclusively on research.?On 3 July 1913, he became a member of the?Prussian Academy of Sciences?in Berlin.?Max Planck?and?Walther Nernst?visited him the next week in Zurich to persuade him to join the academy, additionally offering him the post of director at the?Kaiser Wilhelm Institute for Physics, which was soon to be established.?Membership in the academy included paid salary and professorship without teaching duties at?Humboldt University of Berlin. He was officially elected to the academy on 24 July, and he moved to Berlin the following year. His decision to move to Berlin was also influenced by the prospect of living near his cousin Elsa, with whom he had started a romantic affair. Einstein assumed his position with the academy, and Berlin University,?after moving into his?Dahlem?apartment on 1 April 1914.?As World War I broke out that year, the plan for Kaiser Wilhelm Institute for Physics was aborted. The institute was established on 1 October 1917, with Einstein as its director.?In 1916, Einstein was elected President of the?German Physical Society?(1916–1918).

In 1911, Einstein used his 1907?Equivalence principle?to calculate the deflection of?light from another star?by the Sun's gravity. In 1913, Einstein improved upon those calculations by using?Riemannian space-time?to represent the gravity field. By the fall of 1915, Einstein had successfully completed his general theory of relativity, which he used to calculate that deflection, and the?perihelion precession of Mercury.?In 1919, that deflection prediction was confirmed by Sir?Arthur Eddington?during the?solar eclipse of 29 May 1919. Those observations were published in the international media, making Einstein world-famous. On 7 November 1919, the leading British newspaper?The Times?printed a banner headline that read: "Revolution in Science?– New Theory of the Universe?– Newtonian Ideas Overthrown".

In 1920, he became a Foreign Member of the?Royal Netherlands Academy of Arts and Sciences.?In 1922, he was awarded the 1921?Nobel Prize in Physics?"for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".?While the?general theory of relativity?was still considered somewhat controversial, the citation also does not treat even the cited photoelectric work as an?explanation?but merely as a?discovery of the law, as the idea of photons was considered outlandish and did not receive universal acceptance until the 1924 derivation of the?Planck spectrum?by?S. N. Bose. Einstein was elected a?Foreign Member of the Royal Society (ForMemRS) in 1921.?He also received the?Copley Medal?from the?Royal Society?in 1925.

Einstein resigned from the Prussian Academy in March 1933. Einstein's scientific accomplishments while in Berlin, included finishing the general theory of relativity, proving the?gyromagnetic effect, contributing to the quantum theory of radiation, and?Bose–Einstein statistics.

1921–1922: Travels abroad

Einstein visited New York City for the first time on 2 April 1921, where he received an official welcome by Mayor?John Francis Hylan, followed by three weeks of lectures and receptions.?He went on to deliver several lectures at?Columbia University?and?Princeton University, and in Washington, he accompanied representatives of the?National Academy of Sciences?on a visit to the?White House. On his return to Europe he was the guest of the British statesman and philosopher?Viscount Haldane?in London, where he met several renowned scientific, intellectual, and political figures, and delivered a lecture at?King's College London.

He also published an essay, "My First Impression of the U.S.A.", in July 1921, in which he tried briefly to describe some characteristics of Americans, much as had?Alexis de Tocqueville, who published his own impressions in?Democracy in America?(1835).?For some of his observations, Einstein was clearly surprised: "What strikes a visitor is the joyous, positive attitude to life ... The American is friendly, self-confident, optimistic, and without envy."

In 1922, his travels took him to Asia and later to Palestine, as part of a six-month excursion and speaking tour, as he visited Singapore,?Ceylon?and Japan, where he gave a series of lectures to thousands of Japanese. After his first public lecture, he met the emperor and empress at the?Imperial Palace, where thousands came to watch. In a letter to his sons, he described his impression of the Japanese as being modest, intelligent, considerate, and having a true feel for art.?In his own travel diaries from his 1922–23 visit to Asia, he expresses some views on the Chinese, Japanese and Indian people, which have been described as xenophobic and racist judgments when they were rediscovered in 2018.

Because of Einstein's travels to the Far East, he was unable to personally accept the Nobel Prize for Physics at the Stockholm award ceremony in December 1922. In his place, the banquet speech was made by a German diplomat, who praised Einstein not only as a scientist but also as an international peacemaker and activist.

On his return voyage, he visited?Palestine?for 12 days, his only visit to that region. He was greeted as if he were a head of state, rather than a physicist, which included a cannon salute upon arriving at the home of the British high commissioner,?Sir Herbert Samuel. During one reception, the building was stormed by people who wanted to see and hear him. In Einstein's talk to the audience, he expressed happiness that the Jewish people were beginning to be recognized as a force in the world.

Einstein visited Spain for two weeks in 1923, where he briefly met?Santiago Ramón y Cajal?and also received a diploma from?King Alfonso XIII?naming him a member of the Spanish Academy of Sciences.

From 1922 to 1932, Einstein was a member of the?International Committee on Intellectual Cooperation?of the?League of Nations?in?Geneva?(with a few months of interruption in 1923–1924),?a body created to promote international exchange between scientists, researchers, teachers, artists, and intellectuals.?Originally slated to serve as the Swiss delegate, Secretary-General?Eric Drummond?was persuaded by Catholic activists?Oskar Halecki?and?Giuseppe Motta?to instead have him become the German delegate, thus allowing?Gonzague de Reynold?to take the Swiss spot, from which he promoted traditionalist Catholic values.?Einstein's former physics professor?Hendrik Lorentz?and the Polish chemist?Marie Curie?were also members of the committee.

1925: Visit to South America

In the months of March and April 1925, Einstein visited South America, where he spent about a month in?Argentina, a week in?Uruguay, and a week in?Rio de Janeiro, Brazil.?Einstein's visit was initiated by Jorge Duclout (1856–1927) and Mauricio Nirenstein (1877–1935)?with the support of several Argentine scholars, including?Julio Rey Pastor,?Jakob Laub, and?Leopoldo Lugones. The visit by Einstein and his wife was financed primarily by the Council of the?University of Buenos Aires?and the?Asociación Hebraica Argentina?(Argentine Hebraic Association) with a smaller contribution from the Argentine-Germanic Cultural Institution.

1930–1931: Travel to the US

In December 1930, Einstein visited America for the second time, originally intended as a two-month working visit as a research fellow at the?California Institute of Technology. After the national attention, he received during his first trip to the US, he and his arrangers aimed to protect his privacy. Although swamped with telegrams and invitations to receive awards or speak publicly, he declined them all.

After arriving in New York City, Einstein was taken to various places and events, including?Chinatown, a lunch with the editors of?The New York Times, and a performance of?Carmen?at the?Metropolitan Opera, where he was cheered by the audience on his arrival. During the days following, he was given the keys to the city by Mayor?Jimmy Walker?and met the President of Columbia University, who described Einstein as "the ruling monarch of the mind".?Harry Emerson Fosdick, pastor at New York's?Riverside Church, gave Einstein a tour of the church and showed him a full-size statue that the church made of Einstein, standing at the entrance.?Also during his stay in New York, he joined a crowd of 15,000 people at?Madison Square Garden?during a?Hanukkah?celebration.

Einstein next traveled to California, where he met?Caltech?President and Nobel laureate?Robert A. Millikan. His friendship with Millikan was "awkward", as Millikan "had a penchant for patriotic militarism", where Einstein was a pronounced?pacifist.?During an address to Caltech's students, Einstein noted that science was often inclined to do more harm than good.

This aversion to war also led Einstein to befriend author?Upton Sinclair?and film star?Charlie Chaplin, both noted for their pacifism.?Carl Laemmle, head of?Universal Studios, gave Einstein a tour of his studio and introduced him to Chaplin. They had an instant rapport, with Chaplin inviting Einstein and his wife, Elsa, to his home for dinner. Chaplin said Einstein's outward persona, calm and gentle, seemed to conceal a "highly emotional temperament", from which came his "extraordinary intellectual energy".

Chaplin's film,?City Lights, was to premiere a few days later in Hollywood, and Chaplin invited Einstein and Elsa to join him as his special guests.?Walter Isaacson, Einstein's biographer, described this as "one of the most memorable scenes in the new era of celebrity".?Chaplin visited Einstein at his home on a later trip to Berlin and recalled his "modest little flat" and the piano at which he had begun writing his theory. Chaplin speculated that it was "possibly used as kindling wood by the nazis".

1933: Emigration to the US

In February 1933, while on a visit to the United States, Einstein knew he could not return to Germany with the rise to power of the?nazis?under Germany's new chancellor,?Adolf Hitler.

While at American universities in early 1933, he undertook his third two-month visiting professorship at the?California Institute of Technology?in Pasadena. In February and March 1933, the?Gestapo?repeatedly raided his family's apartment in Berlin.?He and his wife Elsa returned to Europe in March, and during the trip, they learned that the German Reichstag had passed the?Enabling Act?on 23 March, transforming Hitler's government into a?de facto?legal dictatorship, and that they would not be able to proceed to Berlin. Later on, they heard that their cottage had been raided by the nazis and Einstein's personal sailboat confiscated. Upon landing in?Antwerp, Belgium on 28 March, Einstein immediately went to the German consulate and surrendered his passport, formally renouncing his German citizenship.?The nazis later sold his boat and converted his cottage into a?Hitler Youth?camp.

Refugee status

In April 1933, Einstein discovered that the new German government had passed laws barring Jews from holding any official positions, including teaching at universities.?Historian?Gerald Holton?describes how, with "virtually no audible protest being raised by their colleagues", thousands of Jewish scientists were suddenly forced to give up their university positions and their names were removed from the rolls of institutions where they were employed.

A month later, Einstein's works were among those targeted by the?German Student Union?in the?nazi book burnings, with nazi propaganda minister?Joseph Goebbels?proclaiming, "Jewish intellectualism is dead."?One German magazine included him in a list of enemies of the German regime with the phrase, "not yet hanged", offering a $5,000 bounty on his head.?In a subsequent letter to physicist and friend?Max Born, who had already emigrated from Germany to England, Einstein wrote, "...?I must confess that the degree of their brutality and cowardice came as something of a surprise."?After moving to the US, he described the book burnings as a "spontaneous emotional outburst" by those who "shun popular enlightenment", and "more than anything else in the world, fear the influence of men of intellectual independence".

Einstein was now without a permanent home, unsure where he would live and work, and equally worried about the fate of countless other scientists still in Germany. He rented a house in De Haan, Belgium, where he lived for a few months. In late July 1933, he went to England for about six weeks at the personal invitation of British naval officer Commander?Oliver Locker-Lampson, who had become friends with Einstein in the preceding years. Locker-Lampson invited him to stay near his?Cromer?home in a wooden cabin on Roughton Heath in the Parish of?Roughton, Norfolk. To protect Einstein, Locker-Lampson had two bodyguards watch over him at his secluded cabin; a photo of them carrying shotguns and guarding Einstein was published in the?Daily Herald?on 24 July 1933.

Locker-Lampson took Einstein to meet?Winston Churchill?at his home, and later,?Austen Chamberlain?and former Prime Minister?Lloyd George.?Einstein asked them to help bring Jewish scientists out of Germany. British historian?Martin Gilbert?notes that Churchill responded immediately, and sent his friend, physicist?Frederick Lindemann, to Germany to seek out Jewish scientists and place them in British universities.?Churchill later observed that as a result of Germany having driven the Jews out, they had lowered their "technical standards" and put?the Allies' technology ahead of theirs.

Einstein later contacted leaders of other nations, including?Turkey's Prime Minister,??smet ?n?nü, to whom he wrote in September 1933 requesting placement of unemployed German-Jewish scientists. As a result of Einstein's letter, Jewish invitees to Turkey eventually totaled over "1,000 saved individuals".

Locker-Lampson also submitted a bill to parliament to extend British citizenship to Einstein, during which period Einstein made a number of public appearances describing the crisis brewing in Europe.?In one of his speeches he denounced Germany's treatment of Jews, while at the same time he introduced a bill promoting Jewish citizenship in Palestine, as they were being denied citizenship elsewhere.?In his speech he described Einstein as a "citizen of the world" who should be offered a temporary shelter in the UK.?Both bills failed, however, and Einstein then accepted an earlier offer from the?Institute for Advanced Study, in?Princeton, New Jersey, US, to become a resident scholar.

Resident scholar at the Institute for Advanced Study

In October 1933, Einstein returned to the US and took up a position at the Institute for Advanced Study,?noted for having become a refuge for scientists fleeing Nazi Germany.?At the time, most American universities, including Harvard, Princeton and Yale, had minimal or no Jewish faculty or students, as a result of their?Jewish quotas, which lasted until the late 1940s.

Einstein was still undecided on his future. He had offers from several European universities, including?Christ Church, Oxford, where he stayed for three short periods between May 1931 and June 1933 and was offered a five-year studentship,?but in 1935, he arrived at the decision to remain permanently in the United States and apply for citizenship.

Einstein's affiliation with the Institute for Advanced Study would last until his death in 1955.?He was one of the four first selected (along with?John von Neumann?and?Kurt G?del) at the new Institute, where he soon developed a close friendship with G?del. The two would take long walks together discussing their work.?Bruria Kaufman, his assistant, later became a physicist. During this period, Einstein tried to develop a?unified field theory?and to refute the?accepted interpretation?of?quantum physics, both unsuccessfully.

World War II and the Manhattan Project

In 1939, a group of Hungarian scientists that included émigré physicist?Leó Szilárd?attempted to alert Washington to ongoing Nazi atomic bomb research. The group's warnings were discounted. Einstein and Szilárd, along with other refugees such as?Edward Teller?and?Eugene Wigner, "regarded it as their responsibility to alert Americans to the possibility that German scientists might win the?race to build an atomic bomb, and to warn that Hitler would be more than willing to resort to such a weapon."?To make certain the US was aware of the danger, in July 1939, a few months before the beginning of World War II in Europe, Szilárd and Wigner visited Einstein to explain the possibility of atomic bombs, which Einstein, a pacifist, said he had never considered.?He was asked to lend his support by writing?a letter, with Szilárd, to President?Roosevelt, recommending the US pay attention and engage in its own nuclear weapons research.

The letter is believed to be "arguably the key stimulus for the U.S. adoption of serious investigations into nuclear weapons on the eve of the U.S. entry into World War II".?In addition to the letter, Einstein used his connections with the?Belgian Royal Family?and the Belgian queen mother to get access with a personal envoy to the White House's Oval Office. Some say that as a result of Einstein's letter and his meetings with Roosevelt, the US entered the "race" to develop the bomb, drawing on its "immense material, financial, and scientific resources" to initiate the?Manhattan Project.

For Einstein, "war was a disease?... [and] he called for resistance to war." By signing the letter to Roosevelt, some argue he went against his pacifist principles.?In 1954, a year before his death, Einstein said to his old friend,?Linus Pauling, "I made one great mistake in my life—when I signed the letter to President Roosevelt recommending that atom bombs be made; but there was some justification—the danger that the Germans would make them?..."?In 1955, Einstein and ten other intellectuals and scientists, including British philosopher?Bertrand Russell, signed?a manifesto?highlighting the danger of nuclear weapons.

US citizenship

Einstein became an American citizen in 1940. Not long after settling into his career at the Institute for Advanced Study in Princeton, New Jersey, he expressed his appreciation of the?meritocracy?in American culture when compared to Europe. He recognized the "right of individuals to say and think what they pleased", without social barriers, and as a result, individuals were encouraged, he said, to be more creative, a trait he valued from his own early education.

Einstein joined the?National Association for the Advancement of Colored People?(NAACP) in Princeton, where he campaigned for the?civil rights?of African Americans. He considered racism America's "worst disease",?seeing it as "handed down from one generation to the next".?As part of his involvement, he corresponded with civil rights activist?W. E. B. Du Bois?and was prepared to testify on his behalf during his trial in 1951.?When Einstein offered to be a character witness for Du Bois, the judge decided to drop the case.

In 1946, Einstein visited?Lincoln University?in Pennsylvania, a?historically black college, where he was awarded an honorary degree. Lincoln was the first university in the United States to grant college degrees to?African Americans; alumni include?Langston Hughes?and?Thurgood Marshall. Einstein gave a speech about racism in America, adding, "I do not intend to be quiet about it."?A resident of Princeton recalls that Einstein had once paid the college tuition for a black student.?Einstein has said "Being a Jew myself, perhaps I can understand and empathize with how black people feel as victims of discrimination".

Scientific career

Throughout his life, Einstein published hundreds of books and articles.?He published more than 300 scientific papers and 150 non-scientific ones.?On 5 December 2014, universities and archives announced the release of Einstein's papers, comprising more than 30,000 unique documents.?Einstein's intellectual achievements and originality have made the word "Einstein" synonymous with "genius".?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.

1905 –?Annus Mirabilis?papers

The?Annus Mirabilis?papers?are four articles pertaining to the?photoelectric effect?(which gave rise to?quantum theory),?Brownian motion, the?special theory of relativity, and?E = mc2?that Einstein published in the?Annalen der Physik?scientific journal in 1905. These four works contributed substantially to the foundation of?modern physics?and changed views on?space, time, and?matter. The four papers are:

Title?(translated)Area of focusReceivedPublishedSignificance"On a Heuristic Viewpoint Concerning the Production and Transformation of Light"Photoelectric effect18 March9 JuneResolved an unsolved puzzle by suggesting that energy is exchanged only in discrete amounts (quanta).?This idea was pivotal to the early development of quantum theory."On the Motion of Small Particles Suspended in a Stationary Liquid, as Required by the Molecular Kinetic Theory of Heat"Brownian motion11 May18 JulyExplained empirical evidence for the?atomic theory, supporting the application of?statistical physics."On the Electrodynamics of Moving Bodies"Special relativity30 June26?SeptemberReconciled Maxwell's equations for electricity and magnetism with the laws of mechanics by introducing changes to mechanics, resulting from analysis based on empirical evidence that the speed of light is independent of the motion of the observer.?Discredited the concept of a "luminiferous ether"."Does the Inertia of a Body Depend Upon Its Energy Content?"Matter–energy?equivalence27?September21 NovemberEquivalence of matter and energy,?E?=?mc2?(and by implication, the ability of gravity to "bend" light), the existence of "rest energy", and the basis of nuclear energy.

Statistical mechanics

Thermodynamic fluctuations and statistical physics

Einstein's first paper?submitted in 1900 to?Annalen der Physik?was on?capillary attraction. It was published in 1901 with the title "Folgerungen aus den Capillarit?tserscheinungen", which translates as "Conclusions from the capillarity phenomena". Two papers he published in 1902–1903 (thermodynamics) attempted to interpret?atomic?phenomena from a statistical point of view. These papers were the foundation for the 1905 paper on Brownian motion, which showed that Brownian movement can be construed as firm evidence that molecules exist. His research in 1903 and 1904 was mainly concerned with the effect of finite atomic size on diffusion phenomena.

Theory of critical opalescence

Einstein returned to the problem of thermodynamic fluctuations, giving a treatment of the density variations in a fluid at its critical point. Ordinarily the density fluctuations are controlled by the second derivative of the free energy with respect to the density. At the critical point, this derivative is zero, leading to large fluctuations. The effect of density fluctuations is that light of all wavelengths is scattered, making the fluid look milky white. Einstein relates this to?Rayleigh scattering, which is what happens when the fluctuation size is much smaller than the wavelength, and which explains why the sky is blue.?Einstein quantitatively derived critical opalescence from a treatment of density fluctuations, and demonstrated how both the effect and Rayleigh scattering originate from the atomistic constitution of matter.

Special relativity

Einstein's "Zur Elektrodynamik bewegter K?rper"?("On the Electrodynamics of Moving Bodies") was received on 30 June 1905 and published 26 September of that same year. It reconciled conflicts between?Maxwell's equations?(the laws of electricity and magnetism) and the laws of Newtonian mechanics by introducing changes to the laws of mechanics.?Observationally, the effects of these changes are most apparent at high speeds (where objects are moving at speeds close to the?speed of light). The theory developed in this paper later became known as Einstein's special theory of relativity. There is evidence from Einstein's writings that he collaborated with his first wife, Mileva Mari?, on this work. The decision to publish only under his name seems to have been mutual, but the exact reason is unknown.

This paper predicted that, when measured in the frame of a relatively moving observer, a clock carried by a moving body would appear to?slow down, and the body itself would?contract?in its direction of motion. This paper also argued that the idea of a?luminiferous aether—one of the leading theoretical entities in physics at the time—was superfluous.

In his paper on?mass–energy equivalence, Einstein produced?E?=?mc2?as a consequence of his special relativity equations.?Einstein's 1905 work on relativity remained controversial for many years, but was accepted by leading physicists, starting with?Max Planck.

Einstein originally framed special relativity in terms of?kinematics?(the study of moving bodies). In 1908,?Hermann Minkowski?reinterpreted special relativity in geometric terms as a theory of?spacetime. Einstein adopted Minkowski's formalism in his 1915?general theory of relativity.

General relativity

General relativity and the equivalence principle

General relativity (GR) is a?theory of gravitation?that was developed by Einstein between 1907 and 1915. According to?general relativity, the observed gravitational attraction between masses results from the warping of?space and time?by those masses. General relativity has developed into an essential tool in modern?astrophysics. It provides the foundation for the current understanding of?black holes, regions of space where gravitational attraction is so strong that not even light can escape.

As Einstein later said, the reason for the development of general relativity was that the preference of inertial motions within?special relativity?was unsatisfactory, while a theory which from the outset prefers no state of motion (even accelerated ones) should appear more satisfactory.?Consequently, in 1907 he published an article on acceleration under special relativity. In that article titled "On the Relativity Principle and the Conclusions Drawn from It", he argued that?free fall?is really inertial motion, and that for a free-falling observer the rules of special relativity must apply. This argument is called the?equivalence principle. In the same article, Einstein also predicted the phenomena of?gravitational time dilation,?gravitational redshift?and?deflection of light.

In 1911, Einstein published another article "On the Influence of Gravitation on the Propagation of Light" expanding on the 1907 article, in which he estimated the amount of deflection of light by massive bodies. Thus, the theoretical prediction of general relativity could for the first time be tested experimentally.

Gravitational waves

In 1916, Einstein predicted?gravitational waves,?ripples in the?curvature?of spacetime which propagate as?waves, traveling outward from the source, transporting energy as gravitational radiation. The existence of gravitational waves is possible under general relativity due to its?Lorentz invariance?which brings the concept of a finite speed of propagation of the physical interactions of gravity with it. By contrast, gravitational waves cannot exist in the?Newtonian theory of gravitation, which postulates that the physical interactions of gravity propagate at infinite speed.

The first, indirect, detection of gravitational waves came in the 1970s through observation of a pair of closely orbiting?neutron stars,?PSR B1913+16.?The explanation of the decay in their orbital period was that they were emitting gravitational waves.?Einstein's prediction was confirmed on 11 February 2016, when researchers at?LIGO?published the?first observation of gravitational waves,?detected on Earth on 14 September 2015, nearly one hundred years after the prediction.

Hole argument and Entwurf theory

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.

In June 1913, the Entwurf ('draft') theory was the result of these investigations. As its name suggests, it was a sketch of a theory, less elegant and more difficult than general relativity, with the equations of motion supplemented by additional gauge fixing conditions. After more than two years of intensive work, Einstein realized that the?hole argument?was mistaken?and abandoned the theory in November 1915.

Physical cosmology

In 1917, Einstein applied the general theory of relativity to the structure of the universe as a whole.?He discovered that the general field equations predicted a universe that was dynamic, either contracting or expanding. As observational evidence for a dynamic universe was not known at the time, Einstein introduced a new term, the?cosmological constant, to the field equations, in order to allow the theory to predict a static universe. The modified field equations predicted a static universe of closed curvature, in accordance with Einstein's understanding of?Mach's principle?in these years. This model became known as the Einstein World or?Einstein's static universe.

Following the discovery of the recession of the nebulae by?Edwin Hubble?in 1929, Einstein abandoned his static model of the universe, and proposed two dynamic models of the cosmos,?The Friedmann-Einstein universe?of 1931?and the?Einstein–de Sitter universe?of 1932.?In each of these models, Einstein discarded the cosmological constant, claiming that it was "in any case theoretically unsatisfactory".

In many Einstein biographies, it is claimed that Einstein referred to the cosmological constant in later years as his "biggest blunder". The astrophysicist?Mario Livio?has recently cast doubt on this claim, suggesting that it may be exaggerated.

In late 2013, a team led by the Irish physicist?Cormac O'Raifeartaigh?discovered evidence that, shortly after learning of Hubble's observations of the recession of the nebulae, Einstein considered a?steady-state model?of the universe.?In a hitherto overlooked manuscript, apparently written in early 1931, Einstein explored a model of the expanding universe in which the density of matter remains constant due to a continuous creation of matter, a process he associated with the cosmological constant.?As he stated in the paper, "In what follows, I would like to draw attention to a solution to equation (1) that can account for Hubbel's [sic] facts, and in which the density is constant over time" ... "If one considers a physically bounded volume, particles of matter will be continually leaving it. For the density to remain constant, new particles of matter must be continually formed in the volume from space."

It thus appears that Einstein considered a?steady-state model?of the expanding universe many years before Hoyle, Bondi and Gold.?However, Einstein's steady-state model contained a fundamental flaw and he quickly abandoned the idea.