J. Robert Oppenheimer (born Julius Robert Oppenheimer ; April 22, 1904 – February 18, 1967) was an American theoretical physicist who served as the director of the Manhattan Project's Los Alamos Laboratory during World War II. He is often called the "father of the atomic bomb" for his role in overseeing the development of the first nuclear weapons. Born in New York City, Oppenheimer obtained a degree in chemistry from Harvard University in 1925 and a doctorate in physics from the University of Göttingen in Germany in 1927, studying under Max Born. After research at other institutions, he joined the physics faculty at the University of California, Berkeley, where he was made a full professor in 1936. Oppenheimer made significant contributions to physics in the fields of quantum mechanics and nuclear physics, including the Born–Oppenheimer approximation for molecular wave functions; work on the theory of positrons, quantum electrodynamics, and quantum field theory; and the Oppenheimer–Phillips process in nuclear fusion. With his students, he also made major contributions to astrophysics, including the theory of cosmic ray showers, and the theory of neutron stars and black holes. In 1941, Oppenheimer was briefed about nuclear weapon design by Australian physicist Mark Oliphant. In 1942, Oppenheimer was recruited to work on the Manhattan Project, and in 1943 was appointed director of the project's Los Alamos Laboratory in New Mexico, tasked with developing the first nuclear weapons. His leadership and scientific expertise were instrumental in the project's success, and on July 16, 1945, he was present at the first test of the atomic bomb, Trinity. In August, the weapons were used on Japan in the atomic bombings of Hiroshima and Nagasaki, to date the only uses of nuclear weapons in a conflict. In 1947, Oppenheimer was appointed director of the Institute for Advanced Study in Princeton, New Jersey, and chairman of the General Advisory Committee of the new United States Atomic Energy Commission (AEC). He lobbied for international control of nuclear power and weapons in order to avert an arms race with the Soviet Union, and later opposed the development of the hydrogen bomb, partly on ethical grounds. During the Second Red Scare, his stances, together with his past associations with the Communist Party USA, led to an AEC security hearing in 1954 and the revocation of his security clearance. He continued to lecture, write, and work in physics, and in 1963 received the Enrico Fermi Award for contributions to theoretical physics. The 1954 decision was vacated in 2022.
Early life
Childhood and education
Oppenheimer was born Julius Robert Oppenheimer into a non-observant Jewish family in New York City on April 22, 1904, to Ella (née Friedman), a painter, and Julius Seligmann Oppenheimer, a successful textile importer. Robert had a younger brother, Frank, who also became a physicist. His father was born in Hanau, when it was still part of the Hesse-Nassau province of the Kingdom of Prussia, and as a teenager made his way to the United States in 1888, without money, higher education, or English proficiency. He was hired by a textile company and within a decade was an executive there, eventually becoming wealthy. In 1912, the family moved to an apartment on Riverside Drive near West 88th Street, Upper West Side, New York. Their art collection included works by Pablo Picasso, Édouard Vuillard, and Vincent van Gogh. Oppenheimer was initially educated at Alcuin Preparatory School. In 1911, he entered the Ethical Culture Society School, founded by Felix Adler to promote training based on the Ethical movement, whose motto was "Deed before Creed". Oppenheimer's father had been a member of the Society for many years, serving on its board of trustees. Oppenheimer was a versatile student, interested in English and French literature, and particularly mineralogy. He completed third and fourth grades in one year and skipped half of eighth grade. He took private music lessons by famous French flutist Georges Barrère. During his final year of school, Oppenheimer became interested in chemistry. He graduated in 1921, but his further education was delayed a year by an attack of colitis contracted while prospecting in Jáchymov during a family vacation in Czechoslovakia. He recovered in New Mexico, where he developed a love for horseback riding and the southwestern United States. Oppenheimer entered Harvard College in 1922 at age 18. He majored in chemistry; Harvard also required studies in history, literature, and philosophy or mathematics. To compensate for the delay caused by his illness, he took six courses each term instead of the usual four. He was admitted to the undergraduate honor society Phi Beta Kappa and was granted graduate standing in physics on the basis of independent study, allowing him to bypass basic courses in favor of advanced ones. He was attracted to experimental physics by a course on thermodynamics taught by Percy Bridgman. Oppenheimer graduated from Harvard in 1925 with a Bachelor of Arts, summa cum laude, after only three years of study.
Studies in Europe
After being accepted at Christ's College, Cambridge, in 1924, Oppenheimer wrote to Ernest Rutherford requesting permission to work at the Cavendish Laboratory, though Bridgman's letter of recommendation said that Oppenheimer's clumsiness in the laboratory suggested that theoretical, rather than experimental, physics would be his forte. Rutherford was unimpressed, but Oppenheimer went to Cambridge nonetheless; J. J. Thomson ultimately accepted him on the condition that he complete a basic laboratory course. Oppenheimer was very unhappy at Cambridge and wrote to a friend: "I am having a pretty bad time. The lab work is a terrible bore, and I am so bad at it that it is impossible to feel that I am learning anything." He developed an antagonistic relationship with his tutor, Patrick Blackett, a future Nobel laureate. According to Oppenheimer's friend Francis Fergusson, Oppenheimer once confessed to leaving a poisoned apple on Blackett's desk, and Oppenheimer's parents had to convince the university authorities not to expel him. There are no records of either a poisoning incident or probation, though Oppenheimer had regular sessions with a psychiatrist in Harley Street, London: According to Charles Oppenheimer, his grandson, the poison apple story is unsubstantiated and American Prometheus conceded that it was unproven. Oppenheimer was a tall, thin chain smoker, who often neglected to eat during periods of intense concentration. Many friends said he could be self-destructive. Fergusson once tried to distract Oppenheimer from apparent depression by telling him about his girlfriend, Frances Keeley, and how he had proposed to her. Oppenheimer jumped on Fergusson and tried to strangle him. Oppenheimer was plagued by periods of depression throughout his life, and once told his brother, "I need physics more than friends." In 1926, Oppenheimer left Cambridge for the University of Göttingen to study under Max Born; Göttingen was one of the world's leading centers for theoretical physics. Oppenheimer made friends who went on to great success, including Werner Heisenberg, Pascual Jordan, Wolfgang Pauli, Paul Dirac, Enrico Fermi and Edward Teller. He was enthusiastic in discussions to the point of sometimes taking them over. Maria Goeppert presented Born with a petition signed by herself and others threatening a boycott of the class unless he made Oppenheimer quiet down. Born left it out on his desk where Oppenheimer could read it, and it was effective without a word being said. Oppenheimer obtained his Doctor of Philosophy degree in March 1927 at age 23, supervised by Born. After the oral exam, James Franck, the professor administering it, reportedly said, "I'm glad that's over. He was on the point of questioning me." Oppenheimer published more than a dozen papers while in Europe, including many important contributions to the new field of quantum mechanics. He and Born published a famous paper on the Born–Oppenheimer approximation, which separates nuclear motion from electronic motion in the mathematical treatment of molecules, allowing nuclear motion to be neglected to simplify calculations. It remains his most cited work.
Early career
Teaching
Oppenheimer was awarded a United States National Research Council fellowship to the California Institute of Technology (Caltech) in September 1927. Bridgman also wanted him at Harvard, so a compromise was reached whereby he split his fellowship for the 1927–28 academic year between Harvard in 1927 and Caltech in 1928. At Caltech, he struck up a close friendship with Linus Pauling; they planned to mount a joint attack on the nature of the chemical bond, a field in which Pauling was a pioneer, with Oppenheimer supplying the mathematics and Pauling interpreting the results. The collaboration, and their friendship, ended after Oppenheimer invited Pauling's wife, Ava Helen Pauling, to join him on a tryst in Mexico. Oppenheimer later invited Pauling to be head of the Chemistry Division of the Manhattan Project, but Pauling refused, saying he was a pacifist. In the autumn of 1928, Oppenheimer visited Paul Ehrenfest's institute at the University of Leiden in the Netherlands, where he impressed by giving lectures in Dutch, despite having little experience with the language. There, he was given the nickname of Opje, later anglicized by his students as "Oppie". From Leiden, he continued on to the Swiss Federal Institute of Technology in Zurich to work with Wolfgang Pauli on quantum mechanics and the continuous spectrum. Oppenheimer respected and liked Pauli and may have emulated his personal style as well as his critical approach to problems. On returning to the United States, Oppenheimer accepted an associate professorship from the University of California, Berkeley, where Raymond Thayer Birge wanted him so badly that he expressed a willingness to share him with Caltech. Before he began his Berkeley professorship, Oppenheimer was diagnosed with a mild case of tuberculosis and spent some weeks with his brother Frank at a New Mexico ranch, which he leased and eventually purchased. When he heard the ranch was available for lease, he exclaimed, "Hot dog!", and he later called it Perro Caliente ("hot dog" in Spanish). Later, he used to say that "physics and desert country" were his "two great loves". He recovered from tuberculosis and returned to Berkeley, where he prospered as an advisor and collaborator to a generation of physicists who admired him for his intellectual virtuosity and broad interests. His students and colleagues saw him as mesmerizing: hypnotic in private interaction, but often frigid in more public settings. His associates fell into two camps: one saw him as an aloof and impressive genius and aesthete, the other as a pretentious and insecure poseur. His students almost always fell into the former category, adopting his walk, speech, and other mannerisms, and even his inclination for reading entire texts in their original languages. Hans Bethe said of him:
Probably the most important ingredient he brought to his teaching was his exquisite taste. He always knew what were the important problems, as shown by his choice of subjects. He truly lived with those problems, struggling for a solution, and he communicated his concern to the group. In its heyday, there were about eight or ten graduate students in his group and about six Post-doctoral Fellows. He met this group once a day in his office and discussed with one after another the status of the student's research problem. He was interested in everything, and in one afternoon they might discuss quantum electrodynamics, cosmic rays, electron pair production and nuclear physics. Oppenheimer worked closely with Nobel Prize-winning experimental physicist Ernest Lawrence and his cyclotron pioneers, helping them understand the data that their machines were producing at Berkeley's Radiation Laboratory, which eventually developed into today's Lawrence Berkeley National Laboratory. In 1936, Berkeley promoted him to full professor at an annual salary of $3,300 (equivalent to $77,000 in 2025). In return, he was asked to curtail his teaching at Caltech, so a compromise was reached whereby Berkeley released him for six weeks each year, enough to teach one term at Caltech. Oppenheimer repeatedly attempted to get Robert Serber a position at Berkeley but was blocked by Birge, who felt that "one Jew in the department was enough".
Scientific work Oppenheimer did important research in astrophysics (especially as related to general relativity and nuclear theory), nuclear physics, spectroscopy, and quantum field theory, including its extension into quantum electrodynamics. His most significant work involved predictions about neutron stars which were not observed until 1967. Initially, his major interest was the theory of the continuous spectrum. His first published paper, in 1926, concerned the quantum theory of molecular band spectra. He developed a method to carry out calculations of its transition probabilities. He calculated the photoelectric effect for hydrogen and X-rays, obtaining the absorption coefficient at the K-edge. His calculations accorded with observations of the X-ray absorption of the Sun, but not helium. Years later, it was realized that the Sun was largely composed of hydrogen and that his calculations were correct. Oppenheimer made important contributions to the theory of cosmic ray showers. He also worked on the problem of field electron emission. This work contributed to the development of the concept of quantum tunneling. In 1931, he co-wrote a paper, "Relativistic Theory of the Photoelectric Effect," with his student Harvey Hall, in which, based on empirical evidence, he correctly disputed Paul Dirac's assertion that two of the energy levels of the hydrogen atom have the same energy. Subsequently, one of his doctoral students, Willis Lamb, determined that this was a consequence of what became known as the Lamb shift, for which Lamb was awarded the Nobel Prize in Physics in 1955. With Melba Phillips, the first graduate student to begin her PhD under Oppenheimer's supervision, Oppenheimer worked on calculations of artificial radioactivity under bombardment by deuterons. When Ernest Lawrence and Edwin McMillan bombarded nuclei with deuterons they found the results agreed closely with the predictions of George Gamow, but when higher energies and heavier nuclei were involved, the results did not conform to the predictions. In 1935, Oppenheimer and Phillips worked out a theory—subsequently known as the Oppenheimer–Phillips process—to explain the results. This theory is still in use today. As early as 1930, Oppenheimer wrote a paper that essentially predicted the existence of the positron. This was after a paper by Dirac proposed that electrons could have both a positive charge and negative energy. Dirac's paper introduced an equation, later known as the Dirac equation, that unified quantum mechanics, special relativity and the then-new concept of electron spin, to explain the Zeeman effect. Drawing on the body of experimental evidence, Oppenheimer rejected the idea that the predicted positively charged electrons were protons. He argued that they would have to have the same mass as an electron, whereas experiments showed that protons were much heavier than electrons. Two years later, Carl David Anderson discovered the positron, for which he received the 1936 Nobel Prize in Physics. In the late 1930s, Oppenheimer became interested in astrophysics, most likely through his friendship with Richard Tolman, resulting in a series of papers. In the first of these, "On the Stability of Stellar Neutron Cores" (1938), co-written with Serber, Oppenheimer explored the properties of white dwarfs. This was followed by a paper co-written with one of his students, George Volkoff, "On Massive Neutron Cores," which demonstrated that there was a limit, known as the Tolman–Oppenheimer–Volkoff limit, to the mass of stars beyond which they would not remain stable as neutron stars and would undergo gravitational collapse. In 1939, Oppenheimer and another of his students, Hartland Snyder, produced the paper "On Continued Gravitational Contraction", which predicted the existence of what later became termed black holes. After the Born–Oppenheimer approximation paper, these papers remain his most cited, and were key factors in the rejuvenation of astrophysical research in the United States in the 1950s, mainly by John A. Wheeler. Oppenheimer's papers were considered difficult to understand even by the standards of the abstract topics he was expert in. He was fond of using elegant, if extremely complex, mathematical techniques to demonstrate physical principles, though he was sometimes criticized for making mathematical mistakes, presumably out of haste. "His physics was good", said his student Snyder, "but his arithmetic awful." After World War II, Oppenheimer published only five scientific papers, one of them in biophysics, and none after 1950. Murray Gell-Mann, a later Nobelist who, as a visiting scientist, worked with him at the Institute for Advanced Study in 1951, offered this opinion:
He didn't have Sitzfleisch, "sitting flesh," when you sit on a chair. As far as I know, he never wrote a long paper or did a long calculation, anything of that kind. He didn't have patience for that; his own work consisted of little aperçus, but quite brilliant ones. But he inspired other people to do things, and his influence was fantastic.
Private and political life Oppenheimer's mother died in 1931, after which he grew closer to his father, who, though still residing in New York, became a frequent visitor to California. When his father died in 1937, leaving $392,602 (equivalent to $8.6 million in 2024) to be divided between Oppenheimer and his brother Frank, Oppenheimer promptly wrote a will bequeathing his estate to the University of California to fund graduate scholarships.
Politics During the 1920s, Oppenheimer remained uninformed about world affairs. He claimed that he did not read newspapers or popular magazines and only learned of the Wall Street crash of 1929 while he was on a walk with Ernest Lawrence six months after the crash occurred. He once remarked that he never cast a vote until the 1936 presidential election. From 1934 on, he became increasingly concerned about politics and international affairs. In 1934, he earmarked three percent of his annual salary—about $100 (equivalent to $2,400 in 2025)—for two years to support German physicists fleeing Nazi Germany. During the 1934 West Coast Waterfront Strike, he and some of his students, including Melba Phillips and Robert Serber, attended a longshoremen's rally. After the Spanish Civil War broke out in 1936, Oppenheimer hosted fundraisers for the Spanish Republican cause. In 1939, he joined the American Committee for Democracy and Intellectual Freedom, which campaigned against the persecution of Jewish scientists in Nazi Germany. Like most liberal groups of the era, the committee was later branded a communist front. Many of Oppenheimer's closest associates were active in the Communist Party in the 1930s or 1940s, including his brother Frank, Frank's wife Jackie, Kitty, Jean Tatlock, his landlady Mary Ellen Washburn, and several of his graduate students at Berkeley. Whether Oppenheimer was a party member has been debated. Cassidy states that he never openly joined the Communist Party USA (CPUSA), but Haynes, Klehr, and Vassiliev state that he "was, in fact, a concealed member of the CPUSA in the late 1930s". From 1937 to 1942, Oppenheimer was a member at Berkeley of what he called a "discussion group", which fellow members Haakon Chevalier and Gordon Griffiths later said was a "closed" (secret) unit of the Communist Party for Berkeley faculty. The Federal Bureau of Investigation (FBI) opened a file on Oppenheimer in March 1941. It recorded that he attended a meeting in December 1940 at Chevalier's home that was also attended by the Communist Party's California state secretary, William Schneiderman, and its treasurer, Isaac Folkoff. The FBI noted that Oppenheimer was on the executive committee of the American Civil Liberties Union, which it considered a communist front organization. Shortly thereafter, the FBI added Oppenheimer to its Custodial Detention Index, for arrest in case of national emergency. When he joined the Manhattan Project in 1942, Oppenheimer wrote on his personal security questionnaire that he had been "a member of just about every Communist Front organization on the West Coast." Years later, he claimed that he did not remember writing this, that it was not true, and that if he had written anything along those lines, it was "a half-jocular overstatement". He was a subscriber to the People's World, a Communist Party organ, and testified in 1954, "I was associated with the communist movement." In 1953, Oppenheimer was on the sponsoring committee for a conference on "Science and Freedom" organized by the Congress for Cultural Freedom, an anti-communist cultural organization. At his 1954 security clearance hearings, Oppenheimer denied being a member of the Communist Party but identified himself as a fellow traveler, which he defined as someone who agrees with many of communism's goals but is not willing to blindly follow orders from any Communist Party apparatus. According to biographer Ray Monk: "He was, in a very practical and real sense, a supporter of the Communist Party. Moreover, in terms of the time, effort and money spent on party activities, he was a very committed supporter."
Relationships and children In 1936, Oppenheimer became involved with Jean Tatlock, the daughter of a Berkeley literature professor and a student at Stanford University School of Medicine. The two had similar political views; she wrote for the Western Worker, a Communist Party newspaper. In 1939, after a tempestuous relationship, Tatlock broke up with Oppenheimer. In August of that year, he met Katherine ("Kitty") Puening, a former Communist Party member. Kitty's first marriage had lasted only a few months. Her second, common-law, husband from 1934 to 1937 was Joe Dallet, an active member of the Communist Party killed in 1937 in the Spanish Civil War. Kitty returned from Europe to the U.S., where she obtained a Bachelor of Arts degree in botany from the University of Pennsylvania. In 1938 she married Richard Harrison, a physician and medical researcher, and in June 1939 moved with him to Pasadena, California, where he became chief of radiology at a local hospital and she enrolled as a graduate student at the University of California, Los Angeles. She and Oppenheimer created a minor scandal by sleeping together after one of Tolman's parties, and in the summer of 1940 she stayed with Oppenheimer at his ranch in New Mexico. When she became pregnant, Kitty asked Harrison for a divorce and he agreed to it. On November 1, 1940, she obtained a quick divorce in Reno, Nevada, and married Oppenheimer. Their first child, Peter, was born in May 1941, and their second, Katherine ("Toni"), was born in Los Alamos, New Mexico, on December 7, 1944. During his marriage, Oppenheimer rekindled his affair with Tatlock. Later, their continued contact became an issue in his security clearance hearings because of Tatlock's communist associations. Throughout the development of the atomic bomb, Oppenheimer was under investigation by both the FBI and the Manhattan Project's internal security arm for his past left-wing associations. He was followed by Army security agents during a trip to California in June 1943 to visit Tatlock, who was suffering from depression. Oppenheimer spent the night in her apartment. Tatlock killed herself on January 4, 1944, leaving Oppenheimer deeply grieved. At Los Alamos, Oppenheimer began an emotional affair with Ruth Tolman, a psychologist and the wife of his friend Richard Tolman. The affair ended after Oppenheimer returned east to become director of the Institute for Advanced Study but, after Richard's death in August 1948, they reconnected and saw each other occasionally until Ruth's death in 1957. Few of their letters survive, but those that do reflect a close and affectionate relationship, with Oppenheimer calling her "My Love".
Mysticism
Oppenheimer's diverse interests sometimes interrupted his focus on science. He liked things that were difficult and since much of the scientific work appeared easy for him, he developed an interest in the mystical and the cryptic. After going to Harvard, he began to acquaint himself with the classical Hindu texts through their English translations. He also had an interest in learning languages and learned Sanskrit, under Arthur W. Ryder at Berkeley in 1933. He eventually read literary works such as the Bhagavad Gita and Meghaduta in the original Sanskrit, and deeply pondered them. He later cited the Gita as one of the books that most shaped his philosophy of life. He wrote to his brother that the Gita was "very easy and quite marvelous". He later called it "the most beautiful philosophical song existing in any known tongue", and gave copies of it as presents to his friends and kept a personal, worn-out copy on the bookshelf by his desk. He kept referring to it while directing the Los Alamos Laboratory, and quoted a passage from the Gita at the memorial service of President Franklin Roosevelt in Los Alamos. He nicknamed his car Garuda, the mount bird of the Hindu god Vishnu. Oppenheimer never became a Hindu in the traditional sense; he did not join any temple nor pray to any god. He "was really taken by the charm and the general wisdom of the Bhagavad-Gita," his brother said. It is speculated that Oppenheimer's interest in Hindu thought started during his earlier association with Niels Bohr. Both Bohr and Oppenheimer had been very analytical and critical about the ancient Hindu mythological stories and the metaphysics embedded in them. In one conversation with David Hawkins before the war, while talking about the literature of ancient Greece, Oppenheimer remarked, "I have read the Greeks; I find the Hindus deeper." Oppenheimer sat on the Board of Editors of the book series World Perspectives, which published a variety of books on philosophy. During the 1930s, while teaching at Berkeley, Oppenheimer became part of a group in the Bay Area that psychologist Siegfried Bernfeld convened to discuss psychoanalysis. His close confidant and colleague Isidor Isaac Rabi, who had seen Oppenheimer throughout his Berkeley, Los Alamos, and Princeton years, wondering "why men of Oppenheimer's gifts do not discover everything worth discovering", reflected that:
Oppenheimer was overeducated in those fields which lie outside the scientific tradition, such as his interest in religion, in the Hindu religion in particular, which resulted in a feeling for the mystery of the universe that surrounded him almost like a fog. He saw physics clearly, looking toward what had already been done, but at the border he tended to feel there was much more of the mysterious and novel than there actually was ... [he turned] away from the hard, crude methods of theoretical physics into a mystical realm of broad intuition.... In Oppenheimer the element of earthiness was feeble. Yet it was essentially this spiritual quality, this refinement as expressed in speech and manner, that was the basis of his charisma. He never expressed himself completely. He always left a feeling that there were depths of sensibility and insight not yet revealed. These may be the qualities of the born leader who seems to have reserves of uncommitted strength. In spite of this, observers such as physicists Luis Alvarez and Jeremy Bernstein have suggested that if Oppenheimer had lived long enough to see his predictions substantiated by experiment, he might have won a Nobel Prize for his work on gravitational collapse, concerning neutron stars and black holes. In retrospect, some physicists and historians consider this his most important contribution, though it was not taken up by other scientists in his lifetime. The physicist and historian Abraham Pais once asked Oppenheimer what he considered his most important scientific contributions—Oppenheimer cited his work on electrons and positrons, not his work on gravitational contraction. Oppenheimer was nominated for the Nobel Prize in Physics four times, in 1946, 1951, 1955, and 1967, but never won.
Manhattan Project
Los Alamos
During his visit to University of California, Berkeley in September 1941, Australian physicist Mark Oliphant briefed Oppenheimer about the U.K.'s atomic bomb program and its MAUD report. On October 9, 1941, two months before the United States entered World War II, President Franklin D. Roosevelt approved a crash program to develop an atomic bomb. On October 21, Ernest Lawrence brought Oppenheimer into what became the Manhattan Project. Oppenheimer was assigned to take over the project's specific bomb-design research by Arthur Compton at the Metallurgical Laboratory. On May 18, 1942, Gregory Breit resigned due to security concerns and skepticism towards the project. Not long after that, Arthur Compton asked Oppenheimer to take over work on fast neutron calculations from Breit, a task Oppenheimer threw himself into with full vigor. He was given the title "Coordinator of Rapid Rupture"; "rapid rupture" is a technical term that refers to the propagation of a fast neutron chain reaction in an atomic bomb. One of his first acts was to host a summer school for atomic bomb theory in Berkeley. The mix of European physicists and his own students—a group including Serber, Emil Konopinski, Felix Bloch, Hans Bethe, and Edward Teller—kept themselves busy by calculating what needed to be done, and in what order, to make the bomb.
In June 1942, the U.S. Army established the Manhattan Engineer District to handle its part in the atom bomb project, beginning the process of transferring responsibility from the Office of Scientific Research and Development to the military. In September, Brigadier General Leslie R. Groves Jr., was appointed director of what became known as the Manhattan Project. By October 12, 1942, Groves and Oppenheimer had decided that for security and cohesion, they needed to establish a centralized, secret research laboratory in a remote location. Groves selected Oppenheimer to head the project's secret weapons laboratory, although it is not known precisely when. On October 15, 1942, after a meeting in Chicago on the Manhattan Project Groves invited Oppenheimer to join himself, James C. Marshall and Kenneth Nichols on their return trip to New York on the 20th Century Limited. After dinner on the train, they discussed the project. After Oppenheimer left the train, none of the three could name another suitable scientist to head the project. Shortly afterwards Oppenheimer was appointed to head the Los Alamos Laboratory. This decision surprised many, because Oppenheimer had left-wing political views and no record as a leader of large projects. Groves worried that because Oppenheimer did not have a Nobel Prize, he might not have had the prestige to direct fellow scientists, but Groves was impressed by Oppenheimer's singular grasp of the practical aspects of the project and by the breadth of his knowledge. As a military engineer, Groves knew that this would be vital in an interdisciplinary project that would involve not just physics but also chemistry, metallurgy, ordnance, and engineering. Groves also detected in Oppenheimer something that many others did not, an "overweening ambition", which Groves reckoned would supply the drive necessary to push the project to a successful conclusion. Oppenheimer's past associations were not overlooked, but on July 20, 1943, Groves directed that he receive a security clearance "without delay irrespective of the information which you have concerning Mr Oppenheimer. He is absolutely essential to the project." Rabi considered Oppenheimer's appointment "a real stroke of genius on the part of General Groves, who was not generally considered to be a genius". Oppenheimer favored a location for the laboratory in New Mexico, not far from his ranch. On November 16, 1942, he, Groves and others toured a prospective site. Oppenheimer feared that the high cliffs surrounding it would feel claustrophobic, and there was concern about possible flooding. He then suggested a site he knew well: a flat mesa near Santa Fe, New Mexico, which was the site of a private boys' school, the Los Alamos Ranch School. The engineers were concerned about the poor access road and the water supply but otherwise felt that it was ideal. The Los Alamos Laboratory was built on the site of the school, taking over some of its buildings, while many new buildings were erected in great haste. At the laboratory, Oppenheimer assembled a group of the top physicists of the time, whom he called the "luminaries". Los Alamos was initially supposed to be a military laboratory, and Oppenheimer and other researchers were to be commissioned into the Army. He went so far as to order himself a lieutenant colonel's uniform and take the Army physical test, which he failed. Army doctors considered him underweight at 128 pounds (58 kg), diagnosed his chronic cough as tuberculosis, and were concerned about his chronic lumbosacral joint pain. The plan to commission scientists fell through when Rabi and Robert Bacher balked at the idea. James B. Conant, Groves, and Oppenheimer devised a compromise whereby the University of California operated the laboratory under contract to the War Department. It soon turned out that Oppenheimer had hugely underestimated the magnitude of the project: Los Alamos grew from a few hundred people in 1943 to over 6,000 in 1945. Scientists were paid at the salary they were already receiving. However this meant that Oppenheimer, who had been paid by a state university, originally received much less than some of his subordinates. Groves decided to make an exception and increased his salary to be equal to that of the others (without consulting him). Oppenheimer at first had difficulty with the organizational division of large groups but rapidly learned the art of large-scale administration after he took up permanent residence at Los Alamos. He was noted for his mastery of all scientific aspects of the project and for his efforts to control the inevitable cultural conflicts between scientists and the military. Victor Weisskopf wrote:
Oppenheimer directed these studies, theoretical and experimental, in the real sense of the words. Here his uncanny speed in grasping the main points of any subject was a decisive factor; he could acquaint himself with the essential details of every part of the work. He did not direct from the head office. He was intellectually and physically present at each decisive step. He was present in the laboratory or in the seminar rooms, when a new effect was measured, when a new idea was conceived. It was not that he contributed so many ideas or suggestions; he did so sometimes, but his main influence came from something else. It was his continuous and intense presence, which produced a sense of direct participation in all of us; it created that unique atmosphere of enthusiasm and challenge that pervaded the place throughout its time.
Bomb design
At this point in the war, there was considerable anxiety among the scientists that the German nuclear weapons program might be progressing faster than the Manhattan Project. In a letter dated May 25, 1943, Oppenheimer responded to a proposal by Fermi to use radioactive materials to poison German food supplies. Oppenheimer asked Fermi whether he could produce enough strontium without letting too many in on the secret. Oppenheimer continued, "I think we should not attempt a plan unless we can poison food sufficient to kill a half a million men." In 1943, development efforts were directed to a plutonium gun-type fission weapon called "Thin Man". Initial research on the properties of plutonium was done using cyclotron-generated plutonium-239, which was extremely pure but could be created only in tiny amounts. When Los Alamos received the first sample of plutonium from the X-10 Graphite Reactor in April 1944, a problem was discovered: reactor-bred plutonium had a higher concentration of plutonium-240 (five times that of "cyclotron" plutonium). Plutonium-240 decays rapidly, creating many neutrons which could detonate a chain reaction prematurely, making it unsuitable for use in a gun-type weapon. In July 1944, Oppenheimer abandoned the Thin Man gun design in favor of an implosion-type weapon; a smaller version of Thin Man became Little Boy. Using chemical explosive lenses, a sub-critical sphere of fissile material could be squeezed into a smaller and denser form. The metal needed to travel only very short distances, so the critical mass would be assembled in much less time. In August 1944, Oppenheimer implemented a sweeping reorganization of the Los Alamos Laboratory to focus on implosion. He concentrated the development efforts on the gun-type device, but now with a simpler design that only had to work with highly enriched uranium, in a single group. This device became Little Boy in February 1945. After a mammoth research effort, the more complex design of the implosion device, known as the "Christy gadget" after Robert Christy, another student of Oppenheimer's, was finalized as Fat Man in a meeting in Oppenheimer's office on February 28, 1945. In May 1945, an Interim Committee was created to advise and report on wartime and postwar policies regarding the use of nuclear energy. The Interim Committee established a scientific panel consisting of Oppenheimer, Arthur Compton, Fermi, and Lawrence to advise it on scientific issues. In its presentation to the Interim Committee, the panel offered its opinion not just on an atomic bomb's likely physical effects but also on its likely military and political impact. This included opinions on such sensitive issues as whether the Soviet Union should be advised of the weapon in advance of its use against Japan.
Trinity
In the early morning hours of July 16, 1945, near Alamogordo, New Mexico, the work at Los Alamos culminated in the test of the world's first nuclear weapon. Oppenheimer had code-named the site "Trinity" in mid-1944, saying later that the name came from John Donne's Holy Sonnets; he had been introduced to Donne's work in the 1930s by Jean Tatlock, who killed herself in January 1944. Brigadier General Thomas Farrell, who was present in the control bunker with Oppenheimer, recalled:
Dr. Oppenheimer, on whom had rested a very heavy burden, grew tenser as the last seconds ticked off. He scarcely breathed. He held on to a post to steady himself. For the last few seconds, he stared directly ahead and then when the announcer shouted "Now!" and there came this tremendous burst of light followed shortly thereafter by the deep growling roar of the explosion, his face relaxed into an expression of tremendous relief. Oppenheimer's brother Frank recalled Oppenheimer's first words as "I guess it worked."
According to a 1949 magazine profile, while witnessing the explosion Oppenheimer thought of verses from the Bhagavad Gita: "If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendor of the mighty one ... Now I am become Death, the shatterer of worlds." In 1965 he recalled the moment this way:
We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his mult