Julius Robert Oppenheimer (April 22, 1904 – February 18, 1967) was an American theoretical physicist and professor of physics at the University of California, Berkeley. He is best known for his role as the scientific director of the Manhattan Project, the World War II project that developed the first nuclear weapons, for which he is often referred to as the "father of the atomic bomb". In reference to the Trinity test in New Mexico, where the first atomic bomb was detonated, Oppenheimer famously recalled 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." and "Now I am become Death, the destroyer of worlds."

After the war Oppenheimer was a chief advisor to the newly created United States Atomic Energy Commission and used that position to lobby for international control of nuclear power and to avert the nuclear arms race with the Soviet Union. After provoking the ire of many politicians with his outspoken political opinions during the Red Scare, he had his security clearance revoked in a much publicized and politicized hearing in 1954. Though stripped of his direct political influence Oppenheimer continued to lecture, write, and work in physics. A decade later President John F. Kennedy awarded (and Lyndon B. Johnson presented) Oppenheimer the Enrico Fermi Award as a gesture of political rehabilitation.

Oppenheimer's notable achievements in physics include the Born – Oppenheimer approximation, work on electron – positron theory, the Oppenheimer – Phillips process, and the first prediction of quantum tunneling. With his students he also made important contributions to the modern theory of neutron stars and black holes, as well as work on the theory of quantum mechanics, quantum field theory, and the interactions of cosmic rays. As a teacher and promoter of science, Oppenheimer is remembered most for being the chief founder of the American school of theoretical physics while at the University of California, Berkeley, contributing significantly to the rise of American physics to its first era of world prominence in the 1930s. After the second World War, he contributed to American scientific organizations again, as director of the Institute for Advanced Study in Princeton, where he held Einstein's old position of Senior Professor of Theoretical Physics.

J. Robert Oppenheimer was born on April 22, 1904 to Julius S. Oppenheimer, a wealthy Jewish textile importer who had immigrated to the United States from Germany in 1888, and Ella Friedman, a painter. In 1912 the family moved to an apartment on the eleventh floor of 155 Riverside Drive, near West 88th Street. The family's art collection included works by Pablo Picasso and Édouard Vuillard, and at least three original paintings by Vincent van Gogh. He had a younger brother, Frank Oppenheimer, who also became a physicist. Oppenheimer was initially schooled at Alcuin Preparatory School. In 1911 he entered the Ethical Culture Society School. It had been founded by Felix Adler to promote a form of ethical training based on the Ethical Culture movement, whose motto was "Deed before Creed". Julius Oppenheimer had been a member of the Society for Ethical Culture for many years, serving on its board of trustees from 1907 to 1915. Oppenheimer was a versatile scholar, interested in the English and French literature and particularly in mineralogy. He completed the third grade and fourth grade in one year and later skipped half of the eighth grade. During his final year, he became interested in chemistry. He entered Harvard College a year late, at age 18, because he suffered an attack of colitis while prospecting in Joachimstal during a family summer vacation in Europe. To recover he went with Herbert W. Smith, a former English teacher, to New Mexico, where Oppenheimer fell in love with horseback riding and the Southwest United States.

In 1922, Oppenheimer entered Harvard where he studied chemistry. His lecturers included James B. Conant. In addition to an academic major ("concentration"), undergraduates had to study history, literature and philosophy or mathematics. Oppenheimer took a year of English history and two of French literature, and a course on the history of philosophy. He made up for his late start by taking six courses each term, and graduated summa cum laude in three years. He was admitted to Phi Beta Kappa. In his first year at Harvard, Oppenheimer was admitted to graduate standing in physics on the basis of independent study and was exempted from taking the basic classes. In a course on thermodynamics taught by Percy Bridgman Oppenheimer was attracted to experimental physics.

In 1924, Oppenheimer was informed that he had been accepted into Christ's College, Cambridge. He wrote to Ernest Rutherford requesting permission to work at the Cavendish Laboratory. Bridgman provided Oppenheimer with a recommendation, which conceded that Oppenheimer's clumsiness in the laboratory made it apparent his forte was not experimental but rather theoretical physics. Rutherford was unimpressed, but Oppenheimer went to Cambridge in the hope of landing another offer. He was ultimately accepted by J.J. Thomson on condition that he complete a basic laboratory course. Oppenheimer developed an antagonistic relationship with his tutor, Patrick Blackett, who was only a few years his senior. Oppenheimer once doused an apple with noxious chemicals and put it on Blackett's desk; Blackett did not eat the apple, but Oppenheimer was put on probation and ordered to go to London for regular sessions with a psychiatrist.

A tall, thin chain smoker who often neglected to eat during periods of intellectual discomfort and concentration, Oppenheimer was marked by many of his friends as having a self-destructive tendency and, during numerous periods of his life, worried his colleagues and associates with his melancholy and insecurity. When he was studying in Cambridge and had taken a vacation to meet up with his friend Francis Ferguson in Paris, a disturbing event had taken place. During a conversation in which Oppenheimer was narrating his frustration with experimental physics to Ferguson, he had suddenly leapt up and tried to strangle him. Although Ferguson easily fended off the attack, the episode had convinced Ferguson of his friend's deep psychological troubles.

In 1926 he 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 would go on to great success, including Werner Heisenberg, Pascual Jordan, Wolfgang Pauli, Paul Dirac, Enrico Fermi and Edward Teller. However, he was also known for being too enthusiastic in discussions, sometimes to the point of taking over seminar sessions. This irritated some of Born's other pupils so much that Maria Goeppert presented Born with a petition signed by herself and others threatening to boycott the class unless he made Oppenheimer be quiet. 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 the age of 23 at the University of Göttingen, supervised by Born. After the oral exam for his degree, James Franck, the professor administering, reportedly said, "I'm glad that's over. He was on the point of questioning me." Oppenheimer published more than a dozen papers at Göttingen, including many important contributions to the then newly developed 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 in order to simplify calculations. It remains his most cited work.

Oppenheimer applied for a National Research Council fellowship to the California Institute of Technology (Caltech). He was appointed in September 1927, receiving one of 123 postdoctoral fellowships in physics it awarded between 1919 and 1930. However Bridgman also wanted Oppenheimer at Harvard and a compromise was reached whereby Oppenheimer split his fellowship for the 1927 - 1928 academic year between Harvard in 1927 and Caltech in 1928.

At Caltech, Oppenheimer struck a close friendship with Linus Pauling and they planned to mount a joint attack on the nature of the chemical bond, a field in which Pauling was a pioneer — apparently Oppenheimer would supply the mathematics and Pauling would interpret the results. However, this collaboration, and their friendship, was nipped in the bud when Pauling began to suspect that the theorist was becoming too close to his wife, Ava Helen Pauling. Once when Pauling was at work, Oppenheimer had come to their place and blurted out an invitation to Ava Helen to join him on a tryst in Mexico. She flatly refused and reported this incident to Pauling. This, and her apparent nonchalance about the incident, disquieted him, and he immediately cut off his relationship with Oppenheimer. Oppenheimer later invited Pauling to be the head of the Chemistry Division of the Manhattan Project but Pauling refused, saying that he was a pacifist.

In the autumn of 1928, Oppenheimer visited Paul Ehrenfest's institute at the University of Leiden, the Netherlands, where he impressed those there by giving lectures in Dutch despite having little experience with the language. There he was given the nickname of "Opje", which was later Anglicized by his students as "Oppie". From Leiden he continued on to the ETH in Zurich to work with Wolfgang Pauli on problems relating to quantum theory and the continuous spectrum. Oppenheimer highly respected and liked Pauli, and some of his own style and his critical approach to problems was said to be inspired by Pauli.

On returning to the United States, Oppenheimer accepted an associate professorship from the University of California, Berkeley, where Raymond T. Birge wanted him badly enough to offer to share him with CalTech. Before his Berkeley professorship began, Oppenheimer was diagnosed with a mild case of tuberculosis and, with his brother Frank, spent some weeks at a ranch in New Mexico, which he leased and eventually purchased. When he heard the ranch was available for lease, he exclaimed, "Hot dog!" — and later on the name of the ranch became "Perro Caliente", which is a translation of the words "hot" and "dog" into Spanish. Later, Oppenheimer used to say that "physics and desert country" were his "two great loves", loves that would be combined when he directed the atomic bomb project at Los Alamos.

He recovered from his 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. Nobel Prize winner Hans Bethe later said about him:

Probably the most important ingredient Oppenheimer 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.

He also worked closely with Nobel Prize winning experimental physicist Ernest O. Lawrence and his cyclotron pioneers, helping the experimentalists understand the data their machines were producing at the Lawrence's Radiation Laboratory. In 1936, Berkeley promoted Oppenheimer to full professor, at a salary of $3,300 per annum. In return, Oppenheimer was asked to curtail his teaching at CalTech. A compromise was reached whereby he was released for six weeks each year, enough to reach one trimester there.

Oppenheimer developed numerous affectations, seemingly in an attempt to convince those around him — or possibly himself — of his self-worth. He was said to be mesmerizing, hypnotic in private interaction but often frigid in more public settings. His associates fell into two camps: one that saw him as an aloof and impressive genius and an aesthete; another that saw him as a pretentious and insecure poseur. His students almost always fell into the former category, adopting "Oppie's" affectations, from his way of walking to talking and beyond — even trying to replicate his inclination for reading entire texts in their original languages. He became known as a founding father of the American school of theoretical physics, and developed a reputation for his erudition in physics, his eclecticism, his quick mind, his interest in languages and Eastern philosophy, and the eloquence and clarity with which he thought. But he was also emotionally troubled throughout his life, and professed to experiencing periods of depression. "I need physics more than friends", he once informed his brother.

Oppenheimer did important research in theoretical astronomy (especially as it relates to general relativity and nuclear theory), nuclear physics, spectroscopy, and quantum field theory (including its extension into quantum electrodynamics). The formalism of relativistic quantum mechanics also attracted his attention, although because of the then existing well-known problem of the self-energy of the electron, he doubted the validity of quantum electrodynamics at high energies. His work predicted many later finds, which include the neutron, meson, and neutron star.

Oppenheimer made important contributions to the theory of cosmic ray showers and did work that eventually led toward descriptions of quantum tunneling, In 1930 he wrote a paper on the "Relativistic Theory of the Photoelectric Effect", in which, based on empirical evidence, he disagreed with Dirac's assertion that the states of the hydrogen atom must have identical energy states. Subsequently, one of his doctoral students, Willis Lamb, determined that this was a consequence of the Lamb shift, for which Lamb was awarded the Nobel Prize in Physics in 1955.

Oppenheimer worked with his first doctoral student, Melba Phillips, 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 theory. Oppenheimer and Phillips worked out a new theory to explain the results. This became known as the Oppenheimer - Phillips process, and is still in use today.

As early as 1930, Oppenheimer wrote a paper essentially predicting the existence of the positron. This arose from a paper by Paul Dirac published in 1928, that proposed that electrons could have both a positive charge and negative energy. This paper introduced the Dirac equation, a unification of quantum mechanics, special relativity, and the then new concept of electron spin to explain the Zeeman effect. Oppenheimer, drawing on the body of experimental evidence, rejected the idea of these being protons; he argued that they would have to have the same mass as an electron, but the opposite charge. Two years later, Carl David Anderson discovered the positron in 1932, for which he received the 1936 Nobel Prize in Physics.

In the late 1930s, Oppenheimer became interested in astrophysics, probably as a result of his friendship with Richard Tolman, resulting in a series of papers. In the first of these, a 1938 paper co-written with Robert Serber entitled "The Stability of Stellar Neutron Cores", he explored the properties of white dwarves. This was followed by a paper co-written with one of his students, George Volkoff, "On Massive Neutron Cores", in which they demonstrated that there was a size limit, the so called Tolman - Oppenheimer - Volkoff limit, to stars beyond which they would not remain stable as neutron stars, and would undergo gravitational collapse. Finally, in 1939, Oppenheimer and another of his students, Hartland Snyder, produced a paper "on Continued Gravitational Attraction", which predicted the existence of what we today call black holes. After the Born - Oppenheimer approximation paper, these papers remain his most cited, and they were key in the rejuvenation of astrophysical research in the United States in the 1950s, mainly by John Wheeler.

Even beyond the immense abstruseness of the topics he was expert in, Oppenheimer's papers were considered difficult to understand. Oppenheimer was very 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."

Many people thought that Oppenheimer's discoveries and research were not commensurate with his inherent abilities and talents. They still considered him an outstanding physicist, but they did not place him at the very top rank of theorists who fundamentally challenged the frontiers of knowledge. One reason for this could have been his diverse interests, which kept him from completely focusing on any individual topic for long enough to bring it to full fruition. In 1933, he learned Sanskrit and met the Indologist Arthur W. Ryder at Berkeley. He read the Bhagavad Gita in the original language and later he cited it as one of the most influential books that shaped his philosophy of life. His close confidant and colleague, Nobel Prize winner Isidor Rabi, later gave his own interpretation:

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 of mystery of the universe that surrounded him 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 spite of this, some people (such as the Nobel Prize winner physicist Luis Alvarez) have suggested that if he had lived long enough to see his predictions substantiated by experiment, Oppenheimer 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 to be his most important contribution, though it was not taken up by other scientists in his own lifetime. Interestingly, when the physicist and historian Abraham Pais once asked Oppenheimer about what he considered to be his most important scientific contributions, Oppenheimer cited his work on electrons and positrons, but did not mention anything about his work on gravitational contraction. Oppenheimer was nominated for the Nobel Prize three times, in 1945, 1951 and 1967, but never won.

During the 1920s, Oppenheimer kept aloof of worldly matters. He claimed that he did not read newspapers or listen to the radio, and only learned of the Stock Market Crash of 1929 some time after it occurred, and never cast a vote until the 1936 election. However, from 1934 on, he became increasingly concerned about politics and international affairs. In 1934 Oppenheimer earmarked 3 per cent of his salary - about $100 - to support German physicists fleeing from Nazi Germany. During the 1934 West Coast Waterfront Strike, Oppenheimer and some of his students, including Melba Phillips and Bob Server, attended a longshoremen's rally. Oppenheimer repeatedly attempted to get Serber a position at Berkeley but was blocked by Birge, who felt that one Jewish faculty member was enough.

In 1936, Oppenheimer became involved with Jean Tatlock, the daughter of a Berkeley literature professor, who was a student at Stanford University School of Medicine. Like many young intellectuals in the 1930s he became a supporter of social reforms which were later alleged to be communist ideas. Julius Oppenheimer died in 1937 leaving $392,602 to be divided between Robert and his brother Frank. Robert immediately wrote out a will leaving his estate to the University of California for graduate scholarships. He donated to many progressive efforts which were later branded as "left-wing" during the McCarthy era. The majority of his allegedly radical work consisted of hosting fund raisers for the Republican cause in the Spanish Civil War, and other anti-fascist activity. He never openly joined the Communist Party, though he did pass money to liberal causes by way of acquaintances who were alleged to be Party members.

Oppenheimer broke up with Tatlock in 1939. In August that year he met Katherine ("Kitty") Puening Harrison, a radical Berkeley student and former Communist Party member. Harrison had been married three times previously. Her first marriage, to a homosexual musician with drug addiction problems, lasted for only a few months. Her second husband, Joe Dallet, an active member of the Communist party, was killed in the Spanish Civil War. Kitty returned to the United States where she obtained a bachelor of arts degree in botany from the University of Pennsylvania. There she married her third husband, Richard Harrison, a physician and medical researcher in 1938. In June 1939 Kitty and Harrison moved 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. Kitty and Robert created a minor scandal by sleeping together after one of Tolman's parties. In the summer of 1940 she stayed with Robert at his ranch in New Mexico. She finally asked Harrison for a divorce when she found out she was pregnant. When he refused, she obtained an instant divorce in Reno, Nevada, and married Robert on 1 November 1940.

Their first child, Peter was born in May 1941. Oppenheimer joked that the 8-pound (4 kg) baby, born 7 months after their marriage, should be named "Pronto". Their second child, Katherine ("Toni"), was born in Los Alamos, New Mexico, on 7 December 1944. During his marriage, Oppenheimer continued his involvement with Jean Tatlock, and evidently their affair. Later their continued contact became an issue in Oppenheimer's security clearance hearings, due to Tatlock's Communist associations. Many of Oppenheimer's closest associates were active in the Communist Party in the '30s or '40s. They included his brother Frank, Frank's wife Jackie, Kitty, Jean Tatlock, his landlady, Mary Ellen Washburn, and several of his graduate students at Berkeley, including Joe Weinberg, Giovanni Rossi Lomanitz, David Bohm and Philip Morrison.

When he joined the Manhattan Project in 1942, Oppenheimer wrote on his personal security questionnaire that he [Oppenheimer] had been "a member of just about every Communist Front organization on the West Coast." On December 23, 1953, when the Atomic Energy Commission (AEC) was considering whether to revoke his security clearance, Oppenheimer wrote to AEC Chairman K.D. Nichols that he did not remember saying this, that it was not true, and that if he had said anything along those lines, it was "a half-jocular overstatement." On April 15, 1954, during Oppenheimer's AEC security hearing, Roger Robb, counsel for the AEC's Personnel Security Board, asked Oppenheimer, "If you said that to Colonel Lansdale, were you jocular?" Oppenheimer testified, "I don't think I could have been jocular during this interview." He was a subscriber to the People's World, a Communist Party organ, and he testified in 1954, "I was associated with the Communist movement." In 1937 – 42, in the midst of the Great Purge and Hitler - Stalin pact, Oppenheimer was a member at Berkeley of what he called a "discussion group", which was later identified by fellow members, Haakon Chevalier and Gordon Griffiths, as a "closed" (secret) unit of the Communist Party for Berkeley faculty. Oppenheimer was the only member of this group who was not a Communist.

The Federal Bureau of Investigation recorded that J. Robert Oppenheimer attended a meeting in the home of self proclaimed Communist Haakon Chevalier, that Communist Party's California state chairman William Schneiderman, and Isaac Folkoff, West Coast liaison between the Communist Party and Soviet intelligence, attended in Fall 1940, during the Hitler - Stalin pact. Shortly thereafter, the FBI added Oppenheimer to its Custodial Detention Index, for arrest in case of national emergency, and listed him as "Nationalistic Tendency: Communist." Debates over Oppenheimer's Party membership or lack thereof have turned on very fine points; almost all historians agree he had strong socialist sympathies during this time, and interacted with Party members, though there is considerable dispute over whether he was officially a member of the Party. Oppenheimer at his 1954 security clearance hearings 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 the goals of Communism, but without being willing to blindly follow orders from any Communist party apparatus.

On 9 October 1941, after World War II began, President Franklin D. Roosevelt approved a crash program to develop an atomic bomb. To control it, he created a Top Policy Group consisting of himself — although he never attended a meeting — Vice President Henry A. Wallace, Office of Scientific Research and Development (OSRD) Director Vannevar Bush, National Defense Research Committee (NDRC) Chairman James Conant, Secretary of War Henry L. Stimson and Chief of Staff of the Army General George C. Marshall. In May 1942 Conant invited Oppenheimer to take over work on fast neutron calculations, a task that he threw himself into with full vigor. Oppenheimer was given the title "Coordinator of Rapid Rupture", specifically referring to the propagation of a fast neutron chain reaction in an atomic bomb. One of Oppenheimer's first acts was to host a summer school for bomb theory at his building in Berkeley. The mix of European physicists and his own students — a group including Robert Serber, Emil Konopinski, Felix Bloch, Hans Bethe, and Edward Teller — busied themselves calculating what needed to be done, and in what order, to make the bomb. Teller put forward the remote possibility that the bomb would generate enough heat to ignite the atmosphere. Such an event was soon shown to be of near zero probability by Bethe.

In June 1942, the U.S. Army established the Manhattan Engineer District to handle its part in the project, beginning the process of transferring responsibility from the OSRD to the Army. In September, Brigadier General Leslie R. Groves, Jr. was appointed director of what became known as the Manhattan Project. Groves, in turn, selected Oppenheimer to head the project's secret weapons laboratory. Many were surprised at Groves' choice of Oppenheimer, as Oppenheimer was neither known to be politically aligned with the conservative military nor was he known to be an efficient leader of large projects. Groves was also concerned that Oppenheimer did not have a Nobel Prize and might not have the prestige to direct fellow scientists. However Groves was impressed by Oppenheimer's grasp of the practical aspects of designing and constructing an atomic bomb, and by the breadth of Oppenheimer's knowledge. As a military engineer, Groves knew that this would be vital in an interdisciplinary project that would involve not just physics, but chemistry, metallurgy, ordnance and engineering. These were things that Groves found lacking in other scientists. 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. Rabi considered the appointment "a real stroke of genius on the part of General Groves, who was not generally considered to be a genius..."

Oppenheimer and Groves decided that for security and cohesion they needed a centralized, secret research laboratory in a remote location. Scouting for a site in late 1942, Oppenheimer was drawn to New Mexico, not far from his ranch. On 16 November 1942, Oppenheimer, Groves, and others toured a prospective site. Oppenheimer feared that the high cliffs surrounding the site would make his people feel claustrophobic, while the engineers were concerned with the possibility of flooding. Oppenheimer suggested a site that he knew well; a flat mesa near Santa Fe, New Mexico, which was the site of a private boys' school called The Los Alamos Ranch School. Oppenheimer was impressed and expressed a strong preference for the site. 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 hastily built on the site of the school, taking over some of its buildings, but erecting many others in great haste. There Oppenheimer assembled a group of the top physicists of the time, which he referred to as the "luminaries."

Initially Los Alamos was supposed to be a military laboratory with Oppenheimer and other researchers commissioned into the Army. Oppenheimer went so far as to order himself a lieutenant colonel's uniform, and took 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. However, two key physicists, Robert Bacher and Isidor Rabi balked at the idea. Conant, Groves and Oppenheimer devised a compromise was whereby the laboratory was operated by the University of California under contract to the War Department. It soon turned out that Oppenheimer's initial estimates of the size of the effort required were extraordinarily over-optimistic. Los Alamos grew from a few hundred people in 1943 to over 6,000 in 1945.

Oppenheimer at first had difficulty with organizational division of large groups, but soon rapidly learned the art of large-scale administration after he took up permanent residence on the mesa. 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. He was an iconic figure to his fellow scientists, as much a figurehead of what they were working towards as a scientific director. Victor Weisskopf put it thus:

He did not direct from the head office. He was intellectually and even 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.

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 only be created 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, making it unsuitable for use in a gun-type weapon. In July 1944, Oppenheimer abandoned the gun design and in favor of an implosion type. 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.

All the while, Oppenheimer was under investigation by both the FBI and the Manhattan Project's internal security arm for his past left wing associations. He was also followed by Army security agents during an unannounced trip to California in 1943 to meet his former girlfriend, Jean Tatlock, where he spent the night in her apartment. Jean committed suicide on 4 January 1944.

In August 1943, Oppenheimer volunteered to Manhattan Project security agents that three men at Los Alamos National Laboratory had been solicited for nuclear secrets on behalf of the Soviet Union, by a person he did not know who worked for Shell Oil, and who had Communist connections. He gave that person's name: George Eltenton. However, when pressed on the issue in later interviews with Groves, who ordered him to give the names of these men and promised to keep their identity from the FBI, he finally identified the only contact who had approached him, as his friend Haakon Chevalier, a Berkeley professor of French literature who he said had mentioned the matter privately at a dinner at Oppenheimer's house. Oppenheimer would be asked again in 1947 for interviews related to the "Chevalier incident", and he gave contradictory and equivocating statements, telling government agents that actually only one scientist had been approached at Los Alamos, and that person was himself. This was by Chevalier, who at the time had supposedly said that he had a potential conduit through Eltenton for information which could be passed to the Soviets. Oppenheimer claimed to have invented the other contacts in order to conceal the identity of Chevalier, whose identity he believed would be immediately apparent if he named only one contact, but whom he believed to be innocent of any disloyalty.

Groves thought Oppenheimer was too important to the ultimate Allied goals to oust him over this suspicious behavior. On July 20, 1943, he wrote to the Manhattan Engineer District:

In accordance with my verbal directions of July 15, it is desired that clearance be issued to Julius Robert Oppenheimer without delay irrespective of of the information which you have concerning Mr Oppenheimer. He is absolutely essential to the project.

By 1947, Oppenheimer had told two conflicting versions of this story, both of which were taped without his knowledge, and importantly was taped in the latter interview admitting to have told a deliberate lie in the first.

The joint work of the scientists at Los Alamos resulted in the first artificial nuclear explosion near Alamogordo on July 16, 1945, the site of which Oppenheimer named "Trinity". Oppenheimer later said this name was from one of John Donne's Holy Sonnets. According to the historian Gregg Herken, this naming could have been an allusion to Jean Tatlock, who had committed suicide a few months previously and had in the 1930s introduced Oppenheimer to Donne's work. Oppenheimer later recalled that, while witnessing the explosion, he thought of a verse from the Hindu holy book, 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...

Years later he would explain that another verse had also entered his head at that time: namely, the famous verse; "kālo'smi lokakṣayakṛtpravṛddho lokānsamāhartumiha pravṛttaḥ", which Oppenheimer translated as "I am become Death, the destroyer of worlds." Oppenheimer later would be persuaded to quote again in 1965 for a television broadcast:

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 multi-armed form and says, 'Now I am become Death, the destroyer of worlds.' I suppose we all thought that, one way or another.'

According to his brother, at the time Oppenheimer simply exclaimed, "It worked." A contemporary account by Brigadier General Thomas Farrell, who was present in the control bunker at the site with Oppenheimer, summarized his reaction as follows:

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.