The timing of Oppenheimer’s arrival in Göttingen could not have been more ideal. Heisenberg’s breakthrough paper Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen (“Quantum theoretical re-interpretation of kinematic and mechanical relations”) laying the foundations for matrix mechanics had been published in 1925 and Schrödinger’s equation describing the wave function of quantum-mechanical systems had been introduced in January with the 1926 publication Quantisierung als Eigenwertproblem (“Quantization as an Eigenvalue Problem”). Arriving in September, Oppenheimer came to collaborate with Born only months before the third (Born’s) monumental paper on quantum mechanics would be published and a year before the 1927 Fifth Solvay International Conference on Electrons and Photons in which Born and Heisenberg would proclaim quantum mechanics to be “complete and irrevocable”.

Robert found accommodations in a villa owned by a Göttingen physician. Already fluent in German, he for the first time in his life came to feel a pleasant camaraderie with his fellow students and realized that he could actually learn from others, not just from books (Bird & Sherwin, 2005):

“Something which for me — more than most people — is important began to take place, namely, I began to have some conversations, […] something that I probably would not ever have gotten to if I’d been locked up in a room”

Although he still experienced bouts of anxiety and panic, once even fainting and falling to the floor in front of Paul Dirac, on the whole Robert thrived in Göttingen. By the time he arrived in the fall of 1926, his two first research papers had been published by the Cambridge Philosophical Society. The recognition that followed allowed him to engage himself enthusiastically in seminar discussions, often to the chagrin of other students. “He was a man of great talent […] and conscious of his superiority in a way that was embarrassing and led to trouble”.

Among his graduate students, Oppenheimer’s thesis supervisor Max Born was known to make small mistakes in his long calculations and so routinely asked them to recheck his math. When he once gave a set of calculations to Oppenheimer, Robert returned the work and said, apparently un-ironically, “I couldn’t find any mistake — did you really do this alone?” (Bird & Sherwin, 2005).

“Oppenheimer was the only [student] frank and rude enough to say it without joking. I was not offended; it actually increased my esteem for his remarkable personality” — Max Born

In the winter term of 1927, Paul Dirac arrived and found lodgings in the same villa as Robert. As Robert later stated “the most exciting time in my life was when Dirac arrived and gave me the proofs of his paper on the quantum theory of radiation”. Dirac, who was two years older had just completed his seminal paper on the principles of quantum mechanics. Oppenheimer later stated about Dirac that he was “was not easily understood, not concerned with being understood” adding “I thought he was absolutely grand”. Dirac, when later confronted with a statement from Robert that he saw more of him than anyone else in Göttingen, is to have replied with characteristic mercuriality that “That is so. We sometimes went for long walks together, although I had many walks alone” (Pais, 2006).

In the spring of 1927, Oppenheimer submitted his doctoral dissertation entitled Zur Quantentheorie kontinuierlicher Spektren (“On quantum theory of continuous spectra”), which Born recommended be accepted with distinction. The dissertation narrated a complicated calculation for the photoelectric effect in hydrogen and x-rays, later remarked by Nobel Laureate Hans Bethe as “even today still a complicated calculation, beyond the scope of most quantum mechanics textbooks”. Oppenheimer sat for his oral examination on May 11th of the same year. After the defense, the professor administering the session, 1925 Nobel Laureate James Franck reportedly emerged stating “I’m glad that’s over. He was at the point of questioning me.”

Research

In addition to his first two papers written in Cambridge, Oppenheimer would publish another seven papers on quantum physics while still at Göttingen, a incredible output for a 23-year-old graduate student (Bird & Sherwin, 2005). Impressed, Born on one occasion bragged to then-president of MIT Karl Taylor Compton about his student:

“We have here a number of Americans […] One man is quite excellent, Mr. Oppenheimer.”

Over the course of his career, Oppenheimer would publish 73 papers in total, ranging from topics in quantum field theory, particle physics, the theory of cosmic radiations, nuclear physics and cosmology. The following discoveries are amongst his most recognized:

The Born-Oppenheimer approximation (1927)

Prompted by a remark from Heisenberg, Robert in 1927 grew interested in using the new methods of quantum theory to explain, as he put it “why molecules were molecules” (Bird & Sherwin, 2005). Presenting his notes to Born, the professor agreed to collaborate on a paper explaining Oppenheimer’s new idea. In the paper, written while Robert was still in Göttingen, Born and Oppenheimer came to define what is now known as the Born-Oppenheimer approximation, in quantum chemistry and molecular physics the assumption that the motion of atomic nuclei and electrons in a molecule can be treated separately.

Born, M. & Oppenheimer, J.R. (1927). Zur Quantentheorie der Molekeln (“On the Quantum Theory of Molecules“) in Annalen der Physik

Left: Oppenheimer in 1928. Right: Born & Oppenheimer (1927). Zur Quantentheorie der Molekeln in Annalen der Physik.

Following their completion of the paper and Robert’s successful oral defense of his dissertation, he soon after sailed for New York. A rising young star in the newly developed field of quantum theory, Oppenheimer had already been awarded a United States National Research Council fellowship to do post-doctoral work at the California Institute of Technology (Caltech). Before traveling to California, he returned briefly to Harvard in the fall of 1927. There he produced three papers:

Oppenheimer eventually left for Pasadena, California in the beginning of 1928. Despite his teaching duties and the distractions of the California geological landscape (he still collected rocks), Oppenheimer managed to publish six papers his first year there, each dealing with aspects of quantum theory. Among them were

Oppenheimer, J. R. (1928a). On the Quantum Theory of the Ramsauer Effect in Proceedings of the National Academy of Sciences 14 (3) where he addressed the phenomenon of scattering of low-energy electrons by atoms of a noble gas from a quantum perspective, the so-called Ramsauer-Townsend effect; and

On the Quantum Theory of the Ramsauer Effect in Proceedings of the National Academy of Sciences 14 (3) where he addressed the phenomenon of scattering of low-energy electrons by atoms of a noble gas from a quantum perspective, the so-called Ramsauer-Townsend effect; and Oppenheimer, J. R. (1928b). On the Quantum Theory of the Autoelectric Field Currents in Proceedings of the National Academy of Science 14 (5) in which he proposed a theory of field emission, the first example of a quantum effect due to barrier penetration.

In the summer of 1929, Oppenheimer spent a month in Zurich where he worked under Wolfgang Pauli, then Professor of Theoretical Physics at ETH. In March of the same year, Heisenberg and Pauli had completed the first part of their joint work on quantum electrodynamics, the relativistic quantum field theory of electrodynamics. Oppenheimer, once again with impeccable timing, arrived just as the two were commencing with the second part of their work.

Using Pauli and Heisenberg’s young new theory, Oppenheimer commenced with addressing the so-called self-energy problem, to this day one of the most intractable issues in physics. The result was the discovery of a new source of self-energy, a quantum effect without a classical counterpart. He also observed that self-energy effects cause infinite displacements of atomic energy levels (Bird & Sherwin, 2005). These ideas were published in

Oppenheimer, J.R. (1930). Note on the Theory of the Interaction of Field and Matter in Physical Review 35.

Back at Caltech, as a consequence of his considerable research output Oppenheimer began receiving job offers from other prominent American institutions. He eventually turned down an offer from Harvard, but found the prospect of living in Berkeley intriguing, as it reminded him of New Mexico:

"I visited Berkeley and I thought I'd like to go to Berkeley because it was a desert. There was no theoretical physics and I thought it would be nice to try to start something. [...] I liked it enough to want to come back and enough to feel that it was a place where I would be checked if I got too far off base and where I would learn of things that might not be adequately reflected in the published literature" - J. Robert Oppenheimer - Excerpt, J. Robert Oppenheimer: A Life by Abraham Pais (2006)

And so Robert accepted UC Berkeley’s offer, turning down 11 other world-renowned universities. He negotiated to be able to also remain in his position at Caltech, and teach one semester at each school.

“If there ever was a period during which Robert was happy, those were his California years in the 1930s, I would think” — Abraham Pais

In 1931, he co-authored a paper with his student Harvey Hall entitled Relativistic Theory of the Photoelectric Effect in which they based on empirical observations dispute that Dirac’s assertion that two of the energy levels of the hydrogen atoms have the same energy. Another of his doctoral students, Willis Lamb, later determined that this was a consequence of what is now known as the Lamb shift, for which he was awarded the Nobel Prize in 1965.

Left: Oppenheimer in 1931. Right: Oppenheimer (1930). Note on the Theory of the Interaction of Field and Matter in Physical Review

The Oppenheimer-Phillips process (1935)

By 1935, Robert was professor at the University of California, Berkeley. Together with his student Melba Phillips they published the paper Note on the Transmutation Function for Deuterons which appeared in Physical Review 48. The paper describes a type of deuteron-induced nuclear reaction wherein the neutron half of a stable isotope of hydrogen with one proton and one neutron fuses with a target nucleus, the target is transmuted to a heavier isotope while ejecting a proton. The process has since become known as the Oppenheimer-Phillips process, and successfully explains, for instance, the nuclear transformation of carbon-12 to carbon-13. The discovery was made from experiments conducted at the UC Berkeley cyclotron, which showed that some elements became radioactive under deuteron bombardment.

Physicists Paul Dirac (1902–1984), Robert A. Millikan (1868–1953) and Oppenheimer in 1935

Through his friendship with Richard Tolman in the 1930s, Oppenheimer later also grew interested in astrophysics. His co-authored paper On Continued Gravitational Contraction in Physical Review 56 was written in collaboration with his student Hartland Snyder in 1939 and is the first published prediction of the existence of black holes (read: the Tolman-Oppenheimer-Volkoff limit). Of this contribution, largely unrelated to his previous work, physicist Freeman Dyson later wrote:

His theoretical prediction of black holes was by far his greatest scientific achievement, fundamental to the modern development of relativistic astrophysics, and yet he never showed the slightest interest in following it up. So far as I can tell, he never wanted to know whether black holes actually existed. [...] We know that the Oppenheimer-Snyder calculation is correct and describes what happens to massive stars at the end of their lives. It explains why black holes are abundant, and incidentally confirms the truth of Einstein's theory of general relativity. And still, Robert Oppenheimer was not interested. [...] How could he have remained blind to his greatest discovery? [...] Perhaps if the Oppenheimer-Snyder calculation had not happened to coincide with the Bohr-Wheeler theory of nuclear fission and with the outbreak of World War II, Robert would have paid more attention to it. - Excerpt, The Scientist As Rebel (2006) by Freeman Dyson

The Manhattan Project (1942–1945)

“Oppenheimer gave us an example of how large scientific enterprises can be more than the sum of the collaborative effort of their groups. They can be imbued with a creative spirit based upon a common heritage and a common aim” — Victor Weisskopf

If World War I was the “chemist’s war”, then World War II was certainly to become the “physicist’s war”. In the same year Oppenheimer was preoccupied with cosmology and published his paper predicting the existence of black holes, German nuclear physicists Otto Hahn and Fritz Strassmann reported the discovery of uranium fission in a 1939 issue of Die Naturwissenschaften, later identified as nuclear fission by Lise Meitner in the February 11th 1939 issue of Nature. The news of the discovery was brought to America by Niels Bohr and was soon empirically verified in the U.S. by Enrico Fermi at the University of Columbia.

Hungarian physicist Leó Szilárd who at the time was working in the United States, soon realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction, yielding massive amounts of energy for electric power generation, and potentially, atomic bombs. Concerned that German scientists working under the Nazi regime might attempt to exploit the new discovery for bomb-making purposes Szilárd concluded that he should warn the Belgian government, as the Belgian Congo was the best source of uranium ore. He came to this conclusion after conferring with his compatriots Edward Teller and Eugene Wigner. As Albert Einstein — by then a world-renowned scientist and political activist — knew the Belgian Royal Family, Szilárd thought he would be the most suitable person to do this.