What kind of scientist was J. Robert Oppenheimer? Christopher Nolan's film has prompted several one-line pronouncements. “He was no Einstein”—which leaves plenty of room near the top; “He never won a Nobel Prize”—a narrow metric, if undeniably true; “He would have had a Nobel had he lived to see X or Y discovered”—we will never know. Oppenheimer's legacy in research merits a closer look.
When Oppenheimer returned to the United States in 1929 from his tour of the great centers of European physics, he brought a fresh doctorate from Göttingen and a passionate enthusiasm for the new quantum theory that sprang from European soil. His classic work on the quantum theory of molecules, a collaboration with his thesis advisor Max Born, showed the way to systematic approximations in many areas of physics.1 With his zeal, charisma, and intellect, he was, in Nobel Laureate Hans Bethe's assessment, “[m]ore than any other man, … responsible for raising American theoretical physics from a provincial adjunct of Europe to world leadership.”2
At Berkeley, where Ernest O. Lawrence was creating his industrial-strength cyclotron laboratory, Oppenheimer's school of theoretical physics (which migrated seasonally to Caltech, where Oppenheimer also held a faculty position) attracted many brilliant students and postdoctoral scholars and became known for its close engagement with experiment. That style has spread far beyond Berkeley.
We were born too late to meet Oppenheimer or to experience his scientific influence in real time, though we benefited from the atmosphere that grew out of the great school of theoretical physics that he created in Berkeley decades before our arrival. Of the many gifts that Oppenheimer bought to his research and teaching—which were of a piece—probably the greatest was his exquisite taste. His publications during the 1930s reveal a nose for the essential problems of the day or even the day after, together with a wide-ranging curiosity.
Oppenheimer supplied key insight into the implications of Paul Dirac's quantum theory of photons and electrons, with its implication that antimatter must exist. At the end of 1933, in “On the Theory of the Electron and Positive,” he and Wendell Furry spotlighted how “virtual” pairs of electrons and antielectrons (now called positrons) cause deviations from the classical force law between charged particles.3 These effects are crucial to understanding how the forces of nature behave at very high energies (or very short distances).
It's a commonplace now to say that the two great pillars of twentieth-century physics, quantum mechanics and general relativity, are in tension. However, under the right circumstances, they combine to yield extraordinary insight. In “On Massive Neutron Cores,” Oppenheimer and George Volkoff calculated in 1939 that the collapse of a star during a supernova explosion could leave behind a neutron core.4 Coming a decade after Dirac's work on quantum mechanics and a few years after the discovery of the proton's neutral partner, the neutron, this work was ahead of its time. The discovery of pulsars in 1967 made neutron stars real.
Later in 1939, Oppenheimer and Hartland Snyder reported in “On Continued Gravitational Contraction” that when all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse into what we now call—following John Wheeler's coinage—a black hole.5 Using the equations of general relativity, they concluded that such a star would in a finite time—in a single day for an idealized case—shrink indefinitely. The collapsed star, they conclude, “tends to close itself off from any communication with a distant observer.” Only its gravitational influence persists. The same issue of Physical Review contains Bohr and Wheeler's “The Mechanism of Nuclear Fission.”6 On the day those papers were published, the invasion of Poland began World War II and set the stage for Oppenheimer's involvement in the war effort. Evidence for a black hole would not appear for a quarter century.
In the aftermath of the Second World War, Oppenheimer profoundly influenced two invitation-only workshops that helped establish the new prominence of the American scientific community—enriched by illustrious immigrants. At Shelter Island in June 1947 and Pocono Manor in Spring 1948, he served as an animator, defining the agenda, and as an assessor, pronouncing on what had been accomplished and what open issues seemed most urgent.
A hallmark of these now-legendary meetings was the productive conversation between theory and experiment. What emerged was a comprehensive paradigm that to this day is our best description of the world of the very small: relativistic quantum field theory. Though highly serviceable, that framework can be clumsy to apply. Moreover, it has clear limitations when applied to extreme environments, such as black holes. Doing better, in matters of both calculation and principle, is high on the current agenda of fundamental physics.
His leadership of the Los Alamos laboratory aside, Robert Oppenheimer was a major figure in the rise of academic physics in the United States for a third of a century. His two decades as Director of the Institute for Advanced Study saw the development of the multifaceted institution we know today.7 His mighty influence is still felt, through the work of the exceptional cadre of young scientists who gathered around him and, in turn, the contributions of their scientific offspring.8
