The Christopher Nolan film *Oppenheimer* focuses on the major legacy of J. Robert Oppenheimer in the eyes of the public: his work on the atomic bomb project. Yet to understand the man himself, it’s also worth remembering the legacy of Oppenheimer as it relates to his groundbreaking work at the advent of the radical field of quantum theory.

By the mid 1920s, the new era of quantum theory had swept over physics, upending not only theoretical physics but philosophy as well. The clockwork of the Newtonian universe was subsumed by a description that built uncertainty and randomness into the very fabric of the cosmos (and ultimately would lead to the technology of semiconductors and electronic computing).

In 1926 Oppenheimer, then a 22-year-old graduate student at the University of Cambridge, published an advanced treatise that utilized the nascent quantum theory to offer a more complete solution of the two-body problem than those that had been proposed in previous publications by Paul Dirac, Wolfgang Pauli, and Erwin Schrödinger. Oppenheimer’s manuscript caught the attention of Max Born, perhaps the preeminent European theoretical physicist of the day. Oppenheimer was already establishing his scientific reputation as a person concerned with mathematical questions predominantly to the extent that they allowed the prediction of the behavior of actual physical systems.

After joining Born at the University of Göttingen in Germany, Oppenheimer soon outstripped his adviser and his contemporaries alike, often struggling against perceptions of elitism, to address problems emerging with the development of quantum mechanics. Having conquered the two-body atomic problem in his 1926 paper, Oppenheimer induced Born to attack the difficult question of the spectral properties of molecules, an inherently many-body problem. (Variations on the topic continue to fully occupy theorists today.) The resulting Born–Oppenheimer approximation offers an approach for solving the first quantum many-body problem. The approximation scheme remains important to the theory and practice of quantum chemistry (see, for example, *Physics Today*, February 2023, page 16).

By 1929 Oppenheimer was working with Pauli in Zurich just as the physics community, including Dirac and Werner Heisenberg, were grappling with what would come to be called quantum electrodynamics. The theory unites quantum mechanics with the workings of electromagnetic radiation, a phenomenon intimately tied to special relativity. Despite the now commonly held perception that Oppenheimer was insufficiently careful with mathematics, he was able to impress Pauli, perhaps the most meticulous of calculators.

In the summer of 1929, Oppenheimer brought his experience with European quantum electrodynamics to the US, contributing to its foundations and helping it flourish here. He also established what would become the nation’s leading school of theoretical physics at the University of California, Berkeley.

The 1967 Nobel laureate Hans Bethe would later highlight Oppenheimer’s 1930 paper “On the theory of electrons and protons” as having “essentially predicted the positive electron.” Dirac’s relativistic equations had described electrons and another, oppositely charged particle—now known as an antiparticle—simultaneously. Dirac had thought the positively charged particle was the proton, the only other positively charged particle known at the time. Using an ingenious, original argument—an example of what we now call crossing symmetry—Oppenheimer proved that the other particle must have the same mass as the electron. Although he didn’t think his arguments indicated the existence of the positron, Oppenheimer “implicitly predicted” the new particle, in Bethe’s words. Carl Anderson would observe the positron two years later.

Oppenheimer pursued his long-held fascination with astrophysics in the 1930s when he published on an array of topics including cosmic rays and neutron stars. With coauthor Hartland Snyder, he used the equations of general relativity to trace the collapsing mass of a neutron star. That theoretical work predicted black holes. Oppenheimer’s mathematical predictions, of course, have in recent years been borne out by scientific observation of the once-speculative phenomenon.

Oppenheimer’s understanding of the process, procedure, and results of measurement was crucial to his achievements as a scientist. At Berkeley—and throughout his career—he cultivated close relationships with experimentalists. Clumsy in the laboratory himself, Oppenheimer nevertheless keenly appreciated how experimentalists chose materials and experiment designs based on limitations and parameters.

That intellectual dexterity and curiosity distinguished Oppenheimer from other theoreticians. He enjoyed connecting with other scientists and was gifted in synthesizing and extrapolating ideas. His personal style—the mumbling between thoughts, the occasional cutting remarks—may have made him seem an unlikely choice to lead the atomic bomb project. But in hindsight, it’s clear that his theoretical physics interests and lightning-quick ability to grasp new information, together with his intense personal charisma, made him ideally suited to lead such a daunting project.

*Mark Paris is a theoretical physicist at Los Alamos National Laboratory. His research crosses topics in nuclear and particle physics, astrophysics, and cosmology.*