Q: Oppenheimer’s name appears in the early applications of quantum mechanics and the theory of black holes. How good a physicist was he?
A: Well, he was no Einstein. And he’s not even up to the level of Heisenberg, Pauli, Schrödinger, Dirac, the leaders of the quantum revolution of the 1920s. One of the reasons for this was his birth date. He was born in 1904, so he was 3 years younger than Heisenberg, 4 years younger than Pauli. Those few years were enough to place him in the second wave of the quantum revolution and behind the main wave of discovery, in what [philosopher of science] Thomas Kuhn called the “mopping-up operation,” applications of the new theory.
Q: He’s known for the Born-Oppenheimer approximation, which helped extend quantum mechanics from atoms to molecules.
A: That was one of his most cited papers. He wrote that in 1927 while he was in Göttingen [Germany, doing his doctoral work with Max Born]. That same year, Heisenberg presented the uncertainty principle. Bohr and Heisenberg put out the Copenhagen interpretation [of quantum mechanics]. So here’s Oppenheimer doing an application, but a good one because it helped introduce [the method of] quantum perturbation theory.
Q: Even some of his contemporaries said he was a dilettante. How good he was in terms of raw skill?
A: He had the skill and the brilliance. But he didn’t have the focus. He was not absolutely devoted to physics the way one of the great physicists would be. It was just one of his many passions. At the time he was doing physics, he read a lot of literature and languages. Also, in the U.S., the empirical way of approaching physics was predominant [whereas European theorists were pursuing new concepts]. So the theorists’ job was to help experimentalists understand their data. As the physics and the experiments were shifting, his interest shifted, too.
One of his main contributions had only a tenuous connection to observation, and that was black holes. That was an unfortunate situation. In 1939, he and a student, Hartland Snyder, published a paper predicting [collapsing stars could form] black holes, and the whole thing got ignored. They couldn’t pursue it because the war was breaking out. A lot of people just ignored it because it seemed impossible—how could anything collapse to an infinitely dense point?—until [physicist John] Wheeler revived the matter in the 1960s. Not until the 1990s was there any experimental evidence for black holes. I think Oppenheimer would have gotten a Nobel Prize if he was still alive at that point.