My Oppenheimer Reminiscences
Julius Robert Oppenheimer is once more in the news, thanks to the highly publicized film “Oppenheimer.” As a physics graduate student at Princeton University in the early 1960s, I had occasional interactions with Oppenheimer, who was then the director of the Institute for Advanced Study. He was not very friendly to students at this stage of his career. But having suffered more than his share from “the slings and arrows of outrageous fortune,” he remained a sympathetic figure to most students.
Although Oppenheimer was a very good theoretical physicist, it is not easy to be outstanding when your peers are the likes of Enrico Fermi, Werner Heisenberg, Erwin Schrödinger, Hans Bethe, and Edward Teller. Oppenheimer is most noted for his scientific leadership at Los Alamos during World War II. The initial scientific director of the Manhattan Project, Gregory Breit, had difficulties with interpersonal relations. So when Oppenheimer took over in 1943, he had an easy act to follow. Most of the scientists he selected for leadership positions were very successful, notably Hans Bethe as the head of the theoretical division.
But the Manhattan Project was not a result of brilliant theory. What made nuclear weapons possible was not Einstein’s celebrated formula, E = mc², but a rapid-fire series of accidental experimental discoveries in the 1930s, including a celebrated mistake.
Ernest Rutherford and his team demonstrated the first human-caused nuclear reaction about the year 1919. By bombarding nitrogen nuclei with energetic alpha particles (the nuclei of ordinary helium atoms that are emitted during the radioactive decay of heavy elements like uranium or radium), Rutherford’s team produced oxygen isotopes and fast protons, the nucleus of a hydrogen atom.
But positively charged projectiles, like those used by Rutherford and his colleagues, have a hard time impacting the surface of a target nucleus. The positive charge of the nucleus repels the projectile before it can come close enough to “touch” the surface and react. That is why James Chadwick’s surprising discovery, in the year 1932, of an electrically neutral projectile, the neutron, was so important. Neutrons could reach the surface of even the most highly-charged nucleus, uranium, with no hindrance from the nuclear charge.
Chadwick showed that neutrons are produced when you mix alpha-emitting heavy elements like radium with light elements like lithium or beryllium. One of the first to make use of the newly discovered neutrons was the Italian physicist Enrico Fermi. With his research group in Rome, Fermi irradiated many different atoms with neutrons and discovered that the neutrons were readily absorbed, especially if the initially fast neutrons were slowed down by multiple collisions with nuclei of light elements like hydrogen or carbon. Slow neutrons had more time to be absorbed by a target nucleus. Fermi was awarded the Nobel Prize for Physics in 1938 for this work. But both the Nobel Committee and Fermi were profoundly mistaken by the nature of his work.
In his presentation speech in Stockholm, Professor H. Pleijel said:
This general pattern that Fermi has found to be the rule when heavy substances are subjected to irradiation by neutrons, took on special interest when applied by him to the last element in the series of elements, viz. uranium, which has rank number 92. Following this process, the first product of disintegration should be an element with 93 positive electric charges and a new element would thus have been found, lying outside the old series. Fermi’s researches on uranium made it most probable that a series of new elements could be found, which exist beyond the element up to now held to be the heaviest, namely uranium with rank number 92. Fermi even succeeded in producing two new elements, 93 and 94 in rank number. These new elements he called Ausenium and Hesperium.
Fermi certainly deserved to be honored for this and other brilliant work, but he was honored for a mistake. A few months after the Nobel award, the German chemists Otto Hahn and Fritz Strassman discovered that “ausenium” and “hesperium” were not transuranic elements at all, but highly radiative fragments of uranium nuclei, notably neutron-rich isotopes of barium. A few months later, Otto Frisch and Lise Meitner showed that the fission of uranium nuclei produced the barium after neutron absorption. It was soon apparent that the uranium fission also released several additional neutrons and that a nuclear fission chain reaction might be possible. This reaction would release millions of times more energy than the ordinary chemical reactions of high explosives. It was this accidental discovery that led to the Manhattan Project.
Most scientific breakthroughs result from accidents that smart people recognize as important. Breakthroughs are not determined by committees, even Nobel Committees, by the consensus of experts, or by Congressional legislation.