J. Robert Oppenheimer, an American theoretical physicist, is often recognized as the father of the atomic (nuclear) bomb. Christopher Nolan’s latest film centers around Oppenheimer, portrayed by Cillian Murphy. However, the narrative of the bomb’s creation predates Oppenheimer’s leadership at the laboratory involved in the Manhattan Project, the initiative responsible for developing this formidable weapon. The scientific groundwork for the bomb was laid several years before Oppenheimer’s tenure.
A nuclear bomb operates on the principle of fission, involving the splitting of atoms. When atoms are bombarded with neutrons, they undergo a process of division, creating newer atoms and releasing both energy and additional neutrons. Subsequently, these newly released neutrons go on to strike more atoms, causing them to undergo fission as well. This chain reaction amplifies, leading to the release of an astounding amount of energy.
The nuclear bombs deployed on Hiroshima and Nagasaki relied on fission materials, specifically uranium-235 and plutonium-239. The scientific groundwork for these devastating weapons was laid during the Manhattan Project, initiated in 1941-42. At that time, the scientific landscape was relatively new, with the discovery of the neutron in 1932 and the comprehension of fission unfolding only in 1938-39. To truly understand the narrative, one must start with the discovery of the neutron.
The neutron and fission
The neutron and the concept of fission marked a pivotal shift in atomic understanding. Before the discovery of the neutron, the prevailing belief was that the atom comprised only two types of particles: positively charged protons in the nucleus and negatively charged electrons orbiting it. The revelation of the neutron altered this perspective and laid the groundwork for a deeper comprehension of atomic processes, particularly the revolutionary concept of fission.
The neutron, a companion to the proton within the nucleus, was discovered by the English physicist James Chadwick. His groundbreaking work earned him the Nobel Prize for Physics in 1935. The journey to this discovery began in experiments conducted by other scientists from 1930 onward. These experiments involved bombarding beryllium atoms with alpha particles, leading to the emergence of unexplained radiation. Chadwick, through insightful deduction, surmised that this radiation consisted of particles. He determined their size, observed their lack of electric charge, and ultimately identified them as a new type of particle—what we now know as the neutron.
The potential unleashed by the neutron became evident shortly thereafter. Devoid of charge, the neutron could effortlessly penetrate the nucleus of diverse atoms. In theory, through bombardment, it held the capability to transform an atom of one element into an atom of the next higher element in the periodic table. This property highlighted the transformative power of the neutron in altering atomic structures.
Under the leadership of the Italian-American scientist Enrico Fermi (portrayed by Danny Deferrari in the film), a team in his ancestral country undertook experiments involving the bombardment of uranium with neutrons. Initially, they believed they had successfully produced element 93. However, the true revelation was that they had inadvertently initiated a process of fission, a realization that eluded them at the time.
In 1939, German scientists Otto Hahn and Fritz Strassman conducted an experiment where they added non-radioactive barium to uranium, bombarded it with neutrons, and observed radioactivity in the resulting barium. The puzzle was later deciphered by Austrian-born physicist Lise Meitner, Hahn’s former colleague, who had fled to Sweden following Germany’s occupation of Austria. Meitner proposed that the radiation originated from radioactive barium, formed as a result of the uranium bombardment.
The counterintuitive nature of barium being formed from the heavier uranium perplexed scientists. Meitner put forward the idea that the bombardment caused the uranium nucleus to split into two, giving rise to barium and another element (later identified as technetium). This groundbreaking concept shed light on the intricate process of nuclear fission.
Upon reaching the Danish physicist Niels Bohr (depicted by Kenneth Branagh), the theory was presented at an international conference in Washington. Soon after, physicists delved into this novel avenue of experimentation. It didn’t take long for the concept of a chain reaction to emerge. The bombardment of an atom with neutrons, they realized, could unleash a cascade of events—new neutrons being released, striking more atoms, and initiating a continuous cycle that steadily released escalating amounts of energy.
The Hungarian-American physicist Leo Szilard, portrayed by Máté Haumann, foresaw the repercussions of a nuclear weapon falling into Nazi Germany’s hands. Collaborating with colleagues Eugene Wigner and Edward Teller (depicted by Benny Safdie), they managed to persuade the initially hesitant Albert Einstein (played by Tom Conti) to compose a letter to US President Franklin Roosevelt. The letter urged the United States to embark on the development of such a weapon before the Nazis could seize the opportunity.
The letter reached President Roosevelt amid the backdrop of World War II already unfolding in Europe. On December 6, 1941, in response, Roosevelt initiated a project initially known as the Manhattan Engineer District, with the objective of developing an atomic bomb. Remarkably, Japan bombed Pearl Harbor the very next day, marking a critical turning point in the course of history.
Manhattan Project
Oppenheimer, associated with the University of California, Berkeley at the time, was among the scientists who enlisted in the endeavor that would eventually be dubbed the Manhattan Project. Army officer Leslie Groves, portrayed by Matt Damon, overseeing military aspects of the project, selected Oppenheimer to lead weapons development. Opting for Los Alamos, New Mexico, Oppenheimer set the stage for the construction of his laboratory.
Among the hurdles encountered by Oppenheimer and his team was the quest for the ideal element for fission. While uranium was the apparent choice, the challenge lay in the fact that the more prevalent isotope, U-238, was more stable compared to the less abundant U-235. Bohr’s prediction that U-235 would be more prone to fission was validated in subsequent experiments, underscoring the importance of this pivotal insight.
The laboratory required substantial quantities of uranium, demanding a meticulous purification process to avoid wasting neutrons. Although scientists began the production of purified uranium, the snag lay in the fact that less than 1% was U-235, rendering the majority, U-238, incapable of undergoing fission. Eventually, the introduction of a method known as gaseous diffusion proved effective in separating U-235 from U-238, a crucial breakthrough in the pursuit of weapon development.
The subsequent challenge involved moderating the speed of bombardment for improved neutron absorption. The experimenters concluded that slower bombardment was essential. Carbon emerged as the chosen moderator, and further experiments with carbon-graphite mixtures revealed a crucial insight: a chain reaction was only viable if the lump of uranium was sufficiently large. This realization led to the construction of a uranium chain reactor of critical size at the University of Chicago, marking a pivotal milestone in the development process.
Interestingly, U-235 found its application in only one of the two bombs deployed. The other bomb utilized plutonium-239, a product discovered in experiments involving the bombardment of U-238 with neutrons. Recognized for its fission-friendly instability, plutonium-239 was subsequently manufactured at specialized laboratories dedicated to this specific purpose.
The first test of a plutonium-based bomb took place at Jornada del Muerto, New Mexico, on July 16, 1945. Codenamed the Trinity test by Oppenheimer, it marked a significant milestone. The initial nuclear bomb deployed in combat, known as Little Boy, was uranium-based and was dropped on Hiroshima on August 6 of the same year. Subsequently, three days later, the plutonium-based Fat Man was unleashed upon Nagasaki, concluding a devastating chapter in history.
Life after the War
Nolan’s film takes inspiration from the biography “American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer (2005),” penned by Kai Bird and Martin J. Sherwin. The biography delves into Oppenheimer’s life, both within and beyond the scope of the Manhattan Project, offering a holistic exploration of his triumphs and tragedies.
Following the war, Oppenheimer played a role on the advisory committee of the U.S. Atomic Energy Commission (now the Department of Energy). In 1949, the committee, under his guidance, recommended against the construction of a hydrogen bomb. Unlike fission, the hydrogen bomb operates on fusion, where two hydrogen atoms combine into helium under high temperatures. The protons released in this process then fuse with different hydrogen isotopes, creating corresponding helium isotopes. This fusion process, akin to a chain reaction, has the potential to yield a bomb significantly more powerful than those developed during the Manhattan Project.
Oppenheimer’s resistance to develop a hydrogen bomb led to him garnering adversaries. Accusations of being a Soviet spy, fueled by his past communist connections, subjected him to scrutiny by a review board. As a consequence, his security clearance was revoked, limiting his access to classified documents. Undeterred, Oppenheimer persevered by continuing to teach physics. His journey concluded with his passing in 1967.
In 2014, the records from the hearing were declassified, shedding new light on Oppenheimer’s case. Fast forward to 2022, and the Department of Energy acknowledged the unfairness of the proceedings, leading to the reinstatement of Oppenheimer’s security clearance. This reevaluation marked a significant posthumous recognition of his contributions and the rectification of historical judgments.
