Unless you watch Barbie right after, Christopher Nolan’s Oppenheimer will probably leave you with a sense of moroseness. The movie focuses, as the name suggests, on Julius Robert Oppenheimer — a theoretical physicist who acquired the title “the father of the atomic bomb” — and his role in developing the atomic bomb at Los Alamos, New Mexico. 

It might surprise you that the prospect of such a bomb had been looming over different physicists, nations apart, before the bombs that the US dropped on Nagasaki and Hiroshima were even built. The underlying physics of the atomic bomb’s invention constitutes a riveting history that swells with political tumult, prophetic science fiction, and pitiful moral tensions. The events in this quantum history are compressed in such a short time, progressing quickly like a cascading chain reaction that eventually — and inevitably — erupts into chaos. 

1939: Splitting the atom — The basis of nuclear energy

Nuclear technology, like nuclear bombs and power plants, uses nuclear energy, which comes from the interactions in the nucleus of an atom.

An atom is the basic unit of all elements. Every atom contains a nucleus which consists of positively charged particles called protons and particles with no charge called neutrons, surrounded by clouds of negatively charged particles called electrons. 

The protons in the nucleus repel each other. Yet, neutrons and protons are glued together in the nucleus by a strong nuclear force that overrides the protons’ repulsion of each other. 

The strength of the strong nuclear force depends on the ratio of protons and neutrons in the nucleus. When the number of neutrons exceeds the number of protons, the atom can become unstable and needs to fling out energy or certain particles called alpha particles — composed of two neutrons and two protons — and beta particles — composed of a fast-moving electron. These forms of energy are referred to as radiation, and the process of changing the nucleus by emitting radiation is called radioactivity. A radioactive element is unstable and likely to undergo a change in its nucleus. 

In 1938 and 1939, a lab of German scientists including Lise Meitner, Otto Hahn, and Fritz Strassmann experimented with directing a neutron stream at a uranium nucleus. Uranium is an element whose atoms contain 92 protons, which is a relatively large amount of protons compared to other atoms; this makes it a heavy element. Additionally, uranium atoms also contain a higher number of neutrons than protons in their nuclei, which makes uranium radioactive. 

By directing neutrons at the nuclei of uranium, which is the heaviest naturally occurring element, the scientists expected that a new, even heavier element would form. Yet, the lab was not able to identify any such newly created substances. 

Finally, Meitner, along with her nephew Robert Frisch, figured out that if they shot only one neutron, the uranium nucleus — instead of absorbing the neutron — seemed to split apart into two lighter nuclei of different elements, barium and krypton. The singular neutron seemed to tip over the already unstable nucleus so that the strong nuclear force became overridden by the electrostatic repulsion in the nucleus until the uranium atom split. 

Yet, this concept was incredibly confusing. How could a single neutron split the nucleus into two when no one had ever been able to chip off more than a fragment of the nucleus before? Where and how was the large amount of energy required to split the nucleus generated? 

The answers to these questions, of course, lie within Albert Einstein’s famous equation: E = mc2.

1905–1909: Einstein’s energy-mass equivalence and its implications

E = mc2 is a fairly simple equation, but it was truly revolutionary for its implications about energy and mass. Published in Einstein’s 1905 paper, “Does the Inertia of a Body Depend Upon Its Energy-Content?”, the equation says that energy (E) is equal to mass (m) times the speed of light squared (c2). Essentially, Einstein’s equation says that mass and energy are interchangeable. Mass can be viewed as really concentrated energy. 

Furthermore, because mass is multiplied by the speed of light — an enormous magnitude of approximately 1.1 billion kilometres per hour — squared, even the tiniest amount of mass equals an incredible amount of energy. To put this speed into perspective, remember the last time you were on Gardiner Expressway in a car moving at 100 kilometres per hour. Now imagine you were going 11 million times faster than that. 

Finally, imagine that speed multiplied by itself — that is, squared. This is the magnitude multiplied by mass in Einstein’s equation; so even a subatomic mass times this magnitude of speed yields a massive amount of energy. 

In his book E=mc²: A Biography of the World’s Most Famous Equation, David Bodanis writes that mass is “simply the ultimate type of condensed or concentrated energy,” whereas energy is “what billows out as an alternate form of mass under the right circumstances.” What might these circumstances be? 

Well, the splitting of the atom, for one.

In 1909, four years after Einstein’s publication, Meitner attended a conference in which Einstein discussed his findings on the energy-mass equivalence. Meitner recalled: “In the course of [Einstein’s] lecture he… [took] the theory of relativity and from it derive[d] the equation: energy = mass times the square of the velocity of light, and showed that to every radiation [which is a change in the nucleus] must be attributed a… mass. These two facts were so overwhelmingly new and surprising that, to this day, I remember the lecture very well.” 

1938–1939: Nuclear fission

Fast-forward to December 1938: Meitner calculated that the electrostatic repulsion between the protons in the uranium atom had to be around 200 million electron volts (MeV) for them to fly apart from each other and split the nucleus. 

In his previously mentioned book, Bodanis puts the magnitude of this energy into perspective: “In Berkley, California, a building-sized magnet was being planned that might, when charged with more electricity than the whole city of Berkley ordinarily used, power up a particle to 100 MeV.” So, there was something in the infinitesimally small nucleus that was generating double the energy it takes to power a whole city. 

Further studying the reaction, Meitner’s team noticed that its reactants — the uranium atom and the neutron that hit its nucleus — and its products — the krypton and barium nuclei and the extra neutrons released by the reaction — did not have the same mass. This missing mass turned out to be equal to approximately a fifth of a proton, a vanishingly tiny amount. Yet, this missing mass seemed to generate 200 MeV of energy: enough to split a nucleus apart. How? 

From Einstein’s equation, we see that this tiny mass is amplified by the enormous magnitude of the speed of light, squared. 

This tendency of an unstable nucleus to split into two smaller nuclei, releasing energy, is now known as nuclear fission. Although “atomic bomb” is the more popular term, it is the splitting of the nucleus of the atom that releases energy, so “nuclear bomb” is more accurate. 

Tragically, Lise Meitner — due to her status as a Jewish woman — was never given official credit for being the intellectual leader of the team that discovered fission. Instead, Otto Hahn ended up being the one who won a Nobel prize for the revolutionary discovery: a not particularly surprising show of sexism by the Nobel prize committee. 

Fission, named in Meitner’s and Frisch’s 1939 paper titled “Disintegration of Uranium Neutrons: A New Type of Nuclear Reaction,” was revolutionary. The astounding amount of energy available for use in a single atom opened doors to many possibilities — like powering cities in a cleaner and more efficient way. 

But it also nudged the looming possibility of a devastating nuclear weapon. 

1933–1942: Szilard and the nuclear chain reaction

A young physicist called Leo Szilard — who collaborated with Einstein on building safer fridges in the 1920s — was concerned that a nuclear weapon was being developed by the Nazis in light of the discovery of nuclear fission. In fact, he was far ahead of his time and had been worried about such a weapon long before 1939, when Meitner, Hahn, and Strassmann were shooting neutrons at nuclei.

In 1933, Szilard attended a lecture by Ernest Rutherford, a pioneering physicist in nuclear physics. The lecture discussed the H.G. Wells 1914 science fiction novel, The World Set Free, which anticipated an atomic bomb. During the lecture, Rutherford actually dismissed this idea, which deeply irritated Szilard.

While walking home from this lecture, Szilard had an epiphany that if a nucleus was hit by a neutron and in turn released two or more neutrons, those newly released neutrons would hit other nearby nuclei, starting a chain reaction that could release a massive amount of energy. 

With the discovery of fission in 1939, Szilard theorized the possibility of such a self-sustaining chain reaction that would continue the process of fission over and over again, releasing an enormous amount of energy. Szilard recognized that this continuous fission cascade would form the basis of a prospective nuclear bomb. 

When Adolf Hilter came to power in January 1933, Szilard became increasingly concerned about the implications of a possible atomic bomb being developed in Nazi Germany. In October 1939, Szilard convinced Einstein to pen the famous letter to US President Franklin D. Roosevelt to warn about the possible development of atomic weapons based on the nuclear chain reaction in Nazi Germany.

The letter also requested governmental funds to run a large-scale experiment, using a nuclear reactor, that would test whether a sustained nuclear chain reaction was even experimentally possible. 

In 1942, Szilard and a group of 48 other scientists ran a successful nuclear chain reaction in a nuclear reactor, which was designed to contain and control fission. This confirmation of a nuclear chain reaction that could sustain itself started a frantic global race toward developing a nuclear weapon. Under the direction of President Roosevelt, the Manhattan Project was set in motion, led by a shy and arrogant theoretical physicist called Julius Robert Oppenheimer. 

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1942–1945: How the bombs of the Manhattan Project work

The Manhattan Project was a top-secret operation that built the first-ever atomic bombs within a three-year time period, starting in 1942 under President Roosevelt. The project was developed at three main locations: Oak Ridge in Tennessee, Hanford in Washington, and Los Alamos in New Mexico. There were more than 600,000 people involved, including scientists, technicians, engineers, and maintenance staff. Most of these people weren’t told what they were working toward.

The main base of operations for the entire project was at Los Alamos, where scientists and engineers — under the leadership of Oppenheimer — developed and designed the atomic bombs. In just three years, the first atomic bomb was ready to be tested in the Tularosa Basin in New Mexico in July 1945, in what was called the Trinity test. Nearly half a million people lived in Tularosa within about 240 kilometres of the explosion. Even more appalling is the fact that some of them lived about 19 kilometres away from the site of detonation.

The trial ended up being successful, and two bombs — one using uranium as fuel and the other plutonium, which is also radioactive — were prepared to be used in war. 

The bombs developed by the scientists at Los Alamos had two designs: implosion and gun-type. 

The bombs used at the Trinity test and on Nagasaki on August 9, 1945, were both implosion bombs, and were based on a nuclear chain reaction. This chain reaction is a cascade of repeated fission reactions that are sustained on their own. In other words, once a singular fission reaction is set off, no more energy is required to kickstart the resulting fissions. 

However, this cascade can only be self-sustaining at a specific, required minimum mass, called the critical mass. The basic parts of the implosion bomb include a subcritical — slightly less than the critical mass — plutonium core surrounded by standard chemical explosives. When these explosives go off, they compress or implode the core. The compression brings the atoms of the core closer together, increasing its density — the amount of volume something takes up in relation to its mass. 

By bringing the atoms closer, the neutrons released when the core is imploded are able to hit other nuclei more often since they are closer, so the required critical mass for continued nuclear reactions is lower. Therefore, at the higher density that results from imploding the core, the previously subcritical mass is able to reach criticality, triggering the nuclear chain reaction.

Leaving this reaction unchecked and uncontrolled is what caused the huge explosion and release of energy observed at Nagasaki and in the Trinity test. However, nuclear power plants control this same reaction in a reactor, using neutron-absorbing rods to slow it down and stop it. This controlled reaction can then be channelled to generate electricity. 

On the other hand, the Los Alamos scientists considered the gun-type design to be even simpler. The design essentially involved a uranium target and uranium bullet. That bullet would shoot into the target, adding together to total the critical mass and triggering the nuclear chain reaction. On August 6, 1945, a gun-type bomb was used at Hiroshima, and its uranium core ended up being more destructive than the implosion bomb’s plutonium core. 

Einstein’s simple equation was devastatingly proven. Generally, theoretical physicists like Einstein yearn for their equations to be experimentally tested. But at what cost?

The aftermath 

Altogether, just two bombs were responsible for hundreds of thousands of casualties. The specific number of deaths they caused is unknown due to the destructive extent of the bomb. Even many who were not critically injured or dead on the spot at the bombing died from radiation poisoning in the months following. It was a sobering reality: the number of deaths far exceeded any prior estimates. 

Oppenheimer and the other physicists involved were intellectually stimulated enough that they failed to properly evaluate the consequences of their invention. Some sources say that Oppenheimer saw the development of the atomic bomb as a part of his duty in response to the war, and while he progressively expressed some hesitation about continuing the project, his intellectual curiosity about the practical possibility of such a bomb dampened his moral qualms. 

The use and development of the bomb seem to go beyond national duty. Many people have long raised the question of whether it was even necessary to use such a devastating bomb, not once but twice on civilian cities in the name of a war that was pretty much over. 

While Oppenheimer was said to have felt deep remorse for the ramifications of his creation, he never declared the Manhattan Project a mistake. He was even part of the decision-making process that determined that two civilian cities would be the target for the first atomic bomb rather than a military base. In fact, as the first bomb fell on Hiroshima, he declared in a speech at Los Alamos that his only regret was that they were not able to bomb the Germans instead. 

However, after the bombings, Oppenheimer argued against further nuclear development and insisted on global cooperation and regulation in regard to any future nuclear development. 

In fact, a lot of the scientists who contributed — whether intentionally or inadvertently — to the development of the bomb, like Szilard, Oppenheimer, and Einstein, were highly concerned about the implications of Hiroshima and Nagasaki and advocated for nuclear disarmament, especially as the idea of the development of a hydrogen bomb started becoming prominent.

In any case, the histories of nuclear science and nuclear warfare are intertwined — which places a huge moral weight on future scientists working on projects that can be exploited for applications further down the line that they didn’t agree to. Today, global initiatives like the United Nations are in the process of moving toward disarmament and elimination of nuclear warfare, but the mere existence of these weapons holds a latent, ever-present threat. 

Let us hope that, in this case, history does not repeat itself.