The first technological application of quantum physics: When the atomic bomb hit 75 years ago

More than 100 years ago the totality of the First World War shocked people. Millions upon millions of deaths had never before been imagined as the result of a military conflict. This mass destruction of lives had only become possible thanks to new technologies (airplanes, machine guns, tanks, poison gas, etc.). It could not have come worse, people thought. But a much worse horror was awaiting the world: A single bomb that can kill hundreds of thousands. Exactly 75 years ago, on August 6, 1945, the US military dropped the first atomic bomb on the Japanese city of Hiroshima. Three days later a mushroom cloud formed over Nagasaki.

The atomic bomb found its origin in a completely new physical theory at the time that until today represents a synonym for bizarreness: quantum theory. Quantum theory had already caused a huge sensation in the world of physicists for 30 years. It had caused a whole world view, the world view of classical physics (and large parts of that of classical philosophy) to collapse. With the description of the laws in the micro- and nano-cosmos, exciting new technologies were also emerging that were to change the world in the second half of the century (lasers, micro-electronics, medical technology, etc.). But in 1945, the general public was not yet aware of this. This new physics was too complex, too bizarre, and too mathematical. But then it suddenly and completely unexpectedly appeared on the stage of the world public and this with a very loud bang: the first technological application of the quantum physics was the most terrible weapon ever used in military history.

How was this terrible weapon created? Ever since Rutherford’s famous experiment in 1912, which every schoolchild knows about today, physicists have known that the atomic nucleus consists of electrically positively charged elementary particles (protons). But as particles with the same charge repel each other, how is it possible that atomic nuclei are stable? The many protons in the atomic nucleus should fly apart. Another force in the atomic nucleus had to act much more strongly (attracting) than the electric force on the very short distances in the atomic nucleus. But physicists at the time had no idea what this force could be. It was one of many puzzles in the quantum world, in which physicists had just begun to orient themselves.

In 1934, the Italian physicist Enrico Fermi began bombarding uranium atoms with neutrons. His hope was that some of these neutrons would stick to the nucleus of the atom, which would allow the formation of new nuclei not found in nature. To Fermi’s surprise, his experiments produced a large amount of radioactive radiation, the origin of which neither he nor other researchers was able to explain. Four years later, in the summer of 1938, Irène Joliot-Curie, the daughter Marie and Pierre Curies, and her husband Frédéric observed that when uranium is bombarded with neutrons, a completely different element is created, which has a much smaller nucleus than uranium. They were amazed and could not believe that such a large piece could be shot out of the believed to be indivisible nucleus of the uranium atom. In December of the same year, the German researchers Otto Hahn and Lise Meitner also carried out experiments with uranium nuclei to investigate the unknown force in the atomic nucleus. They also bombarded uranium with its 92 protons and – depending on the isotope – 143 or 146 neutrons, and the „ammunition“ here too was slowed neutrons. It turned out that the bombardment produced two completely different elements: barium and krypton. Barium atoms, which could be quickly detected by radiochemistry, have a nuclear charge number of 56 and are therefore almost only half the size of uranium nuclei. With the help of theoretical quantum physics calculations, Meitner concluded that the uranium nucleus had been burst by the neutron bombardment. The fragments absorb a great deal of energy, far more than was generated in any atomic process known to date. But where this energy came from was another mystery. Meitner also calculated that the two nuclei resulting from the fission (plus three neutrons that are released) were in sum slightly lighter than the original atomic nucleus of uranium plus the neutron that caused the fission. What had happened to the missing mass?

The answer to both questions provided Einstein’s famous formula E = mc2, which he had established more than 30 years earlier: According to Meitner’s results the difference in masses before and after the fission corresponded exactly to the energy that the fragments had absorbed. For the first time a process had manifested itself in which the equivalence of energy and mass as formulated by Einstein was directly revealed. At the same time, however, it had also become clear: Inside the atom slumber unimaginable energies! This news ran like wildfire through the scientific world (Otto Hahn, but not Lise Meitner, was awarded the 1944 Nobel Prize in Chemistry for this discovery; however, at the time of the announcement he was still under English military internment together with the leading German nuclear physicists).

The physicists called this energy „nuclear energy“. When the atom is split, a small part of this enormous amount of energy is released, but it is nevertheless millions of times stronger than in conventional chemical reactions. Chance would have it that the fission of a uranium nucleus caused by a neutron releases three more neutrons, which in turn could split uranium nuclei. The physicists realised that an enormous amount of energy could be released in a very short time via a chain reaction.

A lot of energy in a small space that could be released – this quickly aroused the interest of the military in the prevailing wartime period. As early as 1939, Lise Meitner’s nephew, Otto Frisch, together with his British colleague Rudolf Peierls, wrote a memorandum describing the technical construction of a bomb based on nuclear energy. This now made even non-physicists sit up and take notice. Adolf Hitler had shortly before invaded Poland and started the Second World War. As a leading nation in research and technology, National Socialist Germany seemed predestined to be the first country to make military use of nuclear energy and to manufacture atomic bombs. A bomb with such enormous explosive power in the hands of Hitler would have catastrophic consequences for the world, thought not only the two Jews Lise Meitner and Otto Frisch. The Hungarian physicist Leó Szilard had, like Meitner and Frisch, suffered greatly under National Socialist Germany, and the horror of a nuclear-armed Hitler’s Germany forced itself upon him, too. He persuaded Albert Einstein, a strict pacifist until then, to write a letter to the American president Franklin D. Roosevelt and to give him the suggestion to start building an American atomic bomb. The latter took up this suggestion.

Under the highest secrecy (not even the vice president was informed), the American government assembled in 1941 a team of high-ranking scientists and engineers. The goal of the „Manhattan Project“, which turned out to be the most complex and difficult technical project in history, was to build an atomic bomb. Some experience had already been gained with such projects. The Second World War had already become a „war of physicists“, in which technologies such as radar, rocket propulsion and magnetic mine defence had been developed even before the atom bomb.

The first step of the Manhattan Project was to prove that a chain reaction of neutron releases could indeed be initiated and maintained. In December 1942, Enrico Fermi, who had emigrated from Hitler’s ally Italy succeeded in doing so. Below a sports field at the University of Chicago Fermi had constructed the first nuclear reactor in history. This cleared the way for the bomb. The research was centred at a place called Los Alamos in the desert of New Mexico. The scientific leader of the Manhattan Project and therefore later considered the „father of the atomic bomb“ was Robert Oppenheimer, who had received his scientific training under Max Born in Germany. At an early stage, two viable paths had emerged: a first one using the fission of uranium nuclei and a second one using plutonium. Since physicists were not sure which was the more promising path, they decided to pursue both concepts simultaneously. After four years of intensive and strictly secret work, they succeeded in developing both types of bomb. In July 1945 they had completed four devices.

On July 16, 1945, the first atomic bomb in world history exploded on a test site in the desert of New Mexico. Its force exceeded even the most optimistic expectations of the physicists. But when the mighty mushroom cloud appeared on the horizon, a feeling of deepest unease overcame them. Robert Oppenheimer, as he later wrote, quoted inwardly at that moment from the „Bhagavad Gita“, a central Hindu scripture: „Now I have become death, destroyer of worlds. One of his colleagues, the director of the test, Kenneth Bainbridge, expressed it more vividly: „Now we are all sons of bitches.“ The physicists‘ uneasy feeling should prove to be well founded. Only three weeks later, the second mushroom cloud appeared, this time over the skies of war-torn Japan. The third followed just three days later.

In one of the most controversial decisions in US history, the new president Harry Truman, who until shortly before had been completely unaware of even the possibility of the existence of an atomic bomb, decided to use the bomb against Japan. The two drops killed 200,000 people directly. Over the course of the following years, many more followed due to late radioactive damage. Not even seven years had passed from the scientific discovery of the fissionability of the uranium atomic nucleus to the atomic mushrooms of Hiroshima and Nagasaki.

Originally, the American atomic bomb had been intended for Hitler’s Germany, which had however capitulated in May 1945 already. It turned out that Germany had also been running an atomic bomb project. But the so-called Uranium Association (“Uranverein”), led by Werner Heisenberg, had neither the necessary resources nor had it developed the necessary technical methods to actually produce a bomb. To this day there is disagreement among historians as to why the pre-war leading scientific nation did not develop the atomic bomb. Heisenberg himself said that he did not want to put such a bomb into Hitler’s hands. This motivation is still disputed today. Other reasons were certainly that the National Socialist military leadership had simply not recognized the importance of the atomic bomb.

With the atomic bomb, physicists had to realize that their thirst for knowledge could shatter not only a dominant world view, but also the world itself. Many of the scientists who had participated in the project pursued the agonizing question until the end of their lives whether they did not bear a direct responsibility for the death of many people. Robert Oppenheimer plagued his conscience to such an extent that he was later even persecuted by the American secret service, which believed that his remorse could harm the USA in the Cold War against the Soviet Union. With Los Alamos, Hiroshima and Nagasaki, the work of physicists has taken on a dimension that it has not been able to get rid of until today: that of social responsibility.

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