What is the application of quantum physics

The first technological application of quantum physics

When the atomic bomb was used 75 years ago

Over 100 years ago, people were shocked by the totality of the First World War. Up until then, nobody could have imagined millions and millions of deaths as the result of a military conflict. This mass destruction was only made possible by new technologies (airplanes, machine guns, tanks, poison gas, etc.). It couldn't get worse, people thought. But actually a much worse horror was waiting for them: a single bomb that can kill hundreds of thousands of people. 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 was based on a physical theory that was new at the time and which is still a synonym for incomprehensibility among physicists: the quantum theory. In the world of physicists, this had caused a huge sensation since the beginning of the century. It had already collapsed a whole worldview, the worldview of classical physics - and large parts of classical philosophy. With the description of the laws in the micro- and nano-cosmos, exciting new technologies emerged that were to change the world in the second half of the century (lasers, microelectronics, medical technology, etc.). But in the last year of the war, 1945, the general public was not yet aware of this. The new physics was too complex, too bizarre and too mathematical. But then suddenly and completely unexpectedly it appeared on the stage of the world public, and this with a very loud bang: The first technological application of quantum physics was the most terrible weapon that was ever used militarily.

How did this terrible weapon come about?

Since Rutherford's famous experiment in 1912, which every schoolchild knows today, physicists have known that the atomic nucleus consists of positively charged elementary particles (protons). But equally charged particles repel each other. How then 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 have a much stronger (attractive) effect than the electrical force over the very short distances in the atomic nucleus. But the physicists did not know what kind of force that was supposed to be. It was one of many puzzles in the quantum world that physicists had only just begun to look around.

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 atomic nucleus, with which new atomic nuclei that do not occur in nature could be formed. To Fermi's surprise, a large amount of radioactive radiation was generated in his experiments, the origin of which neither he nor other researchers could 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 formed, which has a much smaller nucleus than uranium. They were amazed and couldn't believe that such a large piece could be shot out of the indivisible uranium atomic nucleus.

In December of the same year, the German researchers Otto Hahn and Lise Meitner also carried out experiments with uranium nuclei in order to investigate the unknown force in the atomic nucleus more closely. They, too, bombarded uranium with its 92 protons and - depending on the isotope - 143 or 146 neutrons, and the "ammunition" were also decelerated neutrons. It turned out that the bombardment created two completely different elements: barium and krypton. Barium atoms, which can be quickly detected radiochemically, have an atomic number of 56 and are almost half the size of uranium nuclei.

With the help of theoretical quantum physical calculations, Meitner came to the conclusion that the uranium core had been burst by the neutron bombardment. In doing so, the fragments absorb a great deal of energy, far more than was produced in any known atomic process. But where this energy came from was another mystery at first. Meitner also calculated that the two nuclei that resulted from the fission (plus three neutrons that were released) were slightly lighter in their sum than the original atomic nucleus of uranium plus the neutron that caused the fission. What had happened to the missing mass?

Einstein's famous formula E = mc provided the answer to both questions2that he had set up more than 30 years earlier: The difference between the masses before and after the split corresponded exactly to the energy that the fragments had absorbed, according to Meitner's results. For the first time a process had manifested itself in which the equivalence of energy and mass formulated by Einstein was directly revealed. At the same time, however, it also became clear that unimaginable energies slumber inside the atom! This news ran like wildfire through the scientific world (Otto Hahn, but not Lise Meitner, received the Nobel Prize in Chemistry in 1944 for this knowledge; however, at the time of the announcement, he was still in military internment in England 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 still a million times stronger than in conventional chemical reactions. As luck would have it, when a uranium nucleus is split by a neutron, three more neutrons are released, which in turn were able to split uranium nuclei. The physicists recognized that a chain reaction could release an enormous amount of energy in a very short time.

Manhattan project

A lot of energy in a small space that could be released - that quickly aroused the interest of the military during the ruling war. 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 nuclear bomb. This made non-physicists sit up and take notice. Adolf Hitler had attacked Poland shortly before and started World War II. As a leading nation in research and technology, National Socialist Germany seemed predestined to be the first country to use nuclear energy for military purposes and to manufacture atomic bombs.

A bomb with such enormous explosive power in the hands of Hitler would have catastrophic effects on the world, thought not only the two Jews Lise Meitner and Otto Frisch. Like Meitner and Frisch, the Hungarian physicist Leó Szilárd had suffered greatly under National Socialist Germany, and he too was faced with the horror of a nuclear-armed Hitlerite Germany. He persuaded the hitherto strict pacifist Albert Einstein to write a letter to the American President Franklin D. Roosevelt, encouraging him to start building an American atomic bomb. This took up the impetus.

From 1941 onwards, the American government put together a team of high-ranking scientists and engineers under the utmost secrecy (not even the Vice President was inaugurated). The goal of the "Manhattan Project", which was to be the most complex and difficult technical project in history to date, was to build an atomic bomb. We already had some experience 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 defense had been developed before the atomic bomb.

The first step of the Manhattan project was to prove that a chain reaction of neutron releases could indeed be triggered and sustained. This was achieved in December 1942 by Enrico Fermi, who had emigrated from Italy, which was allied with Hitler. Fermi had constructed the first nuclear reactor in history below a sports field at the University of Chicago. This cleared the way to the bomb. The research was centered at a location called Los Alamos in the New Mexico desert.

The scientific director of the Manhattan Project and therefore later regarded as the "father of the atomic bomb" was Robert Oppenheimer, who had received his scientific training under Max Born in Germany. Two feasible paths had become apparent early on: a first by means of the fission of uranium nuclei and a second with plutonium nuclei. Since the physicists were not sure which was the more promising route, they decided to pursue both concepts at the same time. After four years of intensive and strictly confidential work, they managed to develop both types of bombs. In July 1945 they had completed four atomic bombs.

"Now we're all sons of bitches."

On July 16, 1945, the first atomic bomb in world history exploded on a test site in the desert of New Mexico. Their force exceeded even the most optimistic expectations of physicists. But when the mighty mushroom cloud appeared on the horizon, she was overcome by a feeling of deep discomfort. Robert Oppenheimer quoted, as he later wrote, inwardly from the "Bhagavad Gita", a central script of Hinduism: "Now I have become death, destroyer of the worlds." One of his colleagues, the director of the test, Kenneth Bainbridge, put it more vividly: "Now we're all sons of bitches." The uneasy feeling of the physicists should prove to be justified. Just three weeks later, the second mushroom cloud appeared, this time over the skies of the enemy Japan. The third followed just three days later.

In one of the most controversial and to this day most controversial decisions in US history, the new President Harry Truman, who until recently had been completely unsuspecting even about the possibility of the existence of an atomic bomb, decided to use the bomb against Japan. 200,000 people died immediately in the two drops. In the course of the next few years, many more followed due to radioactive long-term damage. It was less than seven years from the scientific discovery of the fissile nature of the uranium atomic nucleus to the mushroom clouds of Hiroshima and Nagasaki.

The American atomic bomb was originally intended for Hitler's Germany, which, however, had surrendered in May 1945. Germany had also operated an atomic bomb project. But the so-called uranium association under the leadership of Werner Heisenberg had neither had the necessary resources nor developed the necessary technical methods to actually be able to manufacture a bomb. To this day there is disagreement among historians as to why the scientific nation that was by far the leading scientific nation before the war did not develop the atomic bomb. Heisenberg himself said that he did not want to give such a bomb into Hitler's hands. Whether this was really his motivation is still controversial today. Other reasons were certain that the National Socialist military leadership had simply not recognized the importance of the atomic bomb.

With the atomic bomb, the physicists had to realize that their thirst for knowledge can destroy not only the prevailing view of the world, but also the world. Many of the scientists who participated in the project pursued the agonizing question until the end of their lives as to whether they were not directly responsible for the deaths of many people. Robert Oppenheimer plagued his conscience to such an extent that he was later even persecuted by the American secret service, who believed that his remorse could harm the United States 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 to this day: that of social responsibility.

Lars Jaeger studied physics, mathematics, philosophy and history and researched quantum physics and chaos theory for several years. He lives near Zurich, where he has set up two companies of his own that advise institutional financial investors and at the same time maintain regular blogs on the subject of science and current affairs. In September 2019 his book "Daring more future!"

(Lars Jaeger)

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