Leo Szilard was sitting in the lobby of his hotel in London reading the Times. It was Tuesday, September 12th 1933. In that issue, the newspaper featured an in-depth report on a scientific conference where the renowned physicist Ernest Rutherford lectured on the use of energy which was supposedly stored in atoms. Journalists carefully put down every word of Rutherford’s lecture which seemed to annoy Szilard immensely. Rutherford claimed that all the talk about atomic energy was no more than a senseless daydream, and most of the scientists of the time believed him. But Szilard thought otherwise. Rutherford’s statements on the impossibility of the use of nuclear energy made him so angry that he decided to dedicate himself to solving this problem and prove that the famous physicist was wrong. He left the hotel and took a walk through the city to clear his mind. When he was already strolling down one of the nearby streets an idea flashed through his mind. He had thought of a way to release the energy of an atom. This is how he later remembered that moment: “I stopped at the crossing on Southampton Row because the traffic light was red.” In a few moments cars started to slow down and stop and pedestrians were able to cross the road. It was at this very moment that Szilard came to realize how it would be possible to release the enormous amounts of energy, stored in every atom. He would have to create a chain reaction using neutrons.
“Eureka” at the pedestrian crossing
“I was waiting for the traffic light to change and when the green light turned on and I started crossing the road I suddenly realized. If I could find an element which, when colliding with neutrons, would split up or emit two neutrons after absorbing another, a sufficient amount of this element could maintain the chain reaction. At the time, it was not yet clear to me how we could find such an element or what kind of experiments would have to be conducted to do so, but the idea stuck in my mind. In certain circumstances it would be possible to establish a chain reaction, to produce large quantities of energy and build a nuclear bomb. The fact that something like that was really possible became my obsession.”
During the winter, Szilard put his ideas down on paper. The neutron was then a completely new particle, discovered only one year earlier. Of course, the crucial discovery was that neutrons, unlike protons with which they constitute the nucleus of an atom, are not influenced by the repulsive force, so they could penetrate the nucleus and reshape it in such a way that it would become unstable and split into two parts.
Naturally, finding an element which would emit energy and free neutrons after being split up was essential. According to Szilard, this method of nuclear fission was supposed to result in the release of energy, stored in an atomic nucleus, because the combined energy of the newly created nuclei would be smaller than that of the original nucleus, which meant that excess energy would be released in the form of heat. If a couple of additional neutrons that could cause fission of the neighboring nuclei were to be released during this process, a chain reaction would take place.
The most important challenge in achieving this was to find an element which would, during fission, release energy as well as free neutrons. At first, Szilard surmised that the appropriate elements were beryllium and iridium, but this supposition turned out to be wrong. Only later did it become evident that the most suitable element for exploiting nuclear energy was uranium.
A writer’s prediction of the future more accurate than a scientist’s
To finance his research he first turned to a factory-owner. It is interesting that he did not enclose scientific articles with his letter of request, but a passage from the novel by H. G. Wells, The World Set Free. In the novel, a scientist called Holsten invents a way to release the energy stored in atoms. “Of course, all of this is no more than a fantasy,” Szilard wrote in his letter to the factory-owner, intentionally citing Rutherford’s own words, “but I have good reasons to believe that, when it comes to industrial applications of today’s discoveries in the field of physics, it could soon turn out that the predictions of writers are more accurate than the predictions of scientists.” It was also none other than H. G. Wells that invented the term “atom bomb”.
Soon after this, Szilard had to face an important decision. Should he reveal his findings on nuclear energy to the public by publishing them and risk his idea going into the wrong hands, or should he keep it a carefully protected secret? Holsten, the scientist from H. G. Wells’s novel, decided that he was no more than an insignificant instrument in the grand machinery of progress and change. Even if he burned all his papers, someone else would come to the same conclusions in a few years. But Szilard decided otherwise. Rather than describe his idea in a scientific article and publish it in some renowned science magazine, he decided to keep it secret.
He first thoroughly described the details of his discovery of the possibilities of exploiting the energy of atoms. Then, he patented his idea about critical mass and chain reaction, produced with the aid of neutrons. In 1935, he transferred the rights to the patent for the realization of these important ideas to the British Army on condition that everything remains an absolute secret. During the following years, before the Second World War, everyone who new about the idea made an effort to prevent any information revealing the discovery of the exploitation of nuclear energy from reaching Hitler’s scientists. It was only in 1939 that Szilard confirmed his theory by experiment. In February that year, an experiment was carried out in the laboratories of Columbia University in New York which later lead to the realization of the first controlled chain reaction, starting the era of the exploitation of nuclear energy. Szilard built the nuclear reactor on Manhattan with Enrico Fermi, an expert on bombarding atomic nuclei with neutrons. Several years earlier in Rome, Fermi had already discovered that decelerated neutrons fuse better with atomic nuclei than faster neutrons, so it was important to find an appropriate substance that could slow down neutrons. It turned out that one suitable option was heavy water, but it was difficult to obtain it in sufficient quantities. To execute the experiments, a more accessible medium that could perform the task had to be found.
How to beat the Nazis?
As Szilard feared, German scientists were also thinking of ways to build a nuclear reactor. However, they also had difficulties in obtaining sufficient amounts of heavy water, so they were searching for alternative solutions as well. Both were testing graphite, but fortunately the Germans gave it up quickly after they had concluded that graphite absorbed neutrons too well. Szilard, on the other hand, discovered that the problem lied in the addition of boron to the industrial manufactured graphite and not in the graphite itself. It turned out that pure graphite was a very suitable medium for slowing down neutrons in a nuclear reactor. Szilard was much more aware of the gravity of the moment than Fermi. In the summer of 1939, Fermi left for the University of Michigan to study cosmic radiation, so Szilard decided to turn to his old colleague Albert Einstein who, at the time, lived on Long Island some hundred kilometers from New York. He presented his idea which was actually based on the famous equation E=mc2. Einstein replied that he had never thought of anything like that when he was contemplating the possible applications of his infamous equation relating mass to energy.
On his second visit to Einstein, Szilard brought with him a letter he had written to President Roosevelt warning him of the danger that Hitler’s Germany might be researching the use of nuclear fission to create an atomic weapon, and suggesting that the U.S should seriously consider studying the possibility itself. It is reported that Einstein read and signed the letter which later turned out to be extremely important wearing only his dressing gown. The letter began with the words “The recent discoveries of E. Fermi and L. Szilard …” and was dated August 2nd 1939.
Of course, Szilard did not simply send the letter to the President by mail as it might have come into the wrong hands, but asked an acquaintance who was on of the President’s economic advisers to bring it directly into the White House. When Roosevelt finally read the letter he realized that something absolutely had to be done to “prevent the Nazis from blowing us all into the air”. He founded the Uranium Committee, of which Szilard was made member and which set in motion the research that ultimately lead to the creation of the atom bomb, but also nuclear reactors which we use to produce great amounts of electrical energy today, and are, according to many environmentalists, one of the most acceptable energy sources in terms of the effects of global climate change.
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