This paper highlights the contribution of Leo Szilard in the development of atomic age. Szilard and his mentor Einstein were both theorists who shared a practical side. Szilard's work, included publication of a theory in statistical mechanics was recognized a generation later as a seminal paper in information theory. Szilard studied X-ray diffraction. He designed and filed patents for an electron microscope, as well as for a linear accelerator and a cyclotron, which became instruments for probing the structure of an atom. Development was dramatically escalated after the 1942 demonstration of a chain reaction. A chain reaction would require finding an element or isotope with a high probability of absorbing neutrons followed by the release of more free neutrons. The element subsequently would split to form smaller atoms. Szilard launched initiatives for the peaceful use of nuclear energy while trying to stop the arms race and prevent further use of nuclear weapons.
Historians trace the birth of the atomic age to a 1939 letter to President Roosevelt signed by Albert Einstein. It was a month after Germany invaded Poland to begin World War II, the largest conflict in human history. The letter informed Roosevelt that the element uranium might be turned into a new and important source of energy by a chain reaction. It could also lead to an extremely powerful bomb. The letter warned that Germany might have taken over the uranium mines in Czechoslovakia, and added that the most important uranium source was the Belgian Congo.
The energy released by splitting a uranium atom had been defined by Einstein’s famous 1905 equation E=mc2. That and other ideas that transformed science’s understanding of the physical world had made Einstein famous. So the president took note.
Adolph Hitler’s Germany, for reasons that remain uncertain, failed to follow up with the development of a nuclear weapon, but Roosevelt responded to the letter with a modest program to investigate the possibility. It led to the top-secret Manhattan Project, and the 1945 destruction of the Japanese cities of Hiroshima and Nagasaki.
But the man who actually conceived of the possibility of a neutron-based chain reaction was hardly as famous as Einstein. He was an Einstein protégé named Leo Szilard, who probably wrote the letter that went to Roosevelt over Einstein’s name.
Einstein later said he had only served as Szilard s mailbox. And that’s how the relatively obscure Leo Szilard made things happen. Szilard—called “Genius in the Shadows” by his biographer, William Lanouette—once said he might live without any fixed home or position, but could call on Einstein and other associates placed to advance his ideas and concerns for the fate of the world.
Leo Szilard was born in Hungary in 1898. He studied engineering in Budapest until he was drafted for World War I into the Austro-Hungarian Army. The 1918 flu pandemic that killed 20 million people probably saved his life. He was bedridden while his artillery unit was slaughtered.
After the war, he escaped an anti-Semitic regime in Hungary in 1919. He traveled to the University of Berlin, where Albert Einstein, who had achieved fame with his theories of relativity, was a research professor. Szilard was assigned a dissertation topic by Nobel laureate Max Van Laue. He concluded that the problem was unsolvable, and redirected his attention to a contradiction in the Second Law of Thermodynamics known as “Maxwell’s Demon.” It was a paradox defined 50 years earlier by James Clerk Maxwell.
Szilard proceeded to solve it, but it put him in the awkward position of having solved the wrong problem. So Szilard approached Einstein to explain his analysis and dilemma. Einstein responded with a favorable review, and Szilard was awarded a Ph.D.
Szilard and his mentor Einstein were both theorists who shared a practical side. Szilard’s work included publication of a theory in statistical mechanics that would be recognized a generation later as a seminal paper in information theory.
Szilard studied X-ray diffraction. He designed and filed patents for an electron microscope, as well as for a linear accelerator and a cyclotron, which became instruments for probing the structure of an atom.
While Albert Einstein is famous for theoretical physics, he also understood invention. His uncle, Jacob Einstein, was an inventor who patented an electric generator and meter. As a four-year-old in 1883, Albert Einstein watched his father, Hermann, and his uncle install the first electric lighting system for the Octoberfest in Munich.
Albert Einstein experimented with steam engines and other mechanical machines, and hoped to study engineering, but he failed the entrance exams. Repulsed by the militarism he witnessed in Germany, he moved to Switzerland, where he studied physics and received a diploma from the Federal Institute of Technology in Zurich in 1900.
When he failed to get an academic position, his experience with machines and understanding of invention served him. He was hired in 1902 as a clerk in the Swiss Patent Office. He was good at the job. It gave him time for the abstract thinking that led to his revolutionary 1905 publications about relativity, quantum theory, atomic motion, and the equivalence of mass and energy.
Seeking a Better Refrigerator
Physics of the 1920s provided Einstein and Szilard the excitement of discovering mysteries of nature, but seemed to be of little practical value. Meanwhile, the world desperately needed a better refrigerator.
Refrigerators were being introduced with the huge potential benefit of preserving food, but the early models relied on ammonia, which was known to leak into homes and had sometimes killed entire families. Szilard approached Einstein to collaborate on the invention of a safer refrigerator. It was an opportunity to do well financially while doing good for humanity.
They worked together for seven years to produce a heat-driven, absorption-cycle refrigerator. Circulation was achieved by their invention of an electromagnetic pump with no moving parts and minimal risk of leakage. Their refrigerator was licensed to General Electric in Germany. Szilard was hired as a consultant. However, the subsequent inventions in the United States of freon to replace ammonia and of the sealed electric compressor made the Szilard-Einstein refrigerator obsolete.
When Adolph Hitler came to power in 1933, it was time to leave Germany again. Einstein accepted a position at the Institute of Advanced Study in Princeton, where he continued his interest in invention. He collaborated with Gustav Bucky, a physician and inventor of medical equipment, and in 1936 they received a patent for an automatic-exposure camera. It used the photoelectric effect, which Einstein had explained in 1905. “His discovery of the law of the photoelectric effect” was specifically cited when he was presented the Nobel Prize in Physics in 1921.
Szilard, meanwhile, escaped to England, where the neutron had been discovered by John Chadwick in 1932. Pioneering atomic physicists Ernest Rutherford and Niels Bohr proclaimed the neutron to be an important scientific discovery, but did not predict any practical significance, although the science fiction writer H.G. Wells had described an atom bomb in a novel, The World Set Free, published in 1914.
Leo Szilard was crossing a London street in September 1933 when he conceived of the possibility that a neutron released by one atom could penetrate the nucleus of another atom. It could be the basis of a chain reaction. It could release tremendous amounts of nuclear binding energy for peaceful purposes, but could also be the basis for a weapon of mass destruction.
"Szilard ’s new fear was that the bomb would be used against Japan with unacceptable civilian death and destruction, to be followed by a nuclear arms race"
Szilard applied for a secret patent in England. A chain reaction would require finding an element or isotope with a high probability of absorbing neutrons followed by the release of more free neutrons. The element subsequently would split to form smaller atoms. Szilard felt a sense of excitement and despair. He was in the bizarre position of hoping to prove his invention was impossible.
Rutherford rejected Szilard’s request to use his lab at Cambridge, but Szilard received permission from a London hospital. He proceeded to neutron-activate various elements and was relieved at his negative results.
Szilard came to the United States in 1938, and his fear of an atomic bomb returned a year later. He traveled to Princeton to see his old friend Paul Wigner, who would later win a Nobel Prize in Physics. They were both Hungarian natives and had worked together in Germany before escaping the Nazis. Wigner informed him that the German radio-chemist Otto Hahn had used neutrons to split the uranium atom.
Word had come out because Hahn had asked his longtime collaborator, Lise Meitner, who had worked with Szilard before she, too, had left Germany, to explain the creation of barium.
While taking a walk in a park, Meitner and her physicist nephew, Otto Frisch, concluded that the uranium atom had been split, since barium is approximately half the mass of uranium. She used Einstein’s E=mc2 to calculate the energy released. Frisch, who had been working with Niels Bohr, calculated the same energy using a surface tension model of the nucleus. They predicted that a single atom could release enough energy to lift a visible size grain of sand a visible height.
This news reached the United States via Niels Bohr, who attended a conference in Washington in January 1939. It is an accident of history that this apparently obscure discovery, which would ultimately change military power structures throughout the world, corresponded to the start of World War II in Europe.
Szilard traveled to Columbia University to join another refugee, Enrico Fermi, who had taken refuge from Fascist Italy. Fermi had received the 1938 Nobel Prize for performing neutron activation experiments that were similar to those that Szilard had been doing in secret in England. Fermi had discovered that probability of capture was increased if fast neutrons were slowed down by collisions with a non-absorber or moderator. A neutron source was set up next to a pile of uranium with graphite for a moderator. Neutron multiplication was demonstrated. A purer form of graphite and the right fuel might result in a self-sustaining chain reaction.
One option would be to enrich the uranium in the easier-to-split U-235 isotope. The other possibility was to use surplus neutrons in a critical reactor to produce fissionable plutonium 239 from the more abundant U-238 isotope. Szilard was alarmed at the implications.
A German physicist, Paul Harteck, had already informed Hitler that an atomic bomb was possible and had commenced experiments.
Informing the President
Szilard’s dilemma was how to inform President Franklin Roosevelt of the looming danger. Einstein, who at the time was enjoying a sailing vacation on Long Island, might be able to relay the warning. Paul Wigner, who agreed to drive, and Szilard found Einstein, who was unfamiliar with the recent discoveries. Einstein listened and responded that he had never thought of a chain reaction, but agreed with the possibility.
Szilard proceeded to draft a letter to Roosevelt. Once again he needed a ride, this time to get Einsteins signature. He recruited his friend Edward Teller, who was also a refugee from Hungary. Years later, Teller would be known as the father of the hydrogen bomb. He would joke about meeting Einstein as Szilard’s chauffeur.
Einstein compromised his pacifism to sign the letter. Germany invaded Poland in September 1939. The letter was delivered to the president in October by Alexander Sachs, a businessman, biologist, and economist.
Roosevelt called for action. With funding of $6,000, Szilard arranged for the production of graphite that was not contaminated by neutron-capturing boron.
Szilard and Fermi moved to the University of Chicago, and designed a reactor using natural uranium and the purified graphite. A chain reaction was demonstrated in December 1942, a year after the United States had entered the war.
It took four years from the discovery that uranium could be split to the demonstration of a self-sustaining chain reaction. Thirteen years later, after the technology was declassified, Szilard and Fermi were awarded U.S. Patent No 2,708,656 on the basis of this first critical reactor. It was also the basis for their induction into the National Inventors Hall of Fame.
The Manhattan Project
Development was dramatically escalated after the 1942 demonstration of a chain reaction. The combined scientific and industrial program of unprecedented size and secrecy was code-named the Manhattan Project. The director was Gen. Leslie Grove, who had overseen the construction of the Pentagon. The research director was Robert Oppenheimer, a Harvard-educated New York native who had studied theoretical physics in England and Germany.
Huge secret facilities were built in Tennessee, Washington State, and New Mexico for uranium enrichment, plutonium production, assembly, and testing. The two options of enriched uranium 235 and plutonium 239 bred from uranium 238 were pursued. By the spring of 1945, both methods were shown to be workable.
Einstein was briefly recruited to analyze uranium separation, but was then excluded as a security risk. He was made an explosives consultant for the Navy at a fee of $5 per day. Szilard continued in Chicago, where he solved physics and engineering problems related to sustaining the chain reaction, fission products, structure, and cooling for the plutonium production reactors. Meanwhile, he introduced the idea of a breeder reactor for civilian power. It would produce fissionable plutonium faster than it depleted fissionable uranium.
By the spring of 1945, Germany was on the verge of surrender. It was learned that Germany had not made the atom bomb, while the United States would soon have both an enriched uranium and a plutonium bomb. Szilard’s new fear was that the bomb would be used against Japan with unacceptable civilian death and destruction, to be followed by a nuclear arms race that would threaten civilization.
Once again, Szilard shared his concerns with Einstein, who signed one more letter to Roosevelt, dated March 25, 1945. The letter told the president of Szilard’s great concern about lack of contact between the scientists developing a new weapon, and the president and members of the cabinet responsible for formulating policy.
There was no response. Roosevelt died 16 days later. Cabinet members informed Harry Truman about the bomb after he became president.
Szilard organized a petition that was signed by 155 scientists who were working on the project. It was to inform the new president of their moral doubts about the use of the new weapon. General Grove had been unsuccessfully trying to show that Szilard was a spy. He learned of the petition and stopped further circulation.
The fears of Szilard and Einstein were soon realized. President Truman ordered the atomic bombing of the Japanese cities of Hiroshima and Nagasaki in August 1945. Einstein spent much of his remaining 10 years in Princeton trying without success to halt the nuclear arms race and failing to develop a unified theory of forces.
Szilard launched initiatives for the peaceful use of nuclear energy while trying to stop the arms race and prevent further use of nuclear weapons. In 1945, he called for civilian control of nuclear energy. He organized international conferences and the Council for a Livable World. He undertook missions of personal diplomacy.
Szilard also resorted to humor, satire, and science fiction. He wrote the “Voice of Dolphins” and other parables that addressed the arms race, the morality of war, and the mismatch between the modern human’s technical capability and moral capacities.
Szilard shifted his research from physics to the life sciences and became a cell biologist. He studied aging and mutations while inventing laboratory equipment. He befriended Jonas Salk, whose vaccine had eliminated the scourge of polio, and Szilard was named one of the five founding fellows of the Salk Institute for Biological Studies. Another fellow was Francis Crick, who in 1953 had been a co-discoverer of DNA.
In 1951, at the age of 53, Leo Szilard had married his friend Gertrude Weiss, a physician. When he developed bladder cancer, he and Gertrude worked together to develop radiation treatment that was successful. Szilard left his hospital bed to discuss the arms race with Soviet leader Nikita Khrushchev, who was visiting New York and the United Nations. Szilard proposed a direct telephone hotline between Moscow and Washington, but no action was taken.
The Soviet Union and the United States came to the brink of nuclear war during the 1962 Cuban missile crisis. The risk of accidental war was compounded by lack of communications between Khrushchev and President John Kennedy. The hotline that Szilard had been proposing was finally installed. Leo Szilard died in his sleep of a heart attack two years later.