Leo Szilard

1. Lead

Leo Szilard (1898–1964) was a Hungarian-born physicist whose ideas helped to create the atomic age and whose conscience then drove him to campaign against the weapons he had helped to make possible. In 1933, while living in London as a refugee from Nazi Germany, he conceived the idea of the nuclear chain reaction and patented it, the first clear statement that a self-sustaining release of nuclear energy might be achieved.

Six years later, in August 1939, he drafted the letter that Albert Einstein signed and sent to President Franklin D. Roosevelt, warning that Germany might be working on an atomic bomb and urging the United States to begin its own research. That letter is widely regarded as one of the starting points of the Manhattan Project.

Szilard worked on the project at Chicago, where he helped to design the first nuclear reactor, Chicago Pile-1, which achieved the first controlled chain reaction in December 1942. Yet he also became one of the earliest and most persistent critics of the use of the atomic bomb, organising the Franck Report and a petition to the President in 1945 urging that the bomb not be used against Japan without warning.

After the war he left physics for biology, founded the field of molecular biology in part, and devoted his remaining years to arms control, to the Pugwash movement and to inventive schemes for preventing nuclear war. He was a brilliant, restless and often difficult man, remembered both as a prophet of the atomic age and as one of its most troubled consciences.

2. Key Facts

Field	Value
Born	11 February 1898, Budapest, Austria-Hungary
Died	30 May 1964, La Jolla, California, United States
Nationality	Hungarian-born; later American citizen
Role/Rank	Physicist; professor at the University of Chicago; Manhattan Project scientist
Known for	Conception of the nuclear chain reaction; Einstein–Szilard letter of 1939; Chicago Pile-1; Franck Report and 1945 petition against using the bomb
Wars	Second World War (Manhattan Project, 1942–1945)
Awards	Atoms for Peace Award, 1959; Albert Einstein Award, 1960
Fields	Physics, then molecular biology
Family	Married Gertrud Weiss in 1951; no children
Buried	Ashes scattered; memorial at the Salk Institute

3. Early Life and Education

Childhood in Budapest and the Move to Berlin

Leo Szilard was born Leó Szilárd on 11 February 1898 in Budapest, then part of Austria-Hungary, the eldest son of Lajos Szilárd, a civil engineer, and Tekla Vidor. He grew up in a prosperous, cultured Jewish family in a city that was one of the great centres of European science and music.

As a schoolboy he showed an exceptional talent for mathematics and physics, and he won prizes in national competitions. He was also interested in politics and in the future of society, and he liked to imagine inventions and to think about how science might change the world.

He served briefly in the Austro-Hungarian army during the First World War, an experience that left him with a lasting dislike of war and of military life. After the war he studied engineering at the Technical University of Budapest, but the political turmoil of the Hungarian Soviet Republic and the anti-Jewish measures that followed made it impossible for him to continue his studies there.

In 1919 he left Hungary for Berlin, a city that was then the centre of physics in Europe, and he never lived in his homeland again.

From Engineering to Physics

In Berlin, Szilard enrolled at the Technical University and then at the University of Berlin, where he moved from engineering to physics. He attended the lectures of Max Planck, Albert Einstein and Max von Laue, and he became part of the circle of young physicists who gathered around them.

He was a brilliant but undisciplined student, more interested in his own ideas than in formal examinations, and it took him several years to complete his doctorate. His thesis, on thermodynamics and the theory of fluctuations, was supervised by von Laue and completed in 1922.

It contained the germ of an idea that later became known as the Szilard engine, a thought experiment about Maxwell’s demon and the relationship between information and entropy, which is still discussed in physics today. His work on thermodynamics and on the physics of fluctuations showed a mind that was drawn to fundamental questions, and it laid the groundwork for the original thinking that would mark his whole career.

Berlin, Einstein and the Doctorate

Szilard’s years in Berlin were formative in more than one way. He became a close friend of Einstein, who admired his originality and his restless imagination, and the two men worked together on inventions, including a refrigerator without moving parts that they patented jointly.

He also began to think about the social consequences of science, and he wrote essays on how scientists might organise themselves to prevent the misuse of their discoveries. When Hitler came to power in January 1933, Szilard understood immediately what it meant for Jewish scientists in Germany.

He left Berlin within weeks, before most of his colleagues had grasped the danger, and he helped other refugee scientists to find posts abroad. His quick grasp of the political situation was characteristic: throughout his life he prided himself on seeing the consequences of events earlier than other people did.

The friendship with Einstein lasted for the rest of their lives, and it was Einstein who later gave weight to Szilard’s warning about the atomic bomb.

4. Rise to Prominence

The Chain Reaction Idea of 1933

Szilard’s most famous idea came to him in London in September 1933, while he was waiting to cross a street at Southampton Row. He had been reading about the experiments of Ernest Rutherford and about the discovery of the neutron by James Chadwick in 1932.

It occurred to him that if a neutron could split an atom and release two neutrons, those neutrons could split further atoms, producing a chain reaction that would release enormous energy. He realised at once that such a reaction could be used to make a bomb of unprecedented power, and he also realised that the idea must be kept secret.

In 1934 he took out a patent for the chain reaction in Britain, and he assigned it to the British Admiralty so that it would not be published. At the time, most physicists thought that a chain reaction was impossible in practice, because no suitable element was known, and Szilard’s idea was regarded as speculative.

His reputation at this stage rested more on his originality and his range of interests than on any single experimental achievement, and he was known as a man of ideas rather than as a laboratory physicist.

Exile in Britain and the United States

After leaving Germany, Szilard spent two years in Britain, working at Oxford and at St Bartholomew’s Hospital in London, where he carried out experiments on the absorption of neutrons. He was supported by academic refugee organisations, and he worked tirelessly to help other scientists escape from Germany.

In 1938 he moved to the United States, taking up a post at Columbia University in New York, where he worked with Enrico Fermi on neutron experiments. It was at Columbia that the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in Germany in December 1938 reached him, and he immediately understood its significance.

Fission meant that the chain reaction he had imagined in 1933 might now be possible with uranium, and he began experiments with Fermi to test whether uranium would release more neutrons than it absorbed. He worked tirelessly to help other refugee scientists find posts abroad, and his efforts saved many colleagues from the fate that awaited Jews who remained in Germany.

Patents and the Search for Support

Szilard’s work in these years was hampered by his position as a refugee without a permanent post and by his reputation for being difficult to work with. He quarrelled with colleagues over credit and over the direction of experiments, and he was often impatient with the slow pace of academic administration. Yet his insight into the military implications of fission was unmatched.

He persuaded Fermi to delay publication of their results so that the Germans would not learn of them, and he began to look for a way to warn the American government. He understood that the United States had no organisation for military research of this kind, and that a warning would have to come from someone whose name carried weight.

That understanding led directly to his approach to Einstein in the summer of 1939, and to the letter that changed the course of the war. His insight into the military implications of fission was unmatched among his colleagues, and it drove him to seek a hearing at the highest level of government.

5. Leo Szilard’s Role in the War

The Einstein–Szilard Letter of 1939

In July 1939, Szilard visited Einstein at his summer home on Long Island, accompanied by Eugene Wigner, another Hungarian physicist. He explained that fission made an atomic bomb possible and that Germany, which had some of the best physicists in the world and controlled the uranium mines of Czechoslovakia, might already be working on one.

Einstein, who had been a pacifist, agreed to sign a letter to President Roosevelt warning of the danger and urging the United States to support research into uranium. Szilard drafted the letter, and a second, shorter version was sent to the President on 2 August 1939. Roosevelt responded by setting up the Advisory Committee on Uranium, the first step towards the American atomic bomb programme.

Historians debate how much the letter itself influenced Roosevelt, since the programme developed slowly until the attack on Pearl Harbor, but there is no doubt that it was the first high-level warning the President received, and that it framed the argument in terms of a race with Germany.

The Manhattan Project and the Reactor

When the Manhattan Project was created in 1942 under General Leslie Groves, Szilard worked at the Metallurgical Laboratory in Chicago, where Fermi led the effort to build the first nuclear reactor. Szilard had already worked out many of the details of reactor design, including the use of graphite as a moderator and the arrangement of uranium in a lattice, and he held patents on several of the key ideas.

He was not given a senior administrative post, partly because Groves distrusted him and regarded him as a security risk, and partly because of his conflicts with other scientists. Nevertheless, his contribution to the design of the reactor was substantial, and on 2 December 1942, Chicago Pile-1, built under the stands of the university’s football stadium, achieved the first self-sustaining chain reaction in history.

The event proved that Szilard’s idea of 1933 could be realised, and it opened the way to the production of plutonium for the bomb.

Chicago Pile-1 and the Dispute over Credit

The achievement of the first chain reaction was followed by disputes over credit that lasted for the rest of Szilard’s life. Fermi is generally credited as the leader of the reactor project — he had won the Nobel Prize in 1938 for his earlier research on induced radioactivity — but Szilard always maintained that the essential ideas were his.

He filed patent claims with the United States government, and he was eventually awarded a share of the patent rights, but the dispute left him bitter. His defenders argue that his contribution was undervalued because he was an outsider, a refugee without a university chair, while his critics argue that he exaggerated his part and that the reactor was the work of a large team.

The truth is that the reactor required both Szilard’s theoretical insight and Fermi’s experimental leadership, and that neither man could have achieved it alone. The dispute left him bitter, and it shaped his later relations with the scientific establishment, which he regarded as slow to recognise the value of original ideas.

6. Key Decisions

To Warn Roosevelt

The first of Szilard’s great decisions was to warn the American government about the possibility of a German atomic bomb. He could have published his results and left the matter to others, or he could have kept silent and hoped that no one else would see the military implications of fission.

Instead, he chose to act, and he chose Einstein as the messenger because he knew that his own name would not carry enough weight with the President. The decision was not easy for a man who had been a pacifist in the 1920s, and it committed him to a programme that he would later regret.

Yet he believed at the time that the danger from Nazi Germany was so great that the United States had no choice, and that a German bomb would be a catastrophe for the world. His judgement was shaped by his experience as a refugee from Hitler’s Germany, and by his conviction that the Nazis would use any weapon without restraint.

The Franck Report and the Petition

By 1945, Szilard’s views had changed. Germany had been defeated, and Japan was the only remaining enemy, but he believed that the bomb should not be used against Japanese cities without warning. In June 1945 he helped to organise the Franck Report, named after its chairman, James Franck, which argued that a surprise atomic attack on Japan would damage America’s moral standing and would start an arms race with the Soviet Union.

The report proposed a demonstration of the bomb on a deserted island instead. When the report was rejected by the military authorities, Szilard organised a petition signed by more than sixty scientists at Chicago and at Oak Ridge, asking President Harry Truman not to use the bomb without first giving Japan a chance to surrender.

The petition never reached Truman, because it was held up by the military chain of command, and the bombs were dropped on Hiroshima and Nagasaki in August 1945. Szilard always regarded the failure of the petition as one of the great tragedies of his life.

Opposing the Use of the Bomb

Szilard’s opposition to the use of the bomb was based on both moral and political arguments. He believed that killing civilians on a large scale was wrong, even in war, and that the United States would lose its moral authority if it became the first nation to use such a weapon.

He also believed, as a matter of policy, that the use of the bomb would make a future arms race inevitable, because the Soviet Union would now feel compelled to build its own bomb. His critics have argued that he underestimated the difficulty of ending the war with Japan and that an invasion would have cost many more lives than the bombs did.

His defenders reply that Japan was already close to surrender and that a demonstration, or a warning, might have achieved the same result. The debate continues, and Szilard’s part in it remains one of the most discussed aspects of his career.

7. Controversies

Credit for the Chain Reaction

The question of who deserves credit for the chain reaction has been controversial since 1942. Szilard conceived the idea in 1933 and patented it, but the experimental proof came from the work of Fermi and his team at Chicago, and the discovery of fission itself was made by Hahn and Strassmann.

Some historians give Szilard the primary credit for the concept, while others argue that a concept without experimental realisation is only part of the story. Szilard himself was sensitive on the question, and he spent years pursuing patent claims against the United States government. His supporters argue that his contribution was undervalued because he was a refugee and an outsider, and because he did not hold a senior academic post.

His critics argue that he was difficult to work with and that he exaggerated his own part in a collective achievement. The most balanced view is that the chain reaction was the product of many hands, but that Szilard’s insight in 1933 was the essential first step, and that without it the reactor might have come later.

The Bomb, Deterrence and Arms Control

A second controversy concerns Szilard’s later career as an advocate of arms control. After the war he argued that the only way to prevent nuclear war was to create international controls and to educate political leaders about the dangers of the bomb.

He founded the Council for a Livable World in 1962, the first political action committee devoted to arms control, and he took part in the Pugwash Conferences, which brought scientists from East and West together to discuss disarmament. Some critics dismissed his schemes as naive, arguing that he underestimated the political realities of the Cold War and that his proposals, such as his idea for a hotline between Washington and Moscow, were impractical.

Others praised him for seeing earlier than most that nuclear weapons had changed the nature of war, and for insisting that scientists had a responsibility to warn the public. His later work was less influential than his work in physics, but it showed a consistency of purpose that his admirers have always stressed.

Praise and Criticism from Colleagues

Szilard’s colleagues were divided about him in life, and they remain divided in their memoirs. Those who admired him, including Einstein and Wigner, praised his originality, his courage and his willingness to take responsibility for the consequences of his ideas. Those who found him difficult, including Groves and some of his fellow physicists, described him as arrogant, quarrelsome and unwilling to accept the discipline of a large organisation.

Both views contain truth. He was a man of extraordinary intellectual gifts who found it hard to work within institutions, and his career suffered because of it. Yet his very independence allowed him to see what others missed, and to act when others hesitated, and that independence is part of the reason why his story continues to fascinate historians of science.

Yet his very independence allowed him to see what others missed, and to act when others hesitated, and that independence is part of the reason his story still fascinates historians.

8. Later Life and Death

Biology and the Salk Institute

After the war, Szilard left physics and turned to biology, a field in which he had no formal training but in which he made important contributions. He worked at the University of Chicago, where he helped to establish the field of molecular biology, and he carried out research on the ageing of cells and on the control of gene expression.

He was one of the first scientists to argue that biology could be studied with the methods of physics, and he influenced a generation of young biologists, including James Watson and Francis Crick, who acknowledged his encouragement. In 1963 he moved to La Jolla in California, where he became a fellow of the Salk Institute for Biological Studies, founded by Jonas Salk.

He continued to work on biology and on arms control, and he remained active until his health failed. His move from physics to biology was unusual, and it showed the breadth of his curiosity and his refusal to be confined to a single discipline.

Illness, the Voice of the Dolphins, and Death

Szilard’s later years were marked by ill health and by a series of inventive projects that mixed science with science fiction. He wrote a short book, The Voice of the Dolphins, published in 1961, in which he used the device of dolphins who had learned to speak to comment on the politics of the Cold War.

The book was a satire, but it was also a serious argument about the need for international cooperation, and it showed his gift for thinking in unconventional ways. He was diagnosed with bladder cancer in 1960, and he designed his own course of radiation treatment, insisting on a dose that his doctors thought too high.

The treatment was successful for a time, and he lived for four more years, but the cancer returned. He died in his sleep on 30 May 1964 in La Jolla, at the age of sixty-six, and his ashes were scattered over the Pacific Ocean.

Final Years in California

In his final years in California, Szilard continued to write and to campaign for arms control, even as his health declined. He worked on a scheme for a world government and on proposals for reducing the risk of accidental nuclear war, and he remained in touch with the Pugwash movement and with his old friends in physics.

He married Gertrud Weiss in 1951, and she was his companion for the last thirteen years of his life. Those who visited him in La Jolla remembered a man who was still full of ideas, still arguing about the future of science and still convinced that reason could prevail over the dangers that science had created.

His death was mourned by scientists in many countries, and his papers were preserved at the University of California, San Diego, where they remain a valuable source for historians of the atomic age. He remained in touch with the Pugwash movement and with his old friends in physics, and he never stopped arguing that reason could prevail over the dangers science had created.

9. Legacy

Father of the Chain Reaction

Szilard’s scientific legacy rests above all on the chain reaction. The idea he conceived on a London street in 1933 made nuclear power and nuclear weapons possible, and it remains the foundation of reactor physics. The first reactor at Chicago proved the concept, and every nuclear power station in the world today depends on the principle he described.

He is also remembered for the Szilard engine, his thought experiment about information and entropy, which has become important in the modern theory of information. His patents on the chain reaction and on reactor design were eventually recognised by the United States government, and he is now generally regarded as one of the founders of nuclear physics, even though he never received a Nobel Prize.

His story is told in every history of the atomic bomb, and his name is linked permanently with the beginning of the atomic age. His name is linked permanently with the beginning of the atomic age, and his story is told in every history of the Manhattan Project and of nuclear physics.

Arms Control and the Pugwash Spirit

Szilard’s second legacy is in arms control. He was one of the first scientists to argue that those who create a weapon have a responsibility to warn the world about its dangers, and his campaign against the use of the bomb in 1945 set a precedent for the political activity of scientists.

The Council for a Livable World, which he founded in 1962, still exists, and the Pugwash Conferences, in which he took part, received the Nobel Peace Prize in 1995. His idea of a direct communication link between Washington and Moscow was realised in the hotline established after the Cuban Missile Crisis of 1962, and his warnings about the dangers of nuclear war influenced a generation of policymakers.

His legacy in this field is less tangible than his scientific work, but it is no less important, and it is the part of his life that he himself valued most.

Szilard in Popular Memory

In popular memory, Szilard is often portrayed as the archetypal scientist-prophet, the man who saw the atomic bomb coming and then tried to stop it. He appears in histories, in novels and in films about the Manhattan Project, and his letter to Roosevelt is one of the best-known documents of the Second World War.

The portrayal is partly accurate and partly simplified, because it omits his difficulties as a colleague and his sometimes unrealistic political schemes. Yet it captures something essential about him: he was a man who thought about the consequences of his ideas before most other people did, and who was willing to act on what he saw.

That combination of foresight and moral courage is the reason why his name is still remembered, more than half a century after his death.

10. Assessment by Historians

The Scientist as Prophet

Historians of science have generally assessed Szilard as a man of extraordinary originality whose greatest achievement was to see the implications of fission before anyone else did. His conception of the chain reaction in 1933, six years before fission was discovered, is regarded as one of the great acts of scientific imagination of the twentieth century.

His letter to Roosevelt is seen as a decisive moment in the history of the atomic bomb, even if the programme that followed was shaped by many other factors. Historians also note that his contribution to the reactor at Chicago was substantial, and that his patents show how clearly he understood the practical details of reactor design.

On this view, Szilard was a prophet in the literal sense: he saw what was coming and he warned the world, and the world did not listen until it was almost too late.

The Moral Physicist

A second strand of assessment emphasises Szilard’s moral role. Historians who take this view argue that his campaign against the use of the bomb in 1945 was an act of courage, because it put him at odds with the military authorities and with many of his colleagues.

They point to the Franck Report and to the petition as evidence that scientists could, and should, take responsibility for the uses of their work. They also praise his later work on arms control, and they argue that his insistence on international cooperation was ahead of its time.

On this view, Szilard’s importance lies not only in what he discovered but in what he did with the discovery, and in the example he set for later generations of scientists who have tried to control the weapons they helped to create. They also praise his later work on arms control, and they argue that his insistence on international cooperation was ahead of its time and remains relevant today.

Limits of the Legend

More recent scholarship has added a note of caution to the legend. Historians have pointed out that Szilard’s role in the Manhattan Project was smaller than he claimed, and that the reactor was the work of a large team led by Fermi. They have also noted that his political schemes were often impractical, and that his influence on policymakers was limited.

Some have argued that his opposition to the bomb in 1945 came too late, and that he had already accepted the logic of the bomb programme when he wrote to Roosevelt in 1939. The balanced view that has emerged is that Szilard was a brilliant and original thinker whose insight into the chain reaction was genuine and important, whose moral courage in 1945 deserves respect, and whose later career shows both the strengths and the limits of the scientist as a political actor.

He was neither a saint nor a villain, but a complex man whose life illustrates the dilemmas of science in the twentieth century.

Frequently Asked Questions

Who was Leo Szilard?
Leo Szilard was a Hungarian-born physicist who conceived the nuclear chain reaction in 1933, helped to warn President Roosevelt about the atomic bomb in 1939, and worked on the first nuclear reactor at Chicago during the Second World War.

What was the Einstein–Szilard letter?
It was a letter drafted by Szilard and signed by Albert Einstein on 2 August 1939, warning President Franklin D. Roosevelt that Germany might be developing an atomic bomb and urging the United States to begin research into uranium. It led to the creation of the Advisory Committee on Uranium.

What was Chicago Pile-1?
Chicago Pile-1 was the first nuclear reactor, built under the stands of the University of Chicago football stadium. On 2 December 1942 it achieved the first self-sustaining nuclear chain reaction in history, proving that Szilard’s idea of 1933 could be realised.

Why did Szilard oppose the use of the atomic bomb?
He believed that using the bomb against Japanese cities without warning was morally wrong and would damage America’s standing in the world. He also believed it would start an arms race with the Soviet Union. He helped to organise the Franck Report and a petition to President Truman in 1945.

What did Szilard do after the war?
He left physics for biology, helping to found the field of molecular biology at the University of Chicago. He later moved to the Salk Institute in California and devoted much of his time to arms control and to the Pugwash movement.

Why is Szilard important today?
He is remembered as the originator of the chain reaction, as the man who warned Roosevelt about the bomb, and as one of the first scientists to campaign for arms control. His life raises questions about the responsibility of scientists that are still debated today.

Related articles

• August von Mackensen
• Erich Ludendorff
• Ferdinand Foch

Sources

• Leo Szilard, The Voice of the Dolphins and Other Stories, 1961
• William Lanouette and Bela Silard, Genius in the Shadows: A Biography of Leo Szilard, 1992
• Richard Rhodes, The Making of the Atomic Bomb, 1986
• Spencer Weart and Gertrud Weiss Szilard, editors, Leo Szilard: His Version of the Facts, 1978
• James Franck and others, The Franck Report, June 1945
• United States Department of Energy, Manhattan Project history volumes
• Wikipedia, "Leo Szilard"