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Three frames of Albert Einstein and Leo Szilárd from the documentary film “Atomic Power” (1946) for TIME’s The March of Time series

The now famous Einstein-Szilárd letter was written at the initiative of Hungarian nuclear physicist Leó Szilárd with help from Edward Teller and Eugene Wigner in 1939. It was signed by Albert Einstein and sent to the President of the United States, Franklin D. Roosevelt in October 1939. The letter argued that the United States should engage in uranium research. Its writing was motivated by the news of the discovery of uranium fission by Otto Hahn and Fritz Strassmann nine months prior.

The letter prompted Roosevelt to propose the undertaking which would later become the Manhattan Project, producing the first nuclear weapons and — following the atomic bombings of Hiroshima and Nagasaki — leading to the unconditional surrender of Imperial Japan and the conclusion of World War II.

Leo Szilárd’s Role

Hungarian physicist Leó Szilárd (1898–1964) was born in Budapest in 1898 and attended the Palatine Joseph Technical University before enlisting in the Austro-Hungarian Army during World War I. After the war, he resumed engineering studies but due to the chaotic political situation in Hungary eventually left for Berlin in 1919, where he enrolled at the Technische Hochschule, eventually transferring to instead pursue physics at Friedrich Wilhelm University. There, he attended lectures by Planck, Franck, von Laue and Albert Einstein (1879–1955). Szilárd’s doctoral dissertation Über die thermodynamischen Schwankungserscheinungen (“On The Manifestation of Thermodynamic Fluctuations”) was completed in 1922, winning top honors. He was appointed as an assistant to von Laue in 1924 and completed his habilitation in 1927 to become a Privatdozent. Szilárd was given German citizenship in 1930 but was forced to leave the country in 1933 following Adolf Hitler ascent to power.

In the UK, beginning in 1933, Szilárd worked as a physicist in St. Bartholomew’s Hospital, working on radioactive isotopes for medical purposes. By 1938–39, he was working as a visiting researcher in the U.S., eventually settling at Columbia University. His research concerned nuclear chain reactions, a concept he had conceived of in 1933 while reading Ernest Rutherford (1871–1937)’s disparaging remarks about an experiment of his students John Cockcroft (1897–1967) and Ernest Walton (1903–1995). In the experiment — now considered the first man-made splitting of an atom — the two used protons from an accelerator to split lithium-7 into alpha particles. The experiment showed that much greater amounts of energy were produced by the reaction than that which was supplied by the proton. Cockcroft and Walton would be awarded the 1951 Nobel Prize in Physics for the discovery. However, citing “inefficiencies in the process”, Rutherford had dismissed the idea that such a concept could be used to generate power in the future:

“We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms.” — Ernest Rutherford

British physicist James Chadwick (1891-1974) had discoverd the neutron a year before, in 1932. He also determined that the neutron was a new elementary particle, distinct from the proton, albeit of similar mass. Having been trained both as an engineer and a physicist, Szilárd postulated that if instead of a proton, Cockroft and Walton had used neutrons, the process might have been a self-perpertuating chaing reaction capable of producing power without the need for protons or an accelerator.

Szilárd even proposed a patent for the idea the following year, the first for a nuclear reactor. Although the patent was granted in 1936, it was not published until 1949. In his momentous book The Making of the Atomic Bomb* Richard Rhodes describes Szilárd’s moment of inspiration for the nuclear chain reacion in the following way:

In London, where Southampton Row passes Russell Square, across from the British Museum in Bloomsbury, Leo Szilard waited irritably one gray Depression morning for the stoplight to change. A trace of rain had fallen during the night; Tuesday, September 12, 1933, dawned cool, humid and dull. Drizzling rain would begin again in early afternoon. When Szilard told the story later he never mentioned his destination that morning. He may have had none; he often walked to think. In any case another destination intervened. The stoplight changed to green. Szilard stepped off the curb. As he crossed the street time cracked open before him and he saw a way to the future, death into the world and all our woes, the shape of things to come.
— Excerpt, The Making of the Atomic Bomb* by Rhodes (1986)

Over the next few years, beginning in 1935, Szilard attempted to generate nuclear chain reactions using beryllium and indium bombarded with X-rays. Still living in the UK, he had emigrated from Germany after the passing of the Berufsbeamtengesetz (“Law for the Restoration of the Professsional Civil Service”) law and the following “Great Purge of 1933” in which large number of “insufficiently Aryan” civil servants had to leave their jobs. He even wrote Winston Churchill’s scientific advisor Frederic Lindemann (1886–1957) to discuss “the question of whether or not the liberation of nuclear energy can be achieved in the immediate future”, in particular whether “double neutrons“ could be produced “then it is certainly less bold to expect this achievement in the immediate future than to believe the opposite” (Rhodes, 1986).

By the time of the writing of the Einstein-Szilárd letter in 1939, Szilárd was working from Columbia University in New York City, living at King’s Crown Hotel on West 116th Street. The news of the discovery of nuclear fission by Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980), and its theoretical explanation by Lise Meitner (1878–1968) and Otto Frisch (1904–1979), reached New York in early February 1939. Niels Bohr (1885–1962), staying at the Princeton Faculty Center, had arrived with the news from Copenhagen on February 4th. Szilárd learned of it from Eugene Wigner (1902–1995) when he visited Princeton a few days later. Although the implications of Bohr’s news were still unclear, Szilárd’s mind quickly postulated that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction, yielding massive amounts of energy for electric power generation and potentially, atomic bombs. He tried but failed to convince Enrico Fermi (1901–1954) of its potential, and so set out on his own to show experimentally that such was the case.

Szilárd lecturing on nuclear fission in the 1940s (Photo: unknown)

Bohr had arrived with the news from Europe in early February. By the end of February, Szilárd had applied for and obtained permission to use a laboratory at Columbia for three months. He next convinced the postgraduate researcher Walter Zinn (1906–2000) to collaborate with him. Zinn had been conducting experiments bombarding uranium with 2.5MeV neutrons he obtained from a small accelerator. As Rhodes writes, Szilárd suggested that Zinn’s experiments would be more successful if he used lower energy neutrons. Zinn agreed, but didn’t know how to obtain them. Szilárd did, and so suggested the two collaborate on the following experiment:

"All we needed to do," he explained later, "was to get a gram of radium, get a block of beryllium, expose a piece of uranium to the neutrons which come from the beryllium,, and then see by means of the ionization chamber which Zinn had built whether fast neutrons were emitted in the process. Such an experiment need not take more than an hour or two to perform, once the equipment has been built and if you have the neutron source."
— Excerpt, The Making of the Atomic Bomb* by Rhodes (1986)

Szilárd wrote Lindemann at Oxford University for him to ship a cylinder of beryllium he had left in a laboratory there. It arrived on February 18th. The neutron source, two grams of radium sealed in a brass capsule, Szilárd rented using funds borrowed from a fellow inventor named Benjamin Liebowitz. It arrived in early March. From there, the process was simple. In a laboratory at Columbia, Szilárd and Zinn used their radium-beryllium source to bombard a piece of uranium with neutrons

"Everything was ready and all we had to do was to turn a switch, lean back, and watch the screen of a television tube. If flashes of light appeared on the screen, that would mean that neutrons were emitted in the fission process of uranium and this in turn would mean that the large-scale liberation of atomic energy was just around the corner. We turned the switch and saw the flashes. We watched them for a little while and then we switched everything off and went home."
— Excerpt, The Making of the Atomic Bomb* by Rhodes (1986)

As Szilárd later wrote,

“That night, there was very little doubt in my mind that the world was headed for grief”

Szilárd and Zinn roughly estimated the production of neutrons emitted per fission to be two. Szilárd immediately notified his friends Wigner and Teller at Princeton. The latter remembered the moment vividly:

I was at my piano, attempting with the collaboration of a friend and his violin to make Mozart sound like Mozart, when the telephone rang. It was Szilárd, calling from New York. He spoke to me in Hungarian, and he said only one thing: “I have found the neutrons”. — Edward Teller

Although they had found neutrons there, on the seventh floor of Pupin Hall at Columbia University, they had not found a nuclear chain reaction. The result was however significant enough to help Szilárd convince Fermi and his collaborator Herbert L. Anderson (1914–1988) to try a larger experiment, using 230 kg of uranium and a neutron moderator in the form of carbon to slow the neutrons down, maximizing the chance of fission. Szilárd and Fermi collaborated on the design of what would become the first nuclear reactor to maintain a self-sustaining nuclear chain reaction. The design consisted of a pile of uranium oxide blocks interspersed with graphite bricks. Its design is detailed in the publication:

Anderson, H.; Fermi, E.; Szilárd, L. (1939). Neutron Production and Absorption in Uranium. Physical Review. 56 (3): 284–286.

It was published on August 1st, 1939. Fermi and his collaborators went on to achieve the first successful self-sustaining nuclear chain reaction on December 2nd, 1942 at the University of Chicago.

**Left**: The “Chicago Pile-1 “ (CP-1), a diagram of the first nuclear reactor to achieve a self-sustaining chain reaction. **Right**: Szilárd and Norman Hilberry (1899–1986) at the site of CP-1 at the University of Chicago, some years after the war.

Einstein’s Role

Albert Einstein had by the summer of 1939 been involved in the anti-Nazi movement for over six years. Persecuted in Germany, he abandoned his professorship at the University of Berlin in December 1932, months before the enactment of the Berufsbeamtengesetz. As the story goes, Einstein feared for the safety of himself and his family, having been listed in a German magazine as an “enemy of the German regime” with an accompanying illustration marked “not yet hanged” with a $5,000 bounty (Jerome & Taylor, 2006).

Photo of Churchill and Einstein taken at Chartwell during the summer of 1933. From the article “Genius Loves Company” in Airmail

First relocating to the UK, Einstein eventually settled at the newly founded Institute for Advanced Study in Princeton, New Jersey in 1933. From this “quaint and ceremonious village populated by puny demigods on stilts” (his words) Einstein played a pivotal role in the rescue operations that commenced as the persecution of Jewish German scientists intensified. Indeed, it was Einstein who initially motivated Winston Churchill to send his chief scientific advisor Frederick Lindemann (1886-1957) to Germany to help Jewish scholars, including Max Born (1882–1970), find work in England.

The Letter

“Sir: Some recent work […] leads me to expect that the element of uranium may be turned into a new and important source of energy in the immediate future.” — Albert Einstein

Following his confirmation of the viability of a nuclear chain reaction, Szilárd grew even more concerned. That is, he worried that German scientists working under the Nazi regime might attempt to exploit nuclear fission for bomb-making purposes. He concluded, somewhat surprisingly, that among the first on the list to be warned were the Belgian government, because the Belgian Congo was the best source of uranium ore. He came to this conclusion after conferring with his compatriots Teller and Wigner. But who could communicate the warning to the Belgian government? As Rhodes (1986) writes, “it occurred to Szilárd that his old friend Albert Einstein knew the Queen of Belgium”, having met her on a trip to Antwerp in 1929. The three “Martians” (Szilárd, Teller and Wigner) hence concluded that he, by that time a celebrity, would be the most suitable person to warn them.

Einstein and Szilárd’s history went back to when they first met in the early 1920s in Berlin. Einstein had been thoroughly impressed by Szilárd’s doctoral dissertation. The two even designed and patented an Einstein-Szilárd refrigerator pump in 1927, which was later used for the circulation of liquid sodium coolant in nuclear reactors (Robinson, 2015). And so, on July 12th 1939 Szilárd and Wigner got in the latter’s car (Szilárd didn’t own a car, indeed never even learned to drive) and drove to Cutchogue, Long Island where Einstein was vacationing (Rhodes, 1986). Reportedly, the two Hungarians “had no luck soliciting directions to the house”, and at one point, according to Szilárd, “We were at the point of giving up and going back to New York” when “I saw a boy aged maybe seven or eight standing on the curb. I leaned out of the window and I said,

‘Say, do you by any chance know where Professor Einstein lives?’”

The boy knew, and directed them. Szilárd told Einstein about his Columbia experiments, his calculations on chain reactions in uranium and graphite, and the need to warn the Belgian government. Surprised to learn that the great man was not aware of his paper on nuclear chain reactions, Szilárd explained the possibility of generating chain reactions as the result of nuclear fission. Einstein’s reaction was to blurt out

“I never thought of that!”

From his own years of resisting nazism, Einstein was however quick to instinctively share his visitors’ fear that the Nazi’s would use the knowledge to build weapons (Robinson, 2015). He later described Einstein being “very quick to see the implications and perfectly willing to assume responsibility for sounding the alarm even though it was quite possible that the alarm might prove to be a false alarm”. Although hesitant to write the Queen of Belgium directly, Einstein suggested instead contacting an acquaintance who was a member of the Belgian cabinet. Wigner next suggested that the U.S. government should be warned as well, pointing out that the three immigrants were approaching a foreign government without giving the U.S. State Department an opportunity to object.

Reportedly, Einstein dictated a letter to the Belgians and Wigner wrote it down in longhand German. Szilárd drafted a cover letter. Over the next three weeks, the three men went back and fourth with several drafts. At one point an economist named Alexander Sachs (1893–1973) in conversation with Szilárd convinced him that the matters they were discussing first and foremost concerned the White House. He hence proposed that the best thing to do from a practical point of view was to inform the U.S. President Franklin D. Roosevelt (1882–1945) directly. Sachs, having contributed economics texts to Roosevelt’s campaign speeches in 1932, insisted that if Szilárd and Einstein formulated the text, he could communicate it to Roosevelt. Drawing on Einstein’s first draft, Szilárd prepared a new draft of their letter, meant specifically for the President. Szilárd went to see Einstein for his signature on July 30th, this time driven by Teller, who later described being “entered [into] history as Szilárd’s chauffeur”.

Sachs traveled to Washington in October, bringing with him the letter baring Einstein’s name. On October 11th, he presented himself in the late afternoon to Roosevelt in the Oval Office.

“Alex, what are you up to?”

Sachs opened with a story of a young American inventor who wrote a letter to Napoleon (Rhodes, 1986), Robert Fulton (1765–1815), the inventor of the steamboat and submarine. Next, Sach’s tone grew more serious as he cautioned the President to listen carefully. In addition to Einstein and Szilárd’s letter, he read his own summation which he thought more suitable for Roosevelt’s level of understanding of scientific matters. His statement ended with the words:

“Personally I think there is no doubt that sub-atomic energy is available all around us, and that one day man will release and control its almost infinite power. We cannot prevent him from doing so and can only hope that the will not use it exclusively in blowing up his next door neightbor.”

A paragraph to which Roosevelt is said to have responded (Rhodes, 1986):

“Alex, what you are after is to see that the Nazis don’t blow us up”

The rest is history.

The Einstein-Szilárd Letter (August 2nd, 1939)
Sir:
Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element of uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the discussion which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefor that it is my duty to bring to your attention the following facts and recommendations:
In the course of the last four months it has been made probable — through the work of Joliot in France as well as Fermi and Szilárd in America — that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and former Czechoslovakia, wile the most important source of uranium is Belgian Congo.
In view of this situation you may think it is desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America. One possible way of achieving this might be for you to entrust with the task a person who has your confidence and who could perhaps serve in an inofficial capacity. His task might comprise the following:
a) to approach Government Departments, keep them informed of the further development, and put forward recommendations for Government action, giving particular attention to the problem of securing a supply of uranium ore for the United States;
b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contact with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.
I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.
Yours truly,
Albert Einstein

Thank you for reading.

Sincerely,

Jørgen.

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References

Jerome, F. & Taylor, R. 2006. Einstein on Race and Racism*. Rutgers University Press.
Rhodes, R. 1986. The Making of the Atomic Bomb*. Simon & Schuster.
Robinson, A. 2015. Einstein: A Hundred Years of Relativity*. Princeton University Press.

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The Einstein-Szilárd Letter (1939)