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Chain Reaction: From Einstein to the Atomic Bomb

From his theoretical contributions to his influential letter to President Roosevelt, here is the real story of Einstein's indirect yet pivotal role in the atomic bomb's development.

By Walter Isaacson
Mar 18, 2008 12:00 AMFeb 19, 2024 2:11 PM
Atomic Bomb
(Credit: Pixilated Planet/Getty Images)


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In the popular imagination, Albert Einstein is intimately associated with the atom bomb. A few months after the weapon was used against Japan in 1945, Time put him on its cover with an explosion mushrooming behind him that had E = mc2 emblazoned on it. 

In a story overseen by an editor named Whittaker Chambers, the magazine painted a vivid picture of Einstein. It revealed the character of a reserved, almost saint-like, youthful figure with kind brown eyes, the weary expression of someone who has seen too much, and wildly unkempt hair reminiscent of the northern lights. 

Did Albert Einstein Help Build the First Atomic Bomb?

Yet, Albert Einstein was not directly involved in the development of the atomic bomb. He influenced its creation in two major ways: first, by initiating the research that led to the bomb's development in the United States, and second, through his famous equation, E = mc2, which provided the essential theoretical basis for the atomic bomb's possibility.

Newsweek, likewise, did a cover on him, with the headline “The Man Who Started It All.” This was a perception fostered by the U.S. government. It had released an official history of the atom bomb project that assigned great weight to a letter Einstein had written to President Franklin Roosevelt warning of the destructive potential of an atomic chain reaction.

All of this troubled Einstein. “Had I known that the Germans would not succeed in producing an atomic bomb,” he told Newsweek, “I never would have lifted a finger.” He pointed out, correctly, that he had never actually worked on the bomb project. 

And he claimed to a Japanese publication, “My participation in the production of the atom bomb consisted in a single act: I signed a letter to President Roosevelt.” Neither the public image nor the personal protests capture the true, complex story of Einstein and the bomb.

Read More: 5 Interesting Things About Albert Einstein

The Real Story of Einstein and the Atomic Bomb

Contrary to common belief, Einstein knew little about the nuclear particle physics underlying the bomb. On the other hand, as the archives show, Einstein did not merely sign the letter to Roosevelt. He was deeply involved in writing it, revising it, and deciding how to get it to the president.

Leó Szilárd: The Catalyst of Einstein’s Historic Letter

The tale begins with Leó Szilárd, a charming and slightly eccentric Hungarian physicist who was an old friend of Einstein’s. While living in Berlin in the 1920s, they had collaborated on the development of a new type of refrigerator, which they patented but were unable to market successfully. 

After Szilárd fled the Nazis, he made his way to England and then New York, where he worked at Columbia University on ways to create a nuclear chain reaction, an idea he had conceived while waiting at a stoplight in London a few years earlier. When he heard of the discovery of fission using uranium, Szilárd realized that element might be used to produce this phenomenon.

Szilárd discussed the possibility with his friend Eugene Wigner, another refugee physicist from Budapest, and they began to worry that the Germans might try to buy up the uranium supplies of the Congo, which was then a colony of Belgium. 

But how, they asked themselves, could two Hungarian refugees in America find a way to warn the Belgians? Then Szilárd recalled that Einstein happened to be friends with that country’s Queen Elizabeth.“We knew Einstein was somewhere on Long Island, but we didn’t know precisely where,” Szilárd recalled. So he phoned Einstein’s Princeton, New Jersey, office and was told he was renting the house of a Dr. Moore in the village of Peconic. On Sunday, July 16, 1939, they embarked on their mission with Wigner at the wheel (Szilárd, like Einstein, did not drive). But when they arrived, they couldn’t find the house, and nobody seemed to know Dr. Moore. 

Then Szilárd saw a young boy standing by the curb. “Do you, by any chance, know where Professor Einstein lives?” he asked. Like most people in town, the boy did, and he led them up to a cottage near the end of Old Grove Road, where they found Einstein lost in thought.

Sitting at a wooden table on the porch of the sparsely furnished cottage, Szi­lárd explained how an explosive chain reaction could be produced in uranium layered with graphite by the neutrons released from nuclear fission: Those neutrons would split more nuclei, and so on. “I never thought of that!” Einstein interjected. He asked a few questions and quickly grasped the implications. Instead of writing to the Belgian queen, Einstein suggested they should contact a Belgian minister he knew.

Wigner, showing some sensible propriety, suggested that three refugees should not be writing a foreign government about secret security matters without consulting the U.S. State Department. Perhaps, they decided, the proper channel was a letter from Einstein (the only one of them famous enough to be heeded) to the Belgian ambassador, with a cover letter to the State Department. With that plan in mind, Einstein dictated a draft in German. Wigner translated it, gave it to his secretary to be typed, and then sent it to Szilárd.

A few days later, a friend arranged for Szilárd to talk to Alexander Sachs, an economist at Lehman Brothers and a friend of President Roosevelt’s. Showing a bit more savvy than the three theoretical physicists, Sachs insisted that the letter go right to the White House, and he offered to hand-deliver it.

Crafting Einstein’s Letter to President Roosevelt

It was the first time Szilárd had met Sachs, but he found the bold plan appealing. “It could not do any harm to try this way,” he wrote to Einstein. Einstein wrote back asking Szilárd to come back out to Peconic so they could revise the letter. By that point Wigner had gone to California for a visit. So Szilárd enlisted, as driver and scientific sidekick, another friend from the amazing group of Hungarian refugees who were theoretical physicists, Edward Teller.

Szilárd brought with him the original draft from two weeks earlier, but Einstein realized that they were now planning a letter that was far more momentous than one asking Belgian ministers to be careful about Congolese uranium exports. The world’s most famous scientist was about to tell the president of the United States that he should begin contemplating a weapon of almost unimaginable impact. “Einstein dictated a letter in German,” Szilárd recalled, “which Teller took down, and I used this German text as a guide in preparing two drafts of a letter to the president.”

According to Teller’s notes, Einstein’s dictated draft not only raised the question of the Congo’s uranium but also explained the possibility of chain reactions, suggested that a new type of bomb could result, and urged the president to set up formal contact with physicists working on this topic. Szilárd then prepared and sent back to Einstein a 45-line letter and a 25-line version — both dated August 2, 1939 — “and left it up to Einstein to choose which he liked best.” Einstein signed them both in a small scrawl.

The scientists still had to figure out who could best get it into the hands of President Roosevelt. Einstein was unsure Sachs could do the job. When Szilárd sent back to Einstein the typed versions of the letter, he suggested that they use as their intermediary Charles Lindbergh, whose solo transatlantic flight 12 years earlier had made him a celebrity. All three refugee Jews were apparently unaware that the aviator had been spending time in Germany, had been decorated the year before by Hermann Göring with that nation’s medal of honor, and was becoming an isolationist and Roosevelt antagonist.

The Failed Attempt to Involve Charles Lindbergh

Einstein had briefly met Lindbergh a few years earlier in New York, so he wrote a note of introduction, which he included when he returned the signed letters to Szilárd. “I would like to ask you to do me a favor of receiving my friend Dr. Szilárd and think very carefully about what he will tell you,” Einstein wrote. “To one who is outside of science the matter he will bring up may seem fantastic. However, you will certainly become convinced that a possibility is presented here which has to be very carefully watched in the public interest.”

Lindbergh did not respond, so Szilárd wrote him a reminder letter on September 13. Two days later, he realized how clueless he and his colleagues had been when Lindbergh gave a nationwide radio address. It was a clarion call for isolationism. “The destiny of this country does not call for our involvement in European wars,” Lindbergh began. Interwoven were hints of his pro-German sympathies and even some anti-Semitic implications about Jewish ownership of the media. “We must ask who owns and influences the newspaper, the news picture, and the radio station,” Lindbergh said. “If our people know the truth, our country is not likely to enter the war.”

Szilárd’s next letter to Einstein stated the obvious. “Lindbergh is not our man,” he wrote.

The physicists’ other hope was Sachs, who had been given the formal letter to Roosevelt that Einstein signed. But Sachs was not able to find the opportunity to deliver it for almost two months.

By then, events had turned what had been an important letter into an urgent one. At the end of August 1939, the Nazis and Soviets stunned the world by signing a war-alliance pact and proceeded to carve up Poland. That prompted Britain and France to declare war.

Szilárd went to see Sachs in late September and was horrified to discover that he still had not been able to schedule an appointment with Roosevelt. “There is a distinct possibility Sachs will be of no use to us,” Szilárd wrote to Einstein. “Wigner and I have decided to accord him ten days’ grace.” Sachs barely made the deadline. On the afternoon of Wednesday, October 11, he was ushered into the Oval Office carrying Einstein’s letter, Szilárd’s memo, and an 800-word summary he had written on his own.

The president greeted him jovially: “Alex, what are you up to?”

Sachs worried that if he simply left Einstein’s letter and the other papers with Roosevelt, they might be glanced at and then pushed aside. The only reliable way to deliver them, he decided, was to read them aloud. Standing in front of the president’s desk, he read his summation of Einstein’s letter and parts of Szilárd’s memo.

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

“Precisely,” Sachs replied.

“This requires action,” Roosevelt declared to his assistant.

The Impact of Einstein's Letter on the Manhattan Project

The following week, Einstein received a polite and formal thank-you letter from the president. “I have convened a board,” Roosevelt wrote, “to thoroughly investigate the possibilities of your suggestion regarding the element of uranium.” Still, the effort’s slow pace and meager funding prompted Szilárd and Einstein to compose a second letter urging the president to consider whether the American work was proceeding quickly enough.

Despite helping to spur Roosevelt into action, Einstein never worked directly on the bomb project. J. Edgar Hoover, the director of the FBI even back then, wrote a letter to General Sherman Miles, who initially organized the efforts, that described Einstein’s pacifist activities and suggested that he was a security risk. In the end, Einstein played only a small role in the Manhattan Project. He was asked by Vannevar Bush, one of the project’s scientific overseers, to help on a specific problem involving the separation of isotopes that shared chemical traits. Einstein was happy to comply. Drawing on his old expertise in osmosis and diffusion, he worked for two days on a process of gaseous diffusion in which uranium was converted into a gas and forced through filters.

The scientists who received Einstein’s report were impressed, and they discussed it with Bush. In order for Einstein to be more useful, they said, he should be given more information about how the isotope separation fits in with other parts of the bomb-making challenge. Bush refused. He knew that Einstein didn’t have and couldn’t get the necessary security clearance. “I wish very much that I could place the whole thing before him and take him fully into confidence,” Bush wrote, “but this is utterly impossible in view of the attitude of people here in Washington who have studied his whole history.”

Thus the scientist who had explained the need for a bomb-making project was considered too risky to be told about it.

Read More: 10 Famous Scientists and Their Contributions

Frequently Asked Questions About the Atomic Bomb

What Is an Atomic Bomb?

An atomic bomb harnesses extraordinary destructive power through a process known as nuclear fission, which involves breaking apart the core of heavy elements such as plutonium or uranium. With merely 11 to 33 pounds of such enriched materials, it can trigger a colossal explosion, marked by an immense fireball, a powerful shockwave, and dangerous radioactive fallout.

Who Invented the Atomic Bomb?

The atomic bomb was developed by a group of scientists under the Manhattan Project, a research and development undertaking during World War II that produced the first nuclear weapons. While many scientists contributed, J. Robert Oppenheimer is often referred to as the "father of the atomic bomb" for his role as the scientific director of the Manhattan Project.

When Was the Atomic Bomb Dropped?

There were actually 2 atomic bombs that were dropped. The first bomb was detonated over Hiroshima, Japan, on August 6, 1945, followed by the second bomb over Nagasaki, Japan, on August 9, 1945.

Who Dropped the Atomic Bombs?

The atomic bombs were dropped during two different missions by the United States Army Air Forces. The Enola Gay, a B-29 bomber piloted by Colonel Paul W. Tibbets, Jr., dropped the bomb on Hiroshima, while Bockscar, piloted by Major Charles Sweeney, dropped the bomb on Nagasaki.

How Many People Died from the Atomic Bomb?

It is estimated that the atomic bombs caused about 80,000 immediate deaths in Hiroshima and 39,000 in Nagasaki. Thousands more died from injuries later and/or suffered from the effects of radiation sickness, burns, and other trauma in the months and years that followed.

Read More: 20 Brilliant Quotes From Albert Einstein, the Theoretical Physicist Who Became World Famous

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