Albert Einstein signed the letter. Years later he would regret it, calling it the one mistake he had made in his life. But in August 1939, Adolf Hitler’s armies already occupied Czechoslovakia and Austria and his fascist thugs were arresting Jews and political opponents throughout the Third Reich. Signing the letter seemed vital. His friends and fellow physicists, Leo Szilard and Eugene Wigner, had drafted the note he would now send to President Franklin D. Roosevelt.

The scientists had seen their excitement over the recent breakthrough discoveries of the deepest secrets of the atom turn to fear as they realized what unleashing atomic energies could mean. Now the danger could not be denied. The Nazis might be working on a super-weapon; they had to be stopped.

In his famous letter, Einstein warned Roosevelt that in the immediate future, based on new work by Szilard and the Italian physicist Enrico Fermi, “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 radiumlike elements would be generated.” This “new phenomenon,” he said, could lead to the construction of “extremely powerful bombs of a new type.” Just one of these bombs, “carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory.” The Nazis might already be working on such a bomb. “Germany has actually stopped the sale of uranium from Czechoslovakian mines, which she has taken over,” Einstein reported. He urged Roosevelt to speed up American experimental work by providing government funds and coordinating the work of physicists investigating chain reactions.

Roosevelt responded, but tentatively. He formed an Advisory Committee on Uranium to oversee preliminary research on nuclear fission. By the spring of 1940, the committee had allocated only $6,000 to purchase graphite bricks, a critical component of experiments Fermi and Szilard were running at Columbia University. In 1941, however, engineer Vannevar Bush, the president of the Carnegie Institution of Washington and the president’s informal science advisor, convinced Roosevelt to move faster. British Prime Minister Winston Churchill also weighed in, sending the president new, critical studies by scientists in England.

The most important was a memorandum from two German refugee scientists living in England, Otto Frisch and Rudolph Peierls. From their early experiments and calculations, they detailed how vast the potential destructive power of atomic energy could be—and such power’s military implications. Their memo to the British government estimated that the energy liberated from just 5 kilograms of uranium would yield an explosion equal to several thousand tons of dynamite.

This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the center of a big city.

In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effects of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.

Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.

The scientists concluded:

If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realized that no shelters are available that would be effective and that could be used on a large scale. The most effective reply would be a counter-threat with a similar bomb. Therefore it seems to us important to start production as soon and as rapidly as possible.

They did not, at the time, consider actually using the bomb, as “the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country.” Rather, they thought it necessary to have a bomb to deter German use. This was exactly the reasoning of Einstein, Szilard, and others.

Soon after the Frisch-Peierls memo circulated at the highest levels of the British government, a special committee on uranium, confusingly named the MAUD committee for a British nurse who had worked with the family of Danish physicist Niels Bohr, began assessing the two scientists’ conclusions. The MAUD report on “Use of Uranium for a Bomb” would have an immediate impact on the thinking of both Churchill and Franklin Roosevelt in the summer and fall of 1941. It concluded that a “uranium bomb” could be available in time to help the war effort: “the material for the first bomb could be ready by the end of 1943.” Upon meeting with Vannevar Bush and learning of the MAUD committee’s dramatic conclusions on October 9, 1941, Roosevelt authorized the first atomic bomb project.

Bush, then head of the newly formed National Defense Research Committee, asked Harvard President James Conant to direct a special panel of the National Academy of Sciences to review all atomic energy studies and experiments. Though Bush’s committee recommended the “urgent development” of the bomb, the December 1941 attack on Pearl Harbor gave other conventional military concerns greater precedence. It was not until a year later that work began in earnest.

The Manhattan Project, formally the “Manhattan Engineering District,” was created in August 1942 within the Army Corps of Engineers. The laboratory research now became a military pursuit, in part to mask its massive budget. Brigadier General Leslie Groves assumed leadership of the project in September 1942 and immediately accelerated work on all fronts. Historian Robert Norris says of Groves, “Of all the participants in the Manhattan Project, he and he alone was indispensable.”

Groves was the perfect man to direct the massive effort needed to create the raw materials of the bomb, having just finished supervising the construction of the largest office building in the world, the new Pentagon. He needed to find a partner who could mobilize the scientific talent already engaged in extensive nuclear research at laboratories in California, Illinois, and New York. At the University of California at Berkeley, Groves met physicist J. Robert Oppenheimer for the first time and heard his plea for a laboratory purely devoted to work on the bomb itself. Groves thought Oppenheimer “a genius, a real genius,” and soon convinced him to head the scientific effort. Together they chose a remote southwestern mesa as the perfect site for the greatest concentration of applied nuclear brainpower the world had ever seen.


When the young scientists recruited for the Manhattan Project moved into the stark buildings of Los Alamos, New Mexico, surrounded by barbed wire, they understood that they would be working on a top-secret project that could win the war. Most knew that they were there to build the world’s first atomic bomb, but didn’t know much more beyond that. To bring everyone up to speed, physicist Robert Serber gave five lectures in early April 1943 on the scientific and engineering challenges ahead. His lecture notes, mimeographed and given to all subsequent arrivals, became knows as The Los Alamos Primer. Today, it still serves as a valuable guide to the essentials of an atomic bomb.

Serber got right to the point: “The object of the Project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.”

The discovery of fission was new, but the idea of the atom goes back to the early Greek thinkers. In about 400 BCE, Democritus reasoned that if you continuously divided matter, you would eventually get down to the smallest, undividable particle, which he called an atom, meaning “uncuttable.” By the beginning of the twentieth century, scientists realized the atom had an internal structure. In 1908 Ernest Rutherford discovered that atoms had a central core, or nucleus, composed of positively-charged protons, surrounded by the negatively charged electrons detected by J. J. Thompson eleven years earlier. In 1932 James Chadwick discovered that there were particles equal in weight to the proton in the nucleus, but without an electrical charge. He dubbed them neutrons. This led to the atomic model that we are familiar with today, of an atom as a miniature planetary system, with a nucleus of hard, round balls of protons and neutrons with smaller electron balls orbiting around.

Familiar, but not quite right. Danish physicist Niels Bohr, among his many other contributions, found that a large nucleus behaved more like a water droplet. His insight led to a breakthrough discovery in 1939. German scientists Otto Hahn and Fritz Strassman, working with physicist Lise Meitner, had been bombarding uranium, the heaviest element found in nature, with neutrons and observing the new elements that seemed to form. Uranium has an atomic number of 92, meaning it has 92 protons in its nucleus. The scientists thought that the neutrons were being absorbed by the uranium atoms, producing new, man-made elements, but chemical analysis indicated that this was not the case. When Meitner and physicist Otto Frisch applied Bohr’s water droplet model to these experimental results, they realized that under certain conditions the nucleus would stretch and could split in two, like a living cell. Frisch named the process after its biological equivalent: fission.

Three events happen during fission. The least important, it turns out, is that the uranium atom splits into two smaller atoms (usually krypton and barium). Scientists had finally realized the dream of ancient alchemists—the ability to transform one element into another. But it is the other two events that made the discovery really interesting. The two newly created atoms weigh almost exactly what the uranium atom weighed. That “almost” is important. Some of the weight loss is attributable to neutrons flying out of the atom. These are now available for splitting other, nearby uranium nuclei. For every one neutron that splits a uranium nucleus, two more, on average, are generated. Splitting one nucleus can, under the right conditions, lead to the splitting of two additional nuclei, then four, then eight, on up. This is the chain reaction that can start from a single neutron.

The third event is the real payoff. Each fission converts a small amount of the mass of the atom into energy. The first scientists to discover fission applied Einstein’s famous formula, E = mc2, and quickly realized that even this small amount of matter m multiplied by the speed of light squared c2 equals a very large amount of energy E.

Energy at atomic levels is measured in electron volts. Normal chemical reactions involve the forming or breaking of bonds between the electrons of individual atoms, each releasing energies of a few electron volts. Explosives, such as dynamite, release this energy very quickly, but each atom yields only a small amount of energy. Splitting a single uranium nucleus, however, results in an energy release of almost 200 million electron volts. Splitting all 2,580,000,000,000,000,000,000,000 (2.58 trillion trillion) uranium atoms in just one kilogram of uranium would yield an explosive force equal to ten thousand tons of dynamite. This was the frightening calculation behind the Frisch-Peierls memo and Einstein’s letter to Roosevelt. One small bomb could equal the destructive force of even the largest bomber raid.



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