Chapter 17

Einstein and the Bomb

In This Chapter

 A letter to the president

 The basics of nuclear physics and fission

 How the bomb came into being

 Einstein, the pacifist

O f the many applications of the E = mc² equation, perhaps the most dramatic is the atomic bomb. Quantum physics and the equation made it possible.

How did physicists get from E = mc² to chain reactions and the bomb? It wasn’t easy, and it wasn’t planned. The physicists weren’t interested in making a bomb or any other kind of weapon. They were trying to discover the structure of matter, the composition of the atom, the way the universe works. In doing so, they stumbled upon the enormous energies stored in the atomic nucleus and discovered, slowly but steadily, how to unleash them. Political events changed what was an intellectual pursuit into a war effort. And the rest is history.

In this chapter, I explain where the bomb came from and its connection with Einstein’s theory. I discuss Einstein’s letter to President Franklin Delano Roosevelt and his subsequent efforts to discourage war. I retrace the main steps of the scientific history that led from E = mc ² to the bomb. (Yes, the subject matter is nuclear physics, but you’ll be surprised at how understandable it is.) By the end of the chapter, you’ll know how the nuclear bomb works. (But, thank heavens, you still won’t be able to build one!)

Warning the President: Einstein’s Letter

“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 uranium may be turned into a new and important source of energy in the immediate future.” With these now famous words began the letter that Einstein sent to President Roosevelt in 1939.

Einstein didn’t send the letter to brag about his fellow scientists’ work. He wrote to warn the president of the possibility that Nazi Germany was developing a nuclear bomb and to urge him to start a serious effort to see that the United States developed it first.

As I explain later in the chapter, in the section “Remaining a Pacifist,” the author of the letter was actually the physicist Leo Szilard. However, Einstein wrote an early draft, and he signed the final letter as if the words were his own. Einstein was the greatest scientist in the world, and Szilard — a nuclear physicist working in chain reaction and uranium research — thought that the only way the president would pay attention was if Einstein authored the letter. Reluctantly, Einstein signed it (“Yours very truly, A. Einstein”). And in doing so, he made what he called “the greatest mistake” of his life.

The letter doesn’t mention E = mc², relativity, or any of Einstein’s work. The bomb would be a direct application of the equation, and at the time, the physicists involved in developing the bomb knew it. However, Einstein wasn’t involved in this research. Nuclear physics was perhaps the only field in physics that he didn’t know. It had been developed while he was busy trying to unify gravity with the rest of physics (see Chapter 19).

Szilard wasn’t trying to lure Einstein to join the research effort. Instead, he had a sense of urgency in reaching the president. Germany had invaded and conquered Poland and was close to overrunning Belgium. What the scientists were hearing from Germany regarding the development of the bomb wasn’t good, and the president needed to be notified without making too much noise. Szilard wasn’t important enough to be heard. Einstein was.

In the final section of this chapter, I explain how Einstein came to sign the letter and what happened after the president received it. Before I get there, we need to travel the road that led to the development of the nuclear bomb.

Nuclear Physics in a Nutshell

The energy of a nuclear bomb comes from inside the nucleus of the atom. Mass is converted into energy according to E = mc². This energy is the binding energy of the nucleus, the glue that keeps the nucleus of the atom together. (Take a look at Chapter 11 for details about this binding energy.)

Radiating particles

In some cases, the nuclear force is not able to keep a nucleus all together, and the nucleus loses some of its particles. French physicist Henri Becquerel accidentally discovered this effect in 1896. He’d been intrigued by the experiments with x-rays that Wilhelm Roentgen had been doing in Germany (see Chapter 15). Becquerel obtained a uranium salt to see if he could observe these x-rays.

In his laboratory at the Museum of Natural History in Paris (where his father and grandfather had also been physics professors), Becquerel started his experiments by exposing to the sun a photographic plate with the uranium salt sprinkled on it, thinking that sunlight would activate the x-rays. One cloudy day when he couldn’t perform one of his experiments, he placed the photographic plate with the uranium salt in a drawer. A few days later, he went ahead and developed the plate anyway, thinking that he was going to get a faint image. But the image was very sharp, with high contrast. He soon realized that he’d discovered a new type of energetic radiation.

When Pierre and Marie Curie heard of Becquerel’s experiment, they began to search for other elements that could emit similar rays. They found that thorium and uranium emit the same radiation. And in 1898, they discovered two new elements: polonium (named after Marie’s native Poland) and radium. The Curies named the effect radioactivity.

In England, Ernest Rutherford designed experiments to investigate this new radioactivity phenomenon and was able to show that these rays come in two varieties, one more penetrating than the other. The less penetrating one, which he called alpha, has positive electric charge. The Curies in Paris discovered that the other one, called beta, is negatively charged.

Realizing limitations of the nuclear force

Why are these nuclei giving off particles? The nuclear force is supposed to be extremely strong (see Chapter 11). Why isn’t it able to keep all these particles inside the nucleus?

The answer is that the nuclear force has a very short range of action. It’s able to tie in particles that are close to each other. If the particles are too far apart, the force stops working. If the particles happen to be protons, which have positive charges, the electric force acting alone will push them apart (see Figure 17-1).

Figure 17-1: Nuclear particles feel the nuclear force only when they are very close together.

When the nuclear particles are bundled up in a nucleus of an atom, each particle interacts only with its nearest neighbors. In a nucleus with more than 30 particles, a particle in the middle of the nucleus won’t feel the nuclear force of a particle at the edges. For example, in the left image in Figure 17-2, each of the nuclear particles in the cluster feels the nuclear attraction of the other particles in the cluster (its immediate neighbors). However, these particles don’t feel the force of the particle near the edge.

Think of it this way: Imagine that you and a group of several friends are trying to stay together while swimming in rough waters. If you all decide to hold hands, each one of you will be holding on to the two nearest neighbors. The grip of a swimmer at one end of the large chain, no matter how strong it seems to his immediate neighbor, has no influence on a swimmer at the other end. If the water gets too rough, the whole group may break apart, creating small groups of two, three, or maybe four, as shown in the right image of Figure 17-2.

Like the rough waters that break apart your group, the electrical repulsion of the protons tries to break apart a large nucleus. However, in the nucleus, certain helpers try to keep the whole thing together: the neutrons. Neutrons don’t have an electric charge, and the only force they feel is the nuclear attraction. They are the skilled swimmers who won’t be pushed away by the rough waters. If you have enough of them in your group, it will stay together.

Figure 17-2: Nuclear particles in a cluster are attracted to the other clustered particles, just as swimmers are attached to their neighbors.

Studying alpha decay

Like the swimming group with the skilled swimmers, a nucleus with a balanced number of protons and neutrons is stable and stays together. But if a nucleus has too many protons, the total electric repulsion can overwhelm the attraction of the nuclear force, and a piece of the nucleus can fly apart (see Figure 17-3).

Figure 17-3: A nucleus with too many protons can jettison an alpha particle.

The piece that leaves the nucleus is usually in the form of an alpha particle, a cluster of two protons and two neutrons. (This particle is also the nucleus of the helium atom.) It turns out that these four particles are held together very tightly by the nuclear force, so this cluster is a very stable configuration of nuclear particles. These are the particles that Rutherford identified as alpha radiation. Physicists call the effect of the alpha particles leaving the nucleus alpha decay.

Detecting beta decay

It seems as if having a lot of neutrons is good for a nucleus because neutrons don’t feel the electrical repulsion but do feel the nuclear attraction. They are the skilled swimmers in rough waters. However, these skilled swimmers don’t have a lot of stamina. A neutron on its own, away from the nucleus, lasts for only about 15 minutes. After these 15 minutes, it changes into a proton, an electron, and another small particle called the neutrino. This effect is called beta decay.

Inside the nucleus, surrounded by the other particles, neutrons last much longer. When there are enough protons around, a quantum physics effect prevents neutrons from creating more protons. Quantum physics describes it by giving each proton in the nucleus its own space or slot. When there are enough protons, all the slots are taken and no additional protons are allowed (see Figure 17-4).

Figure 17-4: When all the proton slots are taken, no additional protons are allowed in the nucleus.

In a nucleus with too many neutrons, a neutron at the outer edges of the nucleus can decay into a proton because there will be empty slots for this new proton to stay in. Therefore,

A nucleus with too many neutrons is unstable and decays into a proton, an electron, and a neutrino.

The protons created by this decay stay in the nucleus. The electrons don’t belong in the nucleus; there are no slots for them there. The same goes for the neutrinos. Therefore, the electrons and neutrinos are both ejected (see Figure 17-5). Neutrinos are extremely difficult to detect. They can go through the entire Earth and come out at the other end without a single collision. But electrons are easy to detect. These breakaway electrons create the beta rays that the Curies and Rutherford saw.

Figure 17-5: Electrons formed in beta decay are ejected from the nucleus, because there are no slots for them.

In both cases, the alpha and beta decays, the radioactive nucleus changes into the nucleus of another element when it gives off the alpha or the beta particle.

A third type of radioactive decay exists in which the unstable nucleus gives off only very energetic radiation, but no particles are ejected. The radiation is electromagnetic and is called gamma rays. In this case, the nucleus simply gives back some energy that it gained previously, but it doesn’t lose its identity (see Figure 17-6).

Figure 17-6: A nucleus can also emit high-energy electromagnetic radiation in the gamma region of the spectrum.

Discovering Nuclear Fission

In the 1930s, when physicists were applying the rules of quantum physics to the nuclei of atoms and were able to explain the effects I describe in the previous section, they began to look at other possible ways that a heavy nucleus could decay and give off energy. Keep in mind that their goal was to understand the nature of the atom; no one had weapons in mind (yet).

Misreading results

In 1934, in his laboratory in Rome, Italian physicist Enrico Fermi was using neutrons from a radioactive nucleus as projectiles to bombard uranium nuclei. He thought that he was producing new heavier elements — transuranic elements, as he called them. He published a paper in the journal Nature describing his experiments. Scientists around the world became very excited about Fermi’s new elements.

A chemist named Ida Noddack wrote a paper arguing that Fermi hadn’t really proved that he was producing these transuranic nuclei. She thought instead that the experiments were showing that Fermi had actually split the uranium nucleus into two smaller nuclei. She published her paper in the Journal of Applied Chemistry in Germany, a journal that physicists didn’t normally read.

A few physicists did read the article, but it didn’t make sense to them. They all thought that the new element formed in Fermi’s experiments had to be close in mass to the bombarded element. Fermi also read it and, after performing some calculations, decided that Noddack was wrong; the possibility of generating much lighter elements, as she suggested, was very low.

As it turned out, Noddack was ahead of her time. The physicists simply didn’t know enough about the properties of nuclei to realize that what she was proposing was actually happening in Fermi’s experiments.

Realizing that uranium is being split

The final answer to this problem came a few years later, in the middle of World War II. Austrian physicist Lise Meitner and her research group conducted a four-year investigation in her laboratory that not only explained what Fermi had done but, in the process, discovered nuclear fission, the mechanism for the bomb.

Like Noddack and most scientists working in nuclear physics at the time, Meitner was interested in Fermi’s experiments. But unlike Noddack, Meitner and her collaborators initially concluded that Fermi was making new transuranic elements.

In Germany, Meitner began to work on a model that would explain Fermi’s experiment, but things were not falling into place. She couldn’t see how bombarding uranium with one slow neutron, as Fermi was doing, could produce four or five beta decays. In a paper she wrote in 1937, she said that the results were “difficult to reconcile with current concepts of nuclear structure.”

The Curies in Paris also looked at Fermi’s experiment results. From his data, they identified new evidence that he was creating another element, but they couldn’t figure out what it was.

As I explain in the sidebar on Lise Meitner, she was forced to flee Nazi Germany in 1938, and she went to Stockholm to continue her work.

Back in Germany, Meitner’s longtime collaborator Otto Hahn and his assistant identified the element that Fermi was creating as radium, the radioactive element that the Curies had discovered several years earlier. Mail between Stockholm and Berlin was delivered overnight, and Meitner received Hahn’s interpretation right away. She didn’t think Hahn was right. For radium to be produced, two alpha particles needed to be emitted. She didn’t think that the slow neutron in Fermi’s experiment would have enough energy to knock out even a single alpha.

Meitner suggested a new experiment for Hahn’s group to try. “Fortunately, her opinion and judgment carried so much weight with us that we immediately began the . . . experiments,” wrote Hahn’s assistant. The experiment proved that the element being produced wasn’t radium — it was barium, an element much lighter than uranium.

Meitner and Hahn had just discovered nuclear fission. Noddack had been right. Fermi wasn’t producing transuranic elements; he was splitting atoms.

Hahn published the results of the experiments, suggesting that the uranium nucleus had been split in two fragments. Meitner was extremely disappointed that she couldn’t be part of that “beautiful discovery.” But she knew that a “non-Aryan” couldn’t be included in the publication.

Lise Meitner

Lise Meitner was born in Vienna of Jewish descent but was baptized and raised as a Protestant. She became interested in physics when she read about Marie and Pierre Curie’s work. She studied at the University of Vienna, where Ludwig Boltzmann was a professor, and obtained her PhD in physics in 1906.

The next year, she joined the University of Berlin, where she began a collaboration with Otto Hahn, a chemist who was the same age. The two young scientists worked in radioactivity and were very successful; in 1918, they announced the discovery of a new radioactive element. By then, they both had moved to the Kaiser Wilhelm Institute, where she directed the physics lab and he directed the radiochemistry lab.

By 1938, Meitner was considered one of the top nuclear physicists in the world. Einstein called her “our Madam Curie.” An organic chemist at the institute named Kurt Hess, an unknown and envious researcher who became the institute’s first active Nazi, began a campaign to get rid of Meitner. When she mentioned this fact to Hahn, her collaborator and lifelong friend, Hahn went straight to the top administrator at the institute. He was told to fire Meitner.

Hahn fired her. They’d been working together for more than 20 years and knew and respected each other well. “Hahn says I should not come to the Institute anymore. He has . . . thrown me out,” she wrote in her diary.

Hahn might’ve been afraid both for the future of the institute and for his own future. But without Meitner in the picture, he would be the sole recipient of the glory that their success would bring. He would benefit from Meitner’s absence.

In August of 1938, Meitner fled Nazi Germany for Stockholm, Sweden.

Imagining liquid drops

In December of 1938, Meitner traveled to Copenhagen to visit her nephew, Otto Frisch, who was a physicist in Niels Bohr’s institute. Aunt and nephew went out for a walk in the snow one cold winter morning. They began to discuss the results from her former group in Berlin. She’d suggested that the uranium nucleus had split into two similar fragments, one of them a barium nucleus. But how could a slow neutron pack enough energy to accomplish that division?

Frisch’s boss, Niels Bohr, had been advancing the idea that a large liquid drop would be a good model for a heavy nucleus. A large liquid drop is fragile and can be easily split into smaller droplets. If there is enough energy, the drop begins to take on larger and larger elongated shapes, vibrating back and forth until it splits (see Figure 17-7). The new smaller droplets are more difficult to split and are much more stable. The surface tension — the forces keeping the water molecules bound to each other on the surface — is much larger for these smaller drops. (Large soap bubbles are more fragile than small ones for the same reason.)

Figure 17-7: If you add enough energy, you can split a large liquid drop into smaller droplets.

If a heavy nucleus is somehow disturbed, Bohr said, it would behave like this oscillating drop, vibrating until it breaks into two pieces. Although the nuclear force is very strong, it has a short range. Frisch knew that for a nucleus as large as uranium, the surface tension isn’t very large. Meitner did the E = mc² calculation in her head and came up with the value for the huge amount of energy that is released when the uranium nucleus is split. Things were making sense now. Frisch suggested the name fission for the effect.

In Fermi’s experiments, the neutron provides the energy needed to start the vibrations of the uranium nucleus. When the vibrations become large enough, the nucleus breaks up into the two fission fragments (see Figure 17-8).

Figure 17-8: Nuclear fission as explained in the liquid drop model. When the nucleus breaks apart, it releases new neutron projectiles that can cause more fissions.

The new smaller fragments, like the smaller liquid drops, are more difficult to break apart. Their surface tensions are much larger. The nuclear particles — the protons and neutrons — in their nuclei are much more tightly attached to each other. In these smaller nuclei, each particle senses the nuclear force of most of the other particles, because none are too far away (see Figure 17-9). Being more tightly bound, the new nuclei (the fission fragments) have less mass than the larger uranium nucleus, with its more loosely bound particles (see Figure 17-10). The difference in masses is the energy, the E = mc², that keeps them together in the larger nucleus. This energy is released when they break apart.

During that morning walk in the snow, Lise Meitner and her nephew, Otto Frisch, figured out the physics of nuclear fission. They came up with the correct explanation of the experiments that Fermi was doing in Rome and for the experiments that her former research group in Berlin, with Otto Hahn, had done at her suggestion. Hahn’s paper came out in January of 1939 without Meitner’s name. Meitner and Frisch published their interpretation of the phenomenon in Nature a couple weeks later.

Figure 17-9: The smaller fission fragments are much more stable.

Figure 17-10: The fission fragments have a smaller mass than the original uranium nucleus.

Otto Hahn won the Nobel Prize in chemistry in 1944 for the discovery. Alone.

The Nobel committee thus made one of its greatest mistakes. Hahn had contributed substantially to the discovery, but Meitner had guided the research from the beginning through completion. The prize should’ve gone to both. The committee didn’t do in-depth research and failed to see the political reasons behind the omission of Meitner’s name from several key papers.

Making the Bomb

Meitner and her team discovered a way to unleash the enormous energies stored in the nucleus. But they still weren’t thinking about building bombs.

The energies in the nucleus are huge only by comparison. The energy released in the splitting of a single uranium nucleus is only about a trillionth of the energy used in lifting a baseball from the ground to your chest. Even when you consider the enormous number of atoms in a sample of uranium, how can you bombard all of them at once so that you get a large enough amount of energy to be useful? Because if you don’t, the nuclei will give off their tiny amounts of energy at different times — not enough to do anything with.

Creating chain reactions

One scientist was thinking about bombs at the time, but no one was listening. Back in 1932, Leo Szilard came up with the idea that neutrons would make better probes for the nucleus than the alpha particles that Rutherford and the other scientists were using.

Szilard also came up with a way to release the energy all at once. He thought that if you used neutrons, you could perhaps find an element that could release two or more neutrons when bombarded with one neutron. Then, you could use those two neutrons as probes that would generate four more neutrons with which you could bombard four nuclei, and so on. Szilard called the phenomenon a chain reaction (see Figure 17-11).

Figure 17-11: A chain reaction.

In a chain reaction, when an element that’s bombarded with a neutron releases neutrons, these new particles can be used as projectiles to bombard other nuclei. The result is an uncontrolled reaction that may release a great deal of energy.

When scientists were asked to develop a nuclear bomb to help the war effort, Szilard’s idea of a chain reaction took center stage. When he first proposed it, nuclear fission hadn’t been discovered, and no one knew that large nuclei could be split into two by a relatively slow neutron. Szilard had proposed to bombard every element with neutrons until one that would give off additional neutrons was found. That wasn’t going to be practical.

When nuclear fission became understood, the path was clear. Splitting uranium into two fission fragments with one neutron generated three new neutrons that could serve as projectiles for three new possible fissions. These three fissions would generate nine new neutron bullets, which would turn into 27 new neutrons, and so on. Szilard had his chain reaction.

In 1942, Enrico Fermi achieved the first chain reaction in the nuclear physics laboratory that he’d set up at the University of Chicago. (Fermi had moved to the United States from his native Italy to escape Mussolini’s Fascist regime.)

In Fermi’s chain reaction, the mass of the uranium nucleus plus that of the neutron that splits it is much larger than the total mass of the fission fragments produced. Einstein’s E = mc ² tells us that the energy of the uranium nucleus and the energy of the neutron together are greater than the energy of all the fission fragments. This energy is released when the uranium nucleus is split.

Sensing the force

The very limited range of the nuclear force is the reason the nuclear particles in the uranium nucleus are not as tightly bound together as they are in the fission fragments. A particle in a nucleus with a large number of nuclear particles, such as uranium, won’t feel the nuclear force of particles located at the other side of the nucleus. They are beyond the reach of the nuclear force (see the right side of Figure 17-12).

On the other hand, a nuclear particle in a nucleus with very few particles, like helium (which has only four particles) feels the nuclear force of each of the others (see the left side of Figure 17-12). But the total force that it senses is not as large. A nuclear particle sensing the force of all 16 particles in oxygen, for example, is much larger.

Figure 17-12: A particle in a small nucleus senses all the others. A particle near the edge of a large nucleus doesn’t sense particles in the middle.

Identifying the most stable nucleus

There is an maximum size for a nucleus in which all particles sense the presence of all the others. In a nucleus this size, adding one additional particle won’t increase the total force on each one of the other particles. Particles in a smaller nucleus will sense a smaller total force.

When you consider the electric repulsion of the protons, the range of the nuclear force is just enough to keep the 56 nuclear particles of the nucleus of iron all bound to each other. Iron-56, as it’s called, is the most tightly bound, the most stable nucleus there is. A heavy nucleus like uranium-235 (with 235 nuclear particles) has too many particles not bound to too many other particles. When this nucleus is disturbed by an incoming neutron, for example, it easily splits into two much more stable clusters, releasing energy.

Imagining magnets

You can see the reason for the energy release if you imagine, for example, four horseshoe magnets tangled together (see the left side of Figure 17-13). In this configuration, the magnets aren’t paired up, and their mutual attractions are weakened. You could easily separate them.

If you shake the tangle of magnets enough, they’ll pair up in two groups of two magnets, attached to each other with their opposite poles facing (see the right side of Figure 17-13). It will be harder to separate the pairs than it was to separate the tangled magnets.

Figure 17-13: It takes more energy to separate magnets when they’re paired up than when they’re in a tangle.

Imagine that when the magnets are in a tangle, you hold on to only one of them. When you shake it, you’ll feel it pulling on you when it finds its partner. If you imagine doing this experiment with a gigantic magnet, you could get hurt with the energy released by the magnet as it pairs up with its partner.

Splitting a nucleus of uranium-235 releases energy for the same basic reason. The energy released by the gigantic magnets comes from their masses; the conversion of mass into energy according to E = mc². If you had a super-precise balance, you’d find that the magnets weigh slightly less after they pair up than they did when they were in a tangle. The energy released from your magnets is tiny compared with the energy released from a chunk of uranium-235. The nuclear force is just much stronger.

Moving on to nuclear bombs

Fermi’s first chain reaction in 1942 wasn’t allowed to go uncontrolled. In his nuclear reactor at the University of Chicago, he used a neutron absorber to control the rate of the reaction. In a nuclear bomb, the chain reaction is allowed to take place without control.

For a nuclear explosion to happen, you need to have enough uranium to sustain the chain reaction. But just having the right amount is not enough. Generating an uncontrolled chain reaction also depends on the design of the bomb. If you have a good design, 1 kilogram of uranium-235 is enough. That amount is called the critical mass.

If you have the critical mass of uranium-235, it will explode on its own; any stray neutron can start the chain reaction. In a nuclear bomb, then, the critical mass is assembled only at the time of the explosion.

Note that nuclear bomb is a more accurate term than the old name atomic bomb, because the energy comes only from the nucleus of the atom.

In Little Boy, the bomb dropped on Hiroshima, Japan on August 6, 1945, the uranium was kept in two halves at the two ends of a cigar-shaped design (see Figure 17-14). At the time of detonation, an explosive propelled the two pieces together, forming critical mass.

When critical mass is achieved, the uranium explodes in about one microsecond. However, 99.9 percent of the energy in a nuclear bomb is released during the last tenth of a microsecond of the explosion. The sudden release of energy in such a short time is what makes the bomb so powerful.

Figure 17-14: The design for Little Boy, the nuclear bomb dropped on Hiroshima in 1945.

Creating the H bomb

Another, more powerful bomb was invented in the United States several years after the nuclear fission bomb I’ve described so far. This one is called the hydrogen bomb, or H bomb. Another name for it is thermonuclear bomb. Its source of energy is the same as the source of energy that powers the sun, nuclear fusion. And Einstein’s E = mc² equation made it possible as well.

Nuclear fusion is the combination of two light nuclei into a more massive nucleus. Einstein’s E = mc ² says that things that are sticking together have less mass than when they are apart. Before they’re combined, the masses of the two nuclei add up to a larger value than the mass of the final nucleus they create. When the two light nuclei are combined to form a new nucleus, energy is released.

A typical thermonuclear bomb fuses together the nuclei of two varieties of hydrogen, tritium (one proton and two neutrons) and deuterium (one proton and one neutron) to form helium (two protons and two neutrons). The fusion releases one neutron and energy (see Figure 17-15).

The great technological difficulty that needed to be solved in the development of this weapon, the “secret” of the hydrogen bomb, was how to bring these nuclei close enough together that the nuclear force would overcome the extremely large electrical repulsion of the protons in each of the original nuclei. The solution was to surround the hydrogen bomb with a fission bomb and to invent a technique that made possible the use of the gamma rays from the fission bomb to force the nuclei together.

Figure 17-15: Two varieties of hydrogen are fused to form the nucleus of helium with the release of one neutron and energy.

Remaining a Pacifist

Einstein didn’t have anything to do with the development of either nuclear bomb invented in the United States during World War II and the Cold War. As I explain in the following sections, his involvement stopped soon after he signed his famous letter to President Roosevelt.

Fearing a Nazi bomb

After Fermi’s experiments showed the feasibility of a chain reaction, Leo Szilard, who’d been proposing the idea for a long time, went to Princeton, New Jersey, and consulted with the renowned physicist Eugene Wigner. (Like Szilard, Wigner was a refugee from Hungary.) They knew that Belgium controlled large quantities of uranium in the Congo, and Nazi Germany was knocking on Belgium’s door. Both men worried about the possibility that the Germans would get their hands on enough uranium to create a nuclear bomb.

Writing to the president

Szilard knew that Einstein was a good friend of the queen of Belgium and thought that Einstein should write her a letter explaining the urgency of the situation. Wigner and Szilard went to visit Einstein at his house near the Institute of Advanced Studies. They didn’t even get past the porch of Einstein’s house before he agreed to write the letter. But he wanted to send it to the Belgian government instead of to the queen.

Einstein wrote a draft and gave it to Szilard and Wigner. Szilard then spoke with one of his colleagues, who put him in contact with Alexander Sachs, a banker and adviser to President Roosevelt. Sachs recommended taking the letter to the president instead.

With guidance from Sachs, Szilard wrote a draft of the letter that was to be given to Roosevelt. He sent it to Einstein, explaining what had happened. Einstein wanted a second meeting. Wigner was traveling on the west coast, so Szilard came with Edward Teller, a noted physicist who was also from Hungary. At this meeting, Einstein dictated another draft to Teller. A few days later, Szilard sent Einstein two versions of the letter that he’d written based on Einstein’s new draft. Einstein signed both letters and returned them to Szilard.

Sachs personally carried the longer version of the letter to the president on October 11, 1939. Roosevelt understood right away the importance of the letter. “Alex,” he said, “What you are after is to see that the Nazis don’t blow us up.” The president then called his secretary and told him, “This requires action.”

The action that resulted wasn’t what Szilard wanted. The president formed a committee with Fermi, Szilard, Teller, and Wigner. Einstein wasn’t a member. After the committee’s recommendations were sent to the president, the government authorized all of $6,000 for one year’s worth of research on uranium fission and chain reactions.

Szilard went back to Einstein requesting a second letter, which Einstein wrote. After receiving the second letter, the president decided to increase the size of the committee to include Einstein. Einstein declined.

Regretting his support

Einstein’s letters didn’t really inspire the United States to work on building a bomb. The inspiration, instead, was a two-part report written by Otto Frisch, who had fled to England, and the British physicist Rudolf Peierls. The two parts were titled “On the construction of a ‘super-bomb’; based on a nuclear chain reaction in uranium,” and “Memorandum on the properties of a radioactive ‘super-bomb.’” The report was sent to Washington through official channels.

On December 6, 1941, the day before the Japanese attack on Pearl Harbor, President Roosevelt set up the secret Manhattan Engineering District project — the Manhattan Project, as it came to be known. Three days later, Germany declared war on the United States.

The bomb was never used against the Nazis. Nazi Germany collapsed in May of 1945, before the bomb was ready. Instead, bombs were dropped on Hiroshima and Nagasaki and ended the war with Japan.

After the war, Einstein’s letter to the president became well-known. His E = mc² equation, an essential tool in the development of the bomb, was also well known. Some people began to call Einstein the “superfather” of the bomb (J. Robert Oppenheimer, who led the Manhattan Project, was called the father.) Einstein disliked this label very much. “If I had known that the Germans would not succeed in building the atomic bomb, I never would have supported it,” he said.

Striving for peace

“I would unconditionally refuse to do war service, directly or indirectly, and would try to persuade my friends to take the same stand, regardless of the cause of the war.” These were Einstein’s words during an interview in 1921. The fact that he went against this stance by writing a letter to the president in 1939 only shows the magnitude of the threat that Hitler presented to the world. After World War II ended, Einstein returned to his strong pacifist stance.

Einstein had been a pacifist since his youth. He said that his pacifism was an instinctive feeling, “the feeling that possesses me because the murder of men is disgusting.” His pacifism didn’t come from intellectual theories, “but from my deepest antipathy to every kind of cruelty and hatred . . . my pacifism is absolute.”

In 1930, during a visit to the United States, Einstein made a controversial speech in which he said that “if only 2 percent of those called up declared that they would not serve, and simultaneously demanded that all international conflicts be settled in a peaceful manner, governments would be helpless.” The 2 percent included so many people, he thought, that there wouldn’t be enough jails to put them in. The speech appeared in The New York Times the next day and made many people uneasy.

The threat of Hitler forced Einstein to become what he called “a militant pacifist.” During the war, Einstein agreed to serve as a consultant to the Navy. His job was to point out flaws in the designs of new weapons and to present ideas for improvement. This job was easy for Einstein and was somewhat similar to his old job at the Bern patent office.

After the war, and until he died in 1955, Einstein opposed the proliferation of nuclear weapons and campaigned constantly for their abolition. With Szilard, he founded the Emergency Committee of Atomic Scientists in 1946 and served as its president and chairman of trustees. The organization’s goal was to educate the public about the nature of atomic weapons. Besides Einstein and Szilard, the committee included Linus Pauling, Harold Urey, and the famous physicists Hans Bethe and Victor Weisskopf. However, the organization didn’t make the impact that its founders were looking for. The committee was active until 1950.

A similar organization, the Bulletin of the Atomic Scientists, was founded in 1945 by scientists who worked on the Manhattan Project at the University of Chicago. Einstein was one of its first sponsors. The Bulletin continues its mission of education today.