The Physics of War (35 page)

Alan Turing.

Enigma was used by the German navy, army, and air force, but the German high command used another, even more complicated encoder called Lorentz. It was introduced in 1941, and it used twelve wheels. The only way to break its code was with the use of a very large computer, much larger than anything that had ever been built. It would be a huge undertaking, but the information it could supply would be of tremendous value. The design engineer was Tommy Flowers, and the prototype, called Colossus Mark I, was produced in December 1943. It was in operation by February 1944.

With the Colossus, the messages sent by the Lorentz machine could be decoded, and over the next few months a large amount of German intelligence was intercepted and decoded. The Colossus, along with Turing's bombe, no doubt helped to shorten the war.

We saw in the
last chapter
that physics played an important role in many of the weapons of World War II, but it played an even larger role in the greatest weapon of the war—the atomic bomb. Indeed, it played a central role, for the atomic bomb is possible only because of our knowledge of fundamental physical concepts in physics. The subatomic particles that constitute the nucleus within the nucleus are bound together by what is called binding energy, and it is this binding energy that makes the atomic bomb possible.

The development of the atomic bomb is, without a doubt, one of the most impressive and awe-inspiring developments in history. Not it only did take a number of fundamental breakthroughs by a few ingenious thinkers, but it also took a tremendous effort by thousands of people to achieve it. And not only did these people achieve a goal that seemed almost impossible to many at first, but it showed what could be accomplished with enough motivation, determination, and ingenuity.

THE BEGINNING

It's hard to say exactly how it all began, but the experiments of James Chadwick of Cambridge University in England were critical. He was repeating an experiment done earlier by Irene Joliot-Curie and her husband Frederic Joliot-Curie in which a strange particle was able to knock protons out of paraffin. The Joliot-Curies thought the strange particle was a gamma ray. Chadwick showed that it was actually a neutrally charged particle, which he called a neutron.
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This neutral particle was in the nucleus of the atom along with the proton. As it turned out, the neutron had the same mass as the proton. The sum of the atomic weight of the protons and the atomic weight of the neutrons in an atom is approximately equal to the atomic weight (A) of the atom. We also designate the total number of protons in the nucleus as the atomic number (Z). And from these two numbers
we can easily determine the number of neutrons in the nucleus; it is just A – Z. As an example, consider the hydrogen atom, which we know has a nucleus made up of a single proton. It has A = 1 and Z = 1, and since A – Z = 0, it has no neutrons. In the same way, the helium atom has A = 4 and two protons, or Z = 2, and since A – Z = 2, it has 2 neutrons. We can continue in his way through all the elements.

The neutron turned out to be a particularly important research particle because it was neutral. Early physicists tried to learn more about the nucleus by projecting high-speed particles at it to see what would happen. The only known particles available at the time, however, were the proton and the electron, but the electron was too light to have any effect on the nucleus, and the proton was positively charged, as was the nucleus, so the nucleus and the proton repelled one another. Because of this, the proton was also an ineffective projectile. The neutron, however, was not repelled electrically by the electrons or nucleus, so it was an ideal projectile. Before we look into how it was used, however, let's consider Einstein's contribution to the atomic bomb.

EINSTEIN'S ROLE

Einstein is sometimes called the father of the atomic bomb, a title he abhorred, and in reality he had very little to do with it directly. But he did make an important contribution. In a short paper published shortly after he published his famous paper on special relativity in 1905, he showed that energy and mass were related. The title of the paper was “Does the Inertia of a Body Depend on Its Energy Content?” It was only three pages long, but it was one of the most important papers ever published. This paper, along with a paper published a year later, showed an equivalence between mass and energy. In particular, it gave us the equation E = mc
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, where the energy (E) of a given amount of mass is equal to the mass (m) multiplied by the speed of light (c) squared. The speed of light is 186,000 miles per second, and if you square it (multiply it by itself), you obviously get a very large number. This tells us that there is a large amount of energy associated with even a very small amount of mass. Fortunately, it is very difficult to transfer mass directly into energy, but this is, indeed, what happens in an atomic explosion.
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THE ITALIAN BREAKTHROUGH

Most physicists are either experimentalists or theoreticians. Enrico Fermi of the University of Rome, however, was one of very few people who excelled in both areas. He made important theoretical contributions, but at the same time he was a first-rate experimentalist. When the neutron was discovered in 1932, Fermi immediately realized that it would make an ideal projectile. It would not be repelled by the nucleus, and it could easily be projected fast enough so that the surrounding electrons would have no effect on it. The problem was finding a good source of neutrons, and he was soon able to devise an apparatus that would produce a beam of neutrons.
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Enrico Fermi.

One of the hottest areas in physics at the time was radioactive decay. A number of elements were known to spontaneously decay, emitting various types of radiation, referred to as alpha, beta, and gamma rays. A number of people, including Marie Curie, had made important contributions to the area. In 1934, however, Irene Curie and Frederic Joliot announced that they had been able to induce artificial radioactivity. In other words, they had caused a stable element to become radioactive. They had bombarded aluminum nuclei with alpha particles and caused it to become radioactive. They had also found that boron responded in the same way when bombarded with alpha rays.

Fermi was fascinated by the result, and he was sure he could improve on it.
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The alpha particles were big and heavy, and they could easily be stopped, even by a sheet of paper. Furthermore, they were charged. Neutrons would make a much better projectile, and he now had an apparatus for producing them. In addition, he had improved a device that had been invented several years earlier, called the Geiger Counter, which was used for measuring the radiation produced. Fermi and his group began by repeating the experiments of Joliot and Curie, and they quickly verified their results. They then turned to heavier elements. And indeed, many of them became radioactive, but in most cases the radioactivity was short-lived. Some of the elements, in fact, had half-lives (half-life is the time it takes for a mass of radioactive material to fall to half its original value) of less than a minute.
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Fermi and his team tested most of the elements of the periodic table in this way, all the way up to the heaviest known element at the time, namely uranium. And uranium was of particular interest to him. There were no known elements heavier than uranium, so he wondered what would happen if he shot a neutron at the uranium nucleus and it was absorbed. Would a new element be formed? Uranium has an atomic weight of 238 (total number of protons and neutrons in the nucleus); if it absorbed a neutron it should become uranium-239. But this created a new problem: how could they detect uranium-239? This proved to be frustratingly difficult. Finally, however, the team identified a slightly heavier element. Fermi was overjoyed. He had created an element beyond uranium-238. With this he concluded his experiments, but in doing so he failed to make one of the greatest discoveries in history.

In the meantime the world around him was becoming more and more turbulent. Hitler had seized power in Germany and Mussolini had signed a pact with him. Hitler's war against the Jews had already begun, and he demanded that Mussolini cooperate. Jews in Italy were therefore subjected to new legal restrictions. Fermi himself was not in danger, but Laura, his wife, was Jewish, and Fermi knew she might eventually be rounded up by authorities. He wasn't sure what to do. Government officials were unlikely to allow him to leave the country with his wife. Earlier he had been offered several positions at universities in the United States, but he had turned the offers down. He decided to write and ask if they were still interested, and indeed he got an offer from Columbia University. The problem now, however, was getting out of the country without arousing suspicion.

The breakthrough he was waiting for came in the fall of 1938. Fermi was in Copenhagen for a physics meeting when Niels Bohr took him aside and told him he was in line for the Nobel Prize that would be awarded later that year. Fermi was excited, not only at the prospect of winning the prize, but also because it
might provide a possible route out of Italy. Indeed, a few weeks later he received a call informing him that he had won the prize, and that he would have to go to Stockholm, Sweden, to collect it. Furthermore, he was invited to take his family with him to Sweden. Immediately after the Nobel-Prize ceremony, Fermi boarded a plane for England, accompanied by his wife and children. From there they boarded a ship to New York.

HAHN, MEITNER, AND STRASSMANN

Lise Meitner was born into a Jewish family in Vienna, Austria, in 1878. She became interested in physics at an early age, but a scientific vocation was difficult for a woman at that time. Nevertheless, she managed to get a doctoral degree in physics at the University of Vienna. After obtaining it she went to the Kaiser Wilhelm Institute in Berlin and began working as an assistant to the chemist Otto Hahn.
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Initially she worked with no salary, but eventually she became head of a section in chemistry. She worked with Hahn for thirty years, making several important discoveries.
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When Hitler came to power in 1933 she was acting director of the Institute of Chemistry. Although she'd been born into a Jewish family, she had converted to Christianity early in her life, and as an adult she identified herself as Lutheran. Furthermore, she was Austrian by birth. So initially she wasn't worried by Hitler's action against Jews. She buried herself in her work. Others, including the Joliet-Curies in Paris had followed Fermi's lead in bombarding heavy elements, particularly uranium, with neutrons. Hahn and Meitner soon took an interest in their work.

Meitner had barely begun the work, however, when Hitler annexed Austria and issued a proclamation against all Jews, including those from Austria. Although she no longer considered herself a Jew, Meitner knew that that didn't matter to the Nazis. She had to get beyond the German area of influence as soon as possible, but there was a problem. Her visa had expired, and she could not apply for a new one because it would alert the authorities. She was uncertain about what to do, so she wrote Niels Bohr in Copenhagen. He made arrangements for her to get to Holland without a visa. But she still had to get past the Nazi patrols at the border. And, as she feared, a Nazi officer at the border asked her for her visa. She knew it had expired, but she handed it to him. He looked it over carefully as she sat in a state of fright. Finally, after several minutes he handed it back to her without saying anything. Minutes later, much to her relief, she was in Holland.

Bohr got her a position in Stockholm, Sweden, but it came with almost no support, and she was soon quite unhappy. Furthermore, Hahn and his assistant, Fritz Strassmann, were continuing the experiments they had started. Fermi had assumed that when uranium was bombarded with a neutron it would create a heavier, transuranic element, but he hadn't proved it beyond a doubt. But when Hahn and Strassmann did the experiment they were thoroughly confused. They couldn't verify Fermi's result; furthermore, an element, namely barium, with only about half the atomic weight of uranium, appeared to have been produced. It didn't make any sense, but Hahn had run the experiment through several times, getting the same result each time. Knowing that Meitner had a much better knowledge of nuclear physics than he did, he sent her a letter asking her if she had an explanation.

CHRISTMAS 1938

Meitner was amazed by the result, and confused. She had no explanation, but she was sure that Hahn had not made a mistake. If he said there was barium present after the bombardment, it had to be true. But where did it come from? Christmas neared, she pondered the strange result. She had a nephew, Otto Frisch, who was working for Bohr in Copenhagen, and she knew he was single. So she wrote to him to ask whether he would like to spend Christmas with her. He wrote back saying he would be delighted to spend it with her. He had been working on an interesting project related to the magnetic properties of the nucleus, and he was anxious to tell her about it, as she might have some suggestions for him.
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