[1.0] A Little Bomb Like That

released 01 jun 09 / last mod 01 jun 09 / greg goebel / public domain

* The discovery of radioactivity and subatomic particles in the late 19th and early 20th centuries revealed that there were immense energies locked up in the structure of atoms. If such energies could be tapped, they would open the door to seemingly limitless power, as well explosives of previously unimaginable destructive capability. The notion of an "atomic bomb" remained science fiction until the 1930s, but in that decade, the concept of a "chain reaction" -- in which atoms were shattered to toss out particles that shattered a cascade of atoms in turn, suggested that such devices were not so fantastical as had been assumed. With the arrival of World War II, the need for serious consideration of the atomic bomb began to seem increasingly important.

[1.1] DISCOVERING THE ATOM

* The notion that the materials of the Universe are made up of indivisible fundamental particles, or "atoms", goes back to the ancient Greeks, but it didn't become a truly scientific notion until the 19th century, when chemists were able to categorize different fundamental "elements" of matter, defined as materials that could not be chemically broken down into any other materials, and described these elements as being composed of different atoms, with different relative weights and other properties.

By the end of the 19th century, the notion of atoms was well-established in the chemical community, though the physicists had a few reservations. The objections would dealt with in the early years of the 20th century, making the atomic theory universally accepted by the scientific community. Ironically, by that time the atom was no longer seen as indivisible. In 1897, the British physicist Sir Joseph John "J.J." Thomson had shown that atoms contained very small particles with a negative electric charge. These particles were named "electrons" and made up ordinary electric currents.

The heavier atoms had more electrons than the lighter atoms. Atoms are normally electrically neutral; removing electrons left the atom with a positive charge, adding electrons left it with a negative charge. The implication, Thomson realized, was that the atom itself was a matrix of some sort that was positively charged, with the negatively-charged electrons swimming around inside of it and neutralizing the overall positive charge. "Ionizing" the atom by removing or adding electrons gave the atom a net charge. The scheme was called the "plum pudding" model of an atom, since it was reminiscent of a plum pudding with raisins embedded in it. It was not a very accurate image of things, but it was a reasonable first approximation.

In the meantime, the French physicist Henri Becquerel discovered the mysterious energetic emissions of some materials that became known as "radioactivity". The French husband and wife team of Pierre and Marie Curie following up this revelation by discovering the radioactive elements radium and polonium, which would break down into other elements, releasing energy in the process. The atom, despite its name, was not indivisible and immutable. It had components and could break down into other elements, releasing some of those components. The breakdown rate of different unstable atoms proved to be very predictable, with any given collection of such atoms having a distinctive "half life", giving the amount of time it took for half of the atoms to "decay" to other atoms.

Radioactivity decay involved a release of energy much higher than could be maintained by any chemical process. The famous German physicist Albert Einstein's published his theory of special relativity, which had led to the famous "mass-energy relationship":

   energy = mass * speed_of_light^2

In fact, the physicists were still struggling to understand atom and were not yet in any position to exploit the energies locked up within it. In 1910, the New Zealander physicist Ernst Rutherford, working at the Cavendish Laboratory in England, discovered that the atom had a nucleus. Rutherford was conducting an experiment in which he bombarded a gold foil with energetic radiation known as "alpha particles". Every now and then one of the alpha particles bounced back at him, as revealed by flashes on a phosphorescent screen. This made no sense with the Thomson plum pudding model, because in that scenario the positively-charged matrix of the atom was not dense enough to have much effect on the energetic alpha particles. The answer was that the mass of the atom was not uniformly distributed through the atom's volume, but was concentrated in a nucleus. Instead of firing at a piece of paper, he was firing at mostly empty space with a small solid blob in the middle of it. Every now and then an alpha particle would hit a "nucleus" and bounce back.

The First World War put serious research into the atom on hold, but once the conflict was over, Rutherford determined that the nucleus contained a set of particles known as "protons", each of which had a positive charge equal to the negative charge of an electron -- but were about 1,837 times heavier than the electrons. This immediately led to a puzzle. Given that an atom of a specific element had a known number of electrons, it had to have the same number of positively-charged protons to balance out the negative charges of the electrons. However, the masses of atoms were known, and they were as a rule roughly twice as heavy as might be expected from the needed number of protons. One guess as to why this was so was that the nucleus also contained a number of electrons that partly neutralized the set of protons in it.

In addition, just before the war Rutherford's student Frederick Soddy had shown that a specific element could come in a range of masses, varying slightly in mass but otherwise with the same chemical properties -- though they could have different radioactive properties, which was what tipped Soddy off. The existence of "isotopes" for the elements was one of the reasons that relative weights of elements as calculated in the last century seemed to increase by varying steps: the weights calculated were averages of different isotopes.

After the war, in 1919 another one of Rutherford's students, Francis Aston, came up with a scheme to sort out isotopes of elements, inventing a device known as a "mass spectrograph". In its modern form, it features an injection system that strips a sample of an element of an electron or two -- "ionizes" it -- and then electrically accelerates the beam of ions through a magnetic field. The magnetic field forces the paths of the ions to curve, with the curvature greater for lighter isotopes. A row of detectors could then count up the relative concentrations of isotopes in the sample.

* While the experimental physicist Rutherford and his students were probing the atom, theoretical physicists were trying to come up a framework to make the atom understood. The Danish physicist Niels Bohr, working in Rutherford's lab for a time, was able to come up with an ad hoc model of the atom, which explained a number of processes that didn't make sense under classical physics. The essential feature was that the electrons orbiting an atom could only occupy a certain specific set of orbits around the nucleus. This explained features such as specific spectral patterns of light emission from an atom, but the model didn't explain why there were such specific sets of orbits.

Bohr's work was generally sidetracked by WWI, but in the 1920s the theoreticians went back to the task with a vengeance. With Bohr as a senior figure, theorists including the Germans Werner Heisenberg and Max Born, as well as the Austrian Erwin Schroedinger and Swiss Wolfgang Pauli, were able to come up with a much more adequate model of the atom based on "wave mechanics", in which the electron orbits were modeled as a resonant states of waves representing electrons. In this model, the specific sets of orbits correspondent to the possible resonant states of the electrons in orbit. The details are devious and beyond the scope of this outline, but some of the principal players in the effort would be involved in the effort to obtain the energy from the atom.

The experimentalists were overshadowed by the theoreticians for a time, but they soon came back to the fore. In 1932, the British physicist James Chadwick had discovered that the atomic nucleus consisted not only of protons, but also neutral particles otherwise similar to protons, which logically became known as "neutrons". This allowed the clumsy idea of the nucleus containing electrons to be discarded: the additional mass of the nucleus was due to the neutrons. In addition, it explained isotopes: they had differing numbers of neutrons.

The discovery of the neutron led in turn to another discovery. Since particles with the same electric charge repelled each other with a repulsive electromagnetic force, as described by Heisenberg in 1932 there had to be some force stronger than the electromagnetic force to keep the nucleus from flying apart. However, while the electromagnetic force has unlimited range, this "strong force" had to be a close-range force, only acting between neighboring particles in the nucleus; otherwise, it would have effects outside of the nucleus, and none had been observed.

As nuclei incorporated more protons, the long-range electromagnetic forces between the protons tended to make the nucleus more unstable. However, neutrons could be added to a nucleus to provide additional strong-force couplings to keep the nucleus together; since neutrons are electrically neutral, they don't introduce electromagnetic couplings and so can shore up the stability of the nucleus to an extent. As atoms grow heavier, the average ratio of neutrons to protons tends to increase to keep the nucleus from spontaneously breaking up. Of course, the instability still increases, with elements tending to be more unstable as their atomic mass increases, and elements approaching a hundred protons tending to be so unstable as to be difficult to accumulate in quantity. The number of a hundred is not a coincidence: the strong force is about a hundred times stronger than the electromagnetic force, and so a hundred long-range electromagnetic force couplings will overwhelm the short-range strong force.

One of the interesting things that had been discovered in the investigation of the isotopes of atoms was that, even after determining the mass of a particular isotope, it was somewhat less than would be expected by adding up the masses of the appropriate number of individual protons and neutrons needed to make up that nucleus. This was known as the "mass defect" (or equivalently rephrased as the "packing fraction"), and it was quickly realized that it was Einstein's E = MC^2 relationship in operation. In binding together, some of the mass of the nucleons was converted into energy, released, and lost. Not surprisingly, this energy was referred to as the "binding energy".

In terms of the ratio of mass defect to atomic mass, the defect grew larger as elements grew heavier up to iron, and then began to shrink again. This had significant implications in terms of energetics of nucleosynthesis: as a rule, creating heavier elements released energy overall up to iron, but for heavier elements nucleosynthesis absorbed energy. On the other side of the coin, breaking down these heavier elements released energy.

* In any case, the discovery of the neutron completed the basic modern model of an atom, with an atom consisting of an atomic nucleus composed of an agglomeration of protons and neutrons, with a number of electrons matching the number of protons orbiting the nucleus. Elements could have multiple isotopes, differing in their number of neutrons; some isotopes might be stable and others may be unstable, with some elements having no stable isotopes. For example, ordinary hydrogen, the simplest possible and most common atom, has one proton, no neutron, and one electron. Such ordinary hydrogen can be referred to in a shorthand form as:

   H<1/1>

   H<2/1>

   H<3/1>

   He<4/2>

   Be<9/4>     Beryllium-9, a stable isotope.
   Be<10/4>    Beryllium-10, an unstable isotope.
   C<12/6>     Carbon-12, a stable isotope.
   C<14/6>     Carbon-14, a stable isotope.
   Fe<56/26>   Iron-56, a stable isotope.

In the spring of 1933, Hitler passed a law throwing all Jews out of the civil service jobs. That included the state educational system, and all Jewish professors were promptly dismissed, including many who had been raised as Christians and had never been in a synagogue in their lives. That didn't matter: if they had significant Jewish ancestry, they were Jews. Many of the dismissed professors were Hungarian Jews, who had left Hungary for Germany in the early 1920s after a rightist and antisemitic regime came into power, evicting a short-lived Communist government. Now the Hungarians had to move again.

The exodus of Hungarian Jews included a clutch of promising physicists, including Leo Szilard, who had been an associate of Einstein's at the University of Berlin. Britain would welcome many of the refugees, and Szilard quickly made his way to London. He had read THE WORLD SET FREE in 1932 and had found the novel's concept of an "atomic bomb" intriguing. At the back of his mind, Szilard no doubt suspected that it wouldn't be very long before such a terrible weapon would be needed.

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[1.2] NUCLEOSYNTHESIS

* While work on probing the atom continued, new tools were being developed to help with the job. One need was for machinery to accelerate charged particles like electrons or protons to high energies, and by early 1930s a number of "atom smashers", as they were known in the popular press, had been developed. A Norwegian engineer named Rolf Wideroe had developed the first "linear accelerator" or "linac", in which charged particles were accelerated down an evacuated pipe through cylinders that were given alternating electric polarities.

The problem with linacs was that increasing energies meant increasing the length to the point where they wouldn't fit into labs any more. An American physicist named Ernest O. Lawrence of the University of California at Berkeley came up with the idea of accelerating particles with electric fields in a spiral confined by a heavy magnetic field. By 1931 he had his first "cyclotron" working; it would win him the Nobel prize in 1939. He continued his work on bigger and more powerful cyclotrons to obtain higher energies.

* In the early 1930s, the cyclotron was mostly a lab toy, a tool for the future, and for the moment physicists were probing the atom with more modest techniques. The initial clues towards the discovery of the structure of the atom had been provided by radioactive breakdown of nuclei. In 1919, Rutherford had performed experiments that seemed to imply that bombardment of nitrogen (N<14/7>) atoms with alpha particles sometimes to produce oxygen-17 (O<17/8>). This was confirmed in the 1920s; Rutherford had discovered "nucleosynthesis", the artificial creation of heavier atoms from lighter atoms.

Physicists began to tinker with the concept, with one goal being to see if they could synthesize isotopes found in nature. In 1934, the husband and wife team of Frederic Joliot and Irene Curie, daughter of Marie Curie, bombarded an aluminum foil target with alpha particles, producing the radioactive isotope phosphorus-30 (P<30/15>), which had never been observed before. Other physicists began to synthesize other unknown isotopes.

The problem with nucleosynthesis using alpha particles was that the alpha particle was positively charged, and so was repelled by the positively-charged nucleus it was supposed to impact. As nuclei grow heavier up the atomic scale, they grow more positively charged and so alpha particle bombardment becomes that much less effective. Enrico Fermi, a brilliant young Italian physicist working in the laboratory of Orso Mario Corbino at the University of Rome, decided to try bombarding atoms with neutrons instead. Being electrically neutral, they wouldn't be repelled by the positive charge of the nucleus.

The methodical Fermi started out with hydrogen and stepped his way up the periodic table. He had no success and became discouraged, and as one final effort he bombarded fluorine with neutrons on 21 March 1934. He was rewarded with a radioactive isotope of fluorine. Similar experiments on heavier elements also yielded radioactive isotopes.

At first, Fermi and his co-workers believed that fast neutrons would be most effective at nucleosynthesis, but in many case just the opposite proved to be true. They only discovered this by accident, observing that the induced radioactivity was stronger when the neutron source was placed on a wooden table instead of a marble one. Fermi suspected that the wooden table was slowing down the emitted neutrons, and to check out his idea he placed a block of paraffin in the line of the neutron beam during one of his nucleosynthesis experiments. The induced radioactivity increased. Using slow neutrons, Fermi and others in the lab began to perform nucleosynthesis with heavier and heavier atoms.

One of his colleagues, Emilio Segre, left for the United States, where in 1937 he visited Lawrence's lab, bombarding molybdenum with neutrons to obtain a previously unknown element, with atomic number 43. It would eventually be named "technetium" and was the very first "synthetic" element. The reason it wasn't found in nature was because all of its isotopes are radioactive and decay out of detectable existence eventually.

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[1.3] FISSION

* Among the elements bombarded by Fermi with slow neutrons was uranium, then the heaviest element known. The results of the experiments were baffling. A German colleague suggested that the odd results might be due to nuclear fission -- that is, the uranium nuclei were actually being shattered into fragments by the neutron bombardment. Fermi didn't believe it. He was partially deceived because he had covered his uranium samples with aluminum foil to prevent alpha particles released by the natural decay of uranium from confounding his experimental measurements. The aluminum foil also hid the fission fragments.

Fermi had come very close to discovering fission in 1935. It is likely fortunate that he missed, since the discovery might have given the Axis powers a head start on nuclear weapon development. Fermi's discoveries in nucleosynthesis through slow neutron bombardment were still significant enough to reward him with the Nobel Prize in physics in 1938.

By that time, few had any doubts that Europe was headed for violence and chaos. Hitler had continued to put the screws to Germany's Jewry, passing ever more restrictive laws, and in November 1938 Nazi mobs conducted a pogrom called "Kristallnacht" or the "Night of Broken Glass", with Jews vandalized, robbed, arrested, murdered -- leading to a mass exodus. Germany was clearly rearming for war, and in March 1938 German troops had occupied Austria, which was then incorporated into Hitler's Greater German Reich.

Two months later Hitler began to threaten Czechoslovakia, under the pretext that the government there was oppressing ethnic Germans living in the Sudeten regions of the country, bordering Germany. In the face of the threat, the British and French governments caved in at the end of September 1938, signing a "peace treaty" with Hitler at Munich that handed over the Sudetenland to Hitler. Chamberlain got off his airliner in London, waving his umbrella and announcing "peace in our time."

Loss of the Sudetenland stripped Czechoslovakia of the country's border defenses. Hitler took over the rest of Czechoslovakia in the spring of 1939, with the Czech region absorbed into the Greater German Reich and the Slovak region becoming a separate rightist state, closely aligned with Germany. Some of Czechoslovakia's other neighbors also helped themselves to portions.

When Fermi left Rome for Stockholm in the fall of 1938 to accept the Nobel prize, he took with wife Laura and their two children along with him. He was planning to spend seven months after that performing research in America, at Columbia University in New York City. Since he had visited the USA four times before, the Fascist authorities had no reason to believe he would not return. However, Fermi had become increasingly disgusted with the Fascists. Mussolini had decided to throw his lot in with Hitler during the occupation of Austria, and by the summer of 1938 the Italian government was starting to spout antisemitic rhetoric -- much to the shock of most of the citizenry, antisemitism never having been common in Italy. Laura was Jewish. He had no intention of going back. He joined the faculty of Columbia in early 1939, working hard to pick up the trappings and customs of an American.

* As far as fission went, some physicists had suggested that the increasing imbalance between the electromagnetic and strong nuclear forces as the atomic numbers of elements increased was likely to make large nuclei easy to break. Nobody paid too much mind to the idea, until better experimental proof showed the concept was completely on the mark.

In 1937, two chemists, a German named Otto Hahn and an Austrian named Lise Meitner, then collaborating in Germany, decided to methodically follow up Fermi's 1934 experiment, bombarding uranium with neutrons and performing a more careful examination of the end products. Their collaboration was disrupted in 1938 after the annexation of Austria into the German Reich, which turned Meitner into a German citizen. Since she was Jewish, that was far from welcome news. The Dutch agreed to admit her on her old, now obsolete, Austrian passport, so she fled Germany -- suffering through a moment of terror when German soldiers inspected her passport -- to finally end up in Sweden.

Hahn joined forces with another German chemist, Fritz Strassman, to continue the experiments. They finally discovered a radioactive isotope of barium, a much lighter element than uranium, in the products of their experiment. Hahn was startled, writing Meitner: "I don't believe it. There must be some mistake." He decided there wasn't and went public, ensuring him the award of the Nobel prize in 1944. Lise Meitner was keeping up with the effort, and came to the conclusion that fission had occurred. She collaborated with her nephew, the physicist Otto Frisch, who was then working in Niels Bohr's lab in Copenhagen, to write a letter for the British science journal NATURE, and submitted it in early 1939. Frisch told his boss Bohr about the letter, and before January 1939 was out, Bohr had attended a physics conference in Washington DC and spread the news: atomic fission had been performed.

Two researchers at Columbia, Herbert Anderson and John Dunning, worked using advice from Fermi to duplicate the Hahn-Strassman experiment. They bombarded a thin layer of uranium with slow neutrons inside an ionization chamber, this time without the shield of aluminum foil. They quickly discovered fission fragments and confirmed the release of energy. The Columbia group also realized that the fission process released more neutrons -- a matter whose implications were quickly understood.

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[1.4] INVENTING THE CHAIN REACTION

* The discovery of nuclear fission had dramatic consequences. The story began several years previously. On 12 September 1933, Leo Szilard, very annoyed at reading a newspaper article that cited Rutherford's "moonshine" remark, went for a walk through London to think things out. Crossing a street, he realized that nuclear fission might be the key to building an atomic bomb through a "chain reaction": the fission of one atom by neutrons would release more neutrons to perform fission on other atoms, leading to a cascade that released a vast amount of energy. His abstracted walk had produced one of the most literally earth-shaking ideas in history.

The problem was that nobody had even demonstrated fission at the time, and so few in England were enthusiastic about Szilard's big idea: he approached Ernest Rutherford and was sent packing. Szilard was still convinced he was on the right track, though had misgivings. Reflecting on the Japanese occupation of Manchuria in 1934, he wrote: "The discoveries of scientists have given weapons to mankind which may destroy our present civilization if we do not succeed in avoiding further wars." He actually meant bomber aircraft, atomic bombs not having been "given" to mankind just yet, but he could have hardly avoided the dire thought of bombers carrying atomic bombs. He also knew perfectly well that there were plenty of good physicists left in Germany who might also be thinking about atomic bombs, and that his misgivings would have to be swallowed for the duration of the emerging crisis.

Szilard made contact a British physicist, Frederick Lindemann, who became Szilard's patron. The fact that Lindemann was a close confidant of and a scientific advisor to the well-known politician Winston Churchill, at the time out of office but making his voice heard about the ugly events in Germany, was also a plus. Lindemann tipped Szilard off to the fact that patents could be kept secret if they were filed with the British military, and helped prod the British Admiralty into granting a secret patent to Szilard for explosives "very many thousands of times more powerful than ordinary bombs."

Szilard remained in England for the time being, though he was establishing contacts in the United States; he was curious about events on the continent and wanted to see which way events pushed him before jumping across the Atlantic. Following Hitler's occupation of Czechoslovakia, he left for the USA in late 1938, taking a position at Columbia University and eventually becoming an American citizen. There, Szilard accelerated his work on chain reactions. His experiments with beryllium hadn't gone anywhere, but Lise Meitner's paper on nuclear fission in uranium opened the door. Fermi's discovery that slow neutrons provoked fission more easily than fast neutrons opened the door a bit wider.

When Fermi arrived, Szilard pitched the chain reaction to him. Fermi was skeptical that the idea would work, regarding it as a long shot -- but not such a long shot that it wasn't worth investigating, even if just to put the idea to rest. Nuclear research at Columbia accelerated.

There was the question of what material might best support a chain reaction. Szilard was still interested in beryllium, but Niels Bohr, then still in the US and temporarily working at Columbia, considered three alternatives: thorium-232 (Th<232/90>), uranium-235 (U<235/92>), and uranium-238 (U<238/92>). He concluded that uranium-235 was the best candidate -- but it's only 1% of natural uranium, with the rest being uranium-238, and Bohr didn't think it would be practical to separate the two isotopes since their chemical properties were effectively identical, except for a slight difference in atomic weight.

The result of the theoretical and experimental work began to suggest that a chain reaction might well be possible and that an atomic bomb might be practical. Enrico Fermi, watching out from the window of his upper-story office at Columbia over the streets of Manhattan, pretended to hold a ball in his hands and told a colleague: "A little bomb like that, and it would all disappear."

On 17 March 1939, in the wake of the dismemberment of Czechoslovakia, Fermi briefed US Navy officials on the possibility of the atomic bomb. He wasn't completely convinced about the concept himself about the time and didn't pitch it with great enthusiasm. The presentation unsurprisingly fell flat.

Szilard was much hotter about the issue than Fermi. He tried to persuade his colleagues to not publish results of experiments into fission processes, but the physics journals were full of papers on the subject at the time and it was like holding back the tide. In late April, THE NEW YORK TIMES reported on the debate among the physicists over the possibility of an atomic chain reaction, or more colorfully the "probability of some scientist blowing up a sizeable portion of the Earth."

Szilard was working energetically with two long-time colleagues, Eugene Wigner and Edward Teller -- both Hungarian Jewish physicists who had been chased out of Europe by Hitler -- on the basic physics of the chain reaction, with Fermi working in parallel, providing crosschecks. Fermi didn't care much for actually doing experiments in collaboration with Szilard; Szilard was a theoretician and flatly said he didn't like getting his hands dirty, which made a poor impression on Fermi, who was just as or more happy in the lab as in front of a blackboard.

The work gradually converged on the possibility of demonstrating a controlled chain reaction in what would be called an "atomic pile", a layered structure made up of modules of uranium -- to provide the reaction -- embedded in graphite, that is carbon, to act as a "moderator", absorbing neutron emission to prevent the chain reaction from going into "runaway". "Heavy water" -- that is, water incorporating the heavy deuterium isotope of hydrogen instead of ordinary hydrogen -- was also considered as a moderator, though it would prove much less effective. Incidentally, ordinary water wouldn't work, because it had a stronger tendency to absorb neutrons -- had a higher "cross section", as the physicists put it -- and would damp out the chain reaction. In any case, heavy water was regarded as a backup plan, since carbon seemed like the better option.

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[1.5] THE EINSTEIN LETTER

* Progress was being made, but Szilard felt that more progress demanded financial backing from the US government. Unfortunately, the dud presentation Fermi had made to the US Navy left Szilard unsure of what might be done for the moment. He did have another worry, about Belgium of all places -- the Congo was a Belgian colony, uranium was being mined there, and there was the uncomfortable prospect that uranium ore might find its way into Nazi hands via Belgium.

Szilard knew that his old colleague Albert Einstein was on close terms with Queen Elizabeth of Belgium, and thought Einstein might be a good way to pass on a warning about the potential of uranium and the danger of it being passed on to Germany. Szilard made an appointment with Einstein and went to Einstein's summer house in Peconic, Long Island, on Sunday, 16 July 1939, to talk with the great man. Since Szilard didn't have a car, Wigner drove him there; they got lost but a local boy showed them the way to Einstein's house.

Szilard told Einstein about the work on chain reactions; Einstein was intrigued, exclaiming in German: "I never thought of that!" Einstein was perfectly willing to help them get the message about the potential of the atomic bomb, even if there were still doubts that such a weapon could be made to work. As Szilard said later: "The one thing scientists are afraid of is to make fools of themselves. Einstein was free from such a fear and this above all is what made his position unique on this occasion." Wigner did suggest that they "should not approach a foreign government without giving the [US] State Department a chance to object." Einstein dictated a letter to the Belgian ambassador, who he judged more appropriate a contact than the queen, with the letter to be passed on through the State Department.

However, within days a Dr. Alexander Sachs -- a prominent Russian-born American economist from Harvard who was a personal friend of US President Franklin Delano Roosevelt -- got in touch with Szilard. Sachs had got wind of Szilard's desire to get US government backing for atomic bomb research, and Sachs politely suggested that he might be able to make a pitch to Roosevelt for backing. Szilard was taken aback by the offer, but went over to Peconic again on Sunday, 30 July, to talk the matter over with Einstein. Wigner was out of town, so Edward Teller played chauffeur to Szilard instead; Szilard wanted to introduce Teller to Einstein anyway, telling Einstein in advance: "He's very nice."

The meeting that day led to a debate between Szilard and Einstein over the next two weeks on whether Sachs was the best courier for the job -- Sachs himself had suggested a number of alternatives, including the famous aviator Charles Lindbergh, though Roosevelt couldn't stand him -- and how the letter originally intended for the Belgian ambassador should be modified for presentation to Roosevelt. Sachs was judged satisfactory, and in mid-August Szilard handed him the final draft of the letter for Roosevelt.

* Sachs felt he needed an hour of the president's time to give a proper sales pitch -- there was no sense in just handing Roosevelt a letter that would go into the other piles of paper on the desk in the Oval Office -- but Roosevelt was unusually busy at the time. Although Hitler had promised during the Munich crisis that his claims on Czechoslovak territory were to be "the last territorial demand I have to make on Europe", by the summer of 1939 he was making the same sort of loud, threatening noises once more, this time against the Poles. Nobody was going to try to make a deal with Hitler this time around, and he didn't want one anyway: he wanted a war.

On 23 August 1939, about a week after Sachs received the letter, Nazi Germany and the Soviet Union signed a nonaggression pact. The news was an absolute shock to most, since the Nazis and Soviets had been denouncing each other in bitter terms for years. It was a particular shock to Communist parties outside the USSR, who now had to proclaim that Hitler was a friend of the world proletariat -- a turnaround so drastic that it led to the defection of many from the ranks of the Communist International. Astute observers saw it for what it was, a cynical and temporary marriage of convenience between Hitler and Soviet dictator Josef Stalin. Considering Hitler's hostile talk against Poland, the immediate result of the pact was easy to guess: the dismemberment of Poland.

After trumping up a Polish attack on a German town -- with a political prisoner shot to provide "evidence" -- Hitler invaded Poland on 1 September 1939. Britain and France declared war on Germany on 3 September: World War II had begun. Roosevelt was up to his eyebrows in the crisis, in particular lobbying for assistance to Britain. Sachs couldn't even arrange an appointment until a week into September. Szilard and Wigner visited Sachs late in the month and were upset to find out that he hadn't spoken to the president yet, naively unable to understand that getting an audience with an American president wasn't trivial even in calmer times. Sachs promised to go to the White House in a week, and in fact was admitted to the oval office on Wednesday, 11 October 1939.

Roosevelt greeted Sachs with: "Alex, what are you up to?" Sachs gave Roosevelt a carefully prepared little lecture based on Einstein's letter, At the end of his pitch, Roosevelt replied: "Alex, what you are after is to see that the Nazis don't blow us up." Sachs replied: "Precisely." Roosevelt turned to his aide, General Edwin M. "Pa" Watson, and said: "This requires action."

Roosevelt, a politician to the core, was well-known for seeming pleasantly and broadly agreeable to visitors who then would find out that nothing specific actually ended up getting done, but Watson arranged a meeting of the "Advisory Committee for Uranium" on Saturday, 21 October, at the Carleton Hotel in Washington DC. Sachs, Szilard, Teller, and Wigner showed up to present their case; Fermi was still feeling sore about the Navy presentation and absented himself, with Teller acting as his emissary. The foursome met with five government representatives of the US Army, Navy, and National Bureau of Standards (NBS). The government committee was led by Lyman J. Briggs, head of the NBS, and would become known as the "Briggs Committee".

Briggs was not the best choice for heading up the committee, since he was notably conservative and deliberate in his actions. The meeting was somewhat confrontational, with outspoken skepticism from some of the government men -- though Sachs, more used than his physicist associates to government committees, was willing and able to lead the charge. The Army representative even suggested that weapons take decades to come to maturity and so atomic power would not be particularly important for winning the current war, adding that "it isn't weapons that win wars, but the morale of the troops." He went on at length in this vein until Wigner, normally a model of Old World decorum, interrupted to suggest that if morale was the only deciding factor in a war, then the weapons procurement budget of the Army should be promptly cut. The Army man digested this for a moment and then grudgingly answered: "All right, all right, you'll get your money."

Teller only asked for six thousand dollars to obtain graphite for investigation of its use as a moderator. He would soon regret specifying such a pittance, much later commenting that "my friends haven't forgiven me yet." However, on 1 November the Briggs Committee handed a report back to the president, describing the potential of atomic power not only for bombs but also for powering submarines, and recommended "support for a thorough investigation." The report would gather dust in the president's files for several months -- but it wasn't forgotten.

* In the meantime, Poland had fallen to the Nazis. German troops had occupied Warsaw on 1 October, and by 6 October 1939 organized Polish resistance had faded out. The Soviets had cooperated with the destruction of Poland, invading on 27 September and carving out their own chunk, whose borders had been specified by secret provisions of the Nazi-Soviet Non-Aggression Pact. About 100,000 Polish troops escaped, ultimately throwing in their lot with the Allies as "Free Polish" forces.

While the German Wehrmacht -- armed forces -- crushed Poland, the wheels were turning in Germany on investigation of an atomic bomb. Back in the spring of 1939, excitement over the emerging possibilities of nuclear weapons had led to the establishment of a military research office under the direction of Kurt Diebner, a physicist working for the Wehrmacht on explosives research. He obtained a deputy named Erich Bagge, and the two men set up a secret conference on atomic power in Berlin on 16 September 1939.

Prominent physicists, included Otto Hahn, were invited; they seemed at loggerheads on matters, so Bagge contacted Werner Heisenberg, who had sufficient stature to impose order on the herd of physicists, to a second conference on 26 September. The second event had the desired results: in the aftermath, formal research went ahead, with theoretical work to be directed by Heisenberg and experiments to be conducted by a number of other teams. More space was found for the effort at the Kaiser Wilhelm Research Institute.

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