A Physics Café talk given at Explore Booksellers, Aspen, Aug. 28, 2023

Thank you all for coming here today. I imagine that a good number of you have seen the movie, Oppenheimer, which focusses on the remarkable character of J. Robert Oppenheimer, and especially on the confrontations at the 1954 Atomic Energy Commission hearings on his future security clearance. Only hinted at in the movie is the amazing effort to develop atomic bombs, primarily at Los Alamos – the secret town in northern New Mexico – but also at other secret laboratories including Oak Ridge in Tennessee, and Hanford in Washington State, all part of what was euphemistically called the Manhattan Project. What I would like to do in this hour is to give you a flavor of the history of the full development of what the historian Richard Rhodes, author of the monumental book, The Making of the Atomic Bomb, called, “arguably the single most important historic development of the 20th century.”

The movie and indeed the whole history raises many many questions that I will not get into today. The movie has been roundly criticized for what it did not show. To physicists it focusses too heavily on the 1954 hearings rather than the larger story. And more seriously, it does not get into whether dropping bombs on Japan made sense, or could have been avoided. It does not show any victims of the bombing of Hiroshima and Nagasaki. It also does not mention the people in the nearby Tularosa Basin in New Mexico who were irradiated by the Trinity test. And it does not deal significantly with postwar efforts to control atomic weapons. These are points I will also not discuss in this narrative, nor will I discuss the hydrogen bomb.

I have, as a practicing physicist, always been fascinated by the history, and in the mid-80’s took part, as a scientific consultant, in the writing of a technical history of the making of the bomb at Los Alamos. The history is published in a wonderful book called Critical Assembly, issued by Cambridge University Press in 1993. The title has several meanings, including the assembly of the bomb parts and the assembly of the people there. Over the years I got to know many of the principals involved, but not Oppenheimer, who died in 1967.

I also worked rather closely on nuclear and astrophysics with the great Hans Bethe, who appears in the movie, and who was the head of the theoretical physics division at Los Alamos. Bethe is the person who explained in 1938 how stars like the sun generate their energy through nuclear reactions – the process of nuclear fusion, in which hydrogen gets converted to helium, releasing lots and lots of energy. He also played an important role in the early days of the Aspen Center for Physics, where he gave part of his 1967 Nobel Prize money to build the Bethe Hall, appropriately named.

Hans Bethe and Gordon Baym, ca. 1985

Hans Bethe and Gordon Baym, ca. 1985


Let me begin the story by recalling the time frame of the development of the bomb. The U.S. declared war on Japan on Dec. 8, 1941, the day after Pearl Harbor, and on Germany three days later. The Manhattan Project started shortly after, in August 1942. The history of the making of the bomb in Los Alamos – called Project Y – began later that year, under Oppenheimer’s direction, and work there began in earnest April 1, 1943. Just 27 months later, the first bomb produced by Los Alamos was exploded at the Trinity test in the desert in Alamogordo, New Mexico – July 16, 1945. To set the date into context, Germany had surrendered earlier, on May 8, 1945. Los Alamos produced the two weapons that were used in the war against Japan, the one dropped on Hiroshima on August 6 and the second on Nagasaki on August 9. Japan surrendered on August 14. (Notably, the Trinity test was a full two months after the surrender of Germany – this raises the question of why the scientists in Los Alamos, many of whom were refugees of Nazi Germany, continued working on the bomb.) 


Key to the development of an atomic bomb was the discovery in December 1938 that certain heavy atomic nuclei – which are in the center of atoms, like the sun is the center of our solar system – would fission, or break-up. Although fission was discovered in Germany, it was interpreted by Lise Meitner, who being Jewish had shortly before fled to Stockholm. The break-up releases energetic particles called neutrons – one of the two particles that make up the nuclei of all atoms. Examples of fissible nuclei are uranium and plutonium. Shortly thereafter, the great Danish physicist Niels Bohr came to Washington to announce the discovery of fission, and at this time explained, with John Wheeler, how fission works. 

It was soon realized by physicists, including in Nazi Germany, that fission could be employed to make a bomb, and both the U.S. and Britain, as well as Germany – and to an extent Japan – began to think seriously about making a fission weapon. 

The basic idea of making a bomb is that once nuclei begin to fission, a large enough chunk of matter – called a critical mass – can undergo a nuclear chain reaction, in which the neutrons from the breakup or fission of one heavy uranium atom, for example, would trigger reactions in other heavy atoms, which in turn would trigger reactions in still other atoms, until the entire mass of uranium would simply explode. But at this stage no one had ever achieved a nuclear reaction, which would be critical to realize before making an actual weapon.

Fermi’s Pile

The first demonstration of a nuclear chain reaction was at the University of Chicago, in a project which had the public name of the Metallurgical Laboratory or Met Lab. The experiment, really the building of the first nuclear reactor, was carried out by the great Italian physicist, Enrico Fermi (after whom Fermilab in Batavia, Illinois, as well as a gazillion concepts in physics, is named). Built out of the way under the stands of Stagg Field on the University of Chicago campus (a site since demolished – in 1957), Fermi’s reactor, or pile (or CP1 for Chicago Pile) – as it was called – soon achieved the first controlled reaction on December 2, 1942. The experiment was the first achievement of a controlled nuclear reaction, as is used now in all nuclear power plants. The Met Lab eventually grew into Argonne National Laboratory. 

On a personal note, let me recall that after the first chain reaction was achieved, Fermi and his group of researchers drank a bottle of Chianti to celebrate, and they all signed the wicker basket covering the bottle. Albert Wattenberg, who was one of Fermi’s students working on the pile, and one of my important mentors, actually kept the bottle. When Al became a Physics Professor at the University of Illinois he placed it on the mantlepiece in his house, where it stood many many years until he finally donated it to the Smithsonian Museum. 

Now some of you may ask, with all the uranium in Fermi’s pile emitting neutrons, why didn’t the pile explode, like in the bombs that would be developed? The answer is that Fermi realized that one had to control the number of neutrons present – with graphite rods that would slow the neutrons down and make them less likely to trigger further nuclear breakups.


Behind Fermi’s success lies a special person, Leo Szilard, a quite amazing Hungarian physicist, who makes a brief appearance in the movie, where he is called “Silerd.” Szilard made many contributions that are important to the present story: in 1934 he conceived and patented the idea of the neutron-induced nuclear chain reaction; he assigned the patent to the British Admiralty to keep it secret! Then in August 1939 he wrote the famous warning letter to President Roosevelt, signed by Einstein, that Germany might make a fission bomb and that the U.S. should start a nuclear weapons program – planting the seed for the formation of the Office of Scientific Research and Development (the OSRD), led by Vannevar Bush with James Conant as deputy, the program that became the Manhattan Project.
He also invented with Einstein in the late 20’s, a refrigerator with no moving parts, in use today in breeder reactors. And he helped found the Academic Assistance Council in Great Britain to help scientific refugees from Nazi Germany.

Szilard was a genius and at the same time completely flaky – someone we would describe today as totally ADD or ADHD. He apparently never had a permanent residence after he came to the U.S. in 1938, living with others out of a suitacse. General Groves, who directed the Manhattan Project, unfortunately would not allow him to participate in Los Alamos. I asked Bethe what Szilard’s contributions to the war effort were, and he immediately said that Szilard was responsible for the success of Fermi’s pile.

Trained as a chemical engineer in Budapest, Szilard knew that the chemical element Boron was used in making graphite, which would be used to control the chain reaction. Boron, as he also knew, gobbles up neutrons, and if there was any slight contamination by boron in the graphite used to control the chain reaction, the reaction would immediately shut down. To make a reactor work one has to slow down the neutrons, but at the same time make sure that they don’t disappear. So Szilard negotiated with the graphite manufacturers to make graphite without boron, without of course saying why he needed it, and this boron-free graphite was used in Fermi’s reactor.

Interestingly the Germans had realized that boron would be a problem, but not having a Szilard, they rather turned to the idea of controlling a nuclear reaction using heavy water (which is water in which the hydrogen atoms contain an extra neutron). They planned on getting heavy water from an ammonia plant at Ryukan in Norway. But the Norwegian resistance sabotaged the heavy water plant in early 1944, leaving the Germans without a moderator. The Germans never succeeded in even making a simple working reactor.

Einstein, co-signer with Szilard of the letter to Roosevelt that strongly influenced the creation of the Manhattan Project, did not play any further role in the development of atomic weapons. In the movie Oppenheimer shows Einstein a paper with the calculation of whether a bomb would ignite the atmosphere. Such a calculation was indeed done, but no one at Los Alamos believed that there would be any possibility of such a catastrophe. However, there was no way that Oppenheimer would have carried away from Los Alamos a paper with this calculation. Furthermore, Einstein did not have a security clearance.

Types of Bombs

Let me now turn to atomic bombs themselves. Los Alamos developed two types of bombs. The first used the rare isotope of uranium, U-235, which was very difficult to separate from naturally occurring uranium, U-238, but the mechanism of the bomb was relatively simple. One needs enough U-235 to set off a chain reaction, a critical mass. The idea was to have two pieces each less than the critical mass, which would be brought together by a conventional gun firing mechanism. By summer 1945 Los Alamos had accumulated sufficient U-235 only for the bomb that would be dropped on Hiroshima, but not enough to have a second to test whether it would work. Separating out U-235 was very difficult and required lots of electricity. 

The Oak Ridge Laboratory in Tennessee was the site where the U-235 was made, since it could draw on the Tennessee Valley Authority electrical power facilities. The big mass separators that were used, called Calutrons, were a type of cyclotron in which atoms went around in circles guided by magnets. They were named after their origin at Ernest Lawrence’s lab in Berkeley — Lawrence is a main character in the movie. When I visited Oak Ridge a number of years back, I asked if I could see a Calutron, but was reminded that owing to the critical copper shortage during the war – pennies in 1943 were made of zinc, not copper – the magnets in the Calutrons were wound with the silver from Fort Knox, and that after the war all the silver was returned to Fort Knox for safe keeping. Later, a more efficient process called gaseous diffusion was used to separate out the U-235. 

The second type of bomb used an isotope of the element plutonium, Pu-239, which was not naturally occurring, but had to be produced in nuclear reactors by bombarding ordinary uranium with neutrons. Los Alamos would accumulate an adequate supply of plutonium by mid-1945 – from the Hanford reactor in Washington State – to make several plutonium bombs and be able to test one. 

In the plutonium bomb, a shell of plutonium metal is imploded to form a critical mass. The Oppenheimer movie shows vividly the assembly of the pieces to make the bomb. Why the geometry had to be so complicated was an unexpected problem in using Pu-239. In the Spring of 1944, Los Alamos had accumulated enough Pu-239 for the Italian-American physicist, Emilio Segrè to begin to see that a small mass of Pu-239 was fissioning some five times faster than was expected from how fast individual Pu-239 nuclei fissioned. Something was amiss and very very worrisome! 

The culprit turned out to be the isotope Pu-240, which was also made when making Pu-239 in the Hanford reactor, and which was slightly contaminating the Pu-239 metal on hand. (As some of the newly made Pu-239 atoms sit in the reactor, they capture a neutron to become Pu-240.) Now the chances of a given atom of Pu-240 to emit neutrons while decaying are something like once in the age of the Universe – some 14 billion years – yet just a small contamination was enough to explain the more rapid fissioning Segrè saw, The experiments had the slowest data rate ever to date – like one event a month – and the detection of Pu-240 was a remarkable scientific achievement. 

What the contamination also meant was that as one tried to bring the Pu-239 together using the gun assembly that worked for U-235, there would be enough neutrons from Pu-240 to set off a weak nuclear reaction prematurely, creating a fizzle. To overcome this problem, the plutonium would have to brought together by implosion very rapidly and symmetrically. On July 4, 1944 (no day off then) Oppenheimer gave to talk to the entire lab redirecting its efforts toward development of a fast way to trigger a plutonium bomb. Efforts to solve this seemingly impossible problem took up a major fraction of the experimental and theoretical effort at Los Alamos from then on, and the Trinity bomb was tested in just a year. People worked very hard.

Setting Up Los Alamos

As the movie shows, Oppenheimer was enchanted with New Mexico, and had chosen the Los Alamos Ranch School, a private boys school, as the site of Project Y. Oppenheimer and the Berkeley experimental physicist Ernest Lawrence made an incognito visit to the Ranch School in the fall of 1942. Amusingly, the physicist Stirling Colgate, who spent much time at the Aspen Physics Center, and whom many of you may remember, happened to be a seventeen year old student at the school, and in fact recognized Oppenheimer and Lawrence. As Stirling wrote:

“About December that year (1942), two men showed up at school, and we were required to say our yes sirs to a Mr. Jones, who was wearing a fedora, and to a Mr. Smith, who was wearing a porkpie hat. The names were obviously pseudonyms. Not only was everybody showing them great deference, but Mr. Jones seemed most uncomfortable every time someone referred to him by that name. The four of us who were seniors had studied physics. The pictures in our physics textbook made it easy for us to recognize Mr. Jones as Ernest Lawrence and Mr. Smith as Robert Oppenheimer. Furthermore, the discovery of fission had been big news. In fact, we were even aware of the idea of a chain reaction. Clearly, the school was about to be converted to a laboratory to work on a very secret physics project. Why else would top physicists be visiting a place out at the end of nowhere with no water, no roads, no facilities? What was really going on was obvious! We were secretly amused by the pretense” 
Stirling Colgate at Conundrum Hot Springs. Photo courtesy of Craig Wheeler.
Stirling Colgate at Conundrum Hot Springs. Photo courtesy of Craig Wheeler.

How did Oppenheimer become the director of Los Alamos? The job of choosing the leading scientist at Los Alamos fell to Arthur Holly Compton, a renowned experimental physicist, who won the Nobel Prize in 1927 and spent much of his career at Washington University in St. Louis. Compton became head of the Met Lab in Chicago in 1942, and had major responsibility for scoping out the future Manhattan Project.He made two crucial decisions. The first was to reorganize the efforts scattered around the country, and in particular to concentrate research on a plutonium weapon in Chicago; this reorganization included bringing Fermi to Chicago from Columbia University in New York City. The second was to chose Oppenheimer, then at the University of California at Berkeley, to be head of theoretical research on the problem.

Compton knew that Oppenheimer was highly critical of people who did not see things his way, for being somewhat unapproachable, and not in touch with daily details. Compton nonetheless regarded Oppenheimer as the best person to direct the division of the Met Lab that would estimate the amount of plutonium needed for a bomb, and figure out how much energy one could get out of an explosion. Bethe told the historian Richard Rhodes that Oppenheimer was able to direct the effort at Los Alamos so successfully because he was so much smarter than everyone else, and Bethe… included himself in that comparison. Bethe also had the more telling insight, that Oppenheimer before and after the war had been casually cruel to people who made mistakes around him, including Bethe himself, but that “he suspended the hostilities at Los Alamos.”

Compton also felt the need for a more practical person to coordinate the different groups involved at the Met Lab, and his choice for the deputy director of Los Alamos was the somewhat introverted physicist John Manley, who had received who received a bachelor’s degree in physics from the University of Illinois in 1929. As it turned out Manley and Oppenheimer, of quite different personalities, worked well together in establishing the Los Alamos laboratory, and Manley managed the lab until the early 1950’s when he left to become chair of the physics department at the University of Washington in Seattle. As Jennet Conant writes in her lovely book, “Manley recalled that when he met Oppenheimer for the first time, he was ‘somewhat frightened of his evident erudition’ and ‘air of detachment from the affairs of ordinary mortals.’ ”

Final responsibility for choosing the head of Los Alamos fell to General Leslie Groves, the director of the Manhattan project – he chose Oppenheimer in October 1942.

Getting Started at Los Alamos

After choosing the site, the next task in getting Los Alamos going was to recruit people and requisition scientific equipment from around the country – all in secrecy. The Manhattan Project brought to Los Alamos a good fraction of the top physicists in the United States, both established and junior. In addition, physicists from Britain (working on what was known as the Tube Alloys project – another euphemism) joined the effort, as well as such distinguished people as Niels Bohr from Denmark. All in all over 100,000 people were involved in the wartime Manhattan project, with some 4000 at Los Alamos. 

Physicists, both senior and junior, were enticed to come to Los Alamos. Many were involved in other wartime projects, e.g., making radar at the MIT Radiation Laboratory, and did not come. Robert (or Bob) Wilson, the builder of Fermilab, the giant particle accelerator outside Chicago, who was also a frequent visitor to the Aspen Center for Physics, told that when he set off to work at Los Alamos and tried to buy a train ticket from Princeton to Lamy, New Mexico, the closest railroad stop to Los Alamos, the person selling tickets in Princeton advised him definitely not to go there, saying that he had been selling lots of one way tickets –and no one ever came back!!

Within the first three months of 1943 the scientific staff was basically assembled. Oppenheimer felt strongly that the work at Los Alamos should not be compartmentalized, but rather everyone should be aware of others’ problems, to increase cooperative efforts. Groves on the other hand strongly favored compartmentalization (as emphasized in the movie), and prevailed everywhere but at Los Alamos. Oppenheimer, to implement his plan, organized – just after everyone arrived at Los Alamos – what was called the April Conference, in late April 1943, to analyze, and I quote, “the scientific problems of the Los Alamos Laboratory and to define its schedules and its detailed experimental program.” The attendee list reads like a who’s who of physics celebrities, including perhaps a dozen future Nobel Prize winners. (All in all some two dozen Nobel Prize winners, past and future, participated in the wartime effort in Los Alamos.) 

The job of giving a series of lectures at the April Conference on the basic scientific principles of nuclear explosions and weapons fell to Robert (or Bob) Serber. Serber had begun to work with Oppenheimer in Berkeley in the mid- thirties, and then in 1938, despite Oppenheimer wanting to keep him on the faculty at Berkeley he was denied a teaching job there (Berkeley felt that one Jew on the faculty, Oppenheimer, was enough), and Serber joined the University of Illinois faculty instead. It was from Illinois that Oppenheimer recruited Serber into the bomb project, shortly after the U.S. went into World War II. Serber’s lectures were written up and became the first Los Alamos report, known as the Los Alamos Primer, with the appropriate report number LA-1. This report was only declassified in the mid 1960s, and is available as a book now. 

Early on Oppenheimer and John Manley realized that Los Alamos would need particle accelerators (or “atom smashers” as they were known) to study the properties of atomic nuclei, for example, how neutrons would interact with them. And so Manley scoured the country, bringing in equipment from Princeton, the University of Wisconsin, and the University of Illinois, among other places. From Illinois he “borrowed” so to speak, a particular kind of accelerator developed by John Cockroft and Ernest Walton in England in 1932, which they used to split an atomic nucleus for the first time. In order to take pictures of how nuclear chain reactions worked, Los Alamos also requisitioned from the University of Illinois a kind of electron accelerator called a betatron, developed as a very high power electron microscope. 

But getting scientific equipment to Los Alamos, while preserving the secrecy of the Manhattan Project, was not easy. For example, Los Alamos had decided that a cyclotron at Harvard was needed. (I actually worked on a later version of this machine when I was a graduate student at Harvard.) So the military was left to convince Harvard physicists to turn the machine over to them, saying that it was needed for a medical facility in St. Louis, and that they would pay a lot of money for the machine. The Harvard physicists, seeing Bob Wilson – who as I mentioned would be the founder of Fermilab – on the negotiating team, immediately realized what was going on, and offered the machine for almost nothing if it was to be used for studying nuclear fission. But the military stuck to their story, and ended up paying Harvard quite a bit ($1 million) for the machine.

Oppenheimer’s Science

Let me comment on Oppenheimer as a scientist, a topic not explored in the movie. Once he began studying at Göttingen, Germany, Oppenheimer began to show his genius in physics. With his advisor there, Max Born, who won the Nobel Prize for bringing out the probabilistic aspect of the quantum description of the microscopic world – and whose son incidentally was Olivia Newton John’s uncle – Oppenheimer developed in 1927 a very important theory of how solid materials work, thinking analogously of the heavy atomic nuclei as elephants and the electrons as mosquitos, dutifully following the elephants. This is certainly Oppenheimer’s most famous research work. In 1928 he developed the theory of “quantum tunneling,” how a particle can go through a barrier that it could not go through classically. Then in 1930 he discovered theoretically that electrons had to have antiparticles, called positrons, which were discovered experimentally just two years later. 

In the later thirties he became interested in astrophysics, and asking himself what happens to massive stars when they stop burning fuel and realizing that they would completely implode, he proposed the black hole. There is a slight mention of this in the movie. John Wheeler, the physicist who with Niels Bohr worked out the fundamental theory of nuclear fission, and who pioneered and proposed the name “black hole,” eventually agreed that Oppenheimer was the black hole’s real inventor. Oppenheimer also did fundamental work on the super compact stars, neutron stars, which were discovered in the late sixties as pulsars, tiny stars that appear to blink on and off up to 700 times per second. Actually they are rapidly rotating lighthouses. Neutron stars also are one of the main sources of gold in the Universe, but that is another story. 

Oppenheimer did little basic science after the war. All in all, he never realized his true potential as a physicist, but rather became a physics dilettante, as comes through in a number of physicists’ impressions of him.

Physicists’ Impressions of Oppenheimer

First, I. I. Rabi (Isadore Isaac) – whom one sees as Oppenheimer’s close friend in the movie – spent the wartime period developing microwave radar at the MIT Rad Lab (out of which grew our everyday microwave ovens). Rabi came to Los Alamos only occasionally as a consultant, but he was in fact at the Trinity test, as was shown in the movie. Rabi remarked, “I have said of him that he would have been a much better physicist if he had studied the Talmud [the book of Judaic wisdom] rather than Sanskrit… it would have given him a greater sense of himself.” 

And then my advisor Julian Schwinger, who was Rabi’s student, explained why he was not taken with Oppenheimer, 

“He had a quick brain… but the brain must be supplemented by long hours of practice that go into the fluidity and ease.” He continued, “It became clear that since he no longer concerned himself with the (scientific) details of things, it became more and more superficial, which I regretted very much. It was a lesson to me, never to lose completely your touch with the subject, otherwise it’s all over.” 

Consistently, Wolfgang Pauli, who was one of the great theoretical physicists of the twentieth century, remarked,  “Oppenheimer considered physics as an avocation and psychoanalysis as a vocation.”

Portrait of Murray Gell-Mann by Bernice Durand

Portrait of Murray Gell-Mann by Bernice Durand

Murray Gell-Mann, a regular visitor to the Aspen Center for Physics, put it well, “He didn’t have Sitzfleisch[ability to sit and concentrate for long times].  As far as I know, he never wrote a long paper or did a long calculation, anything of that kind. He didn’t have patience for that; his own work consisted of little [perceptions or in Murray’s language] aperçus, but quite brilliant ones. But he inspired other people to do things, and his influence was fantastic.”

Niels Bohr and Werner Heisenberg

I want to switch gears a bit and broaden the story to bring in Niels Bohr – the Danish father of the quantum theory of the microscopic world – and his connections to the bomb project. Bohr’s part in this story began in September 1941 when he was visited in Copenhagen by Werner Heisenberg, his 16 year younger colleague, and the discoverer of the uncertainty principle, which says that you can’t measure with arbitrary accuracy the position and speed of a microscopic object. Bohr and Heisenberg were very close, almost like father and son. At this point though Heisenberg was very actively involved in the German atomic bomb project.

Many of you may have seen the play Copenhagen by Michael Frayne which focussed on this visit. The play is completely uncertain about what actually happened at the meeting, but as it hints, Heisenberg and Bohr discussed Heisenberg making atomic weapons. The details began to emerge only in the late fifties after Robert Jungk published a book, Brighter than a Thousand Suns, on the German bomb project.

The book infuriated Bohr, since it was in large part a revisionist history of Heisenberg’s wartime efforts. And Bohr proceeded to write some eight letters to Heisenberg about the meeting in 1941, which were only released to the public in 2002. Interestingly, he never sent Heisenberg any of the letters, to avoid hurting Heisenberg and his family. He sent only a greeting to Heisenberg on his 60th birthday in 1961. Bohr dictated the letters to his wife Margrethe, his son Aage, and to his scientific assistant, Aage Petersen. The latter Aage and I became friends shortly I arrived as a postdoc in Copenhagen in 1960, and Aage shared with me what Bohr had recently written. I remembered clearly from that time one line from one letter, in which Bohr recalls Heisenberg saying, in effect, that he had been doing nothing for the past two years (1939-41) but thinking about how to make an atomic bomb.

In 1941, the Germans considered Denmark to be a model protectorate, and the country was rather free. The next day, Sept. 18, Bohr was able to go to the Danish Army to have them relay Heisenberg’s revelation of the German bomb project, via Sweden, to the British. The transmission was unfortunately garbled and lost en route!

Bohr was actually asked to work on the U.S. bomb via a secret message sent to Copenhagen in a microfilm hidden in a door key. The request came from John Cockroft, the British physicist who was the first, with Ernest Walton, to split an atom in 1932. Bohr did not act.

But then in October 1943, when the Germans were to round up all the Danish Jews, most of them, including Bohr, his wife and son Aage, slipped out by boat over the sound to Sweden one night. A compassionate German bureaucrat in Copenhagen, Georg Ferdinand Duckwitz, had leaked the information to the Danish underground, and quite amazingly the German patrol boats were quiet that night.

Bohr was immediately picked up by a British fighter plane (hinted at in the movie) and taken to England. (He was in fact unconscious on arrival, having failed to put on an oxygen mask, preferring his favorite pipe.) A short time thereafter he was joined by his then 21 year old son Aage, and the two of them prepared for a trip to Los Alamos. Aage recalled sitting on the floor in a hotel in London taking dictation from Niels, who was sitting in a chair darning socks. The two of them made their way to Washington, Niels under the code name Nicholas Baker, and Aage under Jim Baker. Unfortunately Niels lost his pocket watch, inscribed with his real name, in a department store in Washington, and when Aage went to recover it, the clerk admonished him, “Here in the United States, we use our real names!” (Incidentally, Enrico Fermi was known as Henry Farmer, and Hans Bethe as Howard Battle. Ernest Lawrence was also known as Oscar Wilde (author of The Importance of Being Earnest).

General Groves, worried about a German bomb, was particularly interested in what Heisenberg revealed to Bohr at the meeting in 1941, and actually rode all the way from Chicago to New Mexico by train with the Bohrs, father and son, both to make sure that they got there, but also to find out what he could about the meeting.
Niels Bohr, who was earlier responsible, with John Wheeler, for the idea that Pu-239 would fission very well, did not actually contribute substantiively to the bomb project. But he did make one important contribution. Fermi did not trust the initiator mechanism for the plutonium bomb, and would come in “every second day or so… with a new reason why the initiator wouldn’t work.” So in February 1945, Oppenheimer ask Bohr to assess the initiator program. When Bohr and Aage told Fermi that the device promised to work, it made a lot of difference to Fermi that somebody from outside had said so.

The Plot to Kill Heisenberg

The U.S. was so concerned that the Germans were developing a bomb, that the U.S. Office of Strategic Services, or OSS, the predecessor of the CIA, launched a plot to kill Heisenberg. Heisenberg was scheduled to give a talk in late 1944 at the Swiss physicist Paul Scherrer’s Institute outside of Zurich in Switzerland. Scherrer was actually working for the OSS and let them know of this visit. So the OSS enlisted Moe Berg, a major league baseball catcher and part-time spy to go to Heisenberg’s talk, with a gun under his raincoat to shoot Heisenberg on the spot if he seemed to understand anything about making bombs. It was actually Hans Bethe and Vicky Weisskopf who trained Berg at Los Alamos what to listen for. (Bethe remarked in 1999, ‘It was crazy what we did then, plotting to kill our friends.”)

Berg was quite an amazing character. A Princeton graduate, he was gifted in languages – fluent in German, and speaking Japanese, French, and Latin. Playing on an All-Star baseball team in Japan, 1934, with Lou Gehrig and Babe Ruth, he took movies from a hospital roof of Tokyo skyline, which was used in (then Lieutenant Colonel) Doolittle’s bombing of Tokyo in 1942.

So Heisenberg gave his talk at Scherrer’s institute, with Berg in the audience. Heisenberg did not mention bombs, but talked rather on his recent S-matrix theory of elementary particles. Berg, just to be sure, walked back with Heisenberg to his hotel, and over the next day or so thoroughly sought out, speaking in German, what Heisenberg knew. Heisenberg, learning about the plot to kill him after the War, remarked that he was suspicious that Berg was a spy – but for the German Gestapo. He also remarked how fortunate it was that he chose to talk on a physics topic.

In Closing

Let me say, in closing, that in general physicists emerged from Los Alamos with a tremendous sense of being able to accomplish difficult tasks, especially working in large groups. This “can-do” attitude lead to the U.S. creation of and dominance in “Big Science” after the war, including the development of large particle accelerators, at Fermilab, and Brookhaven, Long Island, and then in Europe and Japan, with hundreds of scientists involved in single experiments. Los Alamos, it is fair to say, changed the way science was done all over the world after the War.

Jeremy Bernstein

Bernstein Jeremy, courtesy AIP Emilio Segre Visual Archives, Physics Today Collection.

Finally, I want to read a few notes I received from Jeremy Bernstein, the physicist and author, as well as staff writer for the New Yorker. and a long-standing Aspen visitor. Jeremy was also a postdoc with Oppenheimer at the Institute for Advanced Study. He wrote, “I once sat next to Oppie (Oppenheimer) on the commute from Princeton to New York. He said that when the trial [the security hearing] was going on he was sure it was happening to someone else,” and then he added, “before he died Oppie had taken part in how he wanted his memorial service to be presented. There was music by Stravinsky and Beethoven, and three talks, by George Kennan, Henry Smyth and Bethe. [Henry Smyth who served on the Atomic Energy Commission was the sole dissenter at the Oppenheimer hearing.] I was invited and indeed have saved my invitation. I talked a good deal to Oppie’s younger brother Frank. He was standing next to Oppie at Trinity and I wanted to know what he really said. I never believed the stuff about the Gita, Frank said; I guess it works. Ken Bainbridge [the Harvard nuclear experimentalist who oversaw the Trinity Test] came up to the group and said, ‘Now we are all sons of bitches.’ ”