Meditations

Feynman and the Bomb

by Alex Wellerstein, published June 6th, 2014

Richard Feynman is one of the best-known physicists of the 20th century. Most of those who know about him know he was at Los Alamos during the Manhattan Project — some of the best "Feynman stories" were set there. But Feynman's own stories about his wartime hijinks were, like most of his stories about himself, half just-for-laughs and half lookit-mee! Feynman's always got to either be a lucky average Joe, or the one brilliant mind in a sea of normals. His Los Alamos antics are mostly just tales of a genius man-child running around a secret laboratory, picking safes and irritating security guards. They aren't very good gauges of what he actually did towards making the bomb. So what did Feynman actually do with regards to making the bomb? And does it matter, for thinking about his later career, especially the work that won him a Nobel Prize two decades later? 

Los Alamos colloquium from 1946, featuring (foreground, from left-to-right), Norris Bradbury, J. Robert Oppenheimer, John Manley, Richard Feynman, and Enrico Fermi. This version is cropped from the scanned copy at the Emilio Segrè Visual Archives. I will note that unlike the more common copies of this photo that have circulated, you can actually get a sense for how many other people were in the room — it looks like a really packed house.

Los Alamos colloquium from 1946, featuring (foreground, from left-to-right), Norris Bradbury, J. Robert Oppenheimer, John Manley, Richard Feynman, and Enrico Fermi. This version is cropped from the scanned copy at the Emilio Segrè Visual Archives. I will note that unlike the more common copies of this photo that have circulated, you can actually get a sense for how many other people were in the room — it looks like a really packed house.

Feynman's own stories of his wartime work are centered around things other than the work itself. So he describes doing calculations, but doesn't really say what they were for. He describes going to Oak Ridge, but only as a pretext for a story about dealing with generals and engineers. He describes the Trinity test, but a lot of that is about his claim to being the only person who saw it without welding glass on. And so on. These give glimpses, but not a very complete picture.

The historian of physics (and my advisor) Peter Galison wrote an article on "Feynman's War" several years back, looking closely at what it actually was that Feynman was doing at the lab, and how it played into the style that he later became famous for in physics.1 Galison argues that Feynman's postwar work is uniquely characterized by being "modular, pictorial, and proudly unmathematical." He contrasts this with the work of several of his contemporaries, including Julian Schwinger, who came up with equivalent solutions to the same physical problems but through a very different (much more mathematical) approach. Feynman's famous diagrams, which had a huge influence on the teaching and practice of postwar physics, exemplify this approach. What in Schwinger and Tomonaga's hands (the other people Feynman shared his Nobel Prize with in 1965) was solvable only through massive, lengthy, laborious math could, in Feynman's hands, be solved through a series of clever diagrams which came up with equivalent results. Feynman's solutions to quantum electrodynamics weren't the only way to do it — but they were easier to comprehend, to teach, and to apply to new questions.

The first Feynman diagram, published in R. P. Feynman, "Space—Time Approach to Quantum Electrodynamics,"Physical Review 76 (1949), 769-789, on 772.

The first published Feynman diagram, from Richard P. Feynman, "Space-Time Approach to Quantum Electrodynamics,"Physical Review 76 (1949), 769-789, on 772.

OK, so what does this have to do World War II? Well, Galison's argument is that you can see the same sort of thinking at work in what Feynman did at Los Alamos, and he argues that it is during the war that he really started applying this mode of physics. He divides Feynman's work into several "projects." They were:

  • Neutron measurements for determining critical mass (including the famous "tickling the dragon's tail" experiment" involving creating brief, barely-subcritical masses)
  • Work on the "Water Boiler" reactor at Los Alamos, which provided further data on nuclear chain reactions
  • Work as a safety supervisor at Oak Ridge, Tennessee, at Oppenheimer's request (which in his own writings is distilled down to a single humorous anecdote where Feynman is simultaneously clueless and brilliant)
  • Developing formulae relating to criticality and implosion efficiency (including the Bethe-Feynman formula)
  • His work on the hydride bomb, an abortive, Teller-inspired approach to make a "cheaper" fission weapon which involved devilishly difficult calculation (because not all of the neutrons produced in the weapon would be of the same energy)

Galison argues that in each of these instances, you can see the germs of his later approaches. He credits this to both Feynman's own personal scientific style and inclinations as a theorist (Feynman didn't seem to like to work with fundamental equations, but with "shortcuts" that lead to quicker, more efficient solutions, for example), but also to the requirements of the wartime goal, where theorists had to come up with tangible, practical results in a very short amount of time.  For example, Galison notes that the formulae relating to how the implosion bomb worked "brought the abstract differential equations to the bottom line: how hot, how fast, how much yield?" The practical needs of the war favored a particular "theoretical style" in general, Galison argues, one that could be most easily meshed with engineering concerns, rapid prototyping, and the lack of time to ruminate on fundamental physics questions.2

The "Water Boiler" reactor at Los Alamos. Source: Los Alamos Archives (12784), via Galison 1998.

The "Water Boiler" reactor at Los Alamos that Feynman worked on. Source: Los Alamos Archives (12784), via Galison 1998, p. 404.

Galison's article is fairly technical. He goes through Feynman's work (what of it that is declassified, anyway) and tries to follow his thinking in a very "internal" way, and then match that up with the requirements imposed by the specific wartime context. If you are well-versed in physics you will probably find the details interesting. I'm more of a big-picture person myself, and I like the structure of Galison's argument even if I don't feel fully capable of digesting all of the physics involved. One small example of Galison's work can probably suffice. Feynman was sent to Oak Ridge, as noted, to serve as a safety supervisor. He was taking over for Robert Christy, who got pneumonia in April 1944. The safety question was not a general one, but a very specific one: how many barrels of uranium (in various degrees of enrichment) could be safely stored in a room without running a risk of a criticality accident? Feynman himself relates the problem like this in his famous bit, "Los Alamos from Below" (reproduced in Surely You're Joking, Mr. Feynman):

It turned out that the army had realized how much stuff we needed to make a bomb — twenty kilograms or whatever it was — and they realized that this much material, purified, would never be in the plant, so there was no danger. But they did not know that the neutrons were enormously more effective when they are slowed down in water. In water it takes less than a tenth — no, a hundredth — as much material to make a reaction that makes radioactivity. It kills people around and so on. It was very dangerous, and they had not paid any attention to the safety at all.

In Feynman's account, he more or less walks in and figures out what the problems were and how to fix them. The story is about ignorance — in particular systemic ignorance due to secrecy — and Feynman's attempts to cut through it.

Feynman's diagrammatic sketch of storage of barrels of uranium at Oak Ridge, prepared for his "Safety Report." Source: Galison 1998, p. 408.

Feynman's diagrammatic sketch of storage of barrels of uranium at Oak Ridge, prepared for his "Safety Report." Source: Galison 1998, p. 408.

Galison's account is more technical. Feynman told them that pretty much any amount of unenriched uranium could be stored safely in the facility, but that 5% enriched and 50% enriched had to be handled fairly carefully. 50% enriched uranium in water, for example, would dangerous at a mere 350 grams of material unless there was a neutron absorbing material (cadmium) present. Feynman developed a series of safety recommendations for all grades of enrichments, and had to use reasonable safety margins to make up for potential errors in the calculations. He became the "point man" for safety questions involving fissionable materials, and developed (as Galison puts it) "visualizable" methods for answering basic (but important) questions about hypothetical systems (e.g. for "Gunk storage tanks," whether they had to be coated with cadmium or not). His methods, Feynman himself emphasizes, were "only approximate, as accuracy has been sacrificed to speed and simplicity in calculation" — the kind of computational "short cut" that was both needed for the practical requirements, but also was common to Feynman's general approach to physics. Galison concludes the section thus:

The admixture of approximation methods, neutron diffusion, nuclear cross sections, floods, fires and wooden walls marked Feynman’s correspondence with the Oak Ridge engineers. From April of 1944 to September 1945, whatever else Feynman was doing, he was also deeply enmeshed in the barely-existing field of nuclear engineering. Out of this interaction came characteristic rules and modular reasoning: visualizable, approximate, from-the-ground-up calculations applied to neutrons, pans, sheds and sludge. Visionary statements reinterpreting established laws of physics ceded to the exigencies of living in a world he had to reach outside the home culture of theoretical physics as he knew it before the war. Now a billion dollar plant was churning out U-235, and only a calculation stood between thousands of workers and nuclear disaster.

Which is a lot more serious-sounding that Feynman's own somewhat jokey accounts of the work. In the latter part of the article, Galison connects all of these methods for thinking — and sometimes even the specific problems — with Feynman's postwar work, showing the influence of his time at Los Alamos.

Diagram of neutron fluctuations from a report by F. de Hoffmann, R.P. Feynman, and R. Serber. Galison notes: "Significantly, Feynman and his collaborators captured the situation in a spacetime diagram drawn with time in the vertical direction and space horizontal. Such an image must be kept in mind when viewing Feynman's early postwar spacetime 'Feynman diagrams,' where again particles are absorbed, emit other particles, and scatter as reckoned by a concatenation of independent algebraic rules." Galison 1998, 405-406.

Diagram of neutron fluctuations from a report by F. de Hoffmann, R.P. Feynman, and R. Serber. Galison notes: "Significantly, Feynman and his collaborators captured the situation in a spacetime diagram drawn with time in the vertical direction and space horizontal. Such an image must be kept in mind when viewing Feynman's early postwar spacetime 'Feynman diagrams,' where again particles are absorbed, emit other particles, and scatter as reckoned by a concatenation of independent algebraic rules." Galison 1998, 405-406.

Feynman stayed at Los Alamos until the fall of 1946, when he relocated to Cornell University. He never worked on weapons again, but he never took a particularly strong stand on it. What did Feynman think about nuclear weapons, and his role in making them? There is some evidence in his private correspondence, much of which was published not too long ago,3 but it is scant. Most of his responses to inquiries were along the lines of "we feared the Nazis would get one first."4That's it. No comment on their use at all, or the end of the war, or any of the other common responses from Los Alamos veterans. When asked in the 1970s about his thoughts on nuclear weapons in general, he demurred: "Problems about the atomic bomb and the future are much more complicated and I cannot make any short  statement to summarize my beliefs here."5

Feynman gave an interview in 1959 where he was asked directly about the bomb. His response was a little lengthier then, but still said very little:

Now, with regard to our own things as human beings, naturally—I myself, for example—worked on the bomb during the war. Now how do I feel about that? I have a philosophy that it doesn't do any good to go and make regrets about what you did before but to try to remember how you made the decision at the time. ...if the scientists in Germany could have developed this thing, then we would be helpless, and I think it would be the end of the civilization at that time. I don't know how long the civilization is going to last anyway. So the main reason why I did work on it at the time was because I was afraid that the Germans would do it first, and I felt a responsibility to society to develop this thing to maintain our position in the war.6

This, of course, ignores the question of "so why did you continue when the Germans were known not to have made much progress?" and much more.

Charles Critchfield, Richard Feynman, J. Robert Oppenheimer, and an unidentified scientist, at Los Alamos. Source: Emilio Segrè Visual Archives, via Los Alamos.

Charles Critchfield, Richard Feynman, J. Robert Oppenheimer, and an unidentified scientist, at Los Alamos. Source: Emilio Segrè Visual Archives, via Los Alamos.

During the interview, Feynman was asked, point blank, whether he worked on any secret projects. Feynman said no, and that this was "out of choice." Pushed further, he elaborated:

I don't want to [do secret work] because I want to do scientific research—that is, to find out more about how the world works. And that is not secret; that work is not secret. There's no secrecy associated with it. The things that are secret are engineering developments which I am not so interested in, except when the pressure of war, or something else like that, makes me work on it. ... Yes, I am definitely anti-working in secret projects. ... I don't think things should be secret, the people developing this. It seems to me very difficult for citizens to make a decision as to what's going on when you can't say what you're doing. And the whole idea of democracy, it seems to me, was that the public, where the power is supposed to lie, should be informed. And when there's secrecy, it's not informed. Now, that's a naive point of view, because if there weren't secrecy, there'd be the Russians who would find out about it. On the other hand, there's some awfully funny things that are secret. It becomes secret that we know what the Russians are keeping secret from us, for instance, or something like that. It seems to me that things go too far in the secrecy.7

After that point in the interview, he steered away from political opinions, explaining that he had strong ones, but he didn't think they were any more valuable than anyone else's opinions.

There were those, of course, who did try to recruit Feynman for military work. John Wheeler, his doctoral advisor at Princeton, and the man who had roped him into Los Alamos in the first place, appealed to him strongly to join the Princeton work on the hydrogen bomb (Matterhorn) in late March 1951. Wheeler had heard the Feynman was trying to spend his sabbatical in Brazil, but Wheeler thought the chance of global war was "at least 40%," and that Feynman's talents might be better spent helping the country. It was a long, emotional letter, albeit a variation on one that Wheeler sent to many other scientists as well. Wheeler tried to win him (and others) with flattery as well, telling Feynman that "You would make percentage-wise more difference there than anywhere else in the national picture." In response to Wheeler's long, many-pointed letter, Feynman simply responded that he didn't want to make any commitments until he found out whether the Brazil idea would work out. End of story. Nothing specific about the hydrogen bomb one way or the other.

High resolution detail of Feynman's Los Alamos security badge photograph. A this resolution you can see a lot more strain on his face than the one I posted awhile back. Source: Los Alamos National Laboratory Archives.

High-resolution detail of Feynman's Los Alamos security badge photograph. At this resolution you can see potentially more strain on his face than the one I posted awhile back. Source: Los Alamos National Laboratory Archives.

What should we make of this? Feynman is a complicated man. Much more complicated than the zany stories let on — and I suspect the stories themselves were some kind of defense mechanism. As Feynman's friend Murray Gell-Mann said at Feynman's memorial service, Feynman "surrounded himself with a cloud of myth, and he spent a great deal of time and energy generating anecdotes about himself." They were stories "in which he had to come out, if possible, looking smarter than anyone else."8 He was not a moralizer, though. His work on the bomb fit into his stories only as a context. He no doubt drew many lessons from his work on nuclear weapons, and he no doubt had many opinions about them in the Cold War, but he kept them, it seems, much to himself.

Oppenheimer famously said that, "In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin..." Perhaps Feynman agreed — perhaps no more humor could be wrung out of the bomb after Hiroshima and Nagasaki. Maybe it's harder to write a zany story about the hydrogen bomb. And maybe he was just truly not interested in them from a technical standpoint, or, as he said in 1959, just didn't think his opinions on these matters, one way or another, were worth a damn, going against the notion held by many of his contemporaries that those who made the bomb had a special knowledge and a special responsibility. To me, he's still something of an enigma, just one that wrapped himself in jokes, rather than riddles.

Notes
  1. Peter Galison, "Feynman’s War: Modelling Weapons, Modelling Nature," Stud. Hist. Phil. Mod. Phys. 29, No. 3 (1998), 391-434. []
  2. This is an argument that Galison also makes at length about wartime work in his 1997 book Image and Logic: A Material Culture of Microphysics, both for Los Alamos and for the MIT Rad Lab, which if you're interested in this kind of thing, is a must-read. []
  3. Michelle Feynman, ed., Perfectly Reasonable Deviations from the Beaten Track: The Letters of Richard P. Feynman (Basic Books, 2005). []
  4. See e.g., Ibid., 268: "I did work on the atomic bomb. My major reason was concern that the Nazi's would make it first and conquer the world." []
  5. Ibid., 305. []
  6. Ibid., 421. []
  7. Ibid., 422-423. []
  8. James Gleick, Genius: The Life and Science of Richard Feynman (Pantheon, 1992), on 11. []

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19 Responses to “Feynman and the Bomb”

  1. Bill Higgins says:

    As it happens, a Dodge Tradesman Feynman once owned is parked in front of Fermilab’s main building right at this moment. He had the van decorated with Feynman diagrams.

    It’s part of a celebration of Feynman-related art, which includes metal Feynman diagram sculptures by Edward Tufte(!) in our gallery. Thus is the Feynman mythos conveyed into the future.

  2. Bill Higgins says:

    Galison’s paper covers Los Alamos and Oak Ridge, but does not discuss Feynman’s work on the isotron. Feynman was recruited by Bob Wilson for a Princeton group developing an accelerator-like isotope-separation device, an effort sponsored by the Office of Scientific Research and Development.

    During 1942, Feynman wrote several isotron documents. (They are listed in the papers of Henry DeWolf Smyth at the American Philosophical Society; several are cited in Wilson’s postwar report on the project.)

    OSRD Report #2: Effects of space charge; use of sine waves
    4: Kinematics of the separator
    7: Formation of bunches under the influence of space charge
    17: Design of the buncher and analyzer
    20: Note on the cascade operation of isotrons
    29: Operation of isotrons in cascade 
    35: Factors which influence the separation

    After the isotron was abandoned many of the Princeton cadre, including Wilson, Cornog, and Feynman, moved to Los Alamos.

  3. Cheryl Rofer says:

    Thanks, Alex. I’m not a fan of the “bad boy” Feynman stories, so it’s good to know what he actually contributed to the Manhattan Project.

    And the criticality calculations indicate one of the things that IAEA inspectors could look for in Iran to check for too much enrichment. Stacking enriched uranium gets tricky.

  4. Nice post, Alex…I too have often wondered about Feynman’s attitude toward the weapons he worked on. While he did not further work, the fact that he never really came out with a firm statement always puzzled me. Even his recollections in the Badash-edited book about Los Alamos spends more time joking and talking about his thumb-the-nose attitudes toward the brass. Some of the sadness about the death of his wife comes through too. But the only real statement is a rather enigmatic few sentences in the book’s closing paragraph. When I read it, it has the same sort of “what could we do?” tone that I find in novels by Vonnegut.

  5. matt pardo says:

    he did do that 1981 interview “the pleasure of finding things out” where he reflects that he worked on the bomb because he wanted to beat the germans to it, and faults himself for immorally continuing to work on it after germany had surrendered. http://www.youtube.com/watch?v=6no328q_VGQ

  6. John Ferngrove says:

    It’s a couple of decades since I read the Feynman fun books, but do I not recall that he went into some kind of depressive period after the bomb had been dropped and around leaving Los Alamos. He fell prey to the conviction that now the thing was out there it was more or less inevitable that sooner or later we’d wipe oorselves out. I recall him describing watching people moving around in a city and all just being gone in an instant or something. Sorry I can’t give references. I let go of the books a while back.

    • JPS says:

      I remember this too, albeit vaguely: Feynmann describing looking at bridges and buildings and thinking how far from a likely ground zero they were; looking at construction and thinking, What’s the point? – i.e., that it was probably going to be wiped out. He shrugs it off with a statement to the effect that he’d been wrong for forty years so far, and was glad others hadn’t given in to such pessimism.

  7. Robert Barron says:

    So what was Oppenheimer’s anecdote?

    (Or where can we find it, if it’s too long to quote)

    • If you mean the anecdote about Oak Ridge, it is in “Los Alamos From Below,” reprinted in Surely You’re Joking, Mr. Feynman. Here’s a sample that gives the flavor of it:

      The people in Oak Ridge didn’t know anything about what it was to be used for; they just knew what they were trying to do. I mean the higher people knew they were separating uranium, but they didn’t know how powerful the bomb was, or exactly how it worked or anything. The people underneath didn’t know at all what they were doing. And the army wanted to keep it that way. There was no information going back and forth. But Segrè insisted they’d never get the assays right, and the whole thing would go up in smoke. So he finally went down to see what they were doing, and as he was walking through he saw them wheeling a tank carboy of water, green water — which is uranium nitrate solution.

      He said, “Uh, you’re going to handle it like that when it’s purified too? Is that what you’re going to do?”

      They said, “Sure why not?”

      “Won’t it explode?” he said.

      Huh! Explode?

      Then the army said, “You see! We shouldn’t have let any information get to them! Now they are all upset.”

      • Robert Barron says:

        Ah, I misunderstood you.

        When you wrote “Work as a safety supervisor at Oak Ridge, Tennessee, at Oppenheimer’s request (which in his own writings is distilled down to a single humorous anecdote where Feynman is simultaneously clueless and brilliant)” I thought “his own writings” referred to Oppenheimer’s writings.

        Thanks.

  8. Steven Trujillo says:

    Great post Alex. My own thoughts about the Feynman MP mythology vs his actual MP work echo Cheryl’s. I’ve always respected his post-war sparseness of “serious” comment on the Project, though I have found myself guilty of assigning possible meaning to what he *didn’t* say about the work – that acknowledging the German threat was in his own way a statement of his feelings about the eventual bombings.

  9. Peter says:

    If you don’t know already, this site got a nice plug today on CNN.com, in a piece about the Goldsboro accident.

  10. Meg says:

    This is an especially great (and well-written) piece. That long quote on secrecy that opposes science with engineering is so rich. I’d love to talk more at some point about what you think about this whole idea that Feynman was more visual in his reasoning than other theoretical physicists. This visual/non-visual thinker thing gets thrown around a lot in differentiating Bohr from Einstein, too, and it’s a place where it is tricky for me to separate lore from practice. All of the archival stuff for all these theorists seems quite rich with diagraming and drawing. I wonder if we need better ways to talk about the different ways that they drew. Just musing. Fantastic post!

    • Meg,

      Thanks so much. Re: visual reasoning, I’ve had a nice e-mail exchange lately with a physicist who wondered about that exact same distinction, arguing that Feynman was obviously as mathematical as anyone else, and that the use of visual analogies is not that interesting in and of itself.

      My response was that I thought Galison was trying to make a distinction between the kind of work Feynman did and the kind of work people like Schwinger did — e.g., Feynman himself did pride himself on finding “tricks” that allowed you to skip huge amounts of purely algebraic manipulation of the sort that Schwinger was exceptionally talented at (but was tedious and difficult to calculate, even by clever people).

      But I also wondered if some part of the fact that we like to highlight that distinction is part of our own outsider bias that assumes that algebraic manipulation is what a theorist “does,” and so we get excited when we see something different. That is, maybe we just find it interesting because we have a misconception about what the normal, “base” state of this kind of work is, given that “visual” calculation methods have obviously been used for ages and ages.

      Personally, I’m so outside of all of their mathematical work that it’s hard for me to offer up any real opinion on it (it’s all Greek to me, and the only difference between Feynman and Schwinger is that Feynman’s diagrams kind of look like little reactions to me, so they superficially make more sense to me than differential equations, but I know I don’t really understand them either).

  11. […] who came under official FBI scrutiny during the Cold War was Richard Feynman.1 Feynman’s work on the bomb at Los Alamos, combined with his fame, penchant for telling stories about safe-cracking, and […]

  12. […] of how well you can hold it together is apparently the essence of the still mostly-classified Bethe-Feynman formula. It is described qualitatively in Samuel Glasstone, “Weapons Activities of Los Alamos […]

  13. […] as is well known. Concerns about criticality excursions at the plants used to enrich uranium were a non-trivial concern. And there were other, more unusal ways to die, as you would expect from any body of people that […]

  14. nc says:

    This IBM work may have been given to Feynman by Oppenheimer after he co-invented the “Bethe-Feynman equation”, used to quickly predict the energy release up to the time the bomb core blows itself apart. Glasstone’s LA-1632 which you’ve uploaded in pdf explains the Bethe-Feynman formula on pp34-36; the bomb core becomes an adiabatically expanding gas with a mean outward acceleration equal to the product of its radius and the square of alpha. The outward force is then simply F=ma, and the pressure is this force divided by the surface area of the sphere. Work energy done by a force causing motion R in the direction of the force is E=FR, and this can be equated with the ideal gas equation, E=PV/(gamma -1) where gamma is about 1.2 for a highly ionised gas. So you can obtain an equation for the effect of core compression on efficiency, etc.