Visions

Dogs in space

by Alex Wellerstein, published June 26th, 2015

Confession: I once told my students something I knew wasn’t true. It was during a lecture on the Space Race, on Sputnik 2, which carried the dog Laika into space in November 1957. I told them about how the Soviets initially said she had lived a week before expiring (it was always intended to be a one-way trip), but that after the USSR had collapsed the Russians admitted that she had died almost immediately because their cooling systems had failed. All true so far.

But then one bright, sensitive sophomore, with a sheen on her eyes and a tremble in her voice, asked, “But did they at least learn something from her death?” And I said, “oh, um, well, uh… yes, yes — they learned a lot.”

Which I knew was false — they learned almost nothing. But what can you do, confronted with someone who is taking in the full reality of the fact that the Soviets sent a dog in space with the full knowledge it would die? It’s a heavy thing to admit that Laika gave her life in vain. (In subsequent classes, whenever I bring up Sputnik, I always preempt this situation by telling the above story, which relieves a little of the pressure.)

A Soviet matchbox with a heroic Laika, the first dog in space. Caption: "First satellite passenger — the dog, Laika." Want it on a shirt, or a really wonderful mug?

A Soviet matchbox with a heroic Laika, the first dog in space. Caption: “First satellite passenger — the dog, Laika.” Want it on a shirt, or a really wonderful mug?

I’m a dog person. I’ve had cats, but really, it’s dogs for me. I just believe that they connect with people on a deeper level than really any other animal. They’ve been bred to do just that, of course, and for a long time. There is evidence of human-dog cohabitation going back tens of thousands of years. (Cats are a lot more recently domesticated… and it shows.) There are many theories about the co-evolution of humans and dogs, and it has been said (in a generalization whose broadness I wince at, but whose message I endorse) that there have been many great civilizations without the wheel, but no great civilizations without the dog.

So I’ve always been kind of attracted to the idea of dogs in space. The “Mutniks,” as they were dubbed by punny American wags, were a key, distinguishing factor about the Soviet space program. And, Laika aside, a lot of them went up and came back down again, providing actually useful information about how organisms make do while in space, and allowing us to have more than just relentlessly sad stories about them. The kitsch factor is high, of course.

A friend of mine gave me a wonderfully quirky and beautiful little book last holiday season, Soviet Space Dogs, written by Olesya Turkina, published by FUEL Design and Publishing. According to its Amazon.com page, the idea for the book was hatched up by a co-founder of the press, who was apparently an aficionado of Mutnikiana (yes, I just invented that word). He collected a huge mass of odd Soviet (and some non-Soviet) pop culture references to the Soviet space dogs, and they commissioned Turkina, a Senior Research Fellow at the State Russian Museum, to write the text to accompany it. We had this book on our coffee table for several months before I decided to give it a spin, and I really enjoyed it — it’s much more than a lot of pretty pictures, though it is that, in spades, too. The narrative doesn’t completely cohere towards the end, and there are aspects of it that have a “translated from Russian” feel (and it was translated), but if you overlook those, it is both a beautiful and insightful book.

Soviet Space Dogs cover

First off, let’s start with the easy question: Why dogs? The American program primarily used apes and monkeys, as they were far better proxies for human physiology than even other mammals. Why didn’t the Soviets? According to one participant in the program, one of the leading scientists had looked into using monkeys, talking with a circus trainer, and found out that monkeys were terribly finicky: the training regimes were harder, they were prone to diseases, they were just harder in general to care for than dogs. “The Americans are welcome to their flying monkeys,” he supposedly said, “we’re more partial to dogs.” And, indeed, when they did use some monkeys later, they found that they were tough — one of them managed to worm his way out of his restraints and disable his telemetric equipment while in flight.

The Soviet dogs were all Moscow strays, picked for their size and their hardiness. The Soviet scientists reasoned that a dog that could survive on the streets was probably inherently tougher than purebred dogs that had only lived a domesticated life. (As the owner of a mutty little rescue dog, I of course am prone to see this as a logical conclusion.)

The Soviet dog program was more extensive than I had realized. Laika was the first in orbit, but she was not the first Soviet dog to be put onto a rocket. Turkina counts at least 29 dogs prior to Laika who were attached to R-1 and R-2 rockets (both direct descendants of the German V-2 rockets), sent up on flights hundreds of miles above the surface of the Earth starting in 1951. An appendix at the back of the book lists some of these dogs and their flights.

Oleg Gazenko, chief of the dog medical program, with Belka (right) and Strelka (left) at a press conference in 1960. Gazenko called this "the proudest moment of his life."

Oleg Gazenko, chief of the dog medical program, with Belka (right) and Strelka (left) at a TASS press conference in 1960. Gazenko called this “the proudest moment of his life.”

Many of them died. Turkina talks of the sorrow and guilt of their handlers, who (naturally) developed close bonds with the animals, and felt personally responsible when something went wrong. Some of the surviving dogs got to live with these handlers when they retired from space service. But when the surviving dogs eventually expired, they would sometimes end up stuffed and in a museum.

I had thought I had heard everything there was to hear about Laika, but I was surprised by how much I learned. Laika wasn’t really meant to be the first dog in space — she was the understudy of another dog who had gotten pregnant just before. Laika’s death was a direct result of political pressures to accelerate the launch before they were ready, in an effort to “Sputnik” the United States once again. The head of the dog medical program, when revealing Laika’s true fate in 2002, remarked that, “Working with animals is a source of suffering to all of us. We treat them like babies who cannot speak. The more time passes, the more I’m sorry about it. We shouldn’t have done it. We did not learn enough from the mission to justify the death of the dog.”

The Soviets did not initially focus on the identity of Laika. Laika was just listed as an experimental animal in the Sputnik 2 satellite. Rather, it was the Western press, specifically American and British journalists, that got interested in the identity, and fate, of the dog. The Soviet officials appear to have been caught by surprise; I can’t help but wonder if they’d had a little less secrecy, and maybe ran this by a few Americans, they’d have realized that of course the American public and press would end up focusing on the dog. It was only after discussion began in the West that Soviet press releases about Laika came out, giving her a name, a story, a narrative. And a fate: they talked about her as a martyr to science, who would be kept alive for a week before being painlessly euthanized.

Staged photo of Belka in a space suit.

Staged photo of Belka in a space suit.

In reality, Laika was already dead. They had, too late, realized that their cooling mechanisms were inadequate and she quickly, painfully expired. The fact that Laika was never meant to come back, Turkina argues, shaped the narrative: Laika had to be turned into a saintly hero, a noble and necessary sacrifice. One sees this very clearly in most of the Soviet depictions of Laika — proud, facing the stars, serious.

The next dogs, Belka and Strelka, came back down again. Belka was in fact an experienced veteran of other rocket flights. But it was Strelka’s first mission. Once again, Belka and Strelka were not meant to be the dogs for that mission: an earlier version of the rocket, kept secret at the time, exploded during launch a few weeks earlier, killing the dogs Lisichka and Chaika. These two dogs were apparently beloved by their handlers, and this was a tough blow. The secrecy of the program, of course, pervades the entire story of the Soviet side of the Space Race, and serves as a marked contrast with the much more public-facing US program (the consequences of which are explored in The Right Stuff, among other places).

When Belka and Strelka came back safely, Turkina argues, they became the first real Soviet “pop stars.” Soviet socialism didn’t really allow valorization of individual people other than Stakhanovite-style exhortations. The achievements of one were the achievements of all, which doesn’t really lend itself to pop culture. But dogs were fair game, which is one reason there is so much Soviet-era Mutnikiana to begin with: you could put Laika, Belka, and Strelka on cigarettes, matches, tea pots, commemorative plates, and so on, and nobody would complain. Plus, Belka and Strelka were cute. They could be trotted out at press conferences, on talk shows, and were the subjects of a million adorable pictures and drawings. When Strelka had puppies, they were cheered as evidence that biological reproduction could survive the rigors of space, and were both shown off and given as prized gifts to Soviet officials. So it’s not just that the Soviet space dogs are cool or cute — they’re also responsible for the development of a “safe” popular culture in a repressive society that didn’t really allow for accessible human heroes. Turkina also argues that Belka and Strelka in particular were seen as paradoxically “humanizing” space. By coming back alive, they fed dreams of an interstellar existence for mankind that were particularly powerful in the Soviet context.

Yuri Gagarin reported to have joked: “Am I the first human in space, or the last dog?” It wasn’t such a stretch — the same satellite that Belka and Strelka rode in could be used for human beings, and gave them no more space. A friend of mine, Slava Gerovitch, has written a lot about the Soviet philosophy of space rocket design, and on the low regard the engineers who ran the program had for human passengers and their propensity for messing things up. Gagarin had about as much control over his satellite as Belka and Strelka did over theirs, because neither were trusted to actually fly a satellite. The contrast between the engineering attitudes of the Soviet Vostok and the American Mercury program is evident when you compare their instrument panels. The Mercury pilots were expected to be able to fly, while poor Gagarin was expected to be flown. 

Soviet Space Dogs is a pretty interesting read. It’s a hard read for a dog lover. But seeing the Soviet space dogs in the context of the broader Soviet Space Race, and seeing them as more than just “biological cargo,” raises them from kitsch and trivia. There is also just something so emblematic of the space age about the idea of putting dogs into satellites — taking a literally pre-historic human technology, one of the earliest and most successful results of millennia of artificial breeding, and putting it atop a space-faring rocket, the most futuristic technology we had at the time.

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Meditations

What remains of the Manhattan Project

by Alex Wellerstein, published June 12th, 2015

What remains of the Manhattan Project? A lot of documents. Some people. A few places. And a handful of artifacts. Maybe less than one might expect, maybe more than one might expect — it was a very large, expensive undertaking, involving a lot of people, so there being some remnants is not surprising. Though given its size, and importance, perhaps one would expect more.

Some of the attending Manhattan Project veterans. Photo by Alex Levy of the Atomic Heritage Foundation.

Some of the attending Manhattan Project veterans. Photo by Alex Levy of the Atomic Heritage Foundation.

The symposium put on by the Atomic Heritage Foundation last week was really excellent — a really important event. The attendance was higher than I would have guessed. At least a dozen Manhattan Project veterans attended, and many children of Manhattan Project veterans (some of whom were born during the war) were there as well. There were also a lot of nuclear historians, scientists, and enthusiasts. I got to spend time talking with a lot of wonderful people who also cared a lot about, and took very seriously, the history of the atomic bomb. Among those who were there included Richard Rhodes (Pulitzer-winning author of The Making of the Atomic Bomb), Stan Norris (biographer of Leslie Groves), Kai Bird and Martin Sherwin (Pulitzer-winning authors of American Prometheus), John Coster-Mullen (major irritant to government censors and author of Atom Bombs), Avner Cohen (author of many books on Israel and the bomb), Ray Smith (the historian at Y-12), and Clay Perkins (physicist and nuclear “collector”), just to name a few. I saw some of my DC friends and former colleagues, and met a lot of nuclear history enthusiasts. All together, it looked like there was well over two hundred people between the two days of it.

There were several themes to the whole event. One was the creation of the Manhattan Project National Historic Park. There were representatives from both the National Park Service and the Department of Energy to talk about the process going forward, and there was also an excellent address by Senator Martin Heinrich of New Mexico.

Richard Rhodes gave the first address of Wednesday by talking about why we save authentic relics of the past. He took a tack I wouldn’t have expected — he started off from the work of the philosopher John Searle on “social reality,” the sorts of facts that exist only by mutual agreement. “When we lose parts of our physical past, we lose part of our social past as well.” Our preference for the originals of things, the “authentic” objects, isn’t just about sentiment, he argued — it is part of what defines us. (And if you don’t believe physical things define you, try losing a wedding ring, or an irreplaceable album of old photographs.) Preserving public memories, spaces of our past, whether positive or negative, help us come to terms with who we are, and what we have done. And he made the point, quite effectively, that we do not just preserve the sites that glorify us — Ford’s Theatre, Manzanar, and the Sand Creek massacre site are all National Historic Sites as well.

Battle deaths in state-based conflicts, 1946-2013, by Max Roser. This is what Rhodes had in mind regarding the decreased amount of deaths from war since World War II. (Note that if WWII was included in this, it would be even more stark: the rate of battle deaths per 100,000 of global population was 300 for the war as a whole.) There are a lot of ways to parse these numbers, as Roser's site makes clear (the raw numbers of wars has been increasing, some of this decline as a unit of population is due to the massive increase in global population), and there are multiple interpretations of the data (whether the bomb has anything to do with it is disputed by scholars), but it is still very interesting. Source: Max Roser, "War and Peace after 1945," OurWorldInData.org

Battle deaths in state-based conflicts, 1946-2013, by Max Roser. This is what Rhodes had in mind regarding the decreased amount of deaths from war since World War II. (Note that WWII’s rate of battle deaths was around 300 per 100,000.) There are a lot of ways to parse these numbers, as Roser’s site makes clear (the raw numbers of wars has been increasing, some of this decline as a unit of population is due to the massive increase in global population), and there are multiple interpretations of the data (whether the bomb has anything to do with it is disputed by scholars), but it is still very interesting. Source: Max Roser, “War and Peace after 1945,” OurWorldInData.org.

Rhodes is no fan of nuclear weapons, and doesn’t believe that the atomic bombs were what caused Japan to surrender in World War II.1 Yet, he argued that when J. Robert Oppenheimer told his recruits that these weapons might end all war, he might not have been wrong. Rhodes noted the marked decrease of deaths by war in the years that followed World War II, paired with the increased risk of a terrible nuclear holocaust. Nuclear weapons, he argued, were the first instance in which science revealed a natural limit to national sovereignty. In Rhodes framing of it, scientists found facts of the natural world which required new political interventions and methods to avoid certain types of war.

As a result, he said, these Manhattan Project sites were potentially among the most significant in the history of the world. It was an interesting way to start things off.

In an event where the participants are present, it is easy to fall into something that feels like just a celebration. And there were those, without a doubt, who felt positively about the Manhattan Project, that it was necessary to end the war, and all of that. There were also those who thought it wasn’t necessary, too. I think my favorite comment came from James Forde, a Manhattan Project veteran who had been employed to clean tubes (later revealed to be gaseous diffusion barriers) near Columbia University during the war. He said that for awhile he felt bad about the atomic bombs, but then he looked more into all of the other damage that non-atomic weapons had done during the war. After that, he lost any enthusiasm for war of any kind. He got solid applause for that.

Age distribution at Los Alamos, May 1945. Top graph is total civilian personnel, bottom is scientific employees only. Keep in mind this was 70 years ago, so anyone in their 20s then would be in their 90s now. Source: Manhattan District History, Book 8, Volume 2, Appendix, Graph 1.

Age distribution at Los Alamos, May 1945. Top graph is total civilian personnel, bottom is scientific employees only. Keep in mind this was 70 years ago, so anyone in their 20s then would be in their 90s now. Source: Manhattan District History, Book 8, Volume 2, Appendix, Graph 1.

The veterans who were there had all been extremely young at the time. This makes sense, of course — if it is the 70th anniversary, almost nobody older than their early twenties is going to still be around today. And at many of the sites, the youth were in abundance. As a result, most of them had fairly small roles, small jobs, though some of them had rubbed shoulders with the giants. There were a few remarkable anecdotes. Isabella Karle talked about working as a chemist at the Metallurgical Laboratory in Chicago, working on plutonium oxide produced at the Oak Ridge X-10 reactor. She had to move it between buildings, and since it was such a small amount, she just carried it in her pocket. Someone found out she was doing this, and required her, a young woman with pigtails, to be escorted between building by burly guards, attracting more attention than she would have otherwise. She also related a story about carrying a radiation counter around with her, and having it go off next to a Coca-Cola machine. Apparently a deliveryman had forgotten some tubing in his car, and borrow a contaminated tube from a Met Lab office, contaminating the machine with who knows what. Fortunately, she said, she had stumbled across this before anyone had used it.

Ben Bederson told some amazing stories about David Greenglass, who he had bunked with as a member of the Special Engineer Detachment. Greenglass, he said, was a “true believer” of a Communist. Bederson pointed out that he, like many New York Jews at the time, had been interested in Communism for awhile (he had grown up in a part of the Bronx that was considered a “Communist neighborhood”) but that most had become disillusioned with it by the time the US was in the war. Greenglass never seemed to take the hint, though, and thought Bederson was a fellow traveler. It was an amusing contrast to people like Klaus Fuchs and Ted Hall, who hid their politics. Bederson eventually asked to be transferred to a different bunk. Later, Greenglass told the FBI he had wanted to try and recruit Bederson but his courier, Harry Gold, told him not to. The reason Greenglass thought Bederson would be a good recruit is because he gave money to the Roosevelt reelection campaign. “That shows you how smart David Greenglass was,” Bederson remarked with sarcasm.

The arming plugs of the Little Boy bomb.

Holding the arming plugs of the Little Boy bomb.

Along with the veterans and the historians, there were some artifactual pieces of the past there. Clay Perkins had brought the arming switches of the Little Boy bomb, which he purchased over a decade ago. The green (“safe”) one was kept plugged into the bomb until after takeoff. While in flight, the assistant weaponeer, Morris Jeppson, climbed into the unpressurized bomb bay of the Enola Gay and removed it. In its place, he put a red (“armed”) plug, making the bomb electrically “live.” (The red plug that Clay has is a spare, of course — the original was destroyed in the explosion over Hiroshima. Jeppson brought multiple spares with him since if he had dropped one during the operation, it would have aborted the mission.) Clay let me hold them, which was moving.

There was also a very surprising artifact brought by one of the veterans: a lucite hemisphere with pieces of Trinitite embedded in it. The Trinitite is not so rare, but the lucite was cast in the same mold that made the plutonium pits for the Trinity and Fat Man bombs, and included the small hold for the neutron initiator. This is an incredible thing to have kept (and I was also allowed to hold this, as well). I am sure its existence is the result of a violation of untold numbers of security rules. John Coster-Mullen, as I expected would, came up immediately afterwards to trace the dimensions. It looked how we all expected it to, but it still amazing to see something like this, knowing how secret it once was, and even now is supposed to be.2

I knew the plugs existed, I did not know the lucite existed. There is something profound about holding artifacts that had such strong connections to history. That historical empathy I spoke of in my most recent post has something to do with it — the brain suddenly makes this connection with this world that often seems so far away. But of course it really isn’t that far away, and the world we have today is largely a product of it. But sometimes even historians need reminders.

Monthly costs of the Manhattan Project, 1943 through 1946. From the Manhattan District History, Volume 5, Appendix A.

Monthly costs of the Manhattan Project, 1943 through 1946. From the Manhattan District History, Volume 5, Appendix A.

My own contribution to the symposium was a talk about the Manhattan Project as a “Crucible for Innovation.” I didn’t choose the title (it is not really my style, even though I do teach at “The Innovation University“), but it was easy enough to roll with: how much innovation took place during the Manhattan Project, and why was it successful? I talked a bit about the secret Manhattan Project patent program as one way to measure its innovation. By the time the AEC took over, the Manhattan Project patent program approved 2,100 separate secret patent applications for filing, and had already filed 1,250 of them with the US Patent Office. As I noted in an article from a few years ago, that latter number represented 1.5% of all of the patent applications filed in 1946. The Manhattan Project was not just the building of a bomb, but the creation an entirely new industry from scratch.

Why did the Manhattan Project succeed? Well, I argued, it almost didn’t — all you would need for it to have been a “failure” (in the sense of having not produced atomic bombs by the end of World War II) would to have been delayed by likely a few months. Which anyone who has ever tried to run even a small project knows is easy enough to do. I always try to emphasize this lack of an inevitability when I talk about the wartime effort, because it is easy to fall into the fallacy of knowing how the story ends and thus seeing it as predestined. The Manhattan Project was an anomaly: it was not innovation as usual, and it was not the natural or obvious path to take. Which is one reason why the US was the only country who actually went down that path with any seriousness during the war. The Manhattan Project still holds the world record for fastest tested or deliverable nuclear weapon after committing to build one: two and a half years.3

A preview of my forthcoming Manhattan Project sites map. Size is a subjective "prominence" rating given by me, the white dots show the actual location of the sites, and the color corresponds to whether it is government/military, educational institution, or private industry. An interactive version will be unveiled this summer, which will give more information about specific sites and permit zooming in, and etc. This only shows sites in the continental US and lower Canada — there are other non-US sites as well in the final version.

A preview of my forthcoming Manhattan Project sites map. Size is a subjective “prominence” rating given by me, the white dots show the actual location of the sites, and the color corresponds to whether it is government/military, educational institution, or private industry. An interactive version will be unveiled this summer, which will give more information about specific sites and permit zooming in, and etc. This only shows sites in the continental US and lower Canada — there are other non-US sites as well in the final version.

Lastly, I also emphasized the size of the project. I’ve talked on here about the immense cost of the work, and the greater-than-most-people-realize manpower requirements. But I also unveiled a screenshot of a work in progress. For a while now, I’ve been trying to make a database of every site where some sort of work on the Manhattan Project was done. I’ve been combing through the Manhattan District History, through archival files on contracts, and through databases of radioactive Superfund sites. I’ve been keeping a tally of any places listed as having some role in the final outcome, however minor. My current list is well over 200 separate sites. Some of these places were research institutions (about 40 are educational institutions of some nature), some were military or government institutions (some created from scratch, some pre-existing), and about half were private industry. Some places produced materials, some just produced paper. (The symposium took place in the Carnegie Institute of Science, which was where Vannevar Bush’s Office of Scientific Research and Development headquarters were during the war, and I delighted in getting to point this out.) Not all sites were equally important, to be sure. But all played some role, even if most of those places probably did not actually know what their role was. The screenshot above is a preview of the map — it is still a work in progress, and the final version will be fully interactive, sortable into different categories, and so on.

It’s a big list. Bigger than I thought when I started it. It just emphasizes again that the Manhattan Project was responsible for the birth of an industry, not just the bomb. Upon learning about the scale of the project in 1944, Niels Bohr told Edward Teller: “I told you it couldn’t be done without turning the whole country into a factory. You have done just that.” It was an apt observation.

Very little of this infrastructure remains. The Manhattan Project National Historic Park is an important step in the right direction for preservation of this history. There is a long road yet to go in terms of figuring out how to make it available to the public, and how to properly present the material. I remain optimistic that it will be an opportunity to talk about history in a productive way, and to build bridges between the ever-changing present and the ever-receding past.

  1. Rhodes is explicitly convinced by the Hasegawa thesis. In his words, though, “I find tragedy but no dishonor in having used atomic bombs to hasten the end of the war,” whether they actually hastened that end or not. It is a nuanced point to make. [↩]
  2. John says it is a bit smaller than the actual pit — the initiator would not have been able to fit in the hole given, and the mass would be off by a few percentage points. So either it was cast in an earlier or alternative pit model, or something happened to it in the meantime. My hypothesis is that it was cast in the actual pit mold, but that it shrunk in some way over time, either because it contracted while it cooled originally (the thermal contraction coefficient for acrylic is around 5X that of uranium, just as a point of comparison), or through some other warping mechanism over the last 70 years. [↩]
  3. The decision to produce nuclear weapons was made in December 1942. The work prior to this was of an exploratory or pilot nature, not a production program. The “Gadget” was ready by mid-July 1945, the other bombs were ready by the end of the month. [↩]

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Meditations

History in the flesh

by Alex Wellerstein, published May 29th, 2015

My main mode of interacting with history is through documents. Memos, reports, letters, telegrams, transcriptions, diagrams — the written word. The sociologist of science Bruno Latour calls these kinds of sources inscriptions, that which got written down, fixed into some kind of reproducible media. In Latour’s work, he emphasizes the act of inscription to highlight the gulf that exists between what gets written down and the anarchy of the raw, natural world. The inscription is a limited product of that raw world, a small subset of its multitudes of activities and phenomena and possibilities, and is always an incomplete lens by which to interrogate the world, but its very incompleteness allows it to be fixed, circulated, and analyzed. Without this act of inscription, science (and history) could not move forward, because there would be no such thing as the necessary “data.”

A small sampling of the sort of "inscriptions" I deal with regularly, the raw stuff of conjuring up the past: a typewritten report later turned into a microfilm entry (later scanned into a PDF file); a typewritten copy of a memo I got from an archive (later photographed by me and turned into a PDF); a hand-drawn diagram (evidence from the Rosenberg trial) that was later deposited into an archive (and later scanned as a TIF file).

A small sampling of the sort of “inscriptions” I deal with regularly, the raw stuff of conjuring up the past: a typewritten report later turned into a microfilm entry (later scanned into a PDF file); a typewritten copy of a memo I got from an archive (later photographed by me and turned into a PDF); a hand-drawn diagram (evidence from the Rosenberg trial) that was later deposited into an archive (and later scanned as a TIF file).

All of which might seem a little obvious once I say it. We need sources, of some sort, to do history? Tell me more. But the reason to point this out is to mention that historians are very aware that there’s lots of things that happened in the past that don’t get inscribed, and that the modes of inscription are not always reliable (even when people are trying to do their best, much less when they aren’t), and that the archive is just a proxy for understanding the past, and not in any way a full representation of the past. The job of the historian, stated in this way, is to piece together a fuller, more synthetic understanding of the past based on what is really a very shallow evidentiary base. We take half a dozen pieces of paper marked with symbols, and use those to try and conjure up an entire lost world. We take scribbles on paper and use them to try and reconstruct the subjective states of other human minds. When you put it like that, it is a pretty wonderfully mystical, kind of medieval, style of knowledge production. Which is why I love it — its flaws are obvious, its possibilities are endless, and it requires a very diverse group of skills that both empirical and creative.

But there are other ways to know the past. Being in the physical places of the past does seem to trigger a different response to it, as opposed to just reading about said places. This is one of the reasons I am so supportive of the Manhattan Project National Historic Park initiative. There is something about witnessing a historical landscape in person, that encourages a different sort of empathy with those who lived past, a seeing with other eyes.

A view from the car window, driving from Albuquerque to Santa Fe. One of about 10 million photographs I took on my trip. Eventually I will post some more. Separately, if you want a Los Alamos Ranch School mug, I was inspired to make them after my trip, and they are based on the actual seal of the school, which I saw for the first time while out there.

A view from the car window, driving from Albuquerque to Santa Fe. One of about 10 million photographs I took on my trip. Eventually I will post some more. Separately, if you want a Los Alamos Ranch School mug, I was inspired to make them after my trip, and they are based on the actual seal of the school, which I saw for the first time while out there.

I haven’t personally been to a lot of these Manhattan Project sites. Sometimes this surprises people, but it’s just been a matter of time and money. For my last Spring Break, though, I had the opportunity to spend a week in New Mexico, teaching a couple classes for my friend Luis Campos at the University of New Mexico (whose book on the history of radium was just published). My wife and I spent a few days in Santa Fe as a guest of the wonderful Cheryl Rofer, who also gave us the best unofficial tour of Los Alamos you could ask for, with assistance from Alan Carr (the Historian for Los Alamos National Laboratory) and the irrepressible Ellen Bradbury Reid, described aptly in an article as the “Eloise of Los Alamos,” which is a phrase I so wish I had come up with.

I had not previously spent too much time in the Southwest before. The landscape out there is stunning and other-worldly. On your left will be nothing but flat, scrubby desert. On your right, a towering mountain. Drive a little further and you find lushness and trees. Drive a little more and you find dryness and rock. Go up high enough and it might be snowing. Look around, feel the vastness of the area. Walk around and see the legacy of the three different peoples who have lived there: the Indians, the Spanish, the Anglos. It is an unusual place that feels as unlike the parts of California I am from as it does the East Coast urban metropolises I have lived for the last ten years. They don’t call it the “Land of Enchantment” for nothing.

All of which deepened, I like to believe, my feeling for Los Alamos during the war. What it must have meant to travel out there, to take the one good road from Santa Fe up onto the Mesa (there is a highway now, of course, but the old road is still there, albeit better paved). The way in which the various Technical Areas were nestled into the tree-lined valleys, and how you could go up those hills and look down on just miles and miles around. Today, of course, Los Alamos is a huge, sprawling laboratory. We didn’t really get to go inside of it to any real extent (there are a lot of rules governing that sort of thing), but Alan Carr took us up a hill which gave us great panoramas of the whole site. The hills around the lab are today full of dead, burned trees, the result of several forest fires that devastated them. But when you drive around the mesas near Bandelier National Monument, you get a sense for that rustic, rocky environment that so appealed to J. Robert Oppenheimer, that seemed such incredible contrast to his secular Jewish upbringing on Manhattan’s Upper West Side.

Nagasaki Mayor Tomihisa Taue, Atomic Heritage Foundation President Cynthia Kelly and Hiroshima Mayor Kazumi Matsui at the meeting in New York, just across from the United Nations building. I was sitting a little out of frame, near Mayor Matsui. Source: Japan Times.

Nagasaki Mayor Tomihisa Taue, Atomic Heritage Foundation President Cynthia Kelly and Hiroshima Mayor Kazumi Matsui at the meeting in New York, just across from the United Nations building. I was sitting a little out of frame, near Mayor Matsui. Source: Japan Times.

About a month afterwards, I had another unusual opportunity to experience history in the flesh. Connected with my work with the Atomic Heritage Foundation (I have joined their Advisory Committee), I was invited to take part in a meeting with the mayors of Hiroshima and Nagasaki, as well as several hibakusha, survivors of the atomic bombings.1 The Japanese delegation was in town for the NPT Review conference, but they wanted to meet with the Atomic Heritage Foundation in a public forum to talk about concerns they had with the Manhattan Project National Historic Park. Most of their concerns were understandable and shared by us: they want the history of the atomic bomb to not be presented in a celebratory mode, and to give credence to the many different perspectives that are held on it. They want the human consequences of the bombings to be made loud and clear. They want these sites to be places where people are encouraged to make up their own minds, rather than simply being told what to think about the past. On this I think everyone was in complete agreement. The details, of course, will be tricky in practice, but such is the nature of these things.

I had never seen hibakusha before, and I was greatly honored to meet them. There are not so many of them left. Many of those who are still alive, as with the remaining Holocaust survivors, were children during World War II. Which points, inadvertently, to the immense human costs of these events, to the innocents swept up into the maw of war. I have of course read much about the Japanese victims of the bomb, but it is another thing to meet them. The incident inspired me to re-read John Hersey’s Hiroshima for the first time in a long while, and the raw humanity of his account hit me in a way that it hadn’t before — the depth of my historical empathy increased measurably.

Which doesn’t tell one how to think about the use of the atomic bomb, I feel compelled to point out. Having sympathy and empathy with the past does not tell one which particular historical point of view one should subscribe to. There are many possible points of view to even non-controversial events, much less intensely controversial ones.2

Several of the still-living Manhattan Project veterans/

Several of the still-living Manhattan Project veterans. It is unclear how many of the nearly half-million people who worked on Manhattan Project are still alive. 

Next week (June 2-3, 2015), as part of a really wonderful symposium on the 70th Anniversary of the Manhattan Project hosted by the Atomic Heritage Foundation in Washington, DC, I will also get to spend some more time with other Manhattan Project veterans. These too are becoming an endangered species, along with all World War II-era veterans. It’s not the specific stories or experiences of these people that I often get the most out of. There’s something about just spending time around these historical actors (as historians like to call our human subjects) that helps you understand their world, remind you of the human content of the past.

If you are in town, the talks will be worth going to. Aside from my own talk (which will be great fun, I assure you), the other committed speakers include Kai Bird, Denise Kiernan, Robert S. Norris, Richard Rhodes, and Martin Sherwin. I have it on good authority that John Coster-Mullen will be in attendance, too. There is still time to register.

In a decade it will be the 80th anniversary of the Manhattan Project and World War II. There will probably not be any veterans to talk to then. There is an advantage to that, for the historian: living historical actors are tricky. They can disagree with you. Their individual perspectives can be intoxicating, charismatic, misleading. They can insist that their perspectives on history take precedence over the synthetic version you have constructed from inscriptions. They aren’t always right on that, as all history students are taught, but that doesn’t mean they can’t make trouble for you — the Smithsonian Enola Gay controversy in 1995 was one in part such a conflict of perspectives. It is in some ways easier to deal with the long-since deceased, because you can regard their inscriptions from something of a remove. But you do miss out on something, and its not just nostalgia. You have to work harder to reconstruct these other worlds, these other subjective states, in the absence of a working, functioning example sitting in front of you. This is why we have to preserve these spaces, and these voices, just as diligently as we have to preserve the documents, the inscriptions.

  1. Among other things, I learned that to an American ear, it is pronounced heh-bak-sha, with the u being essentially silent. [↩]
  2. This was one of the great fallacies of the 1995 Enola Gay controversy — the idea that by talking about the victims of the bombs, you somehow took away from those who thought the bombs were necessary or important. History is complex, and we need to treat the audiences of history as if they were intelligent human beings capable of understanding multiple, possibly contradictory perspectives. If we don’t do that, we are just doing some form of crude activism for one cause or another, and the world has enough of that to go around as it is. [↩]

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What did Bohr do at Los Alamos?

by Alex Wellerstein, published May 11th, 2015

In the fall of 1943, the eminent quantum physicist Niels Bohr managed a dramatic escape from occupied Denmark, arriving first in Sweden, then going to the United Kingdom. He was quickly assimilated into the British part of the Manhattan Project, then well underway. Bohr’s institute in Copenhagen had long been considered the world center of theoretical physics, and in the 1920s, young students from around the world flocked to work with him there. Now, in December 1943, Bohr and his son Aage made their pilgrimage to what was quickly becoming the new, stealth center of nuclear expertise: Los Alamos. At age 59, he would be the oldest scientist on “the Hill,” a place where the average age was 29.

Bohr skiing at Los Alamos, January 1945, seemingly without a care in the world. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr skiing at Los Alamos, January 1945, seemingly without a care in the world. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

This much is a standard part of Manhattan Project lore. Bohr’s contributions are usually spoken of primarily in psychological and moral terms. Bohr inspired the physicists to think about the consequences of their work, and laid the seeds of what would become the effort for postwar international control. He also spoke with both Churchill and Roosevelt, ineffectively, about the need to avoid an arms race. Bohr was a notoriously poor oral communicator, typically being barely audible. His deeply alienated and disturbed Churchill, who thought he might be proposing to tell the Soviets about the weapon. He probably just bored Roosevelt.

Some of the stories of his conduct at Los Alamos are adorably absent-minded. One of my favorite memos in the Manhattan Project archives is a February 1944 letter from Lt. Col. John Lansdale, head of MED security, to Richard Tolman, a physicist who was a good friend of the Bohrs. “Subject: Nicholas Baker,” it starts out, using Bohr’s wartime codename, and explains that in the process of following Bohr around, to make sure he was safe, some, well, deficiencies in his judgment were encountered:

“Both the father and son appear to be extremely absent-minded individuals, engrossed in themselves, and go about paying little attention to any external influences. As they did a great deal of walking, this Agent had occasion to spend considerable time behind them and observe that it was rare when either of them paid much attention to stop lights or signs, but proceeded on their way much the same as if they were walking in the wood. On one occasion, subjects proceeded across a busy intersection against the red light in a diagonal fashion, taking the longest route possible and one of greatest danger. The resourceful work of Agent Maiers in blocking out one half of the stream of automobile traffic with his car prevented their possible incurring serious injury in this instance.”

… I understand that the Bakers will be in Washington in the near future, at which time you will unquestionably see them. If the opportunity should present itself, I would appreciate a tactful suggestion from you to them that they should be more careful in traffic.1

Nobel-Prize winning physicist nearly run over by a car, because he treats American streets like paths in a forest, saved from disaster only by a trailing secret agent blocking the road with his car? You can’t make this stuff up. These kinds of stories reinforce the playful, harmless, “Uncle Nick” character that Bohr has come to represent in this period.

Bohr and General Groves' personal technical advisor, Richard Tolman, attending the opening of the Bicentennial Conference on "The Future of Nuclear Science," circa 1947. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr and General Groves’ personal technical advisor, Richard Tolman, attending the opening of the Bicentennial Conference on “The Future of Nuclear Science,” circa 1947. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

But the truth is a little more complicated. For his part, Bohr would later downplay his role in the actual creation of nuclear weapons. He told another physicist in 1950, for example, that he had spent most of his time while in the United States trying to forestall a nuclear arms race. “That is why I went to America… They didn’t need my help in making the atom bomb,” he later said.2

Did they need Bohr? Probably not — they probably would have managed well enough without him. But this is an odd standard for talking about one’s role in making a weapon of mass destruction. They didn’t need almost any individual who worked on the bomb, in the sense that they could have salvaged on without them.3

And not being “needed” does not really get one off the hook, does it? Which gets at what I think is a key point here: in the postwar, Bohr never relied on his contributions to the bomb as a means of claiming moral superiority, responsibility, or political leverage. He was active in attempts to promote international control and avoid an arms race, but he didn’t do so in a way that ever owned up to his own role in making the bomb. As a result, a lot of people seem to believe that Bohr didn’t really do that much at Los Alamos other than provide the aforementioned moral support and provocative questions.

In fact, Bohr did work on the bomb. And not just on esoteric aspects of the physics, either; one of his role was concerned with the very heart of the “Gadget.”

Niels Bohr (r) conversing animatedly with his son Aage in front of a board full of equations. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Niels Bohr (r) conversing animatedly with his son Aage in front of a board full of equations. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

One of the key parts of the implosion design for the atomic bomb (the same sort of bomb detonated at Trinity and over Nagasaki) is the neutron initiator that sits at the absolute center of the device. It is a deceptively tricky little contraption. At the instance of maximum compression, it needs to send out a small burst of neutrons, to get the whole chain reaction started. It’s not even that many neutrons, objectively speaking — on the order of a hundred or so in the first bombs. But conjuring up a hundred neutrons, at the center of an imploding nuclear assembly, at just the right moment, was a tricky technical problem, apparently.

The details are still classified-enough that figuring out exactly what the nature of the problem is proves a little tough in retrospect. In an interview many years later, the physicist Robert Bacher, head of G (Gadget) Division during the war, recalled that for whatever reason, Enrico Fermi had become particularly focused on the initiator as the lynchpin of the bomb, and maybe his own conscience:

I think Fermi began to be very worried about the fact that this terrific thing that he’d sort of been the father of was going to turn into a great big weapon. I think he was terribly worried about it. … I think he [Fermi] was worried about the whole project, not just the initiator. But focusing on the initiator was the one thing that he thought he could look at. The thing really might not work.

And I think he also felt an obligation to take something that was as hare-brained as this was and try to find a way in which it really wouldn’t work. So he did look into every sort of thing, and I think every second day or so for a period, I’d see him and he’d come up or he’d see Hans [Bethe] and come up with a new reason why the initiator wouldn’t work. …4

Bacher got sick of Fermi’s interference, and eventually went to Oppenheimer to complain. Bacher recalled:

I said, “What I’d like to do is, Uncle Nick is here now, and I’d like to go and explain to him about the initiator and say I’d like his advice and counsel on whether he thinks it will work or not. We’ll answer any question that he puts to us, that we know the answer to.” So we did and he agreed with us and I told him quite frankly, “One of the reasons that we want to do this is that Fermi has so many misgivings about initiators.”

So I talked to him for a long while and then he spent about two days with his son Aage going over every single thing that had been done on this business. I saw him after this and he said, “My that’s very impressive. I think that will work.” I said, “Well now comes the test. Will you talk to Fermi about this? The two of you talk together and give me some counsel of what’s up on this?” So he did. And it made a lot of difference to have Uncle Nick talk to Fermi, because he felt that this wasn’t somebody you had working on some particular model and so on. It was sort of somebody from the outside, and I think it made Fermi feel a lot happier. And it certainly made it a lot easier for us.5

The initiator that “Uncle Nick” convinced Fermi of, the one that they ended up using in the Trinity and Nagasaki bombs, was the “Urchin.”

A schematic of the “Urchin,” as imagined by me, based on a postwar British account.

It was a hollow sphere of beryllium, a mere two centimeters in diameter. The inner side of the sphere was machined with grooves, facing inwards. At the center of these grooves was another sphere of beryllium, centered by pins embedded in the outer shell. On both the inner grooves of the outer shell, and the outer surface of the inner sphere were coated with nickel and gold. Onto the nickel of the inner sphere was a thin film of virulently radioactive polonium. Polonium emits alpha particles; in the non-detonated state of the “Urchin,” these would be absorbed harmlessly by the gold and nickel. But when the bomb came imploding in around it, the beryllium and polonium would be violently mixed, producing a well-known reaction (beryllium + an alpha particle = carbon + neutron) that produced the necessary neutrons.6

Margrethe and Niels Bohr converse in Copenhagen, 1947, in this extremely rare color photo. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Margrethe and Niels Bohr converse in Copenhagen, 1947, in this extremely rare color photo. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

“Urchin” wasn’t the only initiator design on the table. Fermi apparently favored a design with the codename “Grape Nuts.” What was “Grape Nuts”? I have no idea — it’s still classified. Presumably these names meant something, since “Urchin” seems to reference the internal spikes. A topic listing for a May 1945 laboratory colloquium at Los Alamos discussed three initiator designs and their creators: “Urchin,” attributed to James Tuck and Hans Bethe; “Melon-Seed,” attributed to James Serduke; and, lastly, “Nichodemus,” attributed to… Nicholas Baker, the codename for Niels Bohr.7

In the recently-declassified Manhattan District History, there are several paragraphs on Bohr. Most of them describe theoretical work he did on the physics of nuclear fission after arriving at the lab, which “cleared up many questions that were left unanswered before.” His work affected their understanding the nuclear properties of tamper materials, and he apparently gave them ideas for “new and better methods… of alternative means of bomb assembly.” (All of which apparently just pointed to the superiority of implosion, in the end, but still.)

MHD Bohr contributions to bomb

At least one sentence in the Manhattan District History is still completely blacked out. Maybe it refers to the initiator design (which the previous sentence refers to), maybe it refers to something else. It’s interesting that seven decades later, something of what Bohr worked on was still considered too classified to reproduce — evidence that Bohr’s influence on the bomb was less trivial than he would later make it out to be.8

Why does it matter? In Michael Frayn’s Copenhagen, there is, towards the end of the play, an implied asymmetry between Bohr and Heisenberg. Heisenberg is criticized throughout the play for potentially making an atomic bomb for Hitler. The play ultimately says Heisenberg didn’t make an atomic bomb in part because he wasn’t trying to make a bomb. (It does so with perhaps a little bit too much credence to the “he didn’t do it because he was sabotaging it thesis,” which I think there is no evidence for and no reason to believe, but anyway.) Driven by his fears, Bohr goes to the United States and actually does work on the bomb, does contribute to the killing of over a hundred thousand people, and so on. And so there is some irony there, where Heisenberg, supposedly the one in a state of moral jeopardy, is the one who actually contributes to the death of no one, where Bohr, supposedly the moral authority, is the one who helps build the bomb.

Bohr with Elisabeth and Werner Heisenberg in Athens, Greece, 1956. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr with Elisabeth and Werner Heisenberg in Athens, Greece, 1956. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Do Bohr’s contributions to the atomic bomb, however major or minor, weaken his moral authority? I don’t really think so. Bohr’s strongest and most lasting contribution was putting the bug of international control into the heads of people like Oppenheimer. That bug might have come up on its own (when they learned about Bohr’s scheme, Vannevar Bush and James Conant were surprised to find that they had been thinking along almost exactly the same lines, completely independently), but Bohr’s influence on openness, candor, the moral obligation of scientists, and so on had a profound effect on postwar political discourse, even if his dreaded arms race was not avoided. In this light, I think Bohr still comes off pretty well, even if the bomb still does contain traces of his fingerprints.

  1. John Lansdale to Richard Tolman, “Subject: Nicholas Baker,” (5 February 1944), Manhattan Engineer District (MED) records, Records of the Army Corps of Engineers, RG 77, National Archives and Records Administration, College Park, MD, Box 64, “Security.” [↩]
  2. J. Rud Nielson, “Memories of Niels Bohr,” Physics Today 16, no. 10 (Oct. 1963), 28-29. [↩]
  3. I am occasionally drawn into a game of “who is so important that you absolutely couldn’t remove them and still expect it to be successful?” I am inclined to think that almost everyone would be more or less replaceable, as individuals, though there are a few whose contributions were so pivotal that removing them would create serious issues. Someday I will post some concrete thoughts on this on this. [↩]
  4. Robert Bacher interview with Lillian Hoddeson and Alison Kerr (30 July 1984), Robert Bacher papers, Caltech Institute Archives, Pasadena, CA, Box 48, Folder 5. [↩]
  5. Ibid. [↩]
  6. Accounts of the exact dimensions of the “Urchin” vary from source to source. John Coster-Mullen’s book, Atom Bombs, gives what I find to be convincing evidence that it was 0.8 in./2 cm in diameter. There was 20 curies of polonium deposited in them, and they had to be replaced frequently because of polonium’s low half-life. The inner core of the plutonium pit was about 1 in. in diameter, and apparently both the core and the initiator would be expected to expand slightly due to the heat generated by their radioactivity. Apparently James Tuck gave it the name “Urchin,” on account of its inner ridges. There is some question as to how the grooves were machined, whether they were pyramids (as in the British account) or ridges (e.g. like a theatre in the round). It’s always nice to be reminded that there are still a few secret details out there. [↩]
  7. The list of wartime colloquia comes from the Klaus Fuchs FBI File, Part 49 of 111, available on the FBI’s website, starting on page 49 of the PDF. The only other “Nicholas Baker” contribution mentioned in the document is a November 1944 talk on “nuclear reactions of heavy elements and particularly the various results obtained when a neutron comes in contact with heavy nuclei, such as Uranium 238.” [↩]
  8. Manhattan District History, Book 8 (Los Alamos Project), Volume 2 (Technical), pages II-2 to II-3. [↩]

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Visions

Critical mass

by Alex Wellerstein, published April 10th, 2015

When we talk about how nuclear weapons work, we inevitably mention the “critical mass.” This is the amount of fissile material you need to create a self-sustaining nuclear reaction. But it’s a very tricky concept, one often poorly deployed and explained, and the result, I have found while teaching and while talking to people online, is an almost universal confusion about what it means on a physical level.

One of the ways in which the critical mass is visually explained in Glasstone and Dolan's The Effects of Nuclear Weapons (1977 edn.). Want it on a t-shirt? I've got you covered.

One of the ways in which the critical mass is visually explained in Glasstone and Dolan’s The Effects of Nuclear Weapons (1977 edn.). Want it on a t-shirt? I’ve got you covered.

Where does the term come from? In the Smyth Report released in August 1945, the term “critical size” is used almost universally, while “critical mass” is used exactly once (and parenthetically, at that). A more interesting term, the “critical condition,” is used in a few places. The Los Alamos Primer, from 1943, uses critical “radius,” “volume,” “conditions,” in addition to “mass.” The MAUD Report, from 1941, uses critical “size,” “value,” and “amount” — not mass. The Frisch-Peirels memorandum, from 1940, uses critical “radius,” “size,” and “condition.” Leo Szilard’s pre-fission, 1935 patent on chain reacting systems uses the terms critical “thickness,” and “value,” not mass. This is not to imply that people didn’t use the term “critical mass” at the time — but it was one term among many, not the only term. The earliest context I have found it being used extensively comes from a paper in 1941, where it was being used specifically to talk about whether masses of fissile material could be made to explode on demand and not before.1

Why use “critical mass” instead of other terms? For one thing, talking about the mass can help you get a sense of the size of the problem when fissile material is scarce and hard to produce (producing fissile material consumed 80% of the Manhattan Project’s budget). And it can also help you when talking about safety questions — about avoiding a nuclear reaction until you absolutely want on. So you don’t want to inadvertently create a critical mass. And knowing that the critical mass is so many kilograms of fissile material, as opposed to so many tons, was an early and important step in deciding that an atomic bomb was feasible in the first place.

A 5.3 kg ring of 99.96% pure plutonium-239. Under some conditions, this is enough to produce a significant explosive output. In its current form — unreflected, at normal density, in a ring-shape that prevents any neutrons from finding too many atoms to fission with — it is relatively innocuous.

A 5.3 kg ring of 99.96% pure plutonium. Under some conditions, this is enough to produce a significant explosive output. In its current form — unreflected, at normal density, in a ring-shape that prevents any neutrons from finding too many atoms to fission with — it is relatively innocuous.

What I don’t like about the term, though, is that it can easily lead to confusion. I have seen people assert, for example, that you need a “critical mass” of uranium-235 to start a nuclear reaction. Well, you do — but there is no one critical mass of uranium-235. In other words, used sloppily, people seem to often think that uranium-235 or plutonium have single values for their “critical mass,” and that “a critical mass” of material is what you use to make a bomb. But it’s more complicated than that, and this is where I think focusing on the mass can lead people astray.

Put simply, the amount of fissile material you need to start a nuclear reaction varies by the conditions under which it is being considered. The mass of material matters, but only if you specify the conditions under which it is being kept. Because under different conditions, any given form of fissile material will have different critical masses.

I’ve seen people (mostly online) want to talk about how nuclear weapons work, and they look up what “the” critical mass of uranium-235 is, and they find a number like 50 kg. They then say, OK, you must need 50 kg to start a nuclear reaction. But this is wrong. 50 kg of uranium-235 is the bare sphere critical mass of uranium-235. In other words, if you assembled 50 kg of uranium-235 into a solid sphere, with nothing around it, at normal atmospheric conditions, it will start a self-sustaining chain reaction. It probably would not produce an explosion of great violence — the uranium sphere would probably just blow itself a few feet apart (and irradiate anyone nearby). But once blown apart, the reaction would stop. Not a bomb.

The Godiva Device, a "naked" (get it?) critical assembly used as a pulsed nuclear reactor at Los Alamos. A 54 kg near-bare sphere of 93.7% enriched uranium separated into three pieces. At left, it is separated safely — no reaction. At right, you see what happened when the pieces got close enough to start a critical reaction — not a massive explosion (thank goodness), but enough energy output to damage the machine, and to push those pieces of uranium far enough from each other that they could no longer react.

The Godiva Device, a “naked” (get it?) critical assembly used as a pulsed nuclear reactor at Los Alamos. A 54 kg near-bare sphere of 93.7% enriched uranium separated into three pieces. At left, it is separated safely — no reaction. At right, you see what happened when the pieces got close enough to start a critical reaction — not a massive explosion (thank goodness), but enough energy output to damage the machine, and to push those pieces of uranium far enough from each other that they could no longer react. The workers were fortunately a safe distance away.

So does that mean that 50 kg of uranium-235 is a important number in and of itself? Only if you are assembling solid spheres of uranium-235.

Is 50 kg the amount you need for a bomb? No. You can get away with much smaller numbers if you change the conditions. So if you put a heavy, neutron-reflecting tamper around the uranium, you can get away with around 10 to 15 kg of uranium-235 for a bomb — a factor of 3-5X less mass than you thought you needed. If your uranium-235 is dissolved in water, it takes very low masses to start a self-sustaining reaction — a dangerous condition if you didn’t mean to start one! And it may be possible, under very carefully-developed conditions, to make a bomb with even smaller masses. (The bare-sphere critical mass of plutonium is around 10 kg, but apparently one can get a pretty good bang out of 3-4 kg of it, if not less, if you know what you are doing.)2

Conversely, does this mean that you can’t possibly have 50 kg of uranium (or more) in one place without it detonating? No. If your uranium is fashioned not into a solid sphere, but a cylinder, or is a hollow sphere, or has neutron-absorbing elements (i.e. boron) embedded in it, then you can (if you know what you are doing) exceed that 50 kg number without it reacting. And, of course, there are also impurities — the amount of uranium-238 in your uranium-235 will increase the size of any critical mass calculation.

In other words, under different conditions, the mass of fissile material that will react varies, and varies dramatically. These different conditions include different geometries, densities, temperatures, chemical compositions/phases, and questions about whether it is embedded into other types of materials, whether there are neutron-moderating substances (i.e. water) present, enrichment levels, and so on. It’s not a fixed number, unless you also fix all of your assumptions about the conditions under which it is taken place.

Re-creation of Slotin's fatal experiment with the third core. (Source: Los Alamos)

Re-creation of Louis Slotin’s fatal experiment with the third plutonium core. The problem wasn’t the mass of the core, it was that Slotin inadvertently changed the state of the system (by accidentally letting the reflector drop onto it completely when his screwdriver slipped), which took a safe, non-critical assemble of plutonium and moved it into a briefly-critical state. This produced no explosion, but enough radiation to be fatal to Slotin and damaging to others in the room.

The classic example of this, of course, is the implosion bomb design. The bare sphere critical mass of plutonium-239 is 10 kg. The Nagasaki bomb contained 6.2 kg of plutonium as its fuel. At normal, room-temperature densities, a solid sphere of 6.2 kg of plutonium is not critical. Increase its density by 2.5X through the careful application of high explosives, however, and suddenly that is at least one critical mass of plutonium. Even this is something of an oversimplification, because it’s not just the density that matters: the allotropic (chemical) phase of plutonium, for example, affects its critical mass conditions (and plutonium is notorious for having an unusual number of these phases), and the Nagasaki bomb also included many other useful features meant to help the reaction along like a neutron initiator (which gave it a little shot of about 100 neutrons to start things off), and a heavy, natural-uranium tamper.

What I dislike about the term “critical mass,” as well, is that it can serve to obscure the physical process that defines “criticality.” It can make it seem like reactivity is a function of the mass alone, which is wrong. Worse, it can keep people from realizing why the mass matters in the way it does (among other things). And this can lead to confusion on questions like, “how much explosive power does a critical mass release?” The answer is… it has nothing to do with the critical mass per se. That is a question of bomb efficiency, which can seem like a secondary, separate question. But both the question of criticality and efficiency are really one and the same phenomena — if you understand the underlying physical process on an intuitive level.

Criticality, the “critical condition,” is defined as the point at which a chain reacting system becomes self-sustaining. So we can imagine a whole sea of uranium-235 atoms. Neutrons enter the system (either from a neutron source, spontaneous fissioning, or the outside world). If they are absorbed by a uranium-235 nucleus, they have a chance of making it undergo fission. That fission reaction will produce a random number (2.5 on average) of secondary neutrons. To be critical, enough of these neutrons will then have to go on to find other uranium nuclei to keep the overall level of neutrons (the “neutron economy”) constant. If that total number of neutrons is very low, then this isn’t very interesting — one neutron being replenished repeatedly isn’t going to do anything interesting. If we’ve already got a lot of neutrons in there, this will generate a lot of energy, which is essentially how a nuclear reactor works once it is up and running.

Supercriticality, which is what is more important for bomb design (and the initial stages of running a reactor) is when your system produces more than one extra neutron in each generation of fissioning. So if our uranium atom splits, produces 2 neutrons, and each of those go on to split more atoms, we’re talking about getting two neutrons for every one we put into the system. This is an exponentially-growing number of neutrons. Since neutrons move very quickly, and each reaction takes place very quickly (on the order of a nanosecond), this becomes a very large number of neutrons very quickly. Such is a bomb: an exponential chain reaction that goes through enough reactions very quickly to release a lot of energy.

The Trinity Gadget - Sectional View

A sectional view of a rendering of the Trinity “Gadget” I made. The 6.2 kg sphere of plutonium (the second-to-last sphere in the center, which encloses the small neutron initiator) is a safe-to-handle quantity by itself, and only has the possibility of becoming super-critical when the high explosives compress it to over twice its original density. Sizes are to scale.

So what are the conditions that produce these results? Well, it’s true that if you pure enough fissile material in one place, in the right shape, under the right conditions, it’ll become critical. Which is to say, each neutron that goes into the material will get replaced by at least another neutron. It will be a self-sustaining reaction, which is all that “criticality” means. Each fission reaction produces on average 2.5 more neutrons, but depending on the setup of the system, most or all of those may not find another fissile nuclei to interact with. If, however, the system is set up in a way that means that the replacement rate is more than one neutron — if every neutron that enters or is created ends up creating in turn at least two neutrons — then you have a supercritical system, with an exponentially-increasing number of neutrons. This is what can lead to explosions, as opposed to just generating heat.

In a bomb, you need more than just a critical reaction. You need it to be supercritical, and to stay supercritical long-enough that a lot of energy is released. This is where the concept of efficiency comes into play. In theory, the Fat Man’s 6.2 kg of plutonium could have released over 100 kilotons worth of energy. In practice, only about a kilogram of it reacted before the explosive power of the reaction separated the plutonium by enough that no more reactions could take place, and “only” released 20 kilotons worth of energy. So it was about 18% efficient. The relative crudity of the Little Boy bomb meant that only about 1% of its fissile material reacted — it was many times less efficient, even though it had roughly 10X more fissile material in it than the Fat Man bomb. The concept of the critical mass, here, really doesn’t illuminate these differences, but an understanding of how the critical reactions work, and how the overall system is set up, does.

This understanding of criticality is more nuanced than a mere mass or radius or volume. So I prefer the alternative phrasing that was also used by weapons designers: “critical assembly” or “critical system.” Because that emphasizes that it’s more than one simple physical property — it’s about how a lot of physical properties, in combination with engineering artifice, come together to produce a specific outcome.

I’ve been playing around with the scripting language Processing.js recently, in my endless quest to make sure my web and visualization skills are up-to-date. Processing.js is a language that makes physics visualizations (among other things) pretty easy. It is basically similar to Javascript, but takes care of the “back end” of graphics to a degree that you can just say, “create an object called an atom at points x and y; render it as a red circle; when it comes into contact with another object called a neutron, make it split and release more neutrons,” and so on. Obviously it is a little more arcane than just that, but if you have experience programming, that is more or less how it works. Anyway, I had the idea earlier this week that it would be pretty easy to make a simple critical assembly “toy” simulation using Processing, and this is what I produced:

Critical Assembly Simulator

The gist of this application is that the red atoms are uranium-235 (or plutonium), and the blue atoms are uranium-238 (or some other neutron-absorbing substance). Clicking on an atom will cause it to fission, and clicking on the “fire neutron initiator” button will inject a number of neutrons into the center of the arrangement. If a neutron hits a red atom, it has a chance to cause it to fission (and a chance to just bounce off), which releases more atoms (and also pushes nearby atoms away). If it hits a blue atom, it has a chance to be absorbed (turning it purple).

The goal, if one can put it that way, is to cause a chain reaction that will fission all of the atoms. As you will see from clicking on it, in its initial condition it is hard to do that. But you can manipulate a whole host of variables using the menu at the right, including adding a neutron reflector, changing the number of atoms and their initial packing density, the maximum number of neutrons released by the fission reaction, and even, if you care to, changing things like the lifetimes of the neutrons, the likelihood of the neutrons just scattering off of atoms, and whether the atoms will spontaneously fission or not. If you have a reflector added, you can also click the “Implode” button to make it compress the atoms into a higher density.3

The progress of a successful reaction using an imploded reflector. The little yellow parts are a "splitting atom" animation which is disabled by default (because it decreases performance).

The progress of a successful reaction using an imploded reflector. The little yellow parts are a “splitting atom” animation which is disabled by default (because it decreases performance).

This is not a real physical simulation of a bomb, obviously. None of the numbers used have any physically-realistic quality to them, and real atomic bombs rely on the fissioning of trillions of atoms in a 3D space (whereas if you try to increase the number of atoms visible to 1,000, much less 10,000, your browser will probably slow to a crawl, and this is just in 2D space!).4 And this simulator does not take into account the effects of fission products, among other things. But I like that it emphasizes that it’s not just the number of atoms that determines whether the system is critical — it’s not just the mass. It’s all of the other things in the system as well. Some of them are physical constants, things pertaining to the nature of the atoms themselves. (Many of these were constants not fully known or understood until well after 1939, which is why many scientists were skeptical that nuclear weapons were possible to build, even in theory.) Some of them are engineering tricks, like the reflector and implosion.

My hope is that this kind of visualization will help my students (and others) think through the actual reaction itself a bit more, to help build an intuitive understanding of what is going on, as a remedy to the aspects of a prior language that was created by scientists, diffused publicly, and then got somewhat confused. “Critical mass” isn’t a terrible term. It has its applications. But when it can lead to easy misunderstandings, the language we choose to use matters.

  1. E.g. “Can system be controlled safely by dividing mass into two parts? Yes. We believe that it is possible with suitable technical supervision to assemble masses which will be known fractions of the critical mass and which will not explode during the assembly.” The authorship of the report is apparently several members of the Uranium Committee, but their specific names are unlisted. “Fast neutron chain reactions — Summary of discussion on recommendations of the Sub-section on theoretical aspects on October 24, 1941,” (24 October 1941), copy in Bush-Conant File Relating the Development of the Atomic Bomb, 1940-1945, Records of the Office of Scientific Research and Development, RG 227, microfilm publication M1392, National Archives and Records Administration, Washington, D.C., n.d. (ca. 1990), Reel 10, Target 21, Folder 162A, “Reports — Chain Reactions [1941].” [↩]
  2. On the “how low can you go” question, I have found table A.1 in this report useful:International Panel on Fissile Material, “Global Fissile Material Report 2013: Increasing Transparency of Nuclear Warhead and Fissile Material Stocks as a Step toward Disarmament,” Seventh annual report of the International Panel on Fissile Material (October 2013). There is documentary evidence suggesting the Soviets managed to weapons with cores as little as 0.8 kg of plutonium, and got significant (e.g. >1 kiloton) yields from them. [↩]
  3. For those who want it, the source code is here. It is sparsely commented. It is written, again, in Processing.js. [↩]
  4. Just to put this into perspective, 1 kg of plutonium-239 is ~2.5 x 1024 atoms. [↩]

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