Can someone explain the feared runaway nuclear reaction that Oppenheimer presented Einstein in the film? The one where detonating a nuke would’ve exploded the whole world?
Wouldn’t that scenario require many orders of magnitude more energy than the output of the what the first (or current) nuclear weapons were capable of?
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While they were quite sure that it wasn’t a realistic probability (the movie very much played up the probability calculation of this happening) but there was a tiny tiny tiny chance that the nuclear explosion could cause a sufficiently large neutron burst (at sufficiently high pressure and temperature) to trigger an action where two nitrogen atoms (the most common element in earths atmosphere) and a hydrogen atom (hydrogen is everywhere) fuse together, release energy and create a runaway cascade that would set the entire planets atmosphere ablaze.
Both Oppenheimer and his top colleagues considered the chance vanishingly small, but it was (at the time) a non-zero probability. They couldn’t 100% say it wouldn’t happen, but it would require that this reaction was more likely than they had calculated (by several magnitudes) and that the nuclear bombs themselves proved multiple times more powerful than they had previously calculated. Based on how powerful the first bomb turned out to be they could immediately afterwards rule out the possibility of it ever happening (regardless of how powerful they made their bombs), the neutron capture rate just wasn’t high enough.
Very early on in the theoretical research of making an atomic bomb, our understanding of nuclear processes was still incomplete. One of the Manhattan Project scientists (I believe it was Hans Bethe but I might be wrong) realized that they didn’t have enough understanding to 100% rule it out the possibility that the extreme pressures and temperatures from the nuclear explosion could start a runaway fusion process of the nitrogen in the atmosphere. Pretty quickly after that, they gained some more understanding and had the math to be able to calculate that it was actually 100% impossible. Oppenheimer portrays this with some more drama than actually occurred in real life.
>Can someone explain the feared runaway nuclear reaction that Oppenheimer presented Einstein in the film?
A group of Manhattan Project scientists were talking and one jokingly brought up the idea that the fission reaction from setting off the nuke would trigger a chain reaction that would temporarily turn the Earth’s entire atmosphere into a glorified star. The whole nuke worked on the idea that by inciting an atom to break, these two new atoms would then bump into 2 other atoms and repeat trillions of times to create a level of heat and pressure somewhat comparable to taking a chunk out of the sun. What this whole dilemma, introduced, though, was a new aspect that wasn’t previously taken into account; could that chain reaction then carry into the atmosphere until that same level of heat and pressure was being applied by every gas particle on the planet?
>Wouldn’t that scenario require many orders of magnitude more energy than the output of the what the first (or current) nuclear weapons were capable of?
Yes, it would take magnitudes more power than what we have even today, and we’re well aware of that now. But now isn’t then. You have to keep in mind our knowledge of radiation at this time was still juvenile, and as far as we knew then this would be the first time a fission reaction would have even happened on Earth. Every aspect of what they were doing was surrounded by the shroud of the unknown.
That said, though, I’m not sure how the movie portrays it but it really wasn’t that big of a dilemma during the real project. After it being proposed the same scientists almost immediately began running the numbers of it, since they weren’t going to test something that could destroy the whole planet in an instant, and it didn’t take long for them to realize the scenario was so improbable it could be practically labelled as impossible.
As I recall the story, Edward Teller had already started researching how to start a fusion reaction to make a bomb, combining hydrogen to get helium and a lot of energy. He figured that the most likely way to get enough energy to start a fusion reaction was to use a fission reaction, which is what was being developed for the Manhattan Project. In doing his rudimentary calculations, he saw that there was a chance that the atomic bombs being designed would have enough energy to start a fusion reaction, which may continue to fuse all of the hydrogen in the atmosphere making earth a big fireball.
In the movie (and perhaps real life), Oppenheimer had Hans Bethe check the number, since Bethe had a lot of experience with fusion in stellar processes. It was determined that the probability of incinerating the earth was vanishingly small.
It should be noted that Teller continued to advocate for fusion bombs, which eventually were made and were detonated with fission bombs. The fission bombs were usually called atomic bombs and the fusion bombs were called hydrogen bombs.
So just to make one thing clear — in real life, Oppenheimer didn’t go to Einstein for anything, much less on this topic. That was done for dramatic/narrative effect. Einstein was not actually part of the actual work on the bomb in any way.
The actual concern about the “runaway” reaction was that the intense heat of the atomic bomb might cause elements in the Earth’s atmosphere or crust to undergo nuclear fusion. The fear was that these fusion reactions would release more heat, and that this would continue the fusion reaction.
Upon further study, the most “dangerous” reaction — in the sense that it was the most likely to occur out of the all of the elements that might be available in quantity when an atomic bomb was exploding — was a nitrogen-nitrogen fusion reaction. (Air is 78% nitrogen.) They did not know the exact energy output of the atomic bomb, and they did not know the exact “cross-section” of the nitrogen-nitrogen reaction. The “cross-section” is basically the probability that the reaction will happen at a given amount of input energy and density of material.
Even with these unknowns, they made reasonable guesses as to what the answers might be, and then made even more “pessimistic” versions than were reasonable, and found that the amount of energy required to start and sustain the nitrogen-nitrogen reaction in quantity was likely to be several orders of magnitude more than any atomic bomb developed during World War II and probably any nuclear weapon developed ever.
It should be emphasized that they still did not know much about terrestrial nuclear fusion reactions at this stage. They later found out (while trying to invent the hydrogen bomb) that it is even harder to produce a nuclear fusion reaction, even with an atomic bomb as the source of “input” energy, than they thought.
I would also emphasize that there were many “known unknowns” and many “unknown unknowns” involved in deciding that this was not possible. The scientists were aware that they were only just learning about these things. But they concluded that the chance of igniting the atmosphere seemed small-enough to not be a source of worry, even though they did not have the ability to definitively rule it out as a possibility.
Later computer simulations by weapons scientists showed that the only way you could ignite a planet-destroying nuclear fusion reaction on Earth would require greatly increasing the amount of deuterium in the oceans (by a factor of 20X), and then having a bomb that was on the order of 200 teratons of TNT. Which is a very technical way to say, “it’s not actually possible.”
Essentially the conditions required to sustain a nuclear chain reaction are extreme. To quote a [paper](https://sgp.fas.org/othergov/doe/lanl/docs1/00329010.pdf) contemporary to the first tests:
>It is shown that, whatever the temperature to which a section of the atmosphere may be heated, no self-propagating chain of nuclear reactions is likely to be started. The energy losses to radiation always overcompensate the gains due to the reactions.
As the blast radius expands into the atmosphere (or even other mediums like water) the losses through various factors inevitably begin to outweigh the energy generated by the thermonuclear reaction.
Anything that implies any serious scientists with a knowledge of the project thought atmospheric ignition was remotely possible prior to the first test is misleading. I think it stems from a unwillingness for people who live their lives in maths to talk about there being a “zero” possibility of anything, and also the oft mentioned gallows humour that was required to cope with knowing what they were trying to achieve.
Edward Teller and Enrico Fermi were both top physicists who were involved in the Manhattan Project as well as other physics projects. Rumors abound that the two clashed frequently, and that Fermi considered Teller to be something of an “idea man” whose work was not rigorous.
Teller did calculations that suggested that it was possible that a fission blast could trigger fusion of nitrogen in the atmosphere, creating enough energy that more such fusion would occur in a chain reaction. This, he said, showed the possibility that the explosion could be far more devastating than believed, or even could ignite the entire atmosphere.
Other scientists, most notably Fermi, disagreed and even scoffed at this idea. It was suggested that Teller’s work was mathematically flawed, even poorly done, and that the idea had no real merit.
However, there were plenty of people who thought that Teller’s concerns should be addressed, especially as Teller was widely respected. In particular, the non-scientists, not being able to judge for themselves, were concerned that someone with Teller’s reputation said this could happen. This is quite reasonable, as they had no way of knowing who was right, and they were naturally reluctant to risk fusing the entire atmosphere.
As a result, Teller’s work was turned over to other scientists for review to see if there was anything to be concerned about. This review took months and delayed the project.
The result of the review was, “No way. It won’t get within orders of magnitude of happening. We may not know enough to be certain how much energy the bomb will put out, but it can’t possibly be close to doing this. This can’t happen.”
Because of this, the project was allowed to continue. Enrico Fermi thought it was funny to take bets on whether or not the bomb would ignite the atmosphere. Rumors, again unsubstantiated, have arisen that Fermi did it, at least in part, as a way of taunting Teller for his earlier fears.
Elements lighter than iron can release energy if they fuse. Some scientists wanted to know if the power from the nuclear bomb was high enough to make the nitrogen and oxygen in the atmosphere fuse, which would cause a chain reaction
My undergraduate physics professor told a story about this. He said there was one guy whose job was to make sure that the runaway fusion reaction was impossible. He did this by performing the calculation many times until he was satisfied. When it came time for him to observe the first A-bomb test, it was *much* brighter than he had expected. On seeing the surprisingly bright light, his first thought was “Oh NO! I made a mistake!”.
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