July 28, 2023. I haven’t seen Oppenheimer yet, but I’d like to chime in on a fictional scene in the film, in which Oppenheimer asks Einstein whether an atomic detonation might incinerate the entire planet. Although Oppenheimer did indeed fear that terrifying possibility, he did not discuss it with Einstein. In an interview with Dennis Overbye of The New York Times, director Christopher Nolan says he shows Oppenheimer consulting Einstein simply because everyone knows who Einstein is.

Dolan’s fudging of the historical record gives me an excuse to present my own account of the end-of-the-world prediction--or rather, the account of physicist Hans Bethe (1906-2005), who was directly involved in the episode. I first heard about the doomsday hypothesis in 1991 when I was writing a profile of Bethe for Scientific American.

The long-classified story begins in 1942 at the University of California at Berkeley, where Bethe was investigating the theoretical possibility of a fission weapon with Oppenheimer, Edward Teller and other members of the Manhattan Project. Teller, who was already exploring the possibility of a fusion bomb, became worried that a fission explosion might trigger runaway fusion in the atmosphere.

Fission, remember, occurs when heavy elements such as uranium split apart into lighter ones, and fusion when light elements such as hydrogen smash together to form heavier ones. Fusion releases far more energy than fission.

Teller shared his concern with his colleagues, including Bethe, an authority on fusion. Bethe, who ended up heading the theoretical division at Los Alamos Laboratory, won a Nobel prize in 1967 for his work on stellar fusion.

After considering Teller’s conjecture, Bethe and others concluded… But I’ll let Bethe tell the story in his own words. Here is a lightly edited transcript of my 1991 interview with Bethe, which took place at his home in Ithaca, New York.

Horgan: I wonder if you could tell me a little bit about the story of Teller's suggestion that the atomic bomb might ignite the atmosphere around the Earth.

Bethe: It is such absolute nonsense [laughter]… One day at Berkeley, Teller came to the office and said, "Well, what would happen to the air if an atomic [fission] bomb were exploded in the air?" The original idea about the hydrogen [fusion] bomb was that one would explode an atomic bomb and then simply the heat from the atomic bomb would ignite a large vessel of deuterium… and make it react. So Teller said, "Well, how about the air?  There's nitrogen in the air, and you can have a nuclear reaction in which two nitrogen nuclei collide and become oxygen plus carbon, and in this process you set free a lot of energy. Couldn't that happen?"…

Oppenheimer [soon to be appointed head of Los Alamos Laboratory] got quite excited and said, "That's a terrible possibility." And he went to his superior, who was Arthur Compton [director of the Chicago arm of the Manhattan Project], and told him that. Well, I sat down and looked at the problem, about whether two nitrogen nuclei could penetrate each other and make that nuclear reaction, and I found that it was just incredibly unlikely.  And I said so, and I think Teller was very quickly convinced, and so was Oppenheimer when he'd returned from seeing Compton…[Later, Emil Konopinski and another physicist showed that it was impossible] to set the atmosphere on fire. They wrote one or two very good papers on it, and that put the question really to rest…

Just to relieve the tension [at the Trinity test on July 16, 1945, Enrico] Fermi said, "Now, let's make a bet whether the atmosphere will be set on fire by this test." [laughter] And I think maybe a few people took that bet. For instance, in Compton's mind [the doomsday question] was not set to rest. He didn't see my calculations [or] Konopinski’s much better calculations. So it was still spooking [Compton] when he gave an interview [in 1959, see below]. And so it got into the open literature… But by then of course it was absolutely clear, and it was absolutely clear before the Los Alamos test, that nothing like that would happen…

Horgan: What makes it such a fascinating episode… is the idea of doing a calculation on which possibly could rest the fate of the world. [laughter]

Bethe: Right, right.

Horgan: That's obviously an extraordinary kind of calculation to do. Did you have any...  Did you even think about that issue when you saw the Trinity test?

Bethe: No.

Horgan: You were absolutely--

Bethe: Yes.

Horgan: -- completely certain?

Bethe: Yes. The one thing in my mind was that maybe the initiator would not work, because I had a lot to do with its design… No, it never occurred to me that it would set the atmosphere on fire.

Horgan: In a way, this is like a great test of one's belief --

Bethe: … in science. [laughter]

Arthur Compton went public with the doomsday hypothesis in a 1959 interview with writer Pearl Buck for American Weekly. According to Buck, Compton said a runaway nuclear conflagration “would be the ultimate catastrophe. Better to accept the slavery of the Nazis than to run the chance of drawing the final curtain on mankind!”

Compton allegedly told Buck that he approved the Trinity test after calculating the odds of a fusion chain reaction to be “slightly less” than one in three million. Physicist H.C. Dudley cited the Compton-Buck interview in “The Ultimate Catastrophe” in the November 1975 Bulletin of the Atomic Scientists.

Dudley’s article provoked Bethe to respond that there was “no chance whatever” that an atomic blast would “ignite the atmosphere.” Bethe concluded: “There are many excellent reasons against nuclear war… But it is totally unnecessary to add to the many good reasons against nuclear war one which simply is not true.”

I would trust Bethe, as much as any physicist I’ve interviewed, to perform calculations on which the fate of the world depends. Bethe struck me as that rarest of creatures, a wise scientific genius. The terrible irony is that now, in part because of Bethe’s calculations, our fate is in the hands of men like Biden and Putin.

Clarification from Alex Wellerstein, an Actual Expert: My friend and Stevens colleague Alex, an historian specializing in nuclear weapons, posted the following info-packed comment. Alex alludes to the possibility, also raised by Teller, that a nuclear explosion could ignite deuterium in the ocean. The term “Super” refers to a hydrogen fusion bomb.—John Horgan

John, I suspect that by the time you talked to him, especially after the (very silly) Dudley exchange, Bethe was pretty sick of this issue, and probably was downplaying the amount of effort and lingering concerns that existed in 1945. Dan Ellsberg, in The Doomsday Machine, makes an argument that they were more concerned than they let on later, and while I don't totally think he is correct in his whole argument, I think he does a good job of showing that it wasn't quite as dismissible as nonsense at the time, whatever Bethe thought of it.

The Konopinski, Marvin, Teller paper (1946) is a pretty interesting read. They basically conclude that even with very conservative assumptions about how easy it was to fuse nitrogen, too much of the energy released by any fusion that might occur would be "lost" and not translatable into more fusion reactions. The ratio of "lost energy" to "usable energy" is always above 1 (so net loss), although they note that there is a "dangerous temperature" in which that ratio drops to only about 1.6, which they found a bit uncomfortably low given the uncertainties. The only up-shot is that the "dangerous temperature," if I'm reading it correctly, is upwards of many tens of billions of degrees Fahrenheit, much higher than any nuclear weapon they were contemplating could reach (the center of the Sun, by comparison, is only in the tens of millions of degrees). And they were at that point "contemplating" weapons in the 10-100 megaton range.

Interestingly, in 1951, before the first H-bomb test (Ivy Mike), Edward Teller requested that Gregory Breit and his group re-run the atmospheric ignition question from first principles, integrating into it the new understandings of nuclear physics and fusion that had been accumulated since 1946. The Breit group found that there were some better ways to think about it than the Konopinski, et. al, paper, but that the conclusions were essentially the same. I think it is interesting that Teller asked them to re-do the calculations. I do not think he thought it was likely to be dangerous, but it does show he felt it was at least re-checking! It gives this XCKD strip a little more realism than I initially appreciated: https://xkcd.com/809/

In the 1970s, after the Dudley paper came out, Livermore scientists re-ran the atmospheric ignition calculations with their latest computers and understandings of the physics (not out of any worry, but to show that there is literally no chance of it). They found that the only way to create this kind of reaction would be to increase the amount of deuterium in the ocean by 20X, and then to have a 200 teraton weapon (200 million megatons). Which is to say, it's not possible on this planet or with any weapon we could imagine building — and it's not easy!

Some more writings about this are here, though I only learned about the Breit re-do recently and so it is not part of this: https://blog.nuclearsecrecy.com/2018/06/29/cleansing-thermonuclear-fire/

In the film, they use Einstein instead of Compton, and there is a narrative payoff to that, even though it is very historically inaccurate — Einstein was not part of the Manhattan Project and was consulted on nothing by that phase of things. The other inaccuracy regarding this issue is that they have Teller present it at the first Berkeley meeting, which is correct, but they rob it of its context: it came up after Teller presented on the Super (which makes sense), whereas in the film they have Teller proposing the Super much later (perhaps to avoid confusion?). Otherwise they do an OK job of making it clear that this is not something the scientists thought was going to happen, but that a non-scientist (like Groves) might find their probabilistic assessment a little unnerving. —Alex Wellerstein

Further Reading:

The End of War

You’re Not Free If You’re Dead: The Case Against Giving Ukraine F-16s

My Quantum Experiment (free, online book)

Note: This column is an updated, free version of one I posted on ScientificAmerican.com in 2015.