At exactly 5:30 AM on July 16, 1945, the world’s first atomic bomb, codenamed Trinity, detonated over the desert in New Mexico, unleashing in an instant the power of 18,000 tons of TNT. The atomic age had begun. As night turned to day and a fireball 200 metres across rose into the sky, the scientists of the Manhattan Project who had built the bomb reacted in different ways. Some were jubilant, others more somber. J. Robert Oppenheimer, the scientific director of the project, famously recalled a line from the Hindu scripture (ba-ga-vad gee-ta)  Baghavad Gita: “Now I am become death, destroyer of worlds”; while Kenneth Bainbridge, director of the Trinity test, was more blunt, stating: “Now we’re all sons of bitches.” Elsewhere around the test site, money frantically changed hands as scientists settled a series of private bets. Some had wagered that the test would be a dud, or that it would reach just a fraction of its predicted yield. But others, including Italian physicist Enrico Fermi, had wagered on a more disturbing outcome: that the intense heat of the bomb would ignite the atmosphere, setting off an unstoppable chain reaction that would wipe out all life on earth. This apocalyptic bet has since become an infamous part of nuclear lore, but does it have any basis in reality? Could the Trinity test – or any nuclear weapon, for that matter – actually have set earth’s atmosphere ablaze?

Well, let’s dive into it, shall we?

Fear of a nuclear reaction running amok goes back to the earliest days of nuclear weapons research. In his 1969 memoir Inside the Third Reich, former Nazi Minister of Armaments Albert Speer recalled a conversation between Adolf Hitler and physicist Werner Heisenberg, head of the German atomic bomb project:

“Heisenberg had not given any final answer to my question whether a successful nuclear fission could be kept under control with absolute certainty or might continue as a chain reaction. Hitler was plainly not delighted with the possibility that the earth under his rule might be transformed into a glowing star.”

However, the notion of a nuclear weapon setting fire to the atmosphere originated with Hungarian physicist Edward Teller, who would go on to design the even more powerful hydrogen bomb. In a 1991 interview, German physicist Hans Bethe, director of the Manhattan Project’s theoretical division, recalled the origins of the idea:

“One day at Berkeley [in 1942]… Teller came to the office and said, “Well, what would happen to the air if an atomic bomb were exploded in the air?”  The original idea about the hydrogen 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?”  And that caused great excitement. Oppenheimer got quite excited and said, “That’s a terrible possibility.””

When more detailed calculations seemingly confirmed that Teller’s scenario was indeed possible, Oppenheimer tracked down Arthur Compton, director of the Manhattan Project’s metallurgical laboratory, while he was on vacation with his family in Michigan. The pair spent many hours on the shores of Oswego Lake furiously debating Teller’s prediction. In 1959, Compton was interviewed by Pulitzer and Nobel Prize-winning writer Pearl S. Buck for a feature in The Bulletin of the Atomic Scientists titled The Bomb – the End of the World? which helped bring the atmospheric ignition theory to public attention:

“Hydrogen nuclei,” Arthur Compton explained to me, “are unstable, and they can combine into helium nuclei with a large release of energy, as they do on the sun. To set off such a reaction would require a very high temperature, but might not the enormously high temperature of the atomic bomb be just what was needed to explode hydrogen? And if hydrogen, what about hydrogen in sea water? Might not the explosion of the atomic bomb set off an explosion of the ocean itself? Nor was this all that Oppenheimer feared. The nitrogen in the air is also unstable, though in less degree. Might not it, too, be set off by an atomic explosion in the atmosphere?”

“The earth would be vaporized,” I said.

“Exactly,” Compton said, and with that gravity! “It 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!”

Later in the piece, Buck states:

“If, after calculation, [Compton] said, it were proved that the chances were more than approximately three in one million that the earth would be vaporized by the atomic explosion, he would not proceed with the project. Calculation proved the figures slightly less – and the project continued.”

Nonetheless, the possibility of destroying the world continued to weigh on the Manhattan Project scientists’ minds, so over the course of 1943 and 1944, several more individuals tried to put the matter to rest once and for all. As Hans Bethe explained in his 1991 interview:

“Teller at Los Alamos put a very good calculator on this problem, Emil Konopinski, who was an expert on weak interactors, and Konopinski…showed that it was incredibly impossible to set the hydrogen, to set the atmosphere on fire.” 

Further calculations conducted by Hans Bethe and Edward Teller appeared to confirm Konopinski’s results, allaying any lingering doubts and allowing the Trinity test to proceed as planned. Teller and Konopinski, along with Cloyd Marvin, later summarized their findings in a 1946 paper titled Ignition of the Atmosphere With Nuclear Bombs, in which they concluded 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. This is true even with rather extravagant assumptions concerning the reactivity of the nitrogen nuclei of the air. The only disquieting feature is that the “safety factor” i.e. the ratio of losses togas of energy, decreases rapidly with initial temperature, and descends to a value of only about 1.6 just beyond a 10-MeV temperature. It is impossible to reach such temperatures unless fission bombs or thermonuclear bombs are used which greatly exceed the bombs now under consideration. But even if bombs of the required volume (i.e. greater than 1000 cubic meters) are employed, energy transfer from electrons to light quanta by Compton scattering will provide a further safety factor and will make a chain reaction in air impossible.”

For the uninitiated, MeV stands for Mega Electron-Volt, and is a unit of energy equivalent to 1.6×10-13 Joules. Also, Compton scattering, named after Arthur Compton, refers to the elastic transfer of kinetic energy from a photon to an electron – or vice-versa.

Interestingly, the difficulty of igniting a chain reaction in atmospheric nitrogen foreshadowed the difficulties Teller would later encounter in developing his fusion-based ‘Super” or hydrogen bomb. Initially, Teller assumed that the heat and pressure of a conventional nuclear weapon would be sufficient to induce fusion in liquid Deuterium or Heavy Hydrogen. When this turned out not to be the case, Teller and Polish mathematician Stanislaw Ulam were forced to develop an entirely different mechanism known as radiation implosion to initiate a fusion reaction – but that is a subject for another video.

Yet despite Bethe, Teller, and Konopinski’s reassuring calculations, some continued to harbour grave doubts, as Bethe later recalled:

“But of course, it spooked Compton. In [his] mind it was not set to rest.  He didn’t see my calculations.  He even less saw Konopinski’s much better calculations, so it was still spooking in his mind when he gave an interview at some point, and so it got into the open literature, and people are still excited about it.”

But what about Enrico Fermi’s infamous bet in the lead-up to the Trinity test? Well, according to Bethe, this was actually intended as a joke:

“Fermi, of course, didn’t believe that this was possible, but just to relieve the tension at the Los Alamos Trinity test, he said, “Now, let’s make a bet whether the atmosphere will be set on fire by this test.” And I think maybe a few people took that bet.” 

Among those who took Fermi’s bet were U.S. Army soldiers assigned to guard the test site, who apparently needed some serious instruction in logic, given that if that had actually happened, they’d never have been able to collect their winnings. Either way, some of these soldiers were so disturbed by the possibility of incinerating the world that they asked to be relieved from their duties prior to the test – provoking considerable fury from Kenneth Bainbridge.

But in any event, outside of Fermi’s facetious wager, the bets among the Manhattan Project scientists concerned the explosive yield of the test. Edward Teller, for example, predicted 45 kilotons of TNT. Physicist Isidor Rabi chose 18 kilotons – ultimately winning him the betting pool – Hans Bethe 8 kilotons, Robert Oppenheimer 0.3 kilotons, and Ordnance Department leader Norman Ramsey zero kilotons – a complete dud. The latter scenario was the one most feared by Bainbridge, who later wrote:

“My personal nightmare was knowing that if the bomb didn’t go off or hangfired, I, as head of the test, would have to go to the [test] tower first and seek to find out what had gone wrong.”

In the end, however, the test went exactly as planned, and of the more than 2,000 nuclear weapons that have been detonated since July 16, 1945, not one has set the atmosphere on fire. This includes the Tsar Bomba dropped by the Soviet Union on October 30, 1961, which at 50 Megatons remains the largest nuclear weapon ever detonated.

But like all good stories, the idea of atmospheric ignition simply refuses to die. In 1975, Horace C. Dudley, a professor of radiation physics at the University of Illinois Medical Center, published a letter of concern titled The Ultimate Catastrophe in the Bulletin of the Atomic Scientists, in which he reiterated Teller and Compton’s apocalyptic predictions from the 1940s. The letter prompted a rebuttal from Hans Bethe, who stated:

“There was never any possibility of causing a thermonuclear chain reaction in the atmosphere. There was never “a probability of slightly less than three parts in a million,” as Dudley claimed. Ignition is not a matter of probabilities; it is simply impossible…[furthermore] it is totally unnecessary to add to the many good reasons against nuclear war, one which simply is not true.”

Nonetheless, Dudley’s letter attracted considerable attention among U.S. Government policy makers, prompting Roger Batzel, director of the Lawrence Livermore Laboratory in California, to weigh in on the debate:

“I will not comment directly on the several pejorative comments made about nuclear energy production and weapons research. Nor will I attempt to clear up Professor Dudley’s confusion over variable half-lives, the availability of “aether energy,” the earth’s gravitational field, or the reproducibility of large-scale physical phenomena…In summary, extremely conservative calculations have demonstrated that it is completely impossible for either the earth’s atmosphere or sea to sustain fusion reactions of either thermonuclear or nuclear chain reaction type. In particular, such reactions cannot be triggered by the explosion of nuclear weapons, even those having unrealistically high yield and impractically high yield-to-weight.”

In response, Dudley published another letter proposing a number of other – equally unlikely – runaway chain-reaction scenarios. The debate was put to rest by Bernard Felt, editor-in-chief of the Bulletin of the Atomic Scientists, who wrote: 

“However, since Dr. Dudley chose in his rebuttal to give new emphasis to the possibility of a hydrogen plus hydrogen reaction in the ocean, Dr. Bethe would be fully justified in wishing to respond to this, thereby setting off a chain reaction which we could probably not contain.

Rather than risk this contingency, I take the liberty of noting that, contrary to Dr. Dudley’s assertion, the hydrogen plus hydrogen reaction does differ in kind from that of deuterium plus deuterium, to the extent that this reaction requires temperatures and pressures comparable to those occurring in the interior of the Sun. Dr. Bethe’s point about the impossibility of a fusion chain reaction in the oceans therefore remains well-taken.”

Ironically, within just a few years the debate over the effects of nuclear war would shift from one of global incineration to global refrigeration as scientists like Richard Turco and Carl Sagan developed the idea of “Nuclear Winter” – the hypothetical cooling of the earth’s climate due to smoke and dust thrown up by nuclear explosions blotting out the sun. But this, too, is a subject for another video. For now, let us be thankful that the apocalyptic nuclear Sword of Damocles which has hung over humanity’s head for 80 years has yet to fall en masse, and that theories of global incineration and nuclear winter remain firmly in the realm of speculation.