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The true story behind Oppenheimer’s atomic test — and how it just might have ended the world

It turns out there was an “unlikely” chance the first atomic bomb could have ignited the atmosphere — which didn’t stop the Manhattan Project.

A black-and-white photo of Senator Brien McMahon Conversing with Robert Oppenheimer Getty Images
Kelsey Piper is a senior writer at Future Perfect, Vox’s effective altruism-inspired section on the world’s biggest challenges. She explores wide-ranging topics like climate change, artificial intelligence, vaccine development, and factory farms, and also writes the Future Perfect newsletter.

In one trailer for Oppenheimer, the movie about the making of the atomic bomb releasing on Friday, Maj. Gen. Leslie Groves (Matt Damon) asks Manhattan Project leader J. Robert Oppenheimer (Cillian Murphy): “Are we saying there’s a chance that when we push that button we destroy the world?”

The chance, Oppenheimer assures him, is “near zero.”

Groves is not wholly assuaged. “Near zero?” Oppenheimer, frustrated, asks what answer he wanted to hear. Groves, of course, speaks for the audience: “Zero would be nice!”

I would love to tell you that concerns that the first atomic bomb would destroy the world were made up to add some tension to the film. But no, some of the scientists building it were genuinely worried about the possibility. In 1942, Edward Teller, the researcher who later invented the far more powerful hydrogen bomb, gave a presentation in which he observed that an atomic explosion would create temperatures hotter than the sun — and maybe create the conditions under which fusion reactions (which had been discovered only a few years previously and were still poorly understood) could occur.

The upshot: There was a chance they could literally ignite the atmosphere, killing everything that depends on it.

Teller’s presentation caused a stir. Some physicists emphatically rejected the possibility. Other well-pedigreed ones were not as persuaded it could be ruled out, given how much they still didn’t fully understand about how nuclear reactions would happen.

The Manhattan Project at Los Alamos in New Mexico commissioned a secret report, which concluded that this was “unlikely.” This set many fears to rest, but not all of them, and scientists kept rechecking their calculations up to the day of the test. Nobel Prize-winning physicist Arthur Compton — who later said that it would be “better to accept the slavery of the Nazi than run a chance of drawing the final curtain on mankind” — was among those who were less than sure right up to the moment of ignition. As the Manhattan Project’s physicists stood waiting for the test at Trinity site, he proposed, mostly jokingly, they place bets on whether they’d destroy life on Earth.

James Conant, then-president of Harvard University and a witness to the Trinity test, said later that when the flash from the test was unexpectedly much brighter and longer-lasting than they’d predicted, his instantaneous reaction was that they really had ignited the atmosphere and doomed the world.

We now know enough about fusion to know that nuclear bombs cannot ignite the atmosphere. But in his book The Precipice, existential risk researcher Toby Ord argues that the team at the time could not possibly have been wholly confident in their conclusions. Indeed, we know nuclear weapons scientists miscalculated from time to time: in one deadly mistake, the Bravo test of a hydrogen bomb, an explosion was much larger than calculated, exposing hundreds of people to radiation poisoning. (The scientists thought lithium-7 was essentially inert; in the Bravo explosion, a thousand times greater than that of Hiroshima, they learned that it was actually reactive at the right temperatures. Oops!)

It’s hard to feel like we got the Trinity one right — instead of just getting lucky.

How do you end up nervously joking about maybe ending the world?

What in the world moves decent, intelligent, careful, and thoughtful people — and many of the people working on the atomic bomb, including Oppenheimer himself most of the time, were decent, intelligent, careful, and thoughtful — to behavior that from the outside can look gravely irresponsible?

Ordinary people would presumably not agree to a scientific experiment with even a very small chance of destroying the world. That wouldn’t seem like an acceptable risk. We would want researchers to wait until they understood the science better and could be wholly confident that their project wouldn’t ignite the atmosphere.

Much of the answer lies in the geopolitical competition that the Manhattan Project scientists believed themselves to be in with the Nazis. The terrible logic of building the bomb was that if Hitler built it first, he could hold the whole world hostage and spread an ideology of unparalleled evil and destructiveness, so the only thing that mattered was getting there first.

That was the conviction in which the Manhattan Project was initiated. Of course, it eventually became clear that the Nazis were never close to completing an atomic bomb. In fact, by the time of the Trinity test — on July 16, 1945 — Germany had already surrendered. Even if taking risks with the fate of every single person alive was justified to stop Hitler, it had stopped being justified months before the Trinity countdown began.

If Oppenheimer leaves you with more questions than answers, Richard Rhodes’s The Making of the Atomic Bomb is a book I highly recommend to learn more about the Manhattan Project, the extraordinary personalities driving it, and how they made the decisions that eventually introduced atomic weapons to the world. It’s where I found my answer to this question, though it’s far from a fully satisfying one.

That answer is that they were too busy thinking about how to build the bomb to revisit the question of whether they should as the strategic situation changed around them. A project of the scope and scale of the Manhattan Project has stunning inertia. At extraordinary expense and great personal costs, under unimaginable pressure, the researchers had spent years of their lives building something wholly transformative and unprecedented.

Psychologically, they simply didn’t have it in them to quit their life’s work on the brink of completion just because the geopolitical justification they’d originally had was no longer valid, even if there were vague worries about igniting the atmosphere and more concrete worries about permanently changing the world for the worse.

They rechecked and rechecked their calculations, but they seemed to be thinking of matters as “we will go ahead with the test unless we discover that it’ll ignite the atmosphere” instead of “we won’t go ahead with the test unless we know enough about fusion to be absolutely confident it won’t,” much less “do we need to go ahead with the project at all now that the Nazis are beaten?”

Shortly after Franklin Roosevelt’s death in April 1945, new President Harry Truman was briefed for the first time on the bomb. He wrote later that FDR’s close adviser Jimmy Byrnes told him they “were perfecting an explosive great enough to destroy the whole world.” And also, of course, “the bomb might well put us in a position to dictate our own terms at the end of the war.”

A question of when, not whether

One gets the sense the latter consideration loomed larger. The Making of the Atomic Bomb characterizes Truman as impatient with being expected to read the long memos meant to bring him up to speed on the bomb project and laser-focused on its implications for the US/USSR relationship. Everyone moved on to deciding where to drop the bomb, presuming it worked; it’s not clear there was a single meeting in which they sat down and seriously discussed whether to go ahead at all. Aside from a few mavericks like the physicist Leo Szilard, who presciently warned that using the bomb would only encourage the Soviet Union to accelerate its own efforts, it was a question of when, not whether.

And that’s how you get the brightest minds in the world nervously joking that they hope they don’t end all life on Earth.

The people who built the Manhattan Project were absolutely brilliant. And so far, Earth has survived the introduction of their great invention. But this has always felt to me like a cautionary tale, not a triumphant one.

It’s easy to see why those physicists who were completely sure the atmospheric ignition was a fictitious worry went ahead with the test. But what about the ones who weren’t sure, and were joking about it nervously up to the last minute? Did they essentially let themselves get peer pressured into going ahead with a test that they thought might kill every person on the face of the Earth, for a reason (beating the Nazis) that no longer applied? Whose job was it, among genius scientists who were tasked with inventing a superweapon, to call it off if the benefits of a superweapon no longer seemed worth the risks?

If there’s an unrealistic part of Oppenheimer, it’s actually Maj. Gen. Groves, who in the trailer presses the scientist about whether even a small chance should be considered unacceptable. I have found no accounts that he, or anyone else outside the team of scientists who tried to check Teller’s calculations, ever seriously grappled with this worry.

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