In September of 1933, a physicist named Leo Szilard was crossing the intersection where Southampton Row passes Russell Square when he had the idea of a nuclear chain reaction — the key idea behind atomic bombs.

There was, from there, a whole adventure as Szilard tried to figure out what to do with this momentous idea. He went to the more prestigious physicist Isidor Rabi, and Rabi went to the even more prestigious Enrico Fermi. Rabi asked Fermi whether he thought nuclear chain reactions were the real deal, and Fermi sent back a reply:

Nuts!

Rabi asked Fermi what “Nuts!” meant, and Fermi said that it was a remote possibility.

Rabi asked what Fermi meant by “remote possibility,” and Fermi said, “10 percent.”

To which Rabi replied, “10 percent is not a remote possibility if it means that we may die of it.”

Fermi reconsidered.

There are a few different morals one could take away from this story. A moral we don’t take away is, “Every remote possibility is worth worrying about if we may die of it.” There’s nothing “remote” about 10 percent, but if the possibility were sufficiently remote, then it would simply not be worth thinking about.

One moral that we do take from this story: It is sometimes possible to realize that a technology such as a radioactivity cascade is possible, and thus know (before everyone else) that the world is set for some sort of drastic change.

Another moral that we take from this story is that one’s initial intuitions are often not a good guide to anticipating and thinking about drastic changes. Not even if one is a renowned expert in the relevant field, like Enrico Fermi was.

Consider: Where did Fermi even get that “remote possibility” and “10 percent” stuff from in the first place?

Why did Fermi think that you couldn’t get radioactivity to induce more radioactivity in a chain reaction? Was it just that most big ideas don’t pan out?

Replying “Nuts!” seems to be saying something stronger than just that. It seems to reflect a sense that this particular big idea was excessively unlikely to pan out. But why? On what physical argument?

Did it just feel crazy? Yes, the possibility of nuclear weapons would have radical consequences for the world. But reality is not arranged so as to prevent events with large consequences from ever happening.^*

When Fermi first heard Szilard’s idea, he suggested that Szilard publish it and let the whole world know about it — including Germany and its new chancellor, Adolf Hitler.

Fermi lost that argument, and well that it was so, for nuclear weapons turned out to be possible after all. Fermi did ultimately join Szilard’s tiny conspiracy, though he remained a skeptic almost until the moment when he himself oversaw the creation of the first nuclear pile, Chicago Pile-1.

Sometimes, technologies upend the world. If you take for granted that radical new technologies are “nuts,” you can get blindsided by progress, even if you’re one of the smartest scientists in the world. It is a great credit to Fermi, then, that he sat down and had the argument with Szilard. And an even greater credit, that he was persuaded to change his behavior before the technology existed, before he could see it with his own eyes — when there was still time to do something about it.

A very large number of awful things have happened over the course of human history — but some of the awful things that haven’t happened were avoided because somebody sat down and had the conversation. Forced the conversation, in some cases, as Szilard did with Fermi.

Notes