Why Einstein Thought Nuclear Weapons Were Impossible

Now that we have nuclear weapons and nuclear power plants, you might think that it was always inevitable that we would be able to harness the energy inside the nucleus of atoms. But that was far from the case. In fact, serious scientists thought the idea was laughable, like Nobel laureate Robert Millikan, who in 1928 said, "There is no likelihood man can ever tap the power of the atom. The glib supposition of utilizing atomic energy when our coal has run out is a completely unscientific Utopian dream." Or, as Rutherford put it, "Anyone who expects a source of power from the transformation of these atoms is talking moonshine."

Now there was good reason for their pessimism. When Becquerel first observed radioactivity, he thought it was a phenomenon similar to phosphorescence. That's when you shine radiation like light onto an object, and it absorbs that energy and later reradiates it in a different part of the spectrum. Now, uranium ore was known to do this, as I witnessed firsthand. Oh, yeah, X! Yes, fluorescent uranium ore! Yeah!

Here, it's absorbing UV light and reradiating it as visible light. In 1896, Becquerel performed experiments where he placed uranium ore in the sunshine on top of some wrapped-up photographic film. He found that the film was exposed seemingly by invisible rays from the uranium ore that penetrated the paper when the uranium was excited by the sunlight. But one day, when he went to do his experiment, the weather in Paris was lousy, so he put the uranium and the photographic film inside a drawer. A few days later, even though the uranium hadn't seen the sun, he decided to develop the film anyway. And what he found was that the photographic film had been exposed just as before, even though the uranium was not excited by sunlight.

So, this was not a phosphorescence phenomenon; some type of radiation and therefore energy was coming out of a rock unprovoked. But how could a seemingly inert object like a rock give off energy? Where is it getting that energy from? It was a mystery that seemed to violate the law of conservation of energy. That is until Einstein published his famous E=mc², which suggested a source of energy for the might be the mass of the nucleus. Just a tiny bit of mass can give you a lot of energy, and this premise was enough for science fiction writers to let their imaginations run wild.

Like H.G. Wells, who in 1914 published the book The World Set Free, which includes the first mention of the words atomic bomb. He envisioned a uranium-based hand grenade that would continue to explode indefinitely. But to scientists, this was completely detached from reality. As Einstein in 1933 put it, "There is not the slightest indication that nuclear energy will ever be obtainable. It would mean that the atom would have to be shattered at will."

And that's just the thing; people had no ability to make a nucleus do anything. All we were observing was the natural process of radioactive decay: atoms of a particular unstable isotope decaying at random with some characteristic half-life. And the energy given off, although immense on the scale of an atom, is pretty insignificant on the scale of people and the world. The fission of a single uranium atom releases twenty times less energy than the amount required to raise a grain of sand the thickness of a piece of paper.

Now, up until 1932, the only known particle in the nucleus was the proton. So, if you wanted to alter a nucleus, you could conceivably fire a proton at it. But since the nucleus and the proton are both positively charged, they repel. So, you'd have to fire the proton in with such high speed and accuracy to get it to hit and stick to a target. And even then, if you're successful, you've only affected one nucleus, which at best can't even lift a grain of sand. So, you can see why the Nobel Prize winners were saying nuclear weapons not gonna happen.

But then comes the discovery of the neutron, and the neutron changes everything. Because as an uncharged nuclear particle, it can drift ghostly undeflected through matter until it hits a nucleus, transforming it into something else. And this leads to the epiphany of a man named Leo Szilard.

Now Szilard read The World Set Free, so he's already imagined a future in which nuclear energy is harnessed by weaponry. And he remembers the exact moment he comes up with this idea as he's crossing the street in London. He says, "It suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbed one neutron, then such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction."

In other words, the neutron enables us to trigger nuclear reactions at will. And if there's a nucleus which when it splits in this way releases two neutrons, it could trigger more and more fissions at an exponentially increasing rate. The nucleus that has this property is uranium-235. In fact, on average, it releases two and a half neutrons every time it divides.

Now, all of a sudden, you have the possibility of splitting zillions of nuclei simultaneously, releasing incredible amounts of energy all at once. That's an atomic bomb. Now, if you want more control over this release of energy, as in a nuclear power plant, well, then you have to absorb a few neutrons so that the fission of one nucleus only causes the fission of one other nucleus on average. Then you have a steady chain reaction that emits the same amount of energy each instant.

The challenge is that this is like balancing on a knife-edge: absorb too many neutrons and the chain reaction quickly decays to nothing; absorb too few and the rate of reactions increases exponentially, and soon you're back to a bomb or Chernobyl. So, if not for the existence of the neutron, a neutral nuclear particle to trigger reactions that occur in greater numbers relative to protons in the larger nuclei, meaning they're likely to be given off when a large nucleus splits, well, then maybe, as many brilliant scientists suspected, it would be impossible to harness the energy in the nucleus.

But as it is in our universe, the neutron is the hero or the villain of nuclear physics.

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