The history of nuclear bombs

We’ve been developing different ways and levels of harnessing atomic energy in weapons for the past 80 years. But our discovery of the atom and its potential to release massive amounts of energy goes back a lot further than that. Frank Close is a physicist at the University of Oxford. I asked him how we went from the discovery of atoms, to the creation of the most powerful weapons ever made.

弗兰克- 19世纪末,化学was a pretty well-defined science, and the idea was that all matter is ultimately made of atoms. And the belief was that the atoms of elements were all identical, that they were permanently unchanging, and there was nothing smaller than them. But it was in the 1890s when the discovery of what we now call radioactivity occurred completely out of the blue. That the whole of the, what I would call the nuclear age ultimately began. Henri Becquerel in Paris discovered that salts of uranium spontaneously emitted some radiation, which would fog photographic plates that were hidden in the dark without any prior stimulation. And it was the Curies in France who really got to work on this and discovered that there were other elements also, which had this phenomenon of pouring out energy continuously. And it was the Curies who gave the name radioactivity to it. And it's there that the energy locked inside the atomic nucleus being released in radioactivity was the origins of this weird energy that had been seen in radioactivity. And the question of course then emerged as there is indeed this huge reserve of energy buried inside atomic nuclei. Is it possible to extract it and make use of it?

Chris - Who had that insight that said, 'well, there's all this energy locked up in there. We know it shows its hands sometimes with some of these naturally occurring radioactive elements', but who was it who began to say, can we get at it?

Frank - The first measurements that showed that it was indeed huge in magnitude was indeed due to Ernest Rutherford around 1900. When he did the sums, he found that the amount of energy that had been liberated, say by some amount of matter, was millions of times more than chemistry could understand. And so it was Rutherford's measurement of the energy emitted in radioactivity that gave the first clue that there were vast amounts of energy inside atoms. It was only after he had discovered further the existence of the atomic nucleus that then became clear that it was the nucleus where this energy resided.

Chris - That is the turn of the century. We know that the bomb goes off in the 1940s. So what fills that gap? People like Rutherford speculate, we might be able to harness this energy, but how did they go about it?

Frank - By the 1920s, Rutherford and the beginnings of nuclear science had identified that the nuclei of all atoms are made of protons and neutrons. The radioactivity we now know was rays of particles called alpha particles. And by beaming these alpha particles at the nuclei of atoms, they discovered that these nuclei changed. They could break up and in the process liberate some energy. The problem was that it was so slow, there was no obvious way that you could ever make use of it until 1932ish, I think, when at Cambridge they built the first of what began to be known as atom smashers. And in the course of doing that, they discovered that significant amounts of energy were released when you smashed these things apart.

Chris - So who did what next? Because I've been to Argonne National Lab and I'm pretty sure that there's a copy of a letter there, and I think Einstein penned it to the president of the US setting out what could be done with this sort of physics. And what was the thinking that led to him penning that letter and then what had been happening in the background for people to begin to say, 'right, well, let's try and harness this and make this into a weapon'.

Frank - The discovery of nuclear fission took place at the end of 1938. The thing is that to produce, let's call it explosive amounts of energy, ordinary uranium is no good. There are two varieties of uranium for our purposes. One called uranium 238, and one called uranium 235. And it's the 235. That's potentially the explosive sort. The problem is only seven atoms in every thousand tend to be uranium 235. So to make an explosion, you have got to somehow increase the percentage of uranium 235, in the jargon that's known as enrichment. Surprisingly, it seems, nobody asked the question, but if somehow you could make a lump of pure 235, how much would you need to make an explosion? And the first people that did that was Rudolf Peierls and Otto Frisch at the University of Birmingham in 1940. And to their astonishment, they discovered that a lump about the size of a grapefruit would be enough to have an explosive power equivalent in their words, to a thousand tons of dynamite. And they also realised that it would produce radiation, which would be potentially lethal. And this was a weapon that would completely change the nature of warfare. That was when everything moved across to North America into what became known as the Manhattan Project, and the British team went across there as part of that, which eventually led to the development of the atomic bombs at Los Alamos.

Chris - And how does Oppenheimer get involved?

Frank - Oppenheimer is popularly described as the father of the atomic bomb, which in my opinion is the wrong metaphor. I would say he's more like the midwife. It was Oppenheimer by directing this huge program of scientists and engineers in those last years, 1943 to 45, that led eventually to atomic bombs being produced. And so Oppenheimer was in charge of the direction of these teams of scientists and specialists that led to the construction and eventually detonation of these weapons.

Chris - So that was splitting atoms. We dwelled very heavily on splitting atoms and fission so far. So what was the insight that led to someone saying, well, let's fuse them together and make an even more powerful bomb?

弗兰克,你可以这样做was first articulated in a discussion between Edward Teller and Enrico Fermi in the states around 1942. I think it was the conventional, what we call atomic bomb, where you fission uranium. These neutrons that are being produced, which are essential to making the chain reaction happen, they escape the lump before they can really do their stuff. You need a large lump so that the neutrons keep producing energy before the lump is destroyed. A fusion weapon, a hydrogen bomb, if you like, in principle, there is no upper limit to it. I mean the sun, for example, shows if you had enough hydrogen, you could have something burning away like the sun. A big technical question is the sun has been burning for 5 billion years. Can you speed the whole process up so that it liberates its energy in a fraction of a second? It was Enrico Fermi who was present at the Trinity test of the first atomic bomb. He had the insight. In the process of the explosion, the atomic bomb produces a temperature of tens of millions of degrees. And the key thing to making fusion happen is that you need such temperatures. And from his insight was in the moment of the conventional atomic explosion that creates the heat. And so he saw the possibility that an atomic explosion could provide the spark to ignite the fusion bomb. The technical features of making the hydrogen bomb however, are incredibly complex, and there's still a huge amount of secrecy over them. It wasn't until the early 1950s that some of the key steps really were found enabling the thing to be done. And much of that is still secret today.