The bomb (part 1): were nuclear weapons inevitable?

0:00The more you listen to your kids, the closer you'll be. So we asked kids, what do you want your parents to hear? I feel sometimes that I'm not listened to. I would just want you to listen to me more often and evaluate situations with me and lead me towards success. Listening is a form of love. Find resources to help you support your kids and their emotional well-being at SoundItOutTogether.org. That's SoundItOutTogether.org. Brought to you by the Ad Council and Pivotal.

0:36The Economist

0:42On July the 16th, 1945, at 5.29 in the morning, an audacious experiment took place in the deserts of New Mexico. It was the culmination of five decades of theories, arguments and experiments amongst physicists who'd been trying to crack a scientific mystery. What lay inside atoms? On that summer morning, 80 years ago, it had all led to this. First the flash of light, that enormous fireball, the mushroom cloud rising thousands of feet in the sky, and then a long time afterwards, the sound.

1:31The rumble, thunder in the mountains. An explosion bigger than anything anyone had ever seen before. It was the dawn of the nuclear age.

1:59Ever since that day, the world has been grappling with the devastating power of nuclear weapons. Warfare, international security and geopolitics have never been the same. Whatever you think of them, these doomsday weapons are here to stay. For 80 years, they've maintained order around the world, albeit in an uneasy, controversial way. And today, as political tensions rise, so does their importance.

2:34Over the next four weeks, in a special series for Babbage, we'll investigate the past, present and future of nuclear weapons. We'll speak to the people responsible for building America's stockpile of warheads. Find out how those weapons are being modernised. how they're being tested without being exploded, and how, for the first time in decades, new weapons are being developed. What started in the desert with the Manhattan Project in 1945 has now turned into a vast technological enterprise with thousands of scientists and engineers spread across dozens of sites around America. Those facilities include the world's most powerful lasers and its fastest and most capable supercomputer.

3:23And as they push the frontiers of high-energy physics and high-tech materials, America's nuclear scientists are also building tools that have uses beyond weaponry, from fusion to advanced manufacturing.

3:40I'm Alok Jha and this is The Bomb, a special series of Babbage from The Economist. Today, episode one, after the birth of nuclear physics, were doomsday weapons inevitable?

4:02To understand where nuclear weapons came from, you first need to understand the nucleus. You see, this story doesn't start with scientists wanting to make a weapon. It begins instead with a curious and accidental discovery 130 years ago in an academic research lab in Paris. Back then, scientists still didn't really know that much about the most basic building blocks of matter. Well, at that stage, chemistry was pretty well established and the idea that matter was made of atoms was the perceived wisdom, if you like. Frank Close is a physicist and the author of a new book about the birth of nuclear physics called Destroyer of Worlds.

4:46The picture of the atom was that all atoms of a given element were identical, they had no structure, they were permanent, unchanging, like miniature billiard balls, if you like. And the only difference between an atom of one element and another was its mass, that hydrogen atoms were the lightest, they were number one in what became known as Mendeleev's periodic table. helium-2 all the way up to uranium at 92, the heaviest then known. And if you combine atoms of one or more elements together, you've made molecules and compounds. And that was the picture of the world of matter as it was in 1900 or so.

5:26And how complete did scientists think it was? Did they think that that's it now? We know that's what atoms are. We're not going to investigate that any further now. We're just going to look at what compounds they can make and all of that. Or was there hints that that wasn't the base level of matter? Well, in 1896, Henri Becquerel in Paris discovered by accident that minerals that contained uranium spontaneously radiated energy. And he discovered that by having left a mineral on a photographic plate in a darkened drawer, and he had been hoping to illuminate it with sunlight and generate fluorescence, but it was very dark in Paris that February, and after a couple of weeks he decided to expose the plates anyway, and to his astonishment he found this very faint smudge, which showed that uranium was emitting energy even in the dark, which was a sensation.

6:28By that I mean Becquerel had discovered a phenomenon, that atoms were somehow liberating energy, and Marie Curie and her husband immediately picked up on this and they discovered that the ore that they had was emitting radiation more than just the uranium content alone could explain. And so radioactivity was a phenomenon that seemed to say that atoms could emit energy continuously. Ernest Rutherford in Canada discovered that in the process of this radioactivity, elements did indeed change from one to another. So this for the first time showed that atoms were not permanent and unchanging, and indeed that the dream of the alchemists, if you like, had been finally achieved by the ability to change one atom of one element into another.

7:29What was going on, nobody knew, but clearly something was going on.

7:37So how did the researchers go on to investigate why this seemingly magical process of atoms sort of transforming and changing, why that was happening? The first clue, perhaps, was the fact that in 1897 in Cambridge, J.J. Thompson discovered that inside the atoms of all elements are electrically charged particles, electrons, the carriers of electricity, negative electricity. And the fact that atoms overall tend to be electrically neutral showed that there must be some positive charge in there to balance it out. So that was the state of knowledge in the early 1900s. it was clear that atoms could somehow radiate energy and it quickly became clear that the source of radioactivity was in the nucleus.

8:30And in the case of radium, if you could have a gram of radium, that had been emitting energy, as far as they could tell, continuously ever since the start of time.

8:44and so the amount of energy that was contained in that little lump of radium would be enough to drive a ship all the way across the atlantic ocean which indeed is true if you're prepared to spend a hundred years making the journey because the catch of all this was that although atoms are indeed spontaneously liberating this what we now call nuclear energy, it was doing it so slowly that there was no obvious way that one could put it to use. So this was the enigma at the start of the 20th century. Energy is coming out, what's causing it? Is it possible to speed it up and make use of it?

9:30The picture that was emerging was that atoms weren't really like miniature billiard balls, as had once been thought. They contained even smaller particles inside. And these subatomic particles had electrical charges. Since the electron was negatively charged, and atoms were neutral overall, it meant that the radiation-emitting nucleus had to be positively charged. But how were all these different bits of an atom actually arranged? Ernest Rutherford is the person who really started, and he focused on the radiation itself. What he did was he deduced that there were, in fact, two distinct forms of radioactivity, and he named them after the first letters of the Greek alphabet, alpha and beta.

10:21Alpha radiation, he established, was carried by positively charged particles. And the other form of radiation, beta radiation, was nothing other than very flighty, lightweight electrons, negatively charged.

10:41When Rutherford established that alpha particles are nothing other than the positive seeds of helium atoms, and helium is number two in the periodic table, it meant that every time an atom of, let's say, uranium, number 92, if that emitted an alpha particle as radioactivity, it had given up two units, moving uranium down to number 90, which is thorium. So each time an alpha particle would be emitted, you would move downwards in a chain from uranium to thorium and down, eventually ending up at lead at number 82. So over the period of 1900 through 1910, the radioactive ladder, if you like, began to be established that alpha particles would shift you from one element to another, moving you two places up or down the periodic table.

11:34Beta radioactivity turned out to shift you one place up or down the periodic table. Talk me through how Rutherford discovers the nucleus and what is surprising to him about the nature of the nucleus? Rutherford knew there must be positive charge somewhere inside the atomic nucleus. He also knew that alpha radioactivity emits positively charged, relatively massive particles, the alpha particles. And he realised that he could use these, if you like, as atomic bullets to fire at atoms of gold, for example, and see what happened when they arrived. A bit like having a fire hose pointing at some object hidden behind a screen, and from the way that the water then sprays about after hitting the object, you try to work out the shape of the object.

12:35What was he expecting to find and what did he find? Well, I think he was probably expecting that most of the alpha particles would pass through with just a slight deflection. What he found was that when these alphas hit just a very thin piece of gold leaf, which is so thin you can almost see through it, about one in 10 ,000 alpha particles bounced off at about 90 degrees, which is remarkable. ball. And so what was the implication of that? Well, it showed that the positive charge must be very tightly concentrated. Think of the alpha particles as being like a lightweight rubber ball and the atomic nucleus of gold we now realise being like a heavy football.

13:20And so when the light ball hit the football, the light ball bounced off at a violent angle. He said it was as if you had fired a 15-inch shell at a piece of tissue paper and it had come back and hit you. He was a clever man but he wasn't expecting that. So he discovers that there's something positive which is why the alpha particles are being scattered in this way, reflected actually in some cases. So that builds the picture of the atom as an electron sort of floating about the outside and a very dense positive thing in the middle. Is this where the beginning of the children's idea of an atom comes from, which is a sort of mini solar system.

13:56This was indeed where Niels Bohr, the great Danish theorist who was visiting and working with Rutherford in Manchester, had the idea that the atom could be visualised as this miniature solar system of the negative electrons whirling around the positive lump in the middle.

14:17And nobody knew what the positive lump consisted of, but that was the picture that was established by 1913. Okay, so you've got this positive lump, the nucleus was there. What did he do next to try and sort of understand what was inside it? Well, of course, what happened next was that he came to the insight that there is a unit of positive charge in the nucleus, which he called proton. And the idea was that hydrogen, the simplest atom, consists of one proton in the nucleus, entrapping one electron on the outside. and that helium then would have two protons entrapping two electrons. So that was where he was around 1919, but they had also been able to measure the relative masses of these atoms and helium, for example, instead of being twice as heavy as hydrogen, was about four times.

15:16And as they worked at the periodic table, they discovered that although you went from one element to the next and the charge on the nucleus increased by one, the mass of the nucleus increased much faster. And this gave Rutherford the idea in 1920, which he mentioned in lecture at the Royal Society, that there could also be a neutral member in the nucleus. A third particle in the atom. Yes, the electron on the outside, now a proton and a neutron, as he called them in the middle. This might not sound very surprising to us today, but realise that back then, Rutherford has just increased the number of known particles by 50%.

15:55You know, this is quite radical. But that was the idea in 1920. He had this idea of the neutron, but didn't know what it was exactly. And it took about a decade before the neutron's nature was finally discovered. And it was one of Rutherford's students, essentially, one of the people in his laboratory who did it, James Chadwick. But before we get to him, I wonder if you can talk to me about some of the other people involved who also had some thoughts about what the neutron might be and who didn't quite get to the end before Chadwick got there? Well, the story here, I think, really comes alive in the 1930s.

16:30Irene Curie, the daughter of Pierre and Marie Curie, and her husband, Frédéric Joliot, in Paris, fire alpha particles at the nuclei of elements like beryllium and other elements at the lighter end of the periodic table and were saying that photons were coming out of the beryllium nucleus far more energy than you could explain at all.

16:56They were saying photons were coming out. They were saying that photons were coming out. All they really knew was that they fired an alpha particle in which hit beryllium and then downstream something had come off from the beryllium and all they knew about this something was that it was electrically neutral. And so they assumed that it was photons because that was the only electrically neutral thing that they knew of. They hadn't read Rutherford's 1920 lecture proposing the neutron, ironically. But then in Italy, a brilliant student working with Enrico Fermi in Rome, a theorist called Ettore Majorana, said, well, that's the neutron.

17:38They're absolutely stupid. There's no way that you can produce photons out of this, violating the conservation of energy by orders of magnitude. They have produced a massive neutral particle, the neutron, with the same mass as the proton. And Fermi said to him, you've got to publish that. Mayor Arna said, no, no, that's so obvious, so stupid, I don't want to publish that. So he didn't, but actually he was right. And it was then when Chadwick, the assistant of Rutherford at Cambridge, read the paper that the Joliot-Curis had written, in which they reported what they thought to be photons, that Chadwick too realised, just a second, this has to be a massive particle.

18:19And he went to see Rutherford and Rutherford said, they're absolutely wrong. And so Chadwick worked day and night for about three weeks and finally established the existence of the neutron as a massive neutral particle with a mass very similar to that of the proton and announced it in February 1932. And we now know that the neutron indeed is the spark that would light the nuclear fire, both literally and metaphorically.

18:51The neutron would turn out to be critical to the eventual development of nuclear weapons. But when it was discovered in the mid-1930s, scientists were still scratching their heads about how they might actually be able to liberate the enormous energy locked up inside the nucleus. It had only been a couple of decades since scientists had realised that the energy leaking out of the nucleus through radioactivity was potentially vast, but it came out very slowly. Remember, there was enough energy there to propel a ship across the Atlantic Ocean, but it would take over a century. To think about turning it into a useful source of power then, the nucleus would have to release its energy much more quickly.

19:37The path to that really began with Enrico Fermi and his group in Rome in 1934. What had happened was that in Paris, the Jolio Curis had been irradiating atoms with alpha particles and managed to change the nature of those atoms' nuclei. They discovered they could induce radioactivity in atoms that had previously been completely inert, like atoms of aluminium, which had existed for five billion years. Just by firing alpha particles at them? Just by firing alpha particles at them. And I would actually argue that, second only to the discovery of the atomic nucleus itself, this was the real breakthrough that enabled us to get in the atomic nucleus and start liberating the energy.

20:30Now, they were using alpha particles which are positively charged, and positive charges on the nucleus would repel those alpha particles like charges repel. So there's this electrical shield stopping the alphas getting in. So the John O 'Curys were only able to irradiate a few atoms at the lower end of the periodic table. The further you got the periodic table, the bigger the nucleus is, the bigger the electrical shield is, and the other particles just couldn't get in. So alpha particles could induce radioactivity in lighter elements, but it didn't seem that they could do it in heavier ones. That's because heavier atoms have more protons in their nuclei, and therefore more electrical shielding against the positively charged alpha particles.

21:26It was Enrico Fermi who came up with a solution to the problem. Fermi had the idea of using neutrons. The neutron has no charge, there's no electrical barrier at all. Instead of firing alpha particles, which are positively charged at the atoms, why not try using neutrons instead, which have no electrical charge? So Fermi starts irradiating with neutrons, he reproduces the same phenomena that the jolly O 'Curis have done, which is great, But he can now carry on all the way up the periodic table because there's no electrical barrier at all. And he gets to the heaviest element of all, uranium at number 92.

22:06And when he radiates uranium, he finds some sort of signals which he cannot interpret.

22:17Actually, he probably split the uranium in two. But that was 1934, and thankfully, he didn't realise what he had done because that would be the key to atomic weapons. And had it been known five years before the Second World War started, we would not be having this conversation today.

22:38In the late 1930s, many physicists were trying to explain the strange results they'd been seeing when they'd fired neutrons at uranium. We now know that indeed Irene Curie and Pavel Savic, her assistant, had been doing similar experiments in 1937 and also found strange signals. Curie found, she thought, lanthanum, number 57, in the periodic table, far, far, far away from uranium, number 92. Everything that was known about radioactivity was that it shifted you one or two at most. You know, 92 could get you to 90, but not 57. So Irene Cuey didn't understand this and just reported it and did no more.

23:241938, in Berlin, Hahn and Straussmann doing similar experiments and Hahn finds Barium. Now Barium, I think, is number 56 next door to Lanthulham and he doesn't understand it either. The explanation finally came from two of Otto Hahn's long-time collaborators, Lisa Meitner and her nephew Otto Frisch, both Jewish physicists who'd escaped Germany. The thing that had been overlooked in all of this was that the atomic nucleus has a lot of electric charge, positive charge. So if it was deformed very slightly into a sort of dumbbell, the two opposite ends of the dumbbell would each be positively charged and would be repelling one another.

24:13and so the possibility suddenly was that the dumbbell would split in two which can explain why you started with uranium at 92 and you end up with things in the 40s or 50s of the periodic table chance would determine how big the two ends of the dumbbell happened to be but that could be what was going on and they did the calculation and discovered indeed that the amount of energy released was vast that everything about this hypothesis fitted what was being seen. Meitner and Frischer's theory of how atoms split became known as nuclear fission. So in a lump of neutrons and protons making a nucleus, there's a certain amount of energy trapped.

24:57And what nature is trying to do is to lower the amount of energy trapped in there into the most stable form, which means radiating off the excess. And that is what the origins of radioactivity are. But that, we now know, was only the start of it because the idea of what was called the chain reaction was out there, that one neutron coming in would split uranium in two, would also liberate two more neutrons. And you could then imagine those two neutrons hitting two more uranium nuclei, liberating energy, and four and eight and 16, quickly, within about a thousandth of a second, You could have 80 replications of these suddenly explosively coming out.