The Girl Who Fell From the Sky (15 page)

At last there was some reaction from him, some glimmer of shame and anger in his look. ‘And what is that?’

‘You’re a cold fish, Ned. You don’t understand the basic human decencies. You shun the parents, and now you’re shunning me. Soon you’ll have nothing left except your stupid bloody physics.’

There was silence. They stood there in the garden with the wreckage of their relationship between them, like two children looking down on a broken toy. Ned glanced over his shoulder, as though it were all his fault, as though he had smashed the thing in temper and the adults were coming to see what the fuss was all about. But there were no adults around, nobody at all, only the trees in the garden and the blank windows of the houses that surrounded the square.

‘All right, I’ll tell you,’ he said. ‘If that’s the only way you’ll see how important this all is. I’m putting you in danger, even more danger than you were in before, but I’ll tell you. Clément was part of Fred Joliot’s team at the Collège de France, you know that. Well, they were working on the idea of an atomic bomb.’

Time, that flexible dimension, stopped. She thought of Ned’s jokes – death rays, devices that could see in the dark, bombs that could blow up whole cities. And the silly games they’d played – Pig-in-the-middle,
Kriegspiel
, Consequences. ‘An
atomic bomb
? Are you serious?’

He laughed, that little snorting dismissive laugh that so annoyed her. ‘Of course I’m serious.’

‘Clément was working on an atomic bomb?’

‘That’s what I said, isn’t it? He’s still there, still in Paris, and as far as anyone knows, still working at the Collège de France.’

‘And working on a
bomb
?’

‘Who knows if that’s what he’s still doing? But he was.’

‘You mean it could happen? Some sort of super-bomb?’

‘It’s easy. That’s what makes it so frightening.’

‘Easy?’

‘Uranium. You must have heard me talking about it, that Christmas before the war. Everyone was talking about it at the time. If you fire a neutron at a uranium nucleus it splits apart into two new atoms – different elements. Barium and krypton.’

She remembered now. ‘You and Clément came home for the holiday and we all went to the chalet in Megève. We wanted to ski but all you and Clément did was talk about science. I remember no one understanding a word you were saying. Daddy said it sounded like alchemy, turning base metal into gold. The philosopher’s stone. “What will you physicists come up with next?” he kept asking. You got angry with him.’

‘As always, he was missing the point. He thought it was a joke, some kind of esoteric conjuring trick. That’s the trouble with diplomats. They’re all classicists. Not a scientist among them. Any scientist would have realised how fundamental it was.’ Fundamental was one of Ned’s words. He could batter you into submission with it. ‘It was totally unexpected, this splitting. I mean, really amazing. As startling as firing a peashooter at a diamond and – ping! – the diamond splits open … and becomes two new jewels altogether. Ruby and sapphire, say. And at the same time energy is released, a massive amount of energy.’

‘But wasn’t all that done in Germany? What’s it got to do with Clément?’

‘Hahn and Strassmann were the first to publish, in December 1938. Yes, they were in Berlin. But Irène Curie and Pavel Savitch had got exactly the same experimental results a year before at the Radium Institute in Paris, only they hadn’t interpreted them correctly. I must have told you about this at the time.’

‘We were hardly listening, and when we were we didn’t really understand.’

‘It’s not that difficult.’ He looked impatient, almost angry. ‘That’s the trouble with people. They just don’t
try
to understand. You see, atomic nuclei are held together by huge forces, and at the time everyone thought that they couldn’t come apart
like that. But they can. If the nuclei are big enough, they can. And when they do the energy equivalent to those forces is released. Then Fred’s lab showed something more: when this fission takes place – that’s what they call it, fission – as well as the energy it also emits neutrons. These neutrons will then hit
other
uranium atoms and cause them to split as well. If each decaying atom releases at least
two
neutrons then those neutrons could hit two more uraniums, making them split up in turn. You understand the idea? Atomic billiards, but each single collision creating the possibility of
two
further collisions. You’d get a cascade of uranium atoms splitting up, one causing two others, two causing four, four causing eight, and so on. An exponential increase. They call it a chain reaction. Joliot and his team showed that it would happen. Not that it might happen – it
would
happen.’

She was used to conversations like this. Ned had tried to explain his world to her many times. It seemed a bizarre place, of nebulous ideas and cloudy realities. Remember, he’d told her, the atom is mainly nothing at all, a hard nucleus, where all the matter is concentrated, with acres of empty space all around it. If the nucleus were the size of your fist – he’d held up his own – then the outer limits of that one atom, the outer edge of its emptiness, would be about half a mile away. Reality is so much empty space.

‘And this chain reaction makes a bomb?’

‘Think of the energy,’ he said. ‘When a uranium nucleus splits, you’ve suddenly got two nuclei right next door to each other.’ He made a ball of his hands, fingertips touching, and then collapsed the ball into his two fists. ‘But nuclei shouldn’t be close together like that. They should be—’

And suddenly she saw it. ‘Half a mile apart! No, twice that. They should be a mile apart!’ She almost laughed. She saw into his world for the first time: the pure outrage of having two nuclei so close together was something shocking, against nature.

He nodded, as though it was obvious. ‘They should be a mile
apart and instead they are
touching
. So they fly apart at colossal speeds to take up their correct distances. We don’t know exactly how fast they move. Maybe a tenth of the speed of light. The energy involved is vast. We talk of electron volts. Each uranium atom that splits apart like this releases two hundred million electron volts of energy. That’s …’ He seemed to scratch around for a way of saying it. ‘Oh, tiny, useless, enough to move a grain of sand. You can’t do anything with it, not in practical terms. But each kilogram of uranium contains a vast number of atoms – imagine twenty-five with twenty-three zeros after it, that’s the number. If all the atoms split one after another in this chain reaction, you have to multiply the amount of energy released from each nucleus by the total number of atoms. Suddenly you’ve got an immense amount of energy. Do you see what I mean? Potentially unlimited.’

She thought of Clément trying to explain his work to her. It’s exactly like
Kriegspiel
, he had said: groping in the unseen with incomplete information and trying to find out what’s possible. And what was possible was some kind of bomb. She remembered the very last time they’d been together, at Easter time in Paris with her father and Ned, shortly after her seventeenth birthday. They’d walked close together. Occasionally their hands had touched. You mustn’t be frightened, he’d said to her.

‘You’re not listening, are you?’ Ned was saying. ‘You’re not paying attention.’

‘Yes, I am.’

‘If you want to understand, you’ve got to listen.’

‘I
am
listening.’

‘The point is, you must have a sufficiently large mass of uranium. That’s crucial. Remember what I’ve told you before: atoms are mainly nothing at all. The nuclei are like dust motes in an empty room, hard to hit and far apart. Neutrons can go a long way before encountering one, so if you haven’t got enough mass the neutrons simply escape into the air
before they actually hit other atoms. Francis Perrin, another man in Joliot’s team, made an estimate of how much uranium you’d need to guarantee that the chain reaction happens. He called it
la masse critique
. He calculated forty-four tons. Or, with a casing that could reflect escaping neutrons back into the mass, a mere thirteen. He published that in the
Comptes Rendus
so it was completely open to the public. All that I’ve told you was published before the war for anyone to read and work it out for themselves. However, a short while later Joliot’s group filed a secret patent with the Caisse Nationale de la Recherche Scientifique entitled
Perfectionnements des charges explosives
. It’s a patent on how to make an atomic bomb.’

It was a beautiful day. It should have been cold and miserable, threatening a storm. But instead the sun was shining, and leaves were glistening in the light.

‘And all this work was done in Paris?’

‘All in Paris. At the Collège de France, and at their other lab at Ivry. You came to the Collège once with
Papa
, don’t you remember?’

‘Of course I do. We took you and Clément out for lunch.’

‘It wasn’t the happiest of meals.’

‘You got angry over the slightest thing.’

‘I got angry over the way Clément pandered to you.’

She hadn’t understood at the time, but now she did: the intricate complex of Ned’s jealousies. After lunch they’d walked along the
quai
where artists were selling paintings of the usual scenes: the cathedral directly across the river and the Eiffel Tower and nostalgic views of the alleyways of Montmartre. Clément had strolled along beside her, while Ned was condemned to walk ahead with their father. ‘Hurry up,’ he’d complained, looking back at them. ‘We must get back to the lab.’

Clément had ignored him, leaning close to share his thoughts with her, laughing with her, teasing her. ‘I want to pull your leg,’ he said, using the English expression, which seemed to delight
him but sounded outrageous to her, so outrageous that it had made her blush.

‘And what part did Clément play in all this?’

‘He worked on the critical mass problem. Natural uranium is made of two different kinds, different isotopes. Most of it, more than ninety-nine per cent, is uranium 238. A mere 0.3 per cent of natural uranium is the other kind. It’s called uranium 235. Clément worked on the calculations Perrin had used. Mean free paths and cross-sections and probabilities, a whole lot of stuff. Thermal neutrons and slow neutrons. And then he made a crucial observation: if it was only the uranium 235 that was responsible for the fission and if you could obtain a relatively pure sample of 235 then the calculations would be different. A forty-ton, even a twelve-ton atomic bomb is a lot of bomb. An aircraft couldn’t deliver it. But if you can increase the proportion of uranium 235 in your sample, then the value for the critical mass comes down dramatically.’

‘How dramatically?’

‘It depends on the degree of enrichment, and even then the maths is not certain. Clément’s revised calculations were in the order of pounds, not tons. Say ten, maybe even less. No mass at all.’ There was a rockery among the bushes at the centre of the garden. Ned went and picked up two large stones and brought them over. ‘Imagine these are lumps of uranium metal, each one a fraction below the critical mass.’ He handed them to her. ‘Imagine it’s uranium. It’s greyish and shiny. Quite decorative, really. Now smash the two together.’

She did as she was told. A children’s game. Crash! And there was a faint and sulphurous smell of sparks.

‘There! That’s all there is to it. You’ve just blown London off the map and out of history. Vaporised.’

‘Merely by doing that everything
vanishes
?’

‘Merely by doing that. If the two lumps are below the critical mass, as long as you keep them separate nothing happens. Smash them together and the chain reaction begins, fast, almost
as fast as the speed of light. The atoms break up in a cascade, each one causing the next two to split in turn and release their energy. If one kilogram of uranium went like that it would release the equivalent energy of twenty thousand tons of TNT, all detonated in a flash.’

She knew TNT. She knew all the explosives: plastic, Nobel 808, ammonal, gun cotton. She knew how to shape a charge and how to fuse it and how to detonate it. She could break a railway line and put a train out of action, or a car. She might have a go at destroying a bridge, although you’d need to be an expert for that, like Benoît was. But not this, not a whole city, in an instant.

‘This is all theoretical, isn’t it?’

‘It’s as certain as existence itself.’

She tossed the stones back among the bushes and brushed the soil from her hands. ‘I don’t believe you.’

‘That doesn’t make any difference. Whether you believe it or not doesn’t change the facts. It doesn’t depend on belief.’

She looked around. There was the garden with the old plane trees, their camouflage trunks and shivering leaves; and beyond, the buildings of the square, one or two of them hollow shells, but still there; and the city itself, battered by the bombing but still incontrovertibly there. It was beyond imagining that it could all be blown away in an instant simply by banging two lumps of metal together.

‘And now they know all about me and Clément.’ It seemed unreal, circumstance and happenstance and pure coincidence coming together to create a small but terrifying explosion.

She looked at him with an expression that tried to mollify her previous anger. ‘Do you remember playing Pig-in-the-middle, with me in the middle?’

‘We called it “collapsing the wave function”.’

‘It used to make me furious.’

‘But you kept playing, didn’t you? Because of Clément.’

‘That’s what I feel like now. The pig in the middle.’

He smiled, a bitter little smile. ‘You always were,’ he said.

‘And I’ve got to keep playing?’

‘I’m afraid so.’

The Cambridge train was full. All trains were full these days. Soldiers, airmen, men in dark suits carrying significant briefcases, academics in careless tweed jackets and ill-fitting grey flannels.
IS YOUR JOURNEY REALLY NECESSARY
? posters on every platform demanded, but half of England seemed to have reason enough.