‘A school librarian?’ mocked Hugo. ‘That’s pretty funny too.’

Nora ignored him. ‘Do you reckon they’re ghosts? Guiding spirits? Guardian angels? What are they?’

It felt so ludicrous, in the heart of a scientific facility, to be talking like this.

‘They are,’ Hugo gestured, as if trying to pluck the right term from the air, ‘an interpretation.’

‘Interpretation?’

‘I have met others like us,’ Hugo said. ‘You see, I have been in the in-between state for a long time. I have encountered a few other sliders. That’s what I call them. Us. We are sliders. We have a root life in which we are lying somewhere, unconscious, suspended between life and death, and then we arrive in a place. And it is always something different. A library, a video store, an art gallery, a casino, a restaurant . . . What does that tell you?’

Nora shrugged. And thought. Listening to the hum of the central heating. ‘That it’s all bullshit? That none of this is real?’

‘No. Because the template is always the same. For instance: there is always someone else there – a guide. Only ever one person. They are always someone who has helped the person at a significant time in their life. The setting is always somewhere with emotional significance. And there is usually talk of root lives or branches.’

Nora thought about being consoled by Mrs Elm when her dad died. Staying with her, comforting her. It was probably the most kindness anyone had ever shown her.

‘And there is always an infinite range of choices,’ Hugo went on. ‘An infinite number of video tapes, or books, or paintings, or meals . . . Now, I am a scientist. And I have lived many scientific lives. In my original root life, I have a degree in Biology. I have also, in another life, been a Nobel Prize-winning chemist. I have been a marine biologist trying to protect the Great Barrier Reef. But my weakness was always physics. At first I had no idea of how to find out what was happening to me. Until I met a woman in one life who was going through what we are going through, and in her root life she was a quantum physicist. Professor Dominique Bisset at Montpellier University. She explained it all to me. The many-worlds interpretation of quantum physics. So that means we—’

A kind-faced, pink-skinned, auburn-bearded man whose name Nora didn’t know came into the kitchen to rinse a coffee cup, then smiled at them.

‘See you tomorrow,’ he said, in a soft American (maybe Canadian) accent, before padding away in his slippers.

‘Yes,’ said Nora.

‘See you,’ said Hugo, before returning – in a more hushed tone – to his main thread. ‘The universal wave function is real, Nora. That’s what Professor Bisset said.’

‘What?’

Hugo held up a finger. A slightly annoying, wait-a-minute kind of finger. Nora resisted a strong urge to grab it and twist it. ‘Erwin Schr?dinger . . .’

‘He of the cat.’

‘Yes. The cat guy. He said that in quantum physics every alternative possibility happens simultaneously. All at once. In the same place. Quantum superposition. The cat in the box is both alive and dead. You could open the box and see that it was alive or dead, that’s how it goes, but in one sense, even after the box is open, the cat is still both alive and dead. Every universe exists over every other universe. Like a million pictures on tracing paper, all with slight variations within the same frame. The many-worlds interpretation of quantum physics suggests there are an infinite number of divergent parallel universes. Every moment of your life you enter a new universe. With every decision you make. And traditionally it was thought that there could be no communication or transference between those worlds, even though they happen in the same space, even though they happen literally millimetres away from us.’

‘But what about us? We’re doing that.’

‘Exactly. I am here but I also know I am not here. I am also lying in a hospital in Paris, having an aneurysm. And I am also skydiving in Arizona. And travelling around southern India. And tasting wine in Lyon, and lying on a yacht off the C?te d’Azur.’

‘I knew it!’

‘Vraiment?’

He was, she decided, quite beautiful.

‘You seem more suited to strolling the Croisette in Cannes than an Arctic adventure.’

He widened his right hand like a starfish. ‘Five days! Five days I have been in this life. That is my record. Maybe this is the life for me . . .’

‘Interesting. You’re going to have a very cold life.’

‘And who knows? Maybe you are too . . . I mean, if the bear didn’t take you back to your library maybe nothing will.’ He started to fill the kettle. ‘Science tells us that the “grey zone” between life and death is a mysterious place. There is a singular point at which we are not one thing or another. Or rather we are both. Alive and dead. And in that moment between the two binaries, sometimes, just sometimes, we turn ourselves into a Schr?dinger’s cat who may not only be alive or dead but may be every quantum possibility that exists in line with the universal wave function, including the possibility where we are chatting in a communal kitchen in Longyearbyen at one in the morning . . .’

Nora was taking all this in. She thought of Volts, still and lifeless under the bed and lying by the side of the road.

‘But sometimes the cat is just dead and dead.’

‘Sorry?’

‘Nothing. It’s just . . . my cat died. And I tried another life and even in that one he was still dead.’

‘That’s sad. I had a similar situation with a Labrador. But the point is, there are others like us. I have lived so many lives, I have come across a few of them. Sometimes just to say your own truth out loud is enough to find others like you.’

‘It’s crazy to think that there are other people who could be . . . what did you call us?

‘Sliders?’

‘Yep. That.’

‘Well, it’s possible of course, but I think we’re rare. One thing I’ve noticed is that the other people I’ve met – the dozen or so – have all been around our age. All thirties or forties or fifties. One was twenty-nine, en fait. All have had a deep desire to have done things differently. They had regrets. Some contemplated that they may be better off dead but also had a desire to live as another version of themselves.’

‘Schr?dinger’s life. Both dead and alive in your own mind.’

‘Exactement! And whatever those regrets did to our brain, whatever – how would you say? – neurochemical event happened, that confused yearning for death-and-life was somehow just enough to send us into this state of total in-between.’

The kettle was getting noisier, the water starting to bubble like Nora’s thoughts.

‘Why is it always just one person that we see? In the place. The library. Whatever.’

Hugo shrugged. ‘If I was religious, I’d say it was God. And as God is probably someone we can’t see or comprehend then He – or She – or whichever pronoun God is – becomes an image of someone good we have known in our lives. And if I wasn’t religious – which I’m not – I would think that the human brain can’t handle the complexity of an open quantum wave function and so it organises or translates this complexity into something it understands. A librarian in a library. A friendly uncle in a video store. Et cetera.’

Nora had read about multiverses and knew a bit about Gestalt psychology. About how human brains take complex information about the world and simplify it, so that when a human looks at a tree it translates the intricately complex mass of leaves and branches into this thing called ‘tree’. To be a human was to continually dumb the world down into an understandable story that keeps things simple.

She knew that everything humans see is a simplification. A human sees the world in three dimensions. That is a simplification. Humans are fundamentally limited, generalising creatures, living on auto-pilot, who straighten out curved streets in their minds, which explains why they get lost all the time.

‘It’s like how humans never see the second hand of a clock mid-tick,’ said Nora.

‘What?’

She saw that Hugo’s watch was of the analogue variety. ‘Try it. You just can’t. Minds can’t see what they can’t handle.’

Hugo nodded, as he observed his own watch.

‘So,’ Nora said, ‘whatever exists between universes is most likely not a library, but that is the easiest way for me to understand it. That would be my hypothesis. I see a simplified version of the truth. The librarian is just a kind of mental metaphor. The whole thing is.’

‘Isn’t it fascinating?’ said Hugo.

Nora sighed. ‘In the last life I spoke to my dead dad.’

Hugo opened a jar of coffee and scooped out granules into two mugs.

‘And I didn’t drink coffee. I drank peppermint tea.’

‘That sounds terrible.’

‘It was bearable.’

‘Another thing that is strange,’ Hugo said. ‘At any point in this conversation you or I could disappear.’