Beth was sitting beside her pregnant daughter. Now, under a blanket, she took her daughter’s hand, and Mardina squeezed back. Mardina’s eyes were wide in the firelight, her expression blank. This was not a conversation either of them wanted to be part of, Beth was sure.

The ColU was on Chu’s lap, next to Stef. Titus Valerius sat beside the slave boy, listening intently.

And Titus was sceptical. ‘Well, we Romans had no trouble imagining eternity. Or at least, we failed to anticipate an end. Because we never anticipated the Empire to end – do you see? Unbounded and eternal …’

That sounded magnificent in the legionary’s guttural soldier’s Latin, Stef thought. Imperium sine fine.

The ColU said, ‘Our own culture, mine and Stef’s and Beth’s, had its own account of an undying empire – but an empire of scientific logic. We thought we could know the future by looking out at the universe, working out the physical laws that govern it – and then projecting forward the consequences of those laws.

‘The universe only has so much hydrogen – the stuff that stars are made out of. The hydrogen will, or would have, run out when the universe is ten thousand times as old as it is now. No more stars. After the stars there would be an age of black holes and degenerate matter – the compressed, cooling remnants of stars – and the galaxies, huge and dim, would begin to break up. There would be a major transition when protons began to decay – that is, the very stuff of which matter is made … In the end everything would dissolve, and there would be nothing left but a kind of sparse mist, of particles called electrons and positrons – a stuff called positronium – filling an expanding, empty universe. Even so it was possible that minds could survive. Minds more like mine than yours, perhaps. Thoughts carried on the slow wash of electrons – thoughts that might take a million years to complete.’

‘That sounds horrible,’ Mardina said, and Beth could feel the grip of her hand tighten. ‘It doesn’t even make any sense. How could a single thought last a million years? I can’t imagine it.’

‘But experiences of time can differ,’ the ColU said. ‘In my culture there was a Christian scholar called Thomas Aquinas – I wasn’t able to trace him in your history, Titus. He distinguished three kinds of time, or perhaps perceptions of time. Tempus was human time, which we measure by changes in the world around us – the swing of a pendulum, the passage of a season. A Titanian ice giant would experience a slower tempus than a human. Aevus was angel time, measured by internal changes – by the development of thoughts, understanding, moods. For the angels, you see, stood outside the human world. And then there was aeternitas, God’s time, for God and only God could apprehend all of eternity at once. The electron-positron minds would not be God, but in the timeless twilight of the universe they might have been like angels …’

‘Might have been,’ Mardina said, almost bitterly. ‘Might have been.’

The ColU said, ‘The positronium angels will never exist. Our universe won’t last long enough for that. And the reason our universe is not eternal is because of the existence of other universes. And we know they exist because we, all of us, have visited several of them.’

‘Aye, and fought in them,’ Titus said, stirring from his space and pushing back blankets. ‘But in this universe my bladder’s full. Anybody want more tea? Chu, maybe you could put another pot of ice on the fire …’

It took an hour before they were settled again.

When they took their places Beth thought they seemed calmer, more attentive – more ready to take in this strange news from the sky. The break had been a smart bit of people management by Titus Valerius, she thought. Who in the end hadn’t really needed a piss at all.

‘So,’ Titus said now, slurping the last of his tea, ‘as if the fate of this universe wasn’t bad enough, you have to talk about all the other ones.’

Stef smiled. ‘All right, Titus, I know we are leading you on a march you’d rather not be following … It’s all about logic, though. When all else fails, ask a philosopher. Sorry. Old physicist’s joke.

‘Look, we all know from personal experience that other universes exist, with histories more or less similar to this one – or to the one into which each of us was born. And in my culture our philosophers had predicted the existence of those universes. Our laws of nature were well founded, you see, but they did not prescribe how the universe had to be. Many universes were possible – an infinite number. It is just as our science would have predicted the six-fold symmetry of a snowflake, which comes from the underlying geometry of ice crystals, but within that six-fold rule set many individual snowflakes are possible, all different from each other.’

‘Universes as numerous as snowflakes,’ Beth said. ‘That’s wonderful. Scary.’

Stef said, ‘But what are these universes? Where are they? You know that the science of my culture was more advanced than in any other we’ve yet encountered—’

The ColU said, ‘And Earthshine would say that was because we had been the least deflected into efforts to build Hatches for his Dreamers.’

‘We did have some models of the multiverse – I mean, of a super-universe that is a collection of universes. After centuries of study we never came to a definitive answer. We probably never got far out enough into our own universe to be able to map the truth.

‘Still, we believed our universe had expanded from a single point, out of a Big Bang. Expanded, cooled, awash with light at first, atoms and stars and planets and people condensing out later. But our universe was like a single bubble in a bowl of boiling water, like a pot we put on the fire.’ She gestured at the clay pot, within which water was languidly bubbling. ‘You see? There is a substrate, something like the water in the pot. And out of that heated-up substrate emerges, not just one bubble, but a whole swarm of them, expanding, popping … They are the other universes we’ve been visiting.

‘And what’s inside those universes is going to be different, one universe to the next – a little or a lot. Some could differ wildly from the others, not just in historical details. Suppose gravity were stronger – I mean, the force that gives us weight. Then stars would be smaller, and would burn out more quickly. Everything would be different. And if gravity were weaker there might be no stars at all. Of course some universes are going to be more similar than others.’

It seemed to be Chu who understood most readily. Not for the first time Beth wondered what kind of scholar he might have become, given the chance. ‘All the universes we have seen are similar. They all have planets, suns, people. They even have the same people, up to a point.’

‘Yes,’ Stef said eagerly. ‘You’ve got it. When you think about it the differences are pretty small. I mean, whether Rome falls or not would be a big deal for us—’ and she smiled as Titus scowled ferociously ‘—but from Per Ardua, say, you wouldn’t even notice it.’

The ColU said, ‘We believe that the Dreamers can somehow reach out to other universes that are – nearby. There is no good term for it. What is nearness in a multiverse? Beginning in one universe, they reach out into another that is similar, yet which contains a human culture that is more – conducive – to Hatch-building. And we, our small lives, are swept along in the process.’

Beth found herself frowning. ‘But why? Why would they do that?’

Stef said, ‘We need to find that out. In fact I suspect Earthshine may already be learning that secret. What’s important now is that we know the multiverse exists. OK? We’ve been there. Now, the multiverse is big. Surely that’s true. But it can’t be infinite.’