They were passed through, and dropped down into a rest area, with a lavatory, a couple of bunk beds, a small galley.

And here Sir Michael King was waiting for them, loosely strapped to a couch, sipping coffee through a plastic cup with a nozzle. He was wearing a kind of coverall, deep royal blue, cut to fit his squat, heavy frame, that simultaneously looked practical and expensive. When Trant and Stef entered, swimming down from the ceiling, he pushed himself out of his chair. ‘Glad to see you made it down here, Major Kalinski.’

‘Why wouldn’t I, sir?’

‘Most passengers, especially those from Earth, spend most of their time during the accel-decel handover locked in their cabins chucking up.’

‘I’m a veteran of the Earth-moon run. My body’s used to microgravity.’

‘Well, mine isn’t,’ King said. ‘I had to swallow a whole pharmacopeia.’ He grinned, his face pale, sweating. ‘But I wouldn’t miss it for the world.’

Trant nodded at the ISF crewman who, discreet and unspeaking, had followed them in here. ‘Let’s get on with it.’

The crewman opened another hatch, in the floor. Below, Stef saw, was a kind of carpet, speckled with lights that shone bright in a relative gloom. They followed the crewman down through the hatch, and spread out.

There were three, four crew already in this wide chamber, in their jet-black ISF uniforms, swimming over the illuminated carpet, carrying slates, making notes and murmuring to each other. The ‘carpet’ was actually another bulkhead that spanned the width of the hulk, Stef saw now, and the lights that sparkled in the floor, more or less uniformly distributed, were a display. Flux lines swept between the lights, uniting them in a pleasing, swirling geometry – a three-dimensional geometry, Stef saw, as she shifted her head from side to side. The whole was littered with tiny labels, numbers and English letters; the systems sensed whatever she was looking at, and the labels magnified in her vision.

Trant said, ‘This is our engine room. We run everything through this one display. Under acceleration, this is a floor under our feet, but in microgravity it makes more sense to treat it as a vertical wall – you can see there are hand- and footholds . . .’

Sir Michael King was watching Stef intently. ‘This is as close as we can get to the real action. I mean, I understand the display we see here is just a representation of the reality, but . . . Can you feel them, Major? I know you’ve been around kernels for years, but not an array like this. Can you sense them? Can you feel their energies?’

And Stef thought she could, yes, a kind of tide that pulled at her body as she hung there in the air – a tide from the space-time knots of the kernels themselves, perhaps, or maybe a force exerted by the powerful magnetic fields that held them in place, a great wall of them contained not metres from her position. She felt thrilled, viscerally, physically; the pathetic handful of kernels in the lunar labs would have been lost in this huge assembly.

King said, ‘Here you are, confronted by the mystery. Tell me about kernels, Major Kalinski.’

‘I can tell you what we think we know. Which is precious little.’

He pulled a face. ‘And I could tell you how much that inadequate reply has cost me so far.’

‘Sorry.’

He said, ‘I know a kernel is a twisted bit of reality. Like a black hole, right?’

‘That was our first guess. Black holes are similarly twisted bits of space-time, yes, the remnants of imploded giant stars, or maybe relics of the Big Bang. And all black holes radiate; they leak energy from their event horizons, and the smaller they get the hotter they are. But nothing fit. A kernel masses only kilograms, a lot less than any but the most evanescent black hole. And the energy it emits isn’t black-hole Hawking radiation but something much more exotic, a flood of high-energy photons and very high-speed particles, like cosmic rays. Also, the way the energy leaks from a kernel depends on the way you prod it.’

‘You mean with laser beams,’ King said. ‘Well, I know about that. A lot of lives were lost to establish that simple fact. And Mercury gained itself a new crater.’

‘By manipulating it with laser beams you can shape the way a kernel releases its energy store. Get it right and it can even be unidirectional.’

‘Like a little rocket.’

‘A microscopic photon rocket, yes. And that’s what makes them so useful. The kernels carry an electric charge, and are so light that a powerful enough magnetic field can hold a whole bank of them in place, just as in this ship, behind this bulkhead. Fire the control lasers just right and they all open up, and you get a kind of photon rocket.’

‘Driven by a light as bright as the sun,’ King said. ‘Visible across interplanetary distances. Hell of a thing. After all these years, you know, I still can’t get used to the sight. But what I want to know from you, Major, is how the damn things work. Where does all that lovely energy come from?’

‘Well, not from the structure of the kernel itself – it’s not massive enough for that. Our best guess is that a kernel is less like a black hole than a wormhole—’

‘A tunnel in space.’

‘Yes.’

‘I thought wormholes were impossible,’ Trant said. ‘You need some strange kind of matter to keep them open.’

Stef always got irritated when some lay person asked her a question and then started lecturing her about the answer, rather than just listening. She snapped, ‘It may have looked that way according to the kiddie Einstein-relativity stuff you learned at high school, Monica, before you gave up science for engineering. Have you ever heard of a dilaton field? No?’

Monica Trant looked irritated.

King raised luxuriant eyebrows, amused. ‘Well, that’s put you in your place. Let’s get to the basics. A wormhole is a tunnel, right? From here to . . . someplace else.’

‘That’s right.’

‘So the energy that flows out of a kernel, the energy we harness to drive our hulks, doesn’t come from the kernel itself. It comes from someplace else, and is just transmitted through the kernel.’

‘That seems to be true. The ultimate power source must be some very energetic event, somewhere else. A gamma-ray burster, maybe. Could be from the future or the past.’

Trant frowned. ‘What do you mean by that?’

‘The wormhole could connect you to any other point in space and time, sir. Or—’

King waved a hand. ‘Enough, enough. You know, Major, some people are suspicious of the kernels. I mean, their very existence. Your own father was, right? There we were struggling to come up with ways to reach the stars. And now we’ve been handed this magic power source, on a plate, and we’re off to Proxima Centauri. We needed a miracle, and suddenly we had one. The problem is, you see, Major, in business or in politics, hell, in most marriages, if I give you something, it’s generally because I want something of you in return. So what’s the catch?’

‘You’re assuming agency,’ Stef protested. ‘Intervention by some kind of consciousness. It’s better to rule that out until there’s overwhelming evidence. Occam’s razor: you should default to the simplest explanation, and natural causes are the simplest explanation we have for most phenomena. Including the presence of kernels on Mercury.’

Trant said, ‘Wait until we get you to Mercury, Kalinski.’ She shook her head. ‘Occam’s razor. Jesus.’

A gong sounded, echoing around the ship.

King turned to the ladder up to the main hull. ‘They’re about to fire up again. You’ll find me in my couch, until my old bones get used to gravity again . . .’

Stef was allowed to stay down on the control deck, with Trant, the two of them strapped into acceleration harnesses, watching as the highly trained crew went through the process of firing up their laser banks, opening up their tame space-time knots, and allowing their unknown-source energy to stream out.