“I heard what you said, Lieserl.” Louise smiled. “But it’s all a little too mystical for a tired old engineer like me. I’m going to stay here. Help out in the hospital.”

Lieserl frowned at her. “Louise, you’re an engineer, not a doctor. Frankly, I wouldn’t want you treating me.”

Mark smiled. “Besides, we don’t have time for all this self pity, Louise. This is important.”

She sighed. “What is?”

He whispered, in a surprisingly unrealistic hiss, “Didn’t you hear the hull stress noise? Spinner is moving the ship again.”

Think of spacetime as a matrix, Michael Poole whispered. A four-dimensional grid, labeled by distance and duration. There are events: points in time and space, at nodes of the grid. These are the incidents that mark out our lives. And, connecting the events, there are trajectories.

The starbow across the sky broadened, now. That meant her speed had reduced, since the relativistic distortion was lessened. Spinner called up a faceplate display subvocally. Yes: the ship’s velocity had fallen to a fraction over half lightspeed.

Trajectories are paths through spacetime, Poole said. There are timelike trajectories, and there are spacelike trajectories. A ship going slower than light follows a timelike path. And, Spinner, we — all humans, since the beginning of history — work our snail-like way along timelike trajectories into the future. At last, our world lines will terminate at a place called timelike infinity at the infinitely remote, true end of time.

But “spacelike” means moving faster than light. A tachyon — a faster-than-light particle — follows a spacelike path, as does this nightfighter under hyperdrive.

She twisted in her seat. Already the neutron star system had vanished, into the red-shift distance. And directly ahead of her there was a cloud of cosmic string; space looked as if it were criss-crossed by fractures, around which blue shifted star images slid like oil drops.

Poole’s hands, invisible, tightened around hers as the ship threw itself into the cloud of string.

We know at least three ways to follow spacelike paths, Spinner-of-Rope: three ways to travel faster than light. We can use the Xeelee hyperdrive, of course. Or we can use spacetime wormholes. Or, Poole said slowly, we can use the conical spacetime around a length of cosmic string…

Think of the gravitational tensing effect that produces double images of stars around strings. A photon coming around one side of the string can take tens of thousands of years longer to reach our telescopes than a photon following a path on the other side of the string.

So, by passing through the string’s conical deficit, we could actually outrun a beam of light… There was string all around the ship, now, tangled, complex, an array of it receding to infinity. A pair of string lengths, so twisted around each other they were almost braided, swept over her head. She looked up. The strings trailed dazzling highways of refracting star images.

Behind her the huge wings spread wide, exultant.

This damn nightfighter was made for this, she thought.

Under Poole’s guidance, Spinner brought the craft to a dead halt; the discontinuity wings cupped as they tore at space. Then Spinner turned the craft around rapidly — impossibly rapidly — and sent it hurtling at the string pair once more. The nightfighter soared upwards, and this time the two strings passed underneath the ship’s bow.

…And if you can move along spacelike paths, Spinner-of Rope, you can construct closed timelike curves.

The neutron star system was old.

Once the system had been a spectacular binary pair, adorning some galaxy lost in the sky. Then one of the stars had suffered a supernova explosion, briefly and gloriously outshining its parent galaxy. The explosion had destroyed any planets, and damaged the companion star. After that, the remnant neutron star slowly cooled, glitching as it spun like some giant stirring in its sleep, while its companion star shed its life-blood hydrogen fuel over the neutron star’s wizened flesh. Slowly, too, the ring of lost gas formed, and the system’s strange, spectral second system of planets coalesced.

Then human beings had come here.

The humans soared about the system, surveying. They settled on the largest planet in the smoke ring. They threw microscopic wormhole mouths into the cooling corpse of the neutron star, and down through the wormholes they poured devices and — perhaps — human-analogues, made robust enough to survive in the neutron star’s impossibly rigorous environment.

The devices and human-analogues had been tiny, like finely jewelled toys.

The human-analogues and their devices swarmed to a magnetic pole of the neutron star, and great machines were erected there: discontinuity-drives, perhaps powered by the immense energy reserves of the neutron star itself.

Slowly at first, then with increasing acceleration, the neutron star — dragging its attendant companion, ring and planets with it — was forced out of its parent galaxy and thrown across space, a bullet of stellar mass fired at almost light speed.

“A bullet. Yes.” In the pod, Uvarov mused. “An apt term.”

Lieserl stared at the swirling, unresolved pixels inside the Virtual image’s clear tetrahedral frame. “I wonder if there are still people in there,” she said.

Mark frowned. “Where?”

“People-analogues. Inside the neutron star. I wonder if they’ve survived.”

He shrugged, evidently indifferent. “I doubt it. Unless they were needed for maintenance, they would surely have been shut down after their function was concluded.”

Shut down… But these were people. What if they hadn’t been “shut down”? Lieserl closed her eyes and tried to imagine. How would it be, to live her life as a tiny, fish-like creature less than a hair’s-breadth tall, living inside the flux-ridden mantle of a neutron star? What would her world be like?

“A bullet,” Uvarov said again. “And a bullet, fired by our forebears — directly at the heart of this Xeelee construct.”

She opened her eyes.

Mark was frowning. “What are you talking about, Uvarov?”

“Can’t you see it yet? Mark, what do you imagine the purpose of this great engineering spectacle was? We already know from the Superet data, and the fragments provided to us by Lieserl — that the rivalry between humanity and Xeelee persisted for millions of years. More than persisted — it grew in that time, becoming an obsession which — in the end — consumed mankind.”

Lieserl said, “Are you saying that all of this — the discontinuity engines, the hurling of the neutron star across space — all of this was intended as an assault on the Xeelee?”

“But that’s insane,” Mark said.

“Of course it is,” Uvarov said lightly. “My dear friends, we’ve plenty of evidence that humanity isn’t a particularly intelligent species — not compared to its great rivals the Xeelee, at any rate. And I have never believed that humanity, collectively, is entirely sane either.”

“You should know, Doctor,” Mark growled.

“I don’t understand,” Lieserl said. “Humans must have known about the photino birds — damn it, I told them! They must have seen what danger the birds represented to the future of all baryonic species. And they must have seen that the Xeelee — if remote and incomprehensible — were at least baryonic too. So the goals of the Xeelee, if directed against the birds, had to be in the long-term interests of mankind.”

Uvarov laughed at her. “I’m afraid you’re still looking for rational explanations for irrational behavior, my dear. Lieserl, I believe that the Xeelee grew into the position in human souls once occupied by images of gods and demons. But here, at last, was a god who was finite — who occupied the same mortal realm as humans. A god who could be attacked. And attack we did: down through the long ages, while the stars went out around us, all but ignored.”