Gisela looked around the empty scape, angry with herself for losing her temper. Through the window, Chandrasekhar was calmly proceeding to crush s.p.a.cetime out of existence, as it had for the past six billion years.

She said, "And you'd better not be right."

Fifty hours before the Dive, Vikram instructed the probes in the lowest orbits to begin pouring nanomachines through the event horizon. Gisela and Cordelia joined him in the control scape, a vast hall full of maps and gadgets for manipulating the hardware scattered around Chandrasekhar. Prospero was off interrogating Timon, an ordeal Vikram had just been through himself. "Oedipal urges" and "womb/v.a.g.i.n.a symbolism" had figured prominently, though Vikram had cheerfully informed Prospero that as far as he knew, no one in Cartan had ever shown much interest in either organ. Gisela found herself wondering precisely how Cordelia had been created; slavish simulations of flesher childbirth didn't bear thinking about.

The nanomachines comprised only a trickle of matter, a few tonnes per second. Deep inside the hole, though, they'd measure the curvature around them - observing both starlight and signals from the nanomachines following behind - then modify their own collective ma.s.s distribution in such a way as to steer the hole's future geometry closer to the target. Every deviation from free fall meant jettisoning molecular fragments and sacrificing chemical energy, but before they'd entirely ripped themselves apart they'd give birth to photonic machines tailored to do the same thing on a smaller scale.

It was impossible to know whether or not any of this was working as planned, but a map in the scape showed the desired result. Vikram sketched in two counter-rotating bundles of light rays. "We can't avoid having s.p.a.ce collapsing in two directions and expanding in the third - unless we poured in so much matter that it collapsed in all three, which would be even worse. But it's possible to keep changing the direction of expansion, flipping it ninety degrees again and again, evening things out. That allows light to execute a series of complete orbits - each taking about one hundredth the time of the previous one - and it also means there are periods of contraction across the beams, which counteract the de-focusing effects of the periods of expansion."

The two bundles of rays oscillated between circular and elliptical cross-sections as the curvature stretched and squeezed them. Cordelia created a magnifying gla.s.s and followed them "in": forwards in time, towards the singularity. She said, "If the orbital periods form a geometric series, there's no limit to the number of orbits you could fit in before the singularity. And the wavelength is blue-shifted in proportion to the size of the orbit, so diffraction effects never take over. So what's there to stop you doing infinite computation?"

Vikram replied cautiously, "For a start, once colliding photons start creating particle-antiparticle pairs, there'll be a range of energies for each species of particle when it will be travelling so much slower than lightspeed that the pulses will begin to smear. We think we've shaped and s.p.a.ced the pulses in such a way that all the data will survive, but it would only take one unknown ma.s.sive particle to turn the whole stream into gibberish."

Cordelia looked up at him with a hopeful expression. "What if there are no unknown particles?"

Vikram shrugged. "In k.u.mar's model, time is quantised, so the frequency of the beams can't keep rising without limit. And most of the alternative theories also imply that the whole setup will fail eventually, for one reason or another. I only hope it fails slowly enough for us to understand why, before we're incapable of understanding anything." He laughed. "Don't look so mournful! It will be like ... the death of one branch of a tree. And maybe we'll gain some knowledge for a while that we could never even glimpse, outside the hole."

"But you won't be able to do anything with it," Cordelia protested. "Or tell anyone."

"Ah, technology and fame." Vikram blew a raspberry. "Listen, if my Dive clone dies learning nothing, he'll still die happy, knowing that I continued outside. And if he learns everything I'm hoping he'll learn ... he'll be too ecstatic to go on living." Vikram composed his face into a picture of exaggerated earnestness, deflating his own hyperbole, and Cordelia actually smiled. Gisela had been beginning to wonder if morbid grief over the fate of the Divers would be enough to put her off Cartan altogether.

Cordelia said, "What would make it worthwhile, then? What's the most you could hope for?"

Vikram sketched a Feynman diagram in the air between them. "If you take s.p.a.cetime for granted, rotational symmetry plus quantum mechanics gives you a set of rules for dealing with a particle's spin. Penrose turned this inside out, and showed that the whole concept of 'the angle between two directions' can be created from scratch in a network of world lines, so long as they obey those spin rules. Suppose a system of particles with a certain total spin throws an electron to another system, and in the process the first system's spin decreases. If you knew the angle between the two spin vectors, you could calculate the probability that the second spin was increased rather than decreased ... but if the concept of 'angle' doesn't even exist yet, you can work backwards and define it from the probability you get by looking at all the networks for which the second spin is increased.

"k.u.mar and others extended this idea to cover more abstract symmetries. From a list of rules about what const.i.tutes a valid network, and how to a.s.sign a phase to each one, we can now derive all known physics. But I want to know if there's a deeper explanation for those rules. Are spin and the other quantum numbers truly elementary, or are they the product of something more fundamental? And when networks reinforce or cancel each other according to the phase difference between them, is that something basic we just have to accept, or is there hidden machinery beneath the mathematics?"

Timon appeared in the scape, and drew Gisela aside. "I've committed a small infraction - and knowing you, you'll find out anyway. So this is a confession in the hope of leniency."

"What have you done?"

Timon regarded her nervously. "Prospero was rambling on about flesher culture as the route to all knowledge." He morphed into a perfect imitation, and replayed Prospero's voice: "'The key to astronomy lies in the study of the great Egyptian astrologers, and the heart of mathematics is revealed in the rituals of the Pythagorean mystics ... '"

Gisela put her face in her hands; she would have been hard-pressed not to respond herself. "And you said - ?"

"I told him that if he was ever embodied in a s.p.a.ce-suit, floating among the stars, he ought to try sneezing on the face plate to improve the view."

Gisela cracked up laughing. Timon asked hopefully, "Does that mean I'm forgiven?"

"No. How did he take it?"

"Hard to tell." Timon frowned. "I'm not sure that he's capable of grasping insults. It would require imagining that someone could believe that he's less than essential to the future of civilisation."

Gisela said sternly, "Two more days. Try harder."

"Try harder yourself. It's your turn now."

"What?"

"Prospero wants to see you." Timon grinned with malicious pleasure. "Time to have your own mythic essence extracted."

Gisela glanced towards Cordelia; she was talking animatedly with Vikram. Athena, and Prospero, had suffocated her; it was only away from both that she came to life. The decision to migrate was hers alone, but Gisela would never forgive herself if she did anything to diminish the opportunity.

Timon said, "Be nice."

The Dive team had decided against any parting of the clones; their frozen snapshots would be incorporated into the blueprint for Cartan Null without ever being run outside Chandrasekhar. When Gisela had told Prospero this, he'd been appalled, but he'd cheered up almost immediately; it left him all the more room to invent some ritual farewell for the travellers, without being distracted by the truth.

The whole team did gather in the control scape, though, along with Prospero and Cordelia, and a few dozen friends. Gisela stood apart from the crowd as Vikram counted down to the deadline. On "ten", she instructed her exoself to clone her. On "nine", she sent the snapshot to the address being broadcast by an icon for the Cartan Null file - a stylised set of counter-rotating light beams - hovering in the middle of the scape. When the tag came back confirming the transaction, she felt a surge of loss; the Dive was no longer part of her own linear future, even if she thought of the clone as a component of her extended self.

Vikram shouted exuberantly, "Three! Two! One!" He picked up the Cartan Null icon and tossed it into a map of the s.p.a.cetime around Chandrasekhar. This triggered a gamma-ray burst from the polis to a probe with an eight-M orbit; there, the data was coded into nanomachines designed to recreate it in active, photonic form - and those nanomachines joined the stream cascading into the hole.

On the map, the falling icon veered into a "motionless" vertical world line as it approached the two-M sh.e.l.l. Successive slices of constant time in the static frame outside the hole never crossed the horizon, they merely clung to it; by one definition, the nanomachines would take forever to enter Chandrasekhar.

By another definition, the Dive was over. In their own frame, the nanomachines would have taken less than one-and-a-half milliseconds to fall from the probe to the horizon, and not much longer to reach the point where Cartan Null was launched. And however much subjective time the Divers had experienced, however much computing had been done along the way, the entire region of s.p.a.ce containing Cartan Null would have been crushed into the singularity a few microseconds later.

"If the Divers tunnelled out of the hole, there'd be a paradox, wouldn't there?" Gisela turned; she hadn't noticed Cordelia behind her. "Whenever they emerged, they wouldn't have fallen in yet - so they could swoop down and grab the nanomachines, preventing their own births." The idea seemed to disturb her.

Gisela said, "Only if they tunnelled out close to the horizon. If they appeared further away - say here in Cartan, right now - they'd already be too late. The nanomachines have had too much of a head start; the fact that they're almost standing still in our reference frame doesn't make them an easy target if you're actually chasing after them. Even at lightspeed, nothing could catch them from here."

Cordelia appeared to take heart from this. "So escape isn't impossible?"

"Well ... " Gisela thought of listing some of the other hurdles, but then she began to wonder if the question was about something else entirely. "No. It's not impossible."

Cordelia gave her a conspiratorial smile. "Good."

Prospero cried out, "Gather round! Gather round now and hear The Ballad of Cartan Null!" He created a podium, rising beneath his feet. Timon sidled up to Gisela and whispered, "If this involves a lute, I'm sending my senses elsewhere."

It didn't; the blank verse was delivered without musical accompaniment. The content, though, was even worse than Gisela had feared. Prospero had ignored everything she and the others had told him. In his version of events, "Charon's pa.s.sengers" entered "gravity's abyss" for reasons he'd invented out of thin air: to escape, respectively, a failed romance/vengeance for an unspeakable crime/the ennui of longevity; to resurrect a lost flesher ancestor; to seek contact with "the G.o.ds." The universal questions the Divers had actually hoped to answer - the structure of s.p.a.cetime at the Planck scale, the underpinnings of quantum mechanics - didn't rate a mention.

Gisela glanced at Timon, but he seemed to be taking the news that his sole version had just fled into Chandrasekhar to avoid punishment for an unnamed atrocity extremely well; there was disbelief on his face, but no anger. He said softly, "This man lives in h.e.l.l. Mucous on the face plate is all he'll ever see."

The audience stood in silence as Prospero began to "describe" the Dive itself. Timon stared at the floor with a bemused smile. Tiet wore an expression of detached boredom. Vikram kept peeking at a display behind him, to see if the faint gravitational radiation emitted by the inflowing nanomachines was still conforming to his predictions.

It was Sachio who finally lost control and interjected angrily, "Cartan Null is some ghostly image of a scape, full of ghostly icons, floating through the vacuum, down into the hole?"

Prospero seemed more startled than outraged by the interruption. "It is a city of light. Translucent, ethereal ... "

The owl in Sachio's skull puffed its feathers out. "No photon state would look like that. What you describe could never exist, and even if it could it would never be conscious." Sachio had worked for decades on the problem of giving Cartan Null the freedom to process data without disrupting the geometry around it.

Prospero spread his arms in a conciliatory gesture. "An archetypal quest narrative must be kept simple. To burden it with technicalities - "

Sachio inclined his head briefly, fingertips to forehead, downloading information from the polis library. "Do you have any idea what archetypal narratives are?"

"Messages from the G.o.ds, or from the depths of the soul; who can say? But they encode the most profound and mysterious - "

Sachio cut him off impatiently. "They're the product of a few chance attractors in flesher neurophysiology. Whenever a more complex or subtle story was disseminated through an oral culture, it would eventually degenerate into an archetypal narrative. Once writing was invented, they were only ever created deliberately by fleshers who failed to understand what they were. If all of antiquity's greatest statues had been dropped into a glacier, they would have been reduced to a predictable spectrum of spheroidal pebbles by now; that does not make the spheroidal pebble the pinnacle of the artform. What you've created is not only devoid of truth, it's devoid of aesthetic merit."

Prospero was stunned. He looked around the room expectantly, as if waiting for someone to speak up in defence of the Ballad.