Donn found himself standing on a silvered platform, three or four meters across. The Ghost hovered before him. He couldn't see any support for the platform, though gravity felt about normal. They were entirely bathed in pure white light, above, below, all around, an abstraction of a sky. The light was bright, not quite dazzling. And as Donn's eyes adjusted, he gradually made out structure in the light: billows like clouds, all around, slowly evolving, vacuoles boiling.

When he glanced back, he saw that the dodecahedral transit chamber had vanished; somehow he wasn't surprised.

The Amba.s.sador said, "Where do you think you are?"

"In the heart of a star," Five said. "Where else?"

"But not just any star."

"The Boss," Donn said. "But that's impossible. Isn't it, Amba.s.sador?"

"How did you phrase it earlier? 'Evidently not. Or we wouldn't be standing here, would we?'"

The Silverman said, "I understand, Donn Wyman. I am human enough to fear falling. Don't be afraid. Step to the edge. Look down."

Five wouldn't move. She stood there, her hide suit still stained by Ghost blood, bathed in starlight. But Donn stepped to the rim of the floating disk.

And he looked down on a Ghost base in the heart of the star. It was a hollowed-out moon, a rock ball that must have been a thousand kilometers wide, riddled with pa.s.sages and cavities.

The disk began to descend. The motion was smooth, but Five lunged forward and grabbed at Donn's arm.

The moon turned into a complex, machined landscape below them. Ghost ships and science platforms swept over the pocked landscape, tangles of shining net. And Ghosts themselves drifted up from the chambers and machine emplacements, bobbing like balloons, shining in the star's deep light. Behind the moon, there were threads of a more intense brightness, just at the limit of visibility, dead straight. All over the moon's surface, vast cylindrical structures gleamed. The Amba.s.sador said these were intrasystem drives and hyperdrives, systems that had been used to fling this moon into the body of this star and to hold it here.

And there was quagma down there, the Amba.s.sador said, little packets of the primordial stuff, buried in the pits of ancient planetesimal craters. I knew it, knew it, Donn thought. Donn thought.

"The work here is hard," the Amba.s.sador said. "Often lethal. We have poured workers into this mine of light endlessly." And Donn thought of the stream of Ghosts he had seen filing patiently into the transportation booth on Ghostworld. "Few come back, despite all our precautions. But now the work is nearly done."

Five asked, "So how come we aren't all burned up? We're in the middle of a star.'" star.'"

"Perhaps you can see those illuminated threads? Those are refrigeration lasers. By making ourselves hotter even than this star's core, we can dump our heat into it. Of course, what you are seeing is a representation, heavily processed. Starstuff is in fact very opaque."

Donn said, "You are messing with physics again, aren't you, Amba.s.sador?" He thought back to the Coalition's recent observations of the Boss. "We've been observing flares. Are you trying to mend the star, to stop the flares? No, not that. Sink Amba.s.sador, are you destabilizing this star?" are you destabilizing this star?"

The Amba.s.sador rolled. "How would Jack Raoul have put it? 'Guilty as charged.' What do you understand of stellar physics?"

Every star was in equilibrium, with the pressure of the radiation from its fusing core balancing the tendency of its outer layers to fall inward under gravity. A giant star like the Boss, crushed by its own tremendous weight, needed a lot lot of radiation to keep from imploding. So it ran through the hydrogen fuel it used to fuse quickly, and a detritus of helium ash collected in its core. of radiation to keep from imploding. So it ran through the hydrogen fuel it used to fuse quickly, and a detritus of helium ash collected in its core.

"But that 'ash' can fuse too," the Amba.s.sador said. "The fusion process produces such elements as carbon, oxygen, silicon, each of which fuses in turn. The chain ends in iron, which cannot fuse, for if it did so it would absorb absorb energy, not release it. And so an inner core of iron builds up at the heart of a star like this. A core bigger than most worlds, Donn Wyman!" energy, not release it. And so an inner core of iron builds up at the heart of a star like this. A core bigger than most worlds, Donn Wyman!"

Five asked, "So how come it doesn't just collapse?"

"Its components are already crushed together as far as they will go. This is a property of atomic matter. Humans know it as the Pauli exclusion principle. Of course, in time, as the dead zone spreads through the heart of the star, the repulsion will finally be overcome. Electrons will be forced to merge with protons, producing neutrons-a neutron star will be born, smaller and denser than the iron core. And then there will be a collapse of the outer layers, a catastrophic one. But not yet, not for a long time; this star is stable."

"Or it was, was, before you came along," Donn said. "But now you're changing things, aren't you? Planck's constant again?" before you came along," Donn said. "But now you're changing things, aren't you? Planck's constant again?"

"Jack Raoul would be proud of you, Donn. Like you, he was a good guesser."

"If you were to use your moon-machine to reduce Planck in the star's core "

"Then Pauli repulsion would be reduced. The iron core would collapse prematurely."

The Ghost showed them a Virtual representation of what would happen next. The implosion would rapidly mutate into an explosion. Shock waves would form and rebound from the inner layers, and a vast pulse of neutrinos would power further expansion.

"The Boss will be blown apart," said Donn, wondering.

"Yes. A detonation over in seconds, after years of preparation. But the explosion will be asymmetrical, that layer heated by the neutrinos turbulent. This is the key to such explosions, and it is this turbulence we are hoping to control. For the asymmetry will blast the neutron star out of the debris of the Boss it will leave with a significant velocity while releasing a pulse of gravitational wave energy that we would hope to tap and "

"A supernova," Five said. "That's what you're talking about, isn't it? You're going to turn the Boss into a supernova."

"We believe it will be the first artificial artificial detonation of its kind in the evolution of the universe. A supernova used as a cannon to fire out a neutron star, directed as we please! History is watching us, Donn Wyman." detonation of its kind in the evolution of the universe. A supernova used as a cannon to fire out a neutron star, directed as we please! History is watching us, Donn Wyman."

The Silverman comically raised its stump of an arm. "Magnificent!"

Donn paced around. "You're insane."

"Now you do sound like Jack Raoul," said the Amba.s.sador.

"You will devastate worlds!"

"Actually, stars, too," said the Amba.s.sador. "Nearby stars stars will be boiled away." will be boiled away."

"And the Reef," Donn said grimly. "Surely, we're too close to survive."

Five said, "The Reef is ships joined up together, isn't it? I don't remember. You could just fly away."

"We don't have hyperdrive," Donn said. "Confiscated by the Coalition for their Navy ships. I don't imagine they will be handing them back." He turned on the Amba.s.sador. "This is ma.s.s murder. Why are you doing this?"

"Because of the Seer." The new voice was a woman's: Eve Raoul's. Donn heard her words moments before a cloud of pixels popped into existence and coalesced into her thin form.

She stepped to the edge of the platform. "My. Quite a view. Quite a drop, too." She reached out absently, but none of them had a Virtual hand to offer her, and she stepped back.

"I wasn't expecting to see you again," Donn said.

"Well, I didn't expect to be revived again," she said with a trace of bitterness. "At least I'm not in any pain this time. I guess it's good to be useful."

"Useful how?"

The Amba.s.sador said, "Eve is helping us understand an ent.i.ty of our own creation, an ent.i.ty whose wishes have brought us all here today."

Donn's heart thumped. "You mean the Seer."

"Turn around, Five, Donn."

They turned. The Silverman was holding, in his one hand, a box, a tetrahedron ten centimeters or so to an edge. It seemed to have clear walls, and its interior was black and full of stars, stars that swarmed that, at any rate, was Donn's first impression. Five and Donn both stepped closer to look. The "stars" were no more than dust motes, pushed to and fro by random currents in whatever air filled the box.

Donn said, "It's like a toy. What is it?"

"The Seer," Eve said.

The Amba.s.sador said, "The control of the core of a giant star during a catastrophic explosion is ferociously difficult. Even modeling it was beyond our processing resources. So we devised a new generation of AI."

Five said, "This box of dust?"

"This box of dust," Eve said, "is about the most advanced AI we're aware of. For a machine like this, physically you need components that are small enough to be influenced by quantum effects, yet large enough to feel the effects of gravity. A swarm of smart microprobes dust motes."

"A machine like what?"

"A quantum gravity AI."

"On the Miriam, Miriam, we have quantum AIs," Donn said. we have quantum AIs," Donn said.

"Right," Eve said, nodding. "And that gives you an edge. A simple switch can only be in one state at a time on or off. A quantum switch holds information about all all possible states of the switch at any one time. And so you can use it to do parallel processing. Many inputs, many outputs. You get a speed advantage, and a significant one. possible states of the switch at any one time. And so you can use it to do parallel processing. Many inputs, many outputs. You get a speed advantage, and a significant one.

"But a quantum gravity quantum gravity machine goes one step further. You abandon causality altogether." machine goes one step further. You abandon causality altogether."

The blurring of position and velocity in quantum mechanics made traditional causality problematic. And in relativity, too, lightspeed limits ensured that causality was more an aspiration than an iron law.

Donn started to see. "And if you put quantum mechanics and relativity together "

"In a quantum gravity computer, cause and effect are thoroughly mixed up. You don't even need to have input before output, causally. In practice, I think, the Seer is able to glimpse the outline of a solution to a given problem even before it has begun its calculation, and so can guide its processing efficiently to that outcome. In retrospect, its thinking feels like guesswork, an unlikely series of inductive leaps. But it's always right, and very, very fast."

"The Seer really can see the future," Five said, "just as the rumors say."

"But its visions are limited to the outcomes of computing algorithms a few microseconds ahead-or to the furthest future, millennia or more away."

Five glared at the Amba.s.sador. "So why the tetrahedron, fatball? Why is this ultimate brain in a box the shape of the symbol of human freedom?"

"A tetrahedron was the most suitable shape for "

"It's a totem, that's what I think. Some of the Samples believe Ghosts are starting to worship humans, because we're becoming so good at killing you. Hence the Silvermen, walking human statues. Hence the tetrahedral box."

The Amba.s.sador said evenly, "We Ghosts do have a propensity for worshipping that which destroys us, it is true. But you are not yet a G.o.ddess, Sample 5A43."

Donn said sharply, "Enough. Eve, you said how the Seer's thinking feels. feels. How can you know that?" How can you know that?"

"Ah. Good question. Because, not for the first time, the Ghosts had trouble with an AI that ended up not performing quite quite as specified." as specified."

"Like the Silverman."

"Well, yes. And, not for the first time, I, or an avatar of myself, was asked to help interpret for it." She looked at Donn, her gray hair shining in the light of the stellar core. "It sees the future, Donn. And it is afraid."

Donn watched Eve. Her eyes were unfocused, and he thought her representation was degrading, her skin smoothing from lack of definition, a lock of her gray hair flickering. He wondered how it must be to be be her, a representation every bit as sentient as he was, and yet having endured multiple lives already and now locked into a consciousness like no other. her, a representation every bit as sentient as he was, and yet having endured multiple lives already and now locked into a consciousness like no other.

She said, "The Seer is sentient, born of dust into a baffling, acausal universe. But it is a Ghost artifact. And so it shares Ghost values, Ghost a.s.sumptions. The Ghosts survived the death of their world through symbiosis, dissimilar life-forms gathering together. The Ghosts have faith that the life-forms of this era of the universe, a transient age of light- and water-based chemistry, will similarly use cooperation and symbiosis to survive the transition to the new cold age to come when the last star dies."

Five shuddered. "How can you think think like that?" like that?"

"This has happened already, in the universe's history," Eve said. "There are life-forms extant now, in this age of matter, that are survivors of earlier epochs, the age of radiation and of annihilation and of superforces and of "

"Enough," Donn said. Her speech had become rapid, automated, as if she was hypnotized. "Go on, Eve."

"But when this age ends, when dark energy comes to predominate and the fabric of s.p.a.ce-time is torn apart when this happens, and the Seer can see see it there will be no Ghost left alive to witness it, and no symbiotic descendant of the Ghosts." it there will be no Ghost left alive to witness it, and no symbiotic descendant of the Ghosts."

"Why not?"

"Because of us," us," said Five savagely. "Because of humans. said Five savagely. "Because of humans. We win. We win. I don't need a quantum-gravity computer to tell me that!" I don't need a quantum-gravity computer to tell me that!"

"You must understand," the Amba.s.sador said. "The detonation of this sun-we do this because we, this Ghost enclave, have been cut off from our home range by the forces of your Coalition. Billions of individuals, a whole world, trapped behind the lines. We were desperate. We looked for a way to change the parameters the rules of the game. That is our way of resolving problems. We were looking for a way out. Now we see we must do more than that; we must take the Seer and its dreadful counsel to our home ranges. We need time to consider what must be done."

"Such as what?" Five asked.

"Such as escape."

Escape to where? where? Donn wondered. Where could the Ghosts go to escape a rampant, Coalition-led mankind? Out of the Galaxy? Out of the Donn wondered. Where could the Ghosts go to escape a rampant, Coalition-led mankind? Out of the Galaxy? Out of the cosmos cosmos altogether? He tried to focus on his own situation. "Then why have you brought us here? Why tell us this?" altogether? He tried to focus on his own situation. "Then why have you brought us here? Why tell us this?"

"Because of me," said the Silverman. He stepped forward, still cradling the Seer. "You made me smart to punish me. But I am human enough to guess how you would feel about an exploding star."

The Amba.s.sador said, "We did not mean to engineer this star as an act of war, only as a means of escape. We understand now that humans might not see it that way."

"You really don't get human psychology, do you?" Donn said.

"No," said the Silverman cheerfully. "Donn Wyman, you must warn your people. Make them believe, as we could not. Make them flee. And make them believe the Ghosts did not mean war."

"That's a tall order."

"You are our only hope," the Ghost said simply.

Donn felt empty. Could it be true that so much was pivoting on this moment? Because if so, Because if so, he thought, I he thought, I am not strong enough to deal with it. am not strong enough to deal with it.

"You don't have much time," said the Amba.s.sador. It floated toward the lip of the platform.

Donn followed and looked down at the engineered moon. Ghosts swarmed, pinp.r.i.c.ks of dazzling light against the worked regolith. "How long?"

"The mathematics is uncertain."

"There are human colonies scattered through the a.s.sociation," Donn said. "Many of them have hyperdrive, I think. But the main human concentration is the Reef. And we no longer have hyperdrive."

"Ask your father," the Amba.s.sador said.

"What?"

"I, too, approved of the Silverman's wish to contact you personally, Donn Wyman. Because I know that your family has resources. We will send you home now, Donn Wyman."

They pulled back and stood in a row, the Silverman with the tetrahedral box, the looming Ghost, and the Virtual of Eve, gradually disintegrating.