Draq spoke too quickly, made too many weak jokes, and used too many technical phrases Pirius didn’t recognize at all, such as “Mazur-Mottola solutions” and “negative energy density” and “an interior de Sitter condensate phase matched to an exterior Schwarzschild geometry.” But he had a lot of Virtual schematics, immense displays that filled up much of this huge dome’s volume. The special effects were spectacular and entertaining, and Draq, beneath his gaudy creations, gestured like a showman.
A black hole came about if a lump of matter collapsed so far that its surface gravity increased to the point where you would need to travel at lightspeed to escape it. It had happened during the fiery instants after the Big Bang, and could occur nowadays when a giant star imploded. Within its “event horizon,” the surface of no return, still the implosion continued.
Thus the basic geography of a black hole, familiar to every pilot in the Navy. If you fell through the event horizon you could never escape. You would be drawn inexorably into the singularity at the center, a place where the compression forces had exponentiated to the point where spacetime itself was ripped open. Pirius idly watched gaudy displays of exploding stars, Big Bang compression waves, and unlucky smeared-out cartoon pilots.
But now Draq said that the most productive way to think about a black hole was to imagine that the event horizon enclosed a separate universe. After all, nothing within the horizon could ever communicate with anything outside. It was as if a gouge had been ripped out of our spacetime, and another universe patched into the hole. In fact, he said, with an enthusiastic but unwelcome diversion into equations, that was how you dealt with a black hole mathematically.
Inside a conventional black hole, that new baby universe was doomed to implode forever into its singularity. But it didn’t have to be that way. What if that infant universe expanded? After all, that was how the outside universe seemed to behave, and it was possible for gravity to act as a repulsive force: the swelling of the universe itself was being driven by a field of “dark energy” with exactly that property. Draq said that — theoretically anyhow, under certain conditions — the great violence of the collapse of a massive object could shock a region of spacetime into a new configuration. And if that happened, yes, you certainly could create a new baby universe doomed not to collapse, but to expand.
But that expansion was limited. The mass of the collapsing object still drew in matter from the outside world, so there was still an event horizon, the distance of lightspeed escape. But now the horizon was like a stationary shock wave, the place where the infall from the parent universe outside met the expansion of the infant within.
This collision of universes created an “ultra-relativistic fluid,” as Draq called it, like the meniscus on a pond that separated water from air. This exotic stuff was gathered into a shell as thin as a quark, but a spoonful of it would weigh hundreds of tonnes. An unlucky infalling astronaut wouldn’t slide smoothly into the lethal interior, as she would if this were a conventional black hole. Instead, every particle of her mass would have to give up its gravitational energy at the shock front.
This “gravastar” was no black hole; it would blaze brightly with the energy of continual destruction. But even so, Draq said, outlining a paradox Pirius didn’t begin to understand, the temperature of the shell would only be a billionth of a degree above absolute zero.
Pirius failed to see the point of this. But he enjoyed the Virtual fireworks.
Nilis had become increasingly restless as this went on. At last the Commissary lumbered to his feet. “Yes, yes, Commissary Draq, this is all very well. But this is nothing but theory — and antique theory at that. No such ’gravastar’ has ever been observed in nature.”
That was true, Draq conceded. The conditions needed to avoid a simple black-hole collapse were unlikely to occur by chance — an imploding object would need to shed a great deal of entropy to make the gravastar state possible, and nobody knew how that might occur in nature.
Nilis demanded, “Then how do you know the bones of your theory support any meat, eh? And besides, you’re describing spherically symmetric solutions of the equations. If I were to find myself inside a gravastar I would be as cut off and trapped, not to mention doomed to incineration by the shock wave, as if I were in a common black hole! So, Commissary, what use is any of this?”
Draq was clearly nervous, but he fixed his smile like a weapon. “But that’s why we need the Silver Ghosts, Commissary. To go beyond human theory. And to give us experimental verification…”
Nilis joined Draq under his imploding Virtuals, and they launched into a complex and convoluted argument, involving asymptotically matched solutions of partial differential equations and other exotica. Pirius had a pilot’s basic grasp of mathematics, but this was far beyond him.
Mara approached him. She had her hands tucked into the sleeves of her robe. She whispered, “All this is a little rich for my blood, too. Perhaps we should take a walk.” She wouldn’t meet his eyes.
“You don’t want to be with me.”
“No, I don’t,” Mara said. “But it’s my duty to host you. And it’s your duty to understand what we’re doing here on Pluto—”
“Don’t talk to me about duty.”
“ — even if that means you’re going to have to confront your feelings about the Silver Ghosts.”
“Why have I got to ’confront my feelings?’ “ he snapped. “The Ghosts shouldn’t be here. That isn’t a feeling. It’s a fact.”
“What are you scared of?” she asked blandly.
“That’s a stupid question.”
She didn’t react. “It probably is. Will you come?”
He sighed. It was her, or Draq’s partial, differential equations. “All right.”
Suited up, they walked out of the dome. Mara led him perhaps half a kilometer away from the domes of the Christy compound. They didn’t speak.
Once more, the sharp-grained, ultracold frost of Pluto crunched beneath Pirius’s feet, and he tried not to be spooked by the immense mass of Charon poised silently above his head.
They crossed a low ridge, perhaps the worn-down rim of another ancient crater, and approached a new structure. It was an open tangle of cables, wiring, small modules; it looked impractical to Pirius, more like a sculpture. But it seemed oddly familiar, and he dug for the memory, left over from some long-ago training session.
Mara spoke at last. “You understand that the main Ghost reservation, which you saw, is on the far side of the planet. But it was necessary to provide support facilities for the Ghosts who work with us here, at Christy. We decided to take the opportunity to recreate another bit of Ghost technology.”
Then Pirius saw it. “This is a cruiser,” he breathed. “A Ghost cruiser.” Once, millions of ships like this had patrolled the Orion Line, the Ghosts’ great cordon flung across the face of the Galaxy.
The Ghost ship was kilometers long, big enough to have dwarfed the greenship Pirius’s future self would have piloted in the Core. It had nothing like the lines of a human craft. The cruiser was a tangle of silvery rope within which bulky equipment pods were suspended, apparently at random.
And everywhere there were Silver Ghosts, sliding along the silver cables like beads of mercury.
“Of course it’s just a mock-up,” Mara said. “Basically life support. There are no drive units; it can’t fly. And no weapons! I always think it looks more like a forest than a ship. But that’s what it is, in a sense. The Ghosts are like miniature ecologies themselves, and they turned slices of their ecology into their ships. I’ve always thought that was a much more elegant solution than our own clunky mechanical systems.”