Greg Egan: The Planck Dive

Cordelia raised the binoculars and looked sideways, around the hole. “Why can’t I see us?”

“Good question.” Gisela drew a light ray on the map, aimed sideways, leaving the platform just after they’d crossed the horizon. “At the three-M shell, a ray like this would have followed a helix in spacetime, coming back to our world line after one revolution. But here, the helix has been flipped over and squeezed into a spiral—and at best, it only has time to travel halfway around the hole before it hits the singularity. None of the light we’ve emitted since crossing the horizon can make it back to us.

“That’s assuming a perfectly symmetrical Schwarzchild black hole, which is what we’re simulating. And an ancient hole like Chandrasekhar probably has settled down to a fair approximation of the Schwarzchild geometry. But close to the singularity, even infalling starlight would be blue-shifted enough to disrupt it, and anything more massive—like us, if we really were here—would cause chaotic changes even sooner.” She instructed the scape to switch to Belinsky-Khalatnikov-Lifshitz geometry, then restarted time. The stars began to shimmer with distortion, as if seen through a turbulent atmosphere, then the sky itself seemed to boil, red shifts and blue shifts sweeping across it in churning waves. “If we were embodied, and strong enough to survive the tidal forces, we’d feel them oscillating wildly as we passed through regions collapsing and expanding in different directions.” She modified the spacetime map accordingly, and enlarged it for a better view. Close to the singularity, the once-regular cylinders of constant tidal force now disintegrated into a random froth of ever finer, ever more distorted bubbles.

Cordelia examined the map with an expression of consternation. “How are you going to do any kind of computation in an environment like that?”

“We’re not. This is chaos—but chaotic systems are highly susceptible to manipulation. You know Tiplerian theology? The doctrine that we should try to reshape the universe to allow infinite computation to take place before the Big Crunch?”

“Yes.”

Gisela spread her arms to take in all of Chandrasekhar. “Reshaping a black hole is easier. With a closed universe, all you can do is rearrange what’s already there; with a black hole, you can pour new matter and radiation in from all directions. By doing that, we’re hoping to steer the geometry into a more orderly collapse—not the Schwarzchild version, but one that lets light circumnavigate the space inside the hole many times.-

Cartan Null will be made of counter-rotating beams of light, modulated with pulses like beads on a string. As they pass through each other, the pulses will interact; they’ll be blue-shifted to energies high enough for pair-production, and eventually even high enough for gravitational effects. Those beams will be our memory, and their interactions will drive all our computation—with luck, down almost to the Planck scale: ten-to-the-minus-thirty-five meters.”

Cordelia contemplated this in silence, then asked hesitantly, “But how much computation wdl you be able to do?”

“In total?” Gisela shrugged. “That depends on details of the structure of spacetime at the Planck scale—details we won’t know until we’re inside. There are some models that would allow us to do the whole Tiplerian thing in miniature: infinite computation. But most give a range of finite answers, some large, some small.”

Cordelia was beginning to look positively gloomy. Surely she’d known about the Divers’ fate all along?

Gisela said, “You do realize we’re sending in clones? No one’s moving their sole version into Cartan Null!”

“I know.” Cordelia averted her eyes. “But once you are the clone… won’t you be afraid of dying?”

Gisela was touched. “Only slightly. And not at all, at the end. While there’s still a slender chance of infinite computation—or even some exotic discovery that might allow us to escape—we’ll hang on to the fear of death. It should help motivate us to examine all the options! But if and when it’s clear that dying is inevitable, we’ll switch off the old instinctive response, and just accept it.”

Cordelia nodded politely, but she didn’t seem at all convinced. If you’d been raised in a polis that celebrated “the lost flesher virtues,” this probably sounded like cheating at best, and self-mutilation at worst.

“Can we go back now, please? My father will be awake soon.”

“Of course.” Gisela wanted to say something to this strange, solemn child to put her mind at ease, but she had no idea where to begin. So they jumped out of the scape together—out of their fictitious light cones—abandoning the simulation before it was forced to admit that it was offering neither the chance of new knowledge, nor the possibility of death.

When Prospero woke, Gisela introduced herself and asked what he wished to see. She suggested a schematic of Cartan Null; it didn’t seem tactful to mention that Cordelia had already toured Chandrasekhar, but offering him a scape that neither had seen seemed like a diplomatic way of side-stepping the issue.

Prospero smiled at her indulgently. “I’m sure your Falling City is ingeniously designed, but that’s of no interest to me. I’m here to scrutinize your motives, not your machines.”

“Our motives?” Gisela wondered if there’d been a translation error. “We’re curious about the structure of spacetime. Why else would someone dive into a black hole?”

Prospero’s smile broadened. “That’s what I’m here to determine. There’s a wide range of choices besides the Pandora myth: Prometheus, Quixote, the Grail of course… perhaps even Orpheus. Do you hope to rescue the dead?”

“Rescue the dead?” Gisela was dumbfounded. “Oh, you mean Tiplerian resurrection? No, we have no plans for that at all. Even if we obtained infinite computing power, which is unlikely, we’d have far too little information to recreate any specific dead fleshers. As for resurrecting everyone by brute force, simulating every possible conscious being… there’d be no sure way to screen out in advance simulations that would experience extreme suffering—and statistically, they’re likely to outnumber the rest by about ten thousand to one. So the whole thing would be grossly unethical.”

“We shall see.” Prospero waved her objections away. “What’s important is that I meet all of Charon’s passengers as soon as possible.”

“Charon’s….? You mean the Dive team?”

Prospero shook his head with an anguished expression, as if he’d been misunderstood, but he said, “Yes, assemble your ‘Dive team.’ Let me speak to them all. I can see how badly I’m needed here!”

Gisela was more bewildered than ever. “Needed? You’re welcome here, of course… but in what way are you needed?”

Cordelia reached over and tugged at her father’s arm. “Can we wait in the castle? I’m so tired.” She wouldn’t look Gisela in the eye.

“Of course, my darling!” Prospero leant down and kissed her forehead. He pulled a rolled-up parchment out of his robe and tossed it into the air. It unfurled into a doorway, hovering above the ocean beside the pier, leading into a sunlit scape. Gisela could see vast, overgrown gardens, stone buildings, winged horses in the air. It was a good thing they’d compressed their accommodation more efficiently than their bodies, or they would have tied up the gamma ray link for about a decade.

Cordelia stepped through the doorway, holding Prospero’s hand, trying to pull him through. Trying, Gisela finally realized, to shut him up before he could embarrass her further.

Without success. With one foot still on the pier, Prospero turned to Gisela. “Why am I needed? I’m here to be your Homer, your Virgil, your Dante, your Dickens! I’m here to extract the mythic essence of this glorious, tragic endeavor! I’m here to grant you a gift infinitely greater than the immortality you seek!”