Greg Egan: The Planck Dive

“No. She woke as soon as she arrived.”

That seemed even stranger than their excess baggage. If you were traveling with someone, why not delay activation until your companion caught up? Or better yet, package yourselves as interleaved bits?

“But she’s still in the arrival lounge?”

“Yes.”

Gisela hesitated. “Shouldn’t I wait until the other one’s all here? So I can greet them together?”

“No.” The mayor seemed confident on this point. Gisela wished interpolis protocol allowed non-sentient software to play host; she felt woefully ill-prepared for the role herself. But if she started consulting people, seeking advice, and looking into Athena’s culture in depth, the visitors would probably have toured Cartan and gone home before she was ready for them.

She steeled herself, and jumped.

The last person who’d whimsically redesigned the arrival lounge had made it a wooden pier surrounded by gray, windswept ocean. The first of

the two visitors was still standing patiently at the end of the pier, which was just as well; it was unbounded in the other direction, and walking a few kilodelta to no avail might have been a bit dispiriting. Her fellow traveler, still in transit, was represented by a motionless placeholder. Both icons were highly anatomical-realist, clothed but clearly male and female, the unfrozen female much younger-looking. Gisela’s own icon was more stylized, and her surface, whether “skin” or “clothing”—either could gain a tactile sense if she wished—was textured with diffuse reflection rules not quite matching the optical properties of any real substance.

“Welcome to Cartan. I’m Gisela.” She stretched out her hand, and the visitor stepped forward and shook it—though it was possible that she perceived and executed an entirely different act, cross-translated through gestural interlingua.

“I’m Cordelia. This is my father, Prospero. We’ve come all the way from Earth.” She seemed slightly dazed, a response Gisela found entirely reasonable. Back in Athena, whatever elaborate metaphoric action they’d used to instruct the communications software to halt them, append suitable explanatory headers and checksums, then turn the whole package bit-by-bit into a stream of modulated gamma rays, it could never have fully prepared them for the fact that in a subjective instant they’d be stepping ninety-seven years into the future, and ninety-seven light-years from home.

“You’re here to observe the Planck Dive?” Gisela chose to betray no hint of puzzlement; it would have been pointlessly cruel to drive home the fact that they could have seen, everything from Athena. Even if you fetishized realtime data over lightspeed transmissions, it could hardly be worth slipping one-hundred-and-ninety-four years out of synch with your fellow citizens.

Cordelia nodded shyly, and glanced at the statue beside her. “My father, really…”

Meaning what? It was all his idea? Gisela smiled encouragingly, hoping for clarification, but none was forthcoming. She’d been wondering why a Prospero had named his daughter Cordelia, but now it struck her as only prudent—if you had to succumb to a Shakespearean names fad at all—not to put anyone from the same play together in one family.

“Would you like to look around? While you’re waiting for him?”

Cordelia stared at her feet, as if the question was profoundly embarrassing.

“It’s up to you.” Gisela laughed. “I have no idea what constitutes the polite treatment of half-delivered relatives.” It was unlikely that Cordelia did, either; citizens of Athena clearly didn’t make a habit of crossing interstellar distances, and the connections on Earth all had so much bandwidth that the issue would never arise. “But if it was me in transit, I wouldn’t mind at all.”

Cordelia hesitated. “Could I see the black hole, please?”

“Of course.” Chandrasekhar possessed no blazing accretion disk—it was six billion years old, and had long ago swept the region clean of gas and dust—but it certainly left the imprint of its presence on the ordinary starlight around it. “I’ll give you the short tour, and we’ll be back long before your father’s awake.” Gisela examined the bearded icon; with his gaze fixed on the horizon and his arms at his sides, he appeared to be on the verge of bursting into song. “Assuming he’s not running on partial data already. I could have sworn I saw those eyes move.”

Cordelia smiled slightly, then looked up and said solemnly, “That’s not how we were packaged.”

Gisela sent her an address tag. “Then he’ll be none the wiser. Follow me.”

They stood on a circular platform in empty space. Gisela had inflected the scape’s address to give the platform “artificial gravity”—a uniform one gee, regardless of their motion—and a transparent dome full of air at standard temperature and pressure. Presumably all Athena citizens were set up to ignore any scape parameters that might cause them discomfort, but it still seemed like a good idea to err on the side of caution. The platform itself was a compromise, five delta wide—offering some protection from vertigo, but small enough to let its occupants see some forty degrees below “horizontal.”

Gisela pointed. “There it is: Chandrasekhar. Twelve solar masses. Seventeen thousand kilometers away. It might take you a moment to spot it; it looks about the same as the new moon from Earth.” She’d chosen their coordinates and velocity carefully; as she spoke, a bright star split in two, then flared for a moment into a small, perfect ring as it passed directly behind the hole. “Apart from gravitational lensing, of course.”

Cordelia smiled, obviously delighted. “Is this a real view?”

“Partly. It’s based on all the images we’ve received so far from a whole swarm of probes—but there are still viewpoints that have never been covered, and need to be interpolated. That includes the fact that were almost certainly moving with a different velocity than any probe that passed through the same location—so we’re seeing things differently, with different Doppler shifts and aberration.”

Cordelia absorbed this with no sign of disappointment. “Can we go closer?”

“As close as you like.”

Gisela sent control tags to the platform, and they spiraled in. For a while it looked as if there’d be nothing more to see; the featureless black disk ahead of them grew steadily larger, but it clearly wasn’t going to blossom with any kind of detail. Gradually, though, a congested halo of lensed images began to form around it, and you didn’t need the flash of an Einstein ring to see that light was behaving strangely.

“How far away are we now?”

“About thirty-four M.” Cordelia looked uncertain. Gisela added, “Six hundred kilometers—but if you convert mass into distance in the natural way, that’s thirty-four times Chandrasekhar’s mass. It’s a useful convention; if a hole has no charge or angular momentum, its mass sets the scale for all the geometry: the event horizon is always at two M, light forms circular orbits at three M, and so on.” She conjured up a spacetime map of the region outside the hole, and instructed the scape to record the platform’s world line on it. “Actual distances traveled depend on the path you take, but if you think of the hole as being surrounded by spherical shells on which the tidal force is constant—something tangible you can measure on the spot—yon can give them each a radius of curvature without caring about the details of how you might travel all the way to their center.” With one spatial dimension omitted to make room for time, the shells became circles, and their histories on the map were shown as concentric translucent cylinders.