“Snorkel. Why?”
“Did you ever happen to look up while you did, and see something unusual?”
One January after term finals, she’d been among a bunch of cadets from the Academy who’d flown down to the Virgin Islands. What a week they’d had! Wind surfing. Paragliding. Beach volleyball. A day trip to a Mayan ruin on the mainland. And one day they had all gone snorkeling above a reef, the coral vibrant with sparkling whites, warm pastels, and shocking pinks. She remembered the fish, hundreds, sometimes thousands of them, darting and weaving in formation like flocks of birds.
And one giant sea turtle.
She remembered Liam, her boyfriend at the time, pointing upward when Dana looked to see who had grabbed her ankle. The turtle, blue with white speckles, longer nose to tail than she was tall, gliding majestically overhead. The top of the turtle—it was covered in overlapping leather plates rather than by a hard shell—seen as clearly as its underside. The turtle’s back was reflecting off the smooth boundary between sea and sky.
“Total internal reflection.” She thought the startling effect had something to do with the indices of refraction of the neighboring media. Beyond some critical angle dependent on the ratio (if she remembered that correctly), all light striking the boundary bounced off. “Sure, I’ve encountered it. What’s that have to do with our situation?”
“You would be surprised. Whatever you saw while snorkeling involved light in the water unable to cross into the air, never light in the air kept from penetrating into the water. That’s one example of the rule: total internal reflection happens within the medium having the higher index of refraction. That’s the medium with the slower speed of light.”
Dana flexed her drink bulb, almost empty, as she pondered. “Near the string, where light is uncharacteristically fast, starlight bounces off?”
Blake nodded. “Except for incoming light that is all but perpendicular to the string, that’s the case. The closer Endeavour approached the string, the faster the local light speed and the less starlight would reach us.”
So much for a turtle’s back, crystal clear in reflection. Marvin’s challenge had been more like a nighttime plunge into the ocean depths. She tried to imagine following the distant, unseen surface with nothing to guide her but occasional faint glimmers of starlight.
The tepid dregs in her drink bulb couldn’t touch her resurgent chill. She hoped Blake hadn’t noticed her trembling.
Dana said, “Leaving Marvin to infer the location of the cosmic string from the level of incident starlight.” She shifted her empty bulb from hand to hand. “Still, when the string twisted or curved or got denser unexpectedly, when the starlight went away, how did he know which way to veer? The distribution of stars isn’t all that constant.”
“Know? A cynic would say, ‘guess.’” With a sour expression, Blake set down his plate, the food untouched. “Marvin didn’t always know which way to turn, especially as external sensors degraded. Whenever it guessed wrong, whenever by accident it veered the ship toward the string, it needed both drives to back away to safety.”
No wonder they had all but exhausted their supply of deuterium!
Dana said, “I’d call Marvin a freaking genius, except for one thing. We aren’t supposed to be here. Why didn’t we break free from the string after a couple of light-years?” Like we told Marvin to do.
“We were trapped,” Blake said. “Breaking free would have involved prolonged thrusting at right angles to the string. We’d have been broadside to the oncoming radiation all the while. It was bad enough that we got short blasts of radiation with every orbit-maintenance maneuver.”
Dana considered. Endeavour’s only meaningful shielding was fore and aft: what they had retrofitted near the bow, plated over the decks of the bridge and crew quarters, and around the fusion reactor. Perhaps more than the long years of cold sleep were behind how crappy she felt, and Carlos’s deterioration. “So breaking away would have given us a lethal dose.”
“Other than by flying off an end of the cosmic string, yeah. And the attempt would have fried Marvin, too. Its circuits aren’t much happier with radiation than our cells are.”
“So we flew to the end,” she said.
“As fast as the ship could deliver us there.”
“What if the string had been a lot longer?”
“Antonio says this string couldn’t have been too much longer,” Blake said. “Much more mass would have had a visible effect on the local distribution of stars. A longer cosmic string would have been detected centuries ago.”
Dana preferred that answer to the notion that once Endeavour exhausted its deuterium, they would have been done for.
Blake yawned, and Dana followed. It was ridiculous that after forty-five years asleep, her body cried out for a nap. Ridiculous but not to be denied for much longer.
She said, “Bottom-line it for me. Where the hell are we?”
Blake yawned again. “As the interstellar-capable crow flies, about one hundred light-years beyond the Coalsack. Call it seven hundred forty light-years from Sol system. But we took the scenic route: sweeping around the nebula, not through it. If the string had run through the cloud, gravity would have collapsed a big clear channel through the Coalsack long ago.”
“Any idea how far we actually came? As the ship actually flew?”
“Marvin knows how long we accelerated and how hard. It knows how long we coasted. From that, at least a thousand light-years. Jumoke would have been proud of her DED: we peaked out a little above thirty-nine times normal light speed.”
Thirty-nine times? Fuzzy-brained from exhaustion, or radiation, or years in cold sleep, stifling yet another yawn, Dana had to ask, “At least a thousand light-years?”
“Will you quit that, please,” Blake yawned back. “Yeah, at least. The estimates rely on elapsed time as measured by the ship’s clock, and in our own frame of reference, by definition, our clock ticks normally. Did we experience time dilation relative to home? We don’t know.
“So: the estimates are in
