by Greg Egan
Brian couldn’t sleep, so as midnight approached he rose quietly and dressed in the dark. He did his best not to disturb Carol, but he knew that even if he woke her she’d pretend that he hadn’t.
His binoculars were sitting on the table in the hall, and his boots were by the door. He put them on, wincing at the pain in his right knee, then he closed the door gently behind him and strode away from the farmhouse.
It was a perfect night, with no moon and no clouds. Scorpius had just risen in the east, Antares glinting as red as Mars, and from there the whole glorious band of the Milky Way stretched from horizon to horizon.
Brian stopped and sat on an old fencepost, a lone stump of wood that had been there since he was a child, though he had no memory of the larger structure it must once have belonged to. He raised the binoculars and swept them slowly across the dark dust clouds and bright clusters.
Three years before, on a night just like this, he’d spotted a comet no one else had yet seen. When the astronomers calculated its orbit, it had turned out to have a period of ninety thousand years. But no one could be sure that it wasn’t making its one and only appearance; if it had been sent inward by a disturbance in the Oort cloud, it might well suffer another course change, robbing it of a second dalliance with the sun. Even his cosmic namesake might not outlive him as anything more than a frozen corpse.
One of the old dogs, Hera, came limping toward him, whining softly. Brian held out a hand to her, and she nuzzled it. It seemed obvious now that Hera had smelled the cancer in him before he was diagnosed, before he’d even noticed the symptoms. But at the time, he’d assumed that the dog’s melancholy was a symptom of her own declining health.
Hera settled at his feet. Brian turned back to the sky, tracking the binoculars along the ecliptic. Every star and nebula seemed familiar, though he wondered how much detail he really did hold in his memory. The comet had been diffuse enough that there’d been no mistaking it for a star, but he might not have noticed an asteroid in exactly the same place.
He lowered the binoculars and stretched his shoulders. It was cold, and he had to drive to the hospital in the morning.
He stood and looked around, wanting to savor the whole glorious sky one more time before retreating to the warmth of his bed. The Southern Cross was high, a dagger hanging over the celestial pole, while the Small Magellanic Cloud was clipped by the trees along the farm’s boundary.
Some way left of the pole, a pale, steady dot hung in the sky, right above Nu Octantis, about level with Eta Pavonis, and a little brighter than both. Which was not to say much, except that Brian could not recall seeing a star in that position before.
He waited half a minute, expecting the thing to move, but it stayed put, so he lifted the binoculars. What he saw was not a satellite or an aircraft, but a small, tight cluster of stars: dozens at least, all contained within a neat, circular region.
He could have sworn there was no cluster like this in Octans. He’d have to check his Norton’s once he was back in the house, but if this was new . . . what could it be? Dozens of supernovae, all in the same galaxy? All exploding within days of each other—or rather, in some even less likely sequence that brought their light-bursts to Earth in near-perfect synch?
Brian laughed, bemused. He spent a few minutes checking that he hadn’t made some foolish error and ended up disoriented, but he wasn’t mistaken about the location. Then he turned the binoculars on the cluster once more, just in case he’d missed some vital clue that might explain the stars’ shared fate. But if anything, they only seemed more disparate than he’d realized, with none of the sibling resemblance that stars born together sometimes shared.
It was baffling. But he wasn’t going to solve this himself, standing in a paddock getting chilled to the bone. “Come on girl,” he said. “Time to spread the word and get some fresh eyes on this.”
Hera rose, and they set off together for the farmhouse.
* * *
Fatima woke on the second ring and reacted in time to smother the third as she picked up the phone from her bedside table.
“Yes?” she whispered hoarsely, not waiting to check the caller on the screen. Salif hadn’t stirred, but she turned away from him, sandwiching the phone between her ear and the pillow.
“Sorry to wake you, Dr. Benga.” It was Gabrielle, one of her postdocs.
“No problem. What’s happening?” Fatima didn’t think Gabrielle was observing tonight, but she was on the roster for external alerts.
“There’s some kind of transient,” Gabrielle explained. “We really need to check it out. If we could get some time on the AAT—”
“Hang on, where’s this coming from?”
“A farmer in New Zealand. He emailed the department.”
“A farmer saw a light in the sky?”
Gabrielle said, “His name’s Brian Farley. He discovered a comet a few years ago. He’s not some crackpot who saw Venus in his rearview mirror and decided it was a UFO.”
“Okay.” Fatima remembered the comet. “So what’s this transient?”
Gabrielle hesitated. “Multiple stellar-like sources, all in close angular proximity. I’ve taken a look myself through a thirty-centimeter instrument, but I have no idea what to make of it.”
Salif rolled over, muttering incoherently.
“Hang on a sec.” Fatima slipped out of bed and grabbed a robe, then walked into the hallway and headed for her study. “Have you put something on the Astronomer’s Telegram?”
“Not yet. I don’t know how I should describe it.”
“Multiple sources?” Fatima was fully awake now, but that wasn’t making any of this clearer.
“At least sixty. Across about eight arc-minutes. But there’s no structure they could belong to in the catalogs, or on past plates.”
“Taken together, it’s a naked eye object, about magnitude four.”
Fatima booted up her desktop. How did sixty flashbulbs go off together in the same small patch of sky—too close to be any kind of coincidence, too far apart to share a common cause?
“Do you have an image you can mail me?”
When the file came through, it looked like some kind of collage. In the center of the frame, a circular region contained a modestly dense star field—nothing special in itself, but it seemed to have been cut out of an image taken in the star-rich galactic plane, and then pasted into a different one with a substantially sparser background, as befitted the region’s actual coordinates. A sudden shift in line-of-sight density like this could always arise by pure chance. But unless both Farley and Gabrielle were horribly confused, none of the stars in the center here had been present the last time anyone looked.
“I’m sure I can get you time on the AAT,” she told Gabrielle. “We need to take some spectra, for a start.”
“What do you think this is?”
Fatima stared at the image. “Maybe some kind of lensing event?” That didn’t make much sense, though. A chance alignment between a black hole and a distant cluster of stars might magnify and brighten the cluster’s image, but the scale and the geometry weren’t really compatible with that: with this much brightening, whatever was magnified should also have been warped into a pair of arcs centered on the hole. “Put out a telegram just saying what you see; there’s no need to try to interpret it. I’ll make the booking straight away, and send you the details.”
Fatima was able to get an emergency slot starting at two a.m. At least Octans was so close to the pole that it never set over Siding Spring.
She emailed Gabrielle and went back to bed, but then she lay awake pondering the discovery, struggling to piece together some kind of viable hypothesis. What if the gravitational lens responsible was more complex than a single black hole? Maybe two or three foreground galaxies—too dim and distant to show up themselves—were working in concert to produce the image, partly correcting each other’s distortion.
At four o’clock, Fatima got up and checked her emails, but Gabrielle hadn’t sent her anything yet. She walked down to the study and called her.
“I’m still looking at the data,” Gabrielle explained. “I don’t want to make a fool of myself.”
“But what do you have so far?” Fatima pressed her.
“No supernovae. They’re just a whole lot of main sequence stars, with nothing special going on.”
“What about red shift?”
“All less than ten to the minus four.” Gabrielle sounded almost apologetic, as if the phenomenon’s stubbornly inconsistent details were her fault.
“Okay.” So the stars were not in a distant galaxy—and it was unlikely that even a single gravitational lens lay in front of them.
“There’s something else.”
Gabrielle said, “Between the first exposure I took and the latest, seven new stars appeared.”
Fatima considered that. “Where, exactly?”
“On the edge of the central field.”
If this was a lensing event, and the alignment was delicate enough, maybe the image could change in a matter of hours as the magnified region shifted. “And some stars disappeared on the other edge, right?” Fatima asked.
“No,” Gabrielle replied. “The new stars appeared all around the edge. It’s not shifting, it’s growing larger.”
* * *
Fatima’s computer chimed, then brought up a plot of the latest data from Chile: a time series giving each new star’s angular distance from the center of the anomaly. “The growth looks close to linear,” she mused. “About two thirds of an arc-minute per hour.” But almost anything looked linear on a short enough time scale.
“So is the wormhole staying still and getting larger?” Gabrielle wondered. “Or is it a fixed size, but moving closer?”
“I’ve never believed in wormholes,” Fatima confessed. “Take two in relative motion and you’ve got a time machine. And I definitely don’t believe in time machines.”
“Maybe you can believe in just one wormhole at a time,” Gabrielle replied, deadpan.
“There are schemes that could supposedly turn just one into a time machine,” Fatima recalled. “But . . . never mind. Let’s stick to what we can see.”
She glanced out the window of her office; the late-morning sunshine bathing the campus felt jarring, as if she were jet-lagged. Neither she nor Gabrielle had had a chance to sleep before Cerro Tololo took over the observations and fresh data came flooding in, but there’d have to come a point where they worked out a roster that allowed them to take turns resting.
“Suppose this . . . whatever it is . . . has a fixed geometry, and it’s just moving closer,” Gabrielle proposed tentatively.
Fatima said, “Go ahead and try it out.”
Gabrielle did the calculations. If the changes they’d seen were due entirely to the thing’s motion, it should have been visible telescopically for months before Farley spotted it—but an automated wide-field survey had imaged the area three weeks before and found nothing out of the ordinary. More absurdly, though, pursuing the model into the future implied that whatever it was would arrive at the Earth in about eighteen hours—a prediction that lost its apocalyptic potency when Fatima realized that if the true growth was simply linear, it was now eighteen hours since the zero point . . . and however long after that moment they’d chosen to try matching Gabrielle’s model to the data, the model would have forecast a collision an equal time later.
“I love it when the mathematics throws your assumptions back in your face,” Gabrielle joked ruefully.
“So what if we assume this thing is growing from a fixed center?” Fatima suggested.
This time the verdict was more elusive. If the anomaly was a sphere expanding at a constant rate, then merely observing how fast its angular size was changing couldn’t pin down both the distance to its center and the rate of growth. They could plug in any speed they liked and get a matching distance, or vice versa. But to make the distance suitably astronomical demanded a relativistic velocity for the sphere’s border: at a mere twenty light-years, it would need to be expanding at 99.9 percent of light speed.
Fatima’s phone rang. It was Daniel from the press office, nagging her again to draft a statement staking the university’s claim over this epochal discovery.
“We don’t even know what it is!” she insisted. “If we start putting out half-baked theories, we’ll just make fools of ourselves.”
“It’s all over social media,” Daniel warned her. “In an hour or so, when it hits the news outlets, whoever’s on screen explaining exactly where this wormhole is and what it’s doing will own the story.”
“‘Own the story’? What’s that supposed to mean?”
“They’ll be the one that everyone comes to for the definitive answers.”
“You mean the definitive answers that I just told you I don’t have?”
Daniel tried a different tack. “You made the first observations. Who else should do the honors and explain to the public exactly where the current state of knowledge stands?”
“Brian Farley and Gabrielle Chan made the first observations.”
“Is Ms. Chan with you?”
Fatima held the phone to her chest and whispered, “Do you want your thirty seconds of fame now?”
Gabrielle shook her head vehemently.
“Sorry,” Fatima replied. “She was up all night, and I think she came down with the flu.”
“Send me something,” Daniel pleaded.
“I will, I will,” Fatima promised him soothingly. She hung up. “As soon as I have the faintest clue what’s going on.”
* * *
It was dusk when Fatima finally left the campus. As she waited for the bus she looked to the south, but the Pane—as people were now calling it—was still invisible against the pale sky.
She had to accept that there’d probably be no way to determine the Pane’s true distance and motion until more data was in . . . but every time she tried to set one puzzling aspect of the thing aside, a different one resurfaced. She did not believe in wormholes, but if they existed at all she was pretty sure that they ought to bend light. If you arranged for incoming light rays to converge on one mouth of a spherical wormhole, like pins in a pin-cushion, they’d need to emerge from the other mouth in the same configuration, only now they’d be diverging—without having passed through a common central point, or the wormhole would be blocked by an untraversable bottleneck. So the light needed to make a kind of U-turn, even if the two halves of the U lay at different ends of the wormhole.
But the Pane was proving stubbornly unrefractive. Once a new star appeared within the growing circle, its location didn’t shift at all; this window, which should have distorted the view like a thick concave lens, seemed more like an empty frame. There were toy models of cubical wormholes, and other polyhedra, where all the curvature was concentrated along the edges—but even if this wormhole was shaped more like a geodesic dome than a sphere, the images of the stars should have jumped as they crossed behind the dome’s numerous edges.
When Fatima arrived home she could smell dinner cooking. “Now I remember why I married you!” she called out to Salif.
“Don’t tempt fate,” he replied, as she entered the kitchen. “The minister’s still threatening to relocate his whole department to some benighted country town in his electorate. Who’ll cook for you then?”
“You’ll quit and stay here with me, won’t you?”
“I might check the weather in the new location first. There are only so many winters in Canberra I can take.”
He was smiling, but Fatima recalled the promise she’d made: five years here, then she’d move on. Her position at ANU had always been intended as a step along a path that ultimately took them, if not all the way back to Senegal, at least somewhere closer to home.
They sat down to eat, with the television on. The Pane was unavoidable, but Fatima tried not to wince at all the nonsense it was generating. “We’re almost certainly catching our first glimpse of an alien transport network that crisscrosses the galaxy!” a celebrity string theorist from New York enthused.
“So what we’re seeing is like . . . a new subway station under construction?” the interviewer asked.
“That’s the perfect analogy! But as well as seeing the entrance to the station, we’re seeing all the way through the tunnel to the destination at the other end! In this subway, every tunnel is much shorter than the distance between the stations it joins!”
Fatima could understand his boyish excitement, his yearning to believe that the Universe had been delivered to his doorstep. When she was eight years old, her teacher, Mrs. Ndoye, had broken the astonishing news to the class that astronomers had seen a star wobbling from the tug of a planet orbiting around it. Fatima had snuck out of her room that night and looked up into the sky, hoping to witness the stars trembling for herself.
But when she’d told Mrs. Ndoye that she planned to visit the new-found planet, her teacher had gently nudged her expectations a little closer to reality. “When you’re older you might study this world, and a thousand others like it. But light itself takes years to cross these distances. How about leaving something for your grandchildren to do?”
Salif caught the look on her face. “So why isn’t it you there, getting the facts right?”
“Because we don’t have the facts.”
“None at all? You’re not sure of even one thing?”
Fatima said, “I know we’re seeing the light from stars that we weren’t seeing a few days ago, and I know that the stars themselves show no signs that they’ve suddenly brightened. But I don’t know where these stars are, or why we’re seeing them now, or what we’ll see tomorrow.”
Salif nodded soberly. “All right, I can be patient. I won’t plan my trip around the Galaxy just yet.”
* * *
“This has to tell us something!” Gabrielle declared. The Chilean team had just observed a blue supergiant—HD 183582, some four thousand light-years away—disappearing behind the Pane.
Fatima was trying to stay circumspect, but she could agree up to a point: having a new constraint like this ought to reinvigorate their analysis. Apart from setting a maximum distance, the event itself had been much like the appearance of a new star within the circle, in reverse. There had been no blurring, no bending of the light; the now hidden star had just winked out, as sharply as if it had gone behind the Moon.
“I’ll bet you fifty dollars the next one gets occulted as well,” Gabrielle proposed. HD 184039 was less than nine hundred light-years away, and they’d learn the result by noon the next day.
Fatima smiled. “Why would I take that bet? It’d be too much of a coincidence if the Pane was only just close enough to block the first star we could check.”
Gabrielle seemed to like this answer, but she wanted to push it further. “So let’s assume it could be even closer. Say . . . less than a light-year.”
Fatima didn’t protest. She’d entertain anything for the sake of the argument.
Gabrielle said, “What’s the natural speed for a change in geometry to propagate through empty space?”
“So what if the Pane is expanding that fast?”
“Ummm . . . it would have hit us before we saw it coming.” Fatima scrutinized Gabrielle’s face, wondering just how exhausted she was.
Gabrielle shook her head impatiently. “Suppose it’s not spherical, though—suppose it’s circular. If it’s six-tenths of a light-year way, and its radius is growing at the speed of light, that would match the angular growth rate we’re seeing.”
Fatima could see the attraction of the idea. A growing sphere had to grow slower than light or it would have reached the Earth already, but the Pane as they saw it was expanding so rapidly that either the sphere’s border was moving at some arbitrary but still enormous speed, or it would have to be even closer to the Earth than Gabrielle was now proposing.
“Wormhole mouths are spheres, not disks,” she said.
“You don’t believe in wormholes,” Gabrielle retorted.
“I know.” Fatima thought for a moment. “Maybe a disk would make more sense of the optical properties. If you’re just cutting space-time along two flat surfaces and identifying the cuts, you don’t have the same curvature effects . . . though the rim would still be singular, like a cosmic string under negative tension. It’s hard to see how a structure like that could be expanding at the speed of light.”
“I can’t explain the dynamics,” Gabrielle admitted. “And another catch is that we’re seeing a circle, not an ellipse. What are the odds that a disk would be facing us head-on?”
“Maybe you should sleep on it,” Fatima suggested. “I’ve got a class to teach. We can take this up tomorrow.”
The class was meant to be on methods for observing exoplanets, but Fatima decided to cut her students some slack; even brain surgeons and ambulance drivers were probably debating their own theories of the Pane with their colleagues as they worked, so it would be cruel to expect a roomful of budding astronomers to resist the same urge. She gave them free rein to ask questions or just raise ideas, and did her best to keep anyone, herself included, from shouting down even the craziest suggestions.
“If the Pane actually hit us, would we survive passing through?” Leon asked.
“Do we get to take the Sun along, too?” Fatima joked.
“Why not? If the Pane were so tiny it could only fit the Earth, it would have to be so close that someone would have measured its diurnal parallax by now.”
That was a fair point. Fatima said, “Given what we see with the starlight, the curvature doesn’t look extreme—it’s not on the scale of a solar mass black hole, say. But it wouldn’t take that strong a gravitational field to disrupt the Earth’s orbit.”
“So . . . ?”
“We don’t know,” she said. “Maybe we could survive passing through, maybe not. But even if the Pane is big enough to swallow the Solar System, we have no real evidence to show that it’s moving at all, let alone heading straight toward us.”
Copyright © 2019. The Slipway by Greg Egan