“But we’re dealing with a hole of three or four hundred million solar masses.”
Jao burst in: “Why stop at one orbiting black hole? Why not a whole planetary system of them?”
“Exactly,” Jun Davd said. “Of course, a number of additional black holes may exist. In fact, I’d be surprised if they didn’t. And two or more of them may have consolidated to form a larger hole. But nothing big enough to explain what we’re facing here.”
“What, then?”
“What we’ve got here is a binary system, not a planetary system. Both of them have a very rapid spin, they’re very close—only a few diameters apart—and one is at least a fourth the size of the other. In fact, they fit almost perfectly the picture of contact binaries.”
Bram struggled to remember his rusty astronomy. “Contact binaries. When the primordial cloud gains too much angular momentum as it condenses and relieves itself by forming a disk. And the disk condenses into a companion, not a planetary system, because it contains too much material and the lines of magnetic force aren’t strong enough to cause spin-down and the spiraling outward of the disk.” He frowned as if Jun Davd could see him. “But you’re describing the formation of stars.”
Jun Davd’s disembodied voice said, “What, essentially, are black holes?”
Outside the observation wall, the flickering tongue of ionized gas continued to rattle through the spectrum, a slice at a time. How long had Jun Davd said it was? Twenty-five light-years. It had grown another light-year while they were talking.
The viewscreen gave him a better impression of what the tongue was doing. It appeared as a mottled serpent swinging laterally away from them on its whiplash path, swallowing stars as it went.
Bram saw something that must have been a supernova—a wink of brilliant light, that was gone in an instant, on Yggdrasil’s speeded-up time scale. Then another flash and another, each of them bathing space in a cosmic instant of inconceivable radiation, each blowing most of its substance off to add more fuel to whatever was happening here in the heart of the galaxy, leaving its core behind as a neutron star.
Chain reaction!
In the galaxy outside, perhaps forty or fifty days had gone by in the last few minutes. Supernovae weren’t supposed to occur that close together. They were supposed to occur once every century or two.
Now he could see another cosmic jet forming at the hub of whirling gases that marked the central black hole. It whipped around, growing by the light-year, ready to slap Yggdrasil when it was long enough.
“Jun Davd,” Bram said, feeling sick, “how long can these jets grow?”
Jun Davd’s voice was grave. “Millions of light-years in the most extreme cases we know. More often, a few tens of thousands of light-years.”
“And in this case? From what you’ve been able to observe?”
“Long enough to reach the edge of the galaxy. That’s what you wanted to know, isn’t it?”
“And they’re growing at seventy-five percent of the speed of light?”
“They’ll slow down a bit as they proceed outward. Some of them will fall back.”
Bram grasped at a slender hope. “Then there would be time to warn the Father World, wouldn’t there? We could stop broadcasting the Message and use our radio beacon to beam a warning. We know they’re listening to the center of the galaxy. They’d have at least twelve thousand years to get ready. Maybe … maybe build a fleet of ships to migrate. Or … or set up ramjet screens for whole planets.”
“The primary threat isn’t the jets themselves,” Jun Davd said gently. “It’s the wave front of radiation coming from the events that have already begun here in the central region—and that travels at the speed of light.” He went on as if he were giving one of his lectures. “The jets themselves contribute to that wave front—relativistic aberration makes them radiate most intensely in the direction of their motion. But they’re only a part of the story. The supernovae are adding their increment of radiation. But, chiefly, it’s coming from that whirligig of black holes and their accretion disks. The soup of matter that engine is feeding on is getting progressively thicker—it’s now a self-sustaining process—and friction is causing the smaller, four-hundred-million-sun hole to spiral inward at an increasing rate.”
How can he be so calm, Bram wondered.
Jun Davd’s voice remained steady. “The radiation is pulsing outward in a series of shells that at this point are only light-days apart. You’ve noticed the flickering of the viewscreen and the rhythmic surges of acceleration as our engine harvests H-II pushed by that broom of photons. I’ve been taking measurements over the last shipboard hour, and those pulses are coming closer together.”
Bram snatched at another straw. “The inverse-square law! By the time the shells of radiation reach the outskirts of the galaxy—”
“They’ll still be fatal,” Jun Davd said flatly. “In any case, no life anywhere in the galaxy will be able to survive the final event, when the two black holes merge.”
A stunned silence pervaded the bridge. Everybody had been listening, of course—trying not to let it all sink in, taking refuge in Jun Davd’s lucid exposition, clinging to some last atom of hope that out of his calm parade of revelations would come the one final fact that was a reprieve for the Father World and its two races.
Now people avoided looking at one another, as if in some obscure shame.
Out of the silence came a strange sound—one that became a shocking sound a moment later as people on the bridge realized that it was the sound of Jun Davd sobbing.
Bram shocked them further by putting the question that sooner or later would have to be asked.
“Will Yggdrasil survive?”
Jun Davd needed a moment to regain control of his voice, “We are now disposing of more energy than is available to the entire Nar civilization. The fields are still holding. We are protected for the time being. But I don’t know if we can handle something on the scale of a climactic merger of the primary hole and its satellite.”
There was a delayed moan of grief from someone on the bridge. Grief for the loss of the Nar and all their works. These were superb people, Bram’s racing mind said. Personal fear would come later.
Jun Davd took refuge in more pedantry. “When two black holes become one, the resulting event horizon has a greater area than the sum of the areas of the event horizons of the original holes. It does not attain its final shape immediately. During the fraction of a second when the collapse takes place, there is a shifting and complicated topography. The geometry of space-time around it is … irregular. The mathematics to describe it does not exist. And the … distorted … event horizon vibrates. The gravity waves generated by two vibrating masses equivalent to a billion and a half suns will be tremendous. What that will do to the surrounding plasma, you can well imagine. Next, angular momentum will increase abruptly at the same time that queer event horizon expands—and because the conjugate hole is spinning rapidly, the accretion disk will be embedded well below the static limit, transferring mechanical energy and magnetic force. A tremendous explosion of gamma radiation will travel outward at the speed of light, followed by a sphere of stripped matter traveling at relativistic speeds. We can hope to outrun the matter. But our ability to survive the shell of radiation may depend on our distance from the core when it overtakes us.”
There was one last question for Bram to ask.
“How soon before the holes merge?”
“We’ll be lucky to make it around the core.”
“We’re making a run for it, anyway,” Jao said.
“That we are,” Bram agreed. “All we can do is to pour on the gravities.”
He lay propped on the floor pad, feeling the oppressive weight on his chest, his shoulders, his neck as he tried to hold his head up. All down the length of the observation loggia, dozens of people lay similarly sprawled on pads, working prone, their instruments on the floor beside them. Bram had ordered everybody to crawl, not walk, if they absol
utely had to move about. The bulk of the population of the tree were in their quarters, lying down. It was still half a day till periastron, and they would remain there until then.
Jao said, “No, that’s not all we can do. Bram, I’ve got to ask you about something.”
“Ask away.”
“There’s a decision to make about the core maneuver.”
“Why ask me? I’m only year-captain. Discuss it with Jun Davd.”
“I’ve already discussed it with Jun Davd. But it’s not strictly a technical decision. It involves the lives of everyone aboard. Jun Davd says you’re the only one who can make the decision, and I agree.”
With a sinking feeling, Bram said, “Go ahead.”
“We’ve got to get in and out of the galactic core fast. There’s no way we can simply back up, the laws of physics being what they are. We’ve got to use the mass at the center to bend our path in a hyperbola and swing around and out. Now our patron mass turns out to be two masses, very close together—right—but for the purposes of our hyperbolic orbit, we’re treating it as a single mass.”
“Yes,” Bram said, wondering what Jao was driving at.
“Even if there were some magical new law of physics that would let us dump all our inertia at once and come to a dead stop without rattling the glassware, it wouldn’t do us a bit of good. Because we’d have to back out from a standing start and build up all our lovely gamma factor all over again, and that would take us fifty thousand years longer than whipping around the focus of the hyperbola. So we’re committed.”
“Yes, certainly. Everyone understands that.”
Jao tried to wave a leaden hand, gave up the effort. “We also have to add a vector to angle the outward path somewhat above the plane of the galaxy if we want to aim at the Milky Way. That makes it even more tricky, but I won’t go into that now.”
“Yes, yes,” Bram said, wishing Jao would get to the point.
“So, as you said, we decided to pile on all the acceleration that Yggdrasil will bear in the hope of beating the merger of the two black holes and the final explosion by the widest margin possible. But the extra speed means we need an even tighter hyperbola—we’ve got to brush that doomsday engine ever closer, and that has its own dangers.”
“We all decided to take the risk.”
“Otherwise,” Jao said, ignoring the interruption, “we’d escape the galaxy, all right, and save our own skins. But we’d sail on out into intergalactic space and miss our target.” He looked around and lowered his voice. “Of course, there may be some who are frightened enough of the core maneuver to want to do that on purpose.”
“If you’re asking—”
“Bram, there’s a way we can pick up some extra velocity without adding more g’s and taking the risk of cracking Yggdrasil’s branches—to say nothing of our own sacroiliacs.”
“Go on.”
Jao hitched himself closer on his elbows. He took another look around the bridge to make sure that he would not be overheard.
“We use the satellite hole as a gravity machine.”
“What?”
“We’re already going to pick up the rotational energy of the primary hole. It’s got a rotation parameter of point nine nine eight, and it’s doing weird things to the space around it, and the field lines from the accretion disk are going to reach out with magnetic fingers and fling us along by our own magnetic field. Now, what I’m saying is that we can refine our orbit and loop around the satellite hole, and pick up its orbital energy. It’s whizzing around its primary at a ferocious rate at this point. We could pick up fifty thousand g’s of acceleration without it costing us a thing. We wouldn’t even feel it!”
“We’d have to fly between the two holes to make that loop.”
“Uh huh.”
“Jun Davd says they’re only a few diameters apart at this stage. And they’re spiraling closer. The final dive could happen at any moment.”
“He thinks we have enough time to squeak through.”
Bram’s heart was pounding. “He said that at the present high rate of spin, the static limit is well above the edge of the accretion disk. The region between them must be very weird. We could get sucked into the hole with no warning. Into either one of the holes.”
“Yah. That’s why he says it’s your decision.”
Bram was going to ask for an estimate of their chances. But that would be begging the question—asking Jao and Jun Davd to make the decision.
“Do it,” he said.
Jao nodded and crawled off on his elbows and knees to confer with Smeth.
The ghost of a star drifted by, a ball of red so dull as to be at the limits of visibility. The universe outside the long observation wall was no longer blind; it was filled with a meaningless red fog that showed a suggestion of vast billows, specks, twisting sheets. The phantom star cleared a tunnel ahead of it, but that was an illusion; actually, the star was going the other way, and the tunnel, of course, was its wake.
“Can you see it?” Jun Davd’s voice said over the loudspeaker.
“Yes,” Bram called to the directional pickup.
“Interesting,” Jun Davd said. “I wasn’t sure it would be visible. That shows you how chaotic the galaxy’s final act has become. Imagine the odds against encountering a fellow traveler at anything like a fraction of our relativistic speed! I wish I had a window here.”
“There isn’t that much to see, Jun Davd. Just red mist.”
“Yes, a pale reflection of events behind us. Supernova explosions proceeding outward, perhaps. Possibly local turbulence in those expanding jets, sending shock waves in the opposite direction. Puffs of stripped matter, relativistic electrons, moving inward to add to the mischief. The death throes of a galaxy.”
Mim’s hand sought Bram’s. Her floor mat was pulled up close to his. He had tried to make her nap, but she was unable to. The party dress was bedraggled. It had been a long time since she or anyone else had eaten; Marg and her helpers had had to give up when acceleration increased.
“Easy,” Bram said. “Remember, the Father World is still alive at this moment. Nothing will happen to them for almost fifty thousand years.”
“Oh, Bram, it’s so awful! Isn’t there anything we can do?”
He shrugged. “You heard Jun Davd. The first shells of radiation are already on their way. The big one will be right behind us. We can use the Message beacon to send a warning as soon as we swing around the core, but our warning would reach them only a few years before the radiation front did. It would almost be cruel. Then what? Suppose they managed to fit out a few space arks with ramjet drives to save a few thousand Nar and humans. They have no time to build up their gamma to the point we have, and they can’t outrun the expanding shell. They can’t reach another galaxy, anyway, not in a million years of subjective time, because they can’t use the H-II in the inner galaxy to accelerate the way we did.”
“It’s all so ironic,” she said bitterly. “We were supposed to ensure the survival of the Nar species. We came all this way. And now it’s all gone for nothing. Instead of broadcasting their Message, all we can do is to give them their death sentence.”
He squeezed her hand. There was absolutely nothing he could say.
The people around him, fanned out on mats in front of the forward viewscreen, didn’t have much to say, either. Where two people lay together, they might murmur at each other at intervals, but mostly they watched the view painted by Jun Davd’s computer.
Other screens around the bridge, and the screens in the living quarters where the bulk of the people lay waiting it out, had now been plugged into Jun Davd’s program. Some of the people watching with Bram and Mim had access to their own screens, but there seemed to be a compulsion to watch in groups.
“Hold on, everybody,” Jao’s voice said over the loudspeakers. “We just have to give it one more nudge.”
The tree lurched. Tortured wood fibers groaned. Something somewhere gave with a snap. Interior lights flickere
d.
“How did Yggdrasil come through that?” Bram asked the tree systems center through the portable console on the floor next to his head.
The voice of Jao’s granddaughter, Enyd, answered. “We’re all right. We lost a few minor branches—all deadwood, anyway. Yggdrasil’s reacting to the increased g-forces by acting to strengthen its compartments, but it won’t last long enough to cause any serious imbalance.”
“How are you people doing in there?” he asked.
Her voice softened, lost its formality. “We’re all right, Captain,” she said.
He switched off. The view in the forward screen was astonishing. A maelstrom of star-stuff churned around and around, fast enough to see. The private whirlpool of the satellite hole could be seen at this flattened angle of approach as an eddy in the main vortex—an eddy that itself circled around the main swirl of gases, dragging streamers with it. The tilt of its precession gave a peek into its heart of brightness every time it came around. Rags of plasma glittering with embedded stars enclosed the double whirlpool like a disintegrating bird’s nest made of fire.
The background stars were squashed, stretched around the immediate vicinity of the holes, their light bent. A circle of splashed stars followed the smaller hole around, snapping back when it had passed.
Nowhere could Bram see a path through the torrent of radiance. The concentric swirls of gas seemed to make one bean-shaped disk whose irregular contour, twisting with the secondary hole, smoothed out almost to an oval with distance from the dual center.
The sky around that cosmic gullet—if you could call it a sky—was crowded with stars of every possible color, jammed so closely together that in places the blackness of space seemed to be nothing more than pavement showing through. Gigantic irregular blobs of glowing gas wandered among the stars, grazing on them. Every once in a while, the flash of a supernova explosion, speeded up twenty thousand times, could be seen through an en-gorged blob. The blobs had an average drift toward the black holes; the ones closest to the double maelstrom bulged yearningly toward it.
A cloud rushed toward them. “Hold on again,” Jao’s voice said. “We’re going through.”
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