“Then what?”

“It’s yolk,” Anna said. “Yolk, from an egg star.”

“A what?”

Billie sighed with all the seriousness a seven-year-old could muster. “She means,” she said, pronouncing the words carefully, “a neutron star.”

“But it’s like an egg,” Anna said. “The collapsed remains of a supernova. Solid outside and a lot of funny liquids churning around on the inside.”

“And that’s what this stuff is? This Tinkerbell? A droplet of neutron star matter?”

“Only a billion tons or so,” Anna said. “Originally material from the Moon.”

“Tell me what you want with it.”

“We don’t want it” Billie said seriously, and she wiped her nose on Emma’s sleeve.

Anna said, “What we want is what it will become. The degenerate matter is, umm, a fuse. In a moment a fragment of true quark matter will arrive.”

“From where?” Emma asked.

But Anna didn’t answer that. She said, “When the nucleus of quark matter enters the fuse, it will quickly develop an equilibrium strangeness content via weak interactions, and free neutrons will be absorbed as there is no Coulomb barrier—”

“Anna, my dear, I don’t understand a damn word.”

“The fuse will turn into quark matter very rapidly, all of it.”

Emma remembered a briefing Dan Ystebo had prepared for Maura. A neutron star flashing to quark matter. Half its mass being converted to energy in a few seconds. Explosions so vigorous they could be observed from another Galaxy.

“In fact,” the girl said with an element of pride, “the degenerate matter droplet has been shaped so that its collapse will be concentrated. At the very center of the droplet, in a space smaller than a proton, we will reach higher energy densities even than at the hearts of collapsing neutron stars. Higher energy densities than can form anywhere, naturally. Densities that need intelligence, design, to occur.”

“Jesus. Why, Anna? What are you trying to do? Blow up the Moon?”

“Oh, no,” Anna said, a little impatiently. “‘Not just that. The point is not the amount of energy that’s released here, but the precision of its application.”

“Which is why,” Emma said with growing dread, “you are calling this thing a fuse. You’re intending to use this to trigger something else. Something much bigger. Aren’t you?”

Anna smiled happily. “Now you’re starting to understand,” she said brightly.

Seven-year-old Billie turned her sweet, round face up to Emma. She said carefully, “Vacuum collapse. Are you afraid?”

Emma swallowed. “Yes. Yes, I am, Billie. But I don’t know what I’m afraid of.” Now Emma saw that the kid’s lower lip was wobbling. Emma bent, carefully, and leaned toward Billie. “Tell you what,” she said. “It’s okay to cry. But I’ll try not to if you try not to. What do you think?”

And then — suddenly, without warning or fanfare — it began.

Reid Malenfant:

Here was Malenfant, drifting in space.

He remembered how he had grabbed Emma, coaxed her, forced her onto the O ‘Neill to be with him. And he remembered how he had pushed her away, protected her with lies, left her on Earth.

He remembered how he had made love to her in the darkness and silence of space. And he remembered how he had started awake, weightless and disoriented, looking for her, and she had not been there, never had been there.

He remembered how she had come with him on his strange journey through the manifold of universes. And he remembered how he had journeyed alone: lost, frightened, incomplete.

He remembered how she had learned the truth about him at last. He remembered how she had died in his arms. He remembered how much he had missed her, longed to have her back, to tell her.

He remembered how he had wanted it all: his relationship with Emma, to spare her pain, his glorious future vision. And he’d finished with none of it.

The change was done, the timelines rewoven. But, by God, it had cost him.

Malenfant turned his head, refocused his eyes’ new zoom feature, and there was the Moon, swimming alongside the Earth as it always had. Beautiful doomed Earth.

“Shit,” he said. “It’s the end of the world. And all I can think about is myself.”

What else is there?

“… The downstreamers. Are they gods?”

No. They’re just people.

“That’s hard to believe.”

But the human race is very old. They would not recognize you.

“Why not?”

Because your time was very strange. Really, it was still part of the Big Bang, the afterglow. Bright.

“What are they like?”

They are diverse. As diverse as you and me. More. But they have one thing in common. These are the people who chose to live on.

“There were others who chose death? Why?”

Because there are problems with the substrate. It is not infinite in size. No computer can exceed the limits set by the Bekenstein Bound.

“The what?”

It s difficult to talk to you when you know nothing.

“Sorry.”

The uncertainty principle, then. You know about that. Because of the uncertainty principle, a given amount of mass and energy can only assume a finite number of quantum states. So the number of different states achievable is bounded above by the number of states achievable by the whole universe, if all its mass and energy were converted to information, which has not occurred. The number is ten to power ten to power one hundred and twenty-three —

“Ten to power ten to power one hundred and twenty-three, huh. And that’s the number of possible thoughts, inside this computer. Is that what you’re telling me?”

Yes! The substrate is a finite-state machine. It can take only a fixed number of states, and it works in discrete time intervals. A finite-state machine must, after long enough, enter a periodic state. That is —

“They live the same lives,” Malenfant said. “Even think the same thoughts. Over and over. My God, what a fate.” Like autism, he thought. “Why? “

The kid sighed. There was no other way for mind to survive the Heat Death.

The same thoughts over and over, circulating like farts in a space suit. What a destiny, what an end to all hope, what a culmination to all those universes painfully evolving to the point where they could support life and mind, the uncounted years of struggling to survive in this universe… What an end, he thought, to my own grandiose projects.

But Cornelius would have loved it. Sanity, control forever, no change. Just an endless cycle of sameness.

Michael was watching him. You understand.

“Understand what?”

Why the. . . Feynman project was initiated.

“The portals? The messages upstream?”

There are some who do not believe it was meant to be like this. That life, humanity, had a different purpose.

“You’re telling me we have a.purpose!”

Oh, yes. Humans are the most important sentient creatures who have ever existed, or will ever exist.

That sent a shudder down Malenfant’s spine. God damn it, I waited all my life to hear someone tell me that. And now that I have, it terrifies me.

“So these downstreamers of yours have reached back in time and changed things, created another timeline, in which—”

Michael frowned. Your language is like noise. But you are more right than wrong. Yes, I can say that. But there are no such things as timelines. There is a universal wave function that determines a sheaf of paths —

“I heard all that before, and didn’t understand it then… Earth. Do they know what’s going to happen?”

People are, umm, at peace, Malenfant. In a way they weren ‘t in your day.

“Even now, as the lights are going out?”

Even now.

“But, no matter how prosperous and contented and understanding they are, they’re all going to die. All the people on Earth, and the Moon and Mars and wherever the hell else they got to… Tell me about Earth, Michael.”