“Well of course! I did anyway!”

“I know. But I didn’t want you to have any extra encouragement. Because that’s the potentiality cluster with the worst outcomes of all.”

“No!”

“Yes. When you succeed in a reconciliation, and religion dominates science in its earliest phase, you get the deepest and most violent low points in the subsequent histories. This is what Ganymede saw, and this is what he has insisted ever since. When you are burned and become a martyr to science, science more quickly dominates religion, and the subsequent low point is much reduced. It’s bad, but not as bad.”

Galileo thought it over, confused by this newly proliferating vision of the past. “And so,” he said, “what happened after this time, then? This one I am in now?”

“This time is an alternative, as they all are in their time. But this is what you and I, and everyone else in our strand, managed together. An analeptic introjection that made a big change.”

“And is it better?”

She looked him in the eye, smiled very slightly. “I think so.”

Again Galileo considered it. “What happens to the me who burned, then? What happens to that Galileo now?”

She said slowly, explaining again, “All the potentialities exist. When an analepsis creates a new temporal isotopy, it coexists with the others, all of them entangled. All together they make up the manifold, which shifts under the impact of the new potential, and changes, but continues too. Whether we can oxbow a channel and cause it to disappear entirely is an open question. Conceivable in theory, something people claim to have seen, but in practice hard to do. As you might know better than I, I suppose, because of your sessions with Aurora.”

Galileo shook his head doubtfully. “So there is still a world in which Galileo is burned as a heretic?”

“Yes.”

“But no!” Galileo said, rising from the bench to his feet. “I refuse to accept that. I am the sum of all possible Galileos, and all I ever did was say what I saw. None of us should burn for that!”

She regarded him. “It has already happened. What would you do?”

He considered, then said, “Your teletrasporta: I must beg you the use of it. The other box must be there in Rome on that day, I know that already.”

She stood herself and looked down on him, her gaze serious. “You could die there. Both of you.”

“I don’t care,” he said. “All of us are one. I can feel it, they’re in my mind. In my mind I’m burning at the stake. You have the means for a return. So I have to do this.”

Smoke had filled his lungs and was choking him by the time the fire reached his feet. Pain stuffed his consciousness, blasted it until there was nothing but it, and he almost swooned. If he could hold his breath he would faint, but he couldn’t. His feet were catching fire.

Then through the smoke he saw the mass of distended faces break apart at the impact of a man on horseback riding through, his charge smashing people aside so that their roar panicked to a scream. The ring of Dominicans guarding the pyre bunched to repel this helmeted invader, but they all knew what happened when a horse struck men on foot, and before it reached them they broke and ran. The horse reared and twisted before the fire, disappeared behind Galileo. There was a slashing at the chains holding him that made their iron instantly hotter; then he was grabbed around the waist by the horseman, yanked up onto the bucking horse and thrown before the saddle. His ankles were apparently still chained to the stake, so that his feet twisted almost out of their sockets. But then they came free, and he bounced like a long sack on the horse’s flexing shoulders. Everything around him jumbled into a slurry of curses and screams, of a horse’s twisting flank and a sword flashing in smoke. His rescuer roared louder than all of them together as he mastered the horse and charged away. He caught a glimpse of a bearded lower face under the helmet, square mouth red with fury. As he lost consciousness he thought, at least I died dreaming that I saved myself.

And came to in Count da Trento’s cellar in Costozza, moaning. He hurt all over. His companions were still on the stone floor.

“Signor Galilei! Domino Galilei, please, please! Wake up!”

“Qua—? Qua—?”

His mouth would not form words. He could not focus his eyes. They were dragging him by the arms over the rough floor, and he felt his butt scraping over the flagstones as from a great distance, while hearing someone else’s groans, muffled as if through a wall. He wanted to speak but couldn’t. The groans were his.

Hera’s voice, then, in his ear, as he looked down the blasted mountainside of Io, clutching her arm, laid out on the bench.

“You died on the floor of the cellar, that first time, along with your two companions. Now we’ll take the dead body from there and put it back on the stake, to fill your absence in the fiery alternative. Here in Costozza, the rescued one will survive his trauma, and live on. But understand: there will always be this little whirlpool in you, between the worlds.”

“So I live it all again?”

“Yes.”

Galileo groaned. “Do I have to know it?” he asked. “Can you let me forget?”

“Yes, of course. But it will be in you anyway. The potentiality is always there. And sometimes therefore you will remember it, despite the amnestics. Because memory is deep, and always entangled, and while you live, it lives.”

“That’s fine, as long as I don’t remember it.”

“Yes. But even when you don’t, you do. It lies below your feelings.”

“And the others? The other Galileos, in the other potentialities?”

“Please understand. They are always there. There are so many.”

“Will they end? Will it ever end?”

“End? Do things end?”

Galileo groaned again. “So,” he said, “even if I saved myself an infinite number of times, there would still be an infinite number of me that I hadn’t saved. I will live through them again and again. Make the same discoveries and the same mistakes. Suffer the same deaths.”

“Yes. And sometimes you’ll know that. Sometimes you’ll feel it. This is your paradox of the infinities within infinities, which you will have discovered by feeling it in yourself. You live in Galileo’s paradox. You’ll hold your wife and mother apart as they try to kill each other, and it will strike you as horrible, then ridiculous, then beautiful. Something to love. This is the gift of the paradox, the gift of memory’s spiral return.”

“Always in me. Even if I forget.”

“Yes.”

“Then let me forget. Give me the amnestic.”

“Is that what you want? It will mean losing your conscious memory of a lot of this that you have seen out here.” Gesturing at Io’s slaggy grandeur, and at Jupiter’s enormity. And at herself.

“But not really,” Galileo said, “as you have just told me. It will still be in me. So, yes. I have to. I can’t stand to know about the others. I would have to keep going back and trying to change things, like Ganymede. I can’t face that. But I can’t face the bad alternatives either—all the deaths, all the burning. It isn’t right. So—so I need to forget, to go on.”

“As you wish.”

She gave him a pill. He swallowed it. She had slipped another one in the mouth of the Galileo there on the floor of the poisonous cellar, he was sure; a Galileo who would therefore live through all that followed that moment again, in ignorance, just as he had already; or at least until the stranger arrived. When it would all begin again.

“So I didn’t really do anything by rescuing him,” he said. “I didn’t change anything.”

“We made this eddy in time,” she said gently, and touched him.

IN SIENA, WHEN HE CAME OUT of his syncope, he was shaking and white-faced. He stared up at Cartophilus, clutching him by the arm.