“No.”

Cartophilus hesitated for a long time. Just as Galileo thought he had fallen back asleep, he ventured, “Often it seems to me that when one tries to do something based on … knowledge—or even let us say foreknowledge, or a premonition, what the Germans call Schwanung—that whatever you do, it … rebounds. Instead of forestalling it, or fulfilling it, your action has the effect of bringing about exactly the opposite of whatever you might have been trying for. A complementary action, so to speak.”

“You would know better than I, I’m sure.”

“I don’t.”

Galileo lifted his fist again. “Just get your master to me.”

“As soon as I can. In Florence. I promise you.”

Back in Florence, Galileo moved into his newly rented house in Bel-losguardo, the Villa del Segui, a fine establishment overlooking Florence from a hill to the south of the river. He had a real home again, for the first time since Hostel Galilei in Padua. Here he was, back in his gardens, back in La Piera’s care, back in the arms of his girls (or Virginia’s anyway).

He was barely settled in, and had gone out into the garden one night to complete his ablutions, when a movement against the stable wall caused him to flinch.

A black figure emerged from the murk, and he was about to cry out when he saw that it was the stranger. At the sight of that narrow face, the unganymedean face of Ganymede, he experienced a big if vague abreaction; all of the blurred uncertain memories of what had happened to him on the Jovian moons came back to him in force. The memories of his earlier night voyages were like dream memories, with certain moments sticking out more distinctly even than events of the present moment—in particular, in this case, the fire—but the rest fuzzy beyond what was usual for his memory, perhaps because of the dreamy content. They had done things to his mind, he knew that; the woman Hera had helped him to counteract one preparation with another, he recalled. So odd effects were not surprising. In any case, now the earlier voyages had bloomed in him, and all from the sight of the stranger’s hatchet face. Galileo’s heart beat in his chest at the vivid memory of the fire, which had never really left him. “I want to go back,” he demanded. “I have questions to ask.”

“I know,” Ganymede said. “There are questions for you there as well. I have taken steps to secure the device at the other end.”

Galileo snorted. “You hope you have. But I want to see Hera in any case.”

Ganymede frowned. “I don’t think that’s wise.”

“Wisdom has nothing to do with it.”

This time Ganymede merely twisted a knob on a pewter box he was carrying crooked in his elbow, and there they stood, inside one of the green-blue ice caves of Europa.

“Hey,” Galileo said, shocked. “What happened to your teletrasporta?”

Ganymede tilted his head. “All that was done to give you a way to comprehend what was happening. It was felt that if you were bilocated without some way to explain the prolepsis to yourself from within your own frame of reference, you might be excessively disoriented. Some feared you would experience a mental breakdown, or otherwise fail to accept the reality of the prolepsis. Perhaps assume you were dreaming a dream. So we constructed a simulacrum of a translation that would make sense in local terms—in your case, a flight through space. We made the entangler look like something that could cast your vision to us. Then the experience of flight was given to you after you had already been bilocated.”

“You can do that?”

The stranger gave Galileo a pitying look. “Simulated experiences can sometimes be distinguished from real ones, but in data-poor environments, like space, it’s hard to do.”

Galileo gestured at the great ice cavern extending away from them in every direction, its aquamarine roof starred by cracks. “If this cave were not real, how would I tell?”

Ganymede shrugged. “Maybe you couldn’t.”

“I thought not,” Galileo muttered. “These are all dreamscapes.” He thought again of his immolation at the stake. More loudly: “What keeps us warm?”

“Heat.”

“Bah. Where comes the heat? Where comes the air?”

“There are engines creating them.”

“Engines?”

“Machines. Devices.”

“So illuminating!”

“Sorry. The details would mean nothing to you. Very few people here understand them. The heat and air are simple, in any case. It’s protection from Jupiter’s radiation that is difficult. That’s why we stay below the surface most of the time when on Europa. One of the reasons they’ve gone mad, if you ask me. On Ganymede we were out under the sky. On Io, we take advantage of the new bubble fields. But here they have older structures for dealing with the problem.”

“Radiation? Isn’t that another name for heat?”

“Well, but there are vibrations along a spectrum of sizes. What our eyes see are wavelengths of a certain size, but that band of the visible is just part of a range that extends far to either side. Shortest are gamma waves, then longer wavelengths range from braccia to the width of the universe, more or less.”

Galileo stared at him. “And these other waves manifest as?”

“Heat, sometimes. Damage to flesh that can’t be felt. I don’t know exactly how to explain it to you.”

Galileo rolled his eyes. “Then take me to someone who can.”

“We don’t actually have time for that, sorry—”

“Take me to someone who knows! Because you are an idiot.”

Ganymede rolled his eyes. “I forbear—”

“Take me!” Galileo shouted, and shoved the man hard in the chest. At home he would have beat him, so why not here? He wasn’t convinced any of it was real. He kicked Ganymede in the shins, yelling fit to turn all the blues of the place red. “Come on! Someone who knows something. Surely there must be someone who knows something!” He raised his big fist.

“Stop it,” Ganymede complained. He was wispy despite his height, and looked confused to be assaulted. “Quit trying to bully me. We aren’t in one of your downriver alleys here. People will notice what you do, and conclude you aren’t really civilized.”

“Me? It’s you who are uncivilized. You don’t know even the basics of how your machines work.”

“Spare me. No one knows all these things. Could you tell me how every machine of your time worked?”

“Yes, of course. Why not?”

Ganymede pursed his lips. “Well, it is no longer possible.”

“I don’t accept that. The principles at least must be clear, if you make the attempt to understand.”

“You’ll see.” And he muttered to the side, as if to an invisible angel.

“Take me.”

“I’ll take you.”

The gallery they were in was a kind of giant open antechamber to another under-ice city. Broad spaces extended so many miles away from them that in the distance the blue ceiling curved down and met the floor, cutting off any farther sight. Picking out one particular bright silver building ahead of them, just where the ceiling appeared to meet the floor, Galileo found it took only about fifteen or twenty minutes to walk to it. A close horizon. The alleys and strada of this cold town were sometimes crowded with tall graceful people, moving as if in water; at other times the streets were nearly empty. The people wore clothing like Ganymede’s, simple but fine, warm pastel tones making them appear illuminated in the green light.

They continued beyond the silver building for about an hour, he reckoned, passing crowded plazas extending to left and right, some of them open to the black sky, most roofed by ice. As the hour passed, he learned better how to walk in the light downward pull. This strange lightness was suggestive of all kinds of things, including the idea that weight was perhaps proportional to the size of the planet one stood on. Another sign that Europa must be fairly small.