Aurora looked amused that he would presume she was at his service. But he was too hungry for knowledge to be concerned about courtesy, and perhaps she could see that too. “I’ll listen in,” she said. “If I feel I can help, I’ll speak up.”

Her assistants brought the wiry helmet to him, and the alchemical preparation.

Humans sensed only a small part of reality. They were as worms in the earth, comfortable and warm. If God had not given them reason, they would not by their senses know even a minim of the whole.

As it was, however, by the cumulative work of thousands of people, humanity had slowly and painfully built a picture of the cosmos beyond what they could see. And then had found ways to use that knowledge, and move around in the cosmos.

Galileo flew again in the space of ideas, as if through patchy white clouds, following the construction of mathematics’ monumental edifice step by step through the centuries. He was thankful for the velocinestic, because he needed to be quick to apprehend what the machine was saying, and what Aurora added to its speech. This heightened apprehension now took him speedily beyond thought as he was used to it, into some larger realm of understanding, full of feeling and movement, something like a bodily music. He did not just see or sing the music, but became it. Math was his body. Words, symbols, and images all formed in the vague enormous clouds inside him, all moving in a continuous dance of equations and formulas, operations and algorithms, together melding into an ongoing polyphonic chorus. He was singing along and being sung. This meant taking certain things on faith, hoping that his performance of them indicated a subsequent firmer understanding that would grow and hold.

Here Aurora helped him to hew to the main line, reassuring him that he was proceeding just as all the rest of them had at one point or another, enduring confusions to follow a line through them. “No one can know everything,” she said. Galileo found this hard to accept. But in order to keep flying he ignored the bitter taste of his ignorance, of his faith in things he had not mastered. There were more important matters at hand than his sense of complete understanding. Apparently no one got to have that but God.

And so he flew on, diving into the new fields and methods, gauge theory, chromoelectrodynamics, symmetry and supersymmetry, multidimensional topology, manifolds, on and on it went, smaller and bigger, more complex and simpler—and after an extended protraction of his mind he found the long looked-for reconciliation of quantum mechanics and gravity physics. It came only very late in the story, when they got down into the very finest grain of things, regarding sizes that were so small that Galileo marveled there could be any knowledge of them whatsoever. But apparently it had been done.

As the generations of scientists had succeeded each other, each step of comprehension had served as scaffolding on which to stand and erect the next level. At every step of the way, quantum mechanics had proved itself accurate and useful. And so one aspect of it, Pauli’s exclusion principle, could be combined with the speed of light to establish minimum lengths and times: these were true minimums, because further division would break either the speed of light or the exclusion principle. The minimum width established by this principle turned out to be

of a meter, and traveling at the speed of light a photon would cross this distance in of a second—a second being about the equivalent of a pulse, which Galileo measured as the speed of his calm heart beating. The ultimate minim of time, in other words, was a billionth of a billionth of a billionth of a billionth of a heartbeat, more or less. That was brief! The universe was very fine-grained indeed. Just thinking about it gave Galileo a shiver. It was stunning to feel in himself that fine grain, the dense texture of the glossy plenum—to sense in that density also God’s sense of artistry, His meticulousness or pulitezza. His love of math.

He flew on, doing his best to catch up to Aurora, who was continuing as if the minimum units were not stupendously, unimaginably small. She was used to the idea of them, and moving on to the question of how physicists had dealt with the idea that all space and time might be created out of the vibration of objects of the absolute minimum size and duration. Their most powerful experimental machines would have to be 10²⁰ times more powerful than they were to be able to investigate these minimal particles or events; in other words, an accelerator ring large enough to create the energies needed would have to be as big around as the galaxy. The particles they sought were so small that if one of them were expanded to the size of the Earth, the nucleus of an atom, to stay proportional, would have to be expanded to ten times the size of the universe.

Galileo laughed at this. He said, “It’s the end of physics then.”

For it meant that a stupendous abyss lay between humanity and the fundamental reality that would explain things at all the larger scales. They couldn’t cross that abyss. Physics was therefore stumped.

And indeed, for a long time mathematical physics and cosmology skittered around and appeared to stall, as physicists struggled to concoct scaffolding that they could cast all the way across the abyss in a single throw—that would give them even questions to ask.

“To an extent we are still there,” Aurora said. “But a mathematician named Bao made a bridge that seems to have held, and allowed us to build from it. Let’s go there now.”

Before Bao’s time, Galileo saw, which was precisely the beginning of the period known later as the Accelerando, the goal of physicists was to explain everything. He recognized that; it was the reductio ad absurdum of science: to know everything. The unspoken desire in that urge was the hope that, knowing everything, humanity would also know what to do. The blank that was their sense of purpose would perhaps also be filled.

But to know everything was asking too much. “They want to be like God!” he said.

“Maybe God is only a prolepsis,” Aurora said. “Our image of what we could be, imagined by contemplating our future.”

“Which would make it an analepsis, no?”

She laughed as they flew. “You like paradoxes, but of course this one is just entanglement all over again. We are extensive in time. Fly on and you will see.”

So they flew. Progress in physics struggled on. Theories of what occurred at the minimum and in postulated extra dimensions were elaborated, considered, challenged, refined. Predictions were made that could sometimes be checked against observation, or involved findings just beyond the realm of current observation. Ideas thus drove technologies. Slowly progress was made. But the unbridgeable abyss made every theory speculative. The wind from Galileo’s flight could have knocked down some of these houses of cards, and the collapsed theories he flew through had perhaps been knocked down in just that way, by the offhand remark of some observer like Bao, surveying the whole landscape and taking a completely new line over it.

It wasn’t until the twenty-eighth century that a theoretical structure accomplished a substantial part of what had been begun so long before. It was a physics based on Bao’s bridge to the minim, and on experiments spanning the solar system—controversial experiments that had entrained significant portions of the system’s total potential energy. Bao’s work had clarified the ten-dimensional manifold of manifolds theory that had been proposed since the time of Kaluza and Klein. Bao’s version had created many cosmological and subatomic questions and predictions that had given them experiments to try, observations to make, the results of which then gave them corrections and surprises, but mainly confirmations, a sense that they were on the right track at last—and in some ways had been all along, if one made allowances for the usual eddies and dead ends. Each generation had served as scaffolding for the generations that followed, and the work continued through collapses and reversals, almost one might say mindlessly. “It’s like watching ants building a mound,” Galileo observed as he flew through the elaborations. “The mass just keeps grinding.”