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Cass said haltingly, "No, I’m sorry. I can’t join you." So much for feeling smugly unshockable for daring to contemplate cross-modal sex. She joked, "I draw the line at any implementation where I experience detectable weight changes every time I learn something." Femtomachines shuffled binding energies equivalent to a significant portion of their own mass; it would be like gaining or losing half a kilogram several times a second, from the sheer gravity of your thoughts.

Rainzi smiled. "I thought you’d say no. But it would have been discourteous not to ask."

"Thank you. I appreciate that."

"But you’d see it as a kind of death?"

Cass scowled. "I’m embodied, not deranged! If a copy of my mind experiences a few minutes' consciousness, then is lost, that’s not the death of anyone. It’s just amnesia."

Rainzi looked puzzled. "Then I don’t understand. I know you prefer embodiment, for the sake of having honest perceptions of your surroundings, but we’re not talking about immersing you in some comforting simulation of being back on Earth. Your experiment should last almost six picoseconds. Running on a strong-force substrate, you’d have a chance to watch the data coming in, in real time. Of course, you’ll receive a useful subset of the same information eventually, but it won’t be as detailed, or as immediate. It won’t be as real."

He smiled provocatively. "Suppose the ghost of Sarumpaet came to you in your sleep, and said: I’ll grant you a dream in which you witness the decay of the Diamond Graph. You’ll travel back in time, shrink to the Planck scale, and see everything with your own eyes, exactly as it happened. The only catch is, you won’t remember anything when you wake. You say you don’t believe that the dreamer would be dying. So wouldn’t you still want the dream?"

Cass let go of one handhold and swiveled away from the wall. There wasn’t much point objecting that he was offering her a view billions of times coarser than that, of a much less significant event. It wasn’t a ringside seat at the birth of the universe, but it was still the closest she could hope to get to an event for which she’d already sacrificed seven hundred and forty-five years of her life.

She said, "It’s not the fact that I wouldn’t remember the experience. If you’ve lived through something, you’ve lived through it. What worries me is all the other things I’d have to live through. All the other people I’d have to become."

Cass dated the advent of civilization to the invention of the quantum singleton processor. The Qusp. She accepted the fact that she couldn’t entirely avoid splitting into multiple versions; interacting with any ordinary object around her gave rise to an entangled system — Cass plus cloud, Cass plus flower — and she could never hope to prevent the parts that lay outside her from entering superpositions of different classical outcomes, generating versions of her who witnessed different external events.

Unlike her hapless ancestors, though, she did not contribute to the process herself. While the Qusp inside her skull performed its computations, it was isolated from the wider world — a condition lasting just microseconds at a time, but rigidly enforced for the duration — only breaking quarantine when its state vector described one outcome, with certainty. With each operating cycle, the Qusp rotated a vector describing a single alternative into another with the same property, and though the path between the two necessarily included superpositions of many alternatives, only the final, definite state determined her actions.

Being a singleton meant that her decisions counted. She was not forced to give birth to a multitude of selves, each responding in a different way, every time she found her conscience or her judgment balanced on a knife edge. She was not at all what Homo sapiens had actually been, but she was close to what they’d believed themselves to be, for most of their history: a creature of choice, capable of doing one thing and not another.