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Mark_Friedenbach comments on [LINK] Could a Quantum Computer Have Subjective Experience? - Less Wrong Discussion
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The act of writing on the paper was an irreversible action. And yes, looking at it is comunication, in the physical sense. Specifically, the photon interaction with the paper and with your eyes is not reversible. Any act of extracting information from the computational process in a way where the information or anything causally dependent on that information is not also reversed when the computation is run backwards, must be an irreversable action.
What does a universe look like where a computation has been run forwards, and then run backwards in a fully reversible way? Like it never happened at all.
I think the confusion here is about what "fully quantum whole brain emulation" actually means.
The idea is that you have a box (probably large), within which is running a closed system calculation which is equivalent to simulating someone sitting in a room trying to write a theorem (all the way down to the quantum level). You are not interacting with the simulation, you are running the simulation. At every stage of the simulation, you have perfect information about the full density matrix of the system (i.e., the person being simulated, the room, the atoms in the person's brain, the movements of the pencil, etc.)
If you have this level of control, then you are implementing the full unitary time evolution of the system. The time evolution operator is reversible. Thus, you can just run the calculation backwards.
So, to the person in the room writing the proof, as far as they know, the photon flying from the paper hitting their eye and being registered by their brain is an irreversible interaction--they don't have complete control over their environment. But to you, the simulation runner, this action is perfectly reversible.
Now, the contention may be that this simulated person wasn't actually ever conscious during the course of this ultra-high-fidelity experiment. Answering that question either way seems to have strange philosophical implications.
What you describe is all true, however useless as described. The earlier poster wanted the simulation to output data (e.g. by writing it on paper -- the paper being outside of the simulation), and then reverse the simulation. Sorry, you can't do that. "Reversible" has very specific meaning in the context of statistical and quantum physics. Even if the computation itself can be reversed, once it has output data that property is lost. We'd no longer be talking about a reversible process, because once the computation is reversed, that output still exists.
I'm not sure who you're talking about because I'm the person above referring to someone writing on paper--and the paper was meant to also be within the simulation. The simulator is "reading the paper" by nature of having perfect information about the system.
"Reversible" in this context is only meant to describe the contents of the simulation. Computation can occur completely reversibly.
Sorry, got mixed up with cameroncowan. Anyway, to the original point:
You said "Once they've finished the proof, you record what's written on their paper, and reverse the entire simulation... Looking at what they wrote on the paper doesn't mean you have to communicate with them."
My interpretation--which may be wrong--is that you are suggesting that the person running the simulation record the state of the simulation at the moment the problem is solved, or at least the part of the simulator state having to do with the paper. However the process of extracting information out of the simulation -- saving state -- is irreversable, at least if you want it to survive rewinding the simulation.
To put differently, if the simulation is fully reversible, then you run it forwards, run it backwards, and that the end you have absolutely zero knowledge about what happened inbetween. Any preserved state that wasn't there at the beginning would mean that the process wasn't fully reversed.
Looking at the paper is communicating with the simulation. It maybe be a one-way communication, but that is enough.
I'm suggesting that the person running the simulation knows the state of the simulation at all times. If this bothers you, pretend everything is being done digitally, on a classical computer, with exponential slowdown.
Such a calculation can be done reversibly without ever passing information into the system.
What do you mean by "knows the state of the simulation"? What is the point of this exercise?
Yes the machine running the simulation knows the current state of the simulation at any given point (ignoring fully homomorphic encryption). It must however forget this intermediate state when the computation is reversed, including any copies/checkpoints it has. Otherwise we're not talking about a reversible process. Do we agree on this point?
My original post was:
How does your setup of a simulated person performing mathmatics, then being forgotten as the simulation is run backwards address this concern?
I would like to know that as well because I think there is an effect if it is conscious to make it fully reversible I think denies a certain consciousness.
That's what Scott's blog is about :)