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:
Giving answers is an irreversible operation. The whole “is a fully reversible computer conscious?” thing doesn’t really make sense to me—for the computer to actually have an effect on the world requires irreversible outputs. So I have trouble imagiing scenarios where my expectactions are different but the entire process remains reversible...
How does your setup of a simulated person performing mathmatics, then being forgotten as the simulation is run backwards address this concern?
I disagree that “giving answers is an irreversible operation”. My setup explicitly doesn’t “forget” the calculation (the calculation being simulating someone proving the Riemann hypothesis, and us extracting that proof from the simulation), and my setup is explicitly reversible (because we have the full density matrix of the system at all times, and can in principle perform unitary time evolution backwards from the final state if we wanted to).
Nothing is ever being forgotten. I’m not sure where that came from, because I’ve never claimed that anything is being forgotten at any step. I’m not sure why you’re insisting that things be forgotten to satisfy reversibility, either.
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 disagree that “giving answers is an irreversible operation”. My setup explicitly doesn’t “forget” the calculation (the calculation being simulating someone proving the Riemann hypothesis, and us extracting that proof from the simulation), and my setup is explicitly reversible (because we have the full density matrix of the system at all times, and can in principle perform unitary time evolution backwards from the final state if we wanted to).
Nothing is ever being forgotten. I’m not sure where that came from, because I’ve never claimed that anything is being forgotten at any step. I’m not sure why you’re insisting that things be forgotten to satisfy reversibility, either.