perhaps think some more about how you are going to correct all errors and prevent an error catastrophe upon reversal
The normal way. This generates waste heat, but at a rate which depends on the error rate of your components. Under our current understanding of physics, this can be driven essentially to zero in the long run. Even if it can't, it can at least be driven down until we encounter some as-yet-unknown fundamental physical limitation. If we imagine people living in a reversible CA, or any other laws of physics which we can understand, then we can see how they could build an error-free computer once they had a theory of everything. Do you suspect our universe is more complicated, so that such an understanding is impossible? What do you think determines the quantitative bound on achievable error rate?
and about how you are going to propagate the "reverse now" signal in a reversible manner.
I don't understand this objection. I can write down reversible circuits which coordinate with only a constant factor penalty (it is trivial if I don't care about the constant--just CNOT in a 'reverse' bit to each gate from a central controller, and then make each gate perform its operation in reverse if the bit is set, tripling the number of gates). What fundamental non-idealness of reality are you appealing to here, that would prevent a straightforward solution?
perhaps think some more about how you are going to correct all errors and prevent an error catastrophe upon reversal
The normal way. This generates waste heat, but at a rate which depends on the error rate of your components. Under our current understanding of physics, this can be driven essentially to zero in the long run.
It seems to me that with hardware error correction, you have to pay for your error rate with heat. If you want to recover your machine by running it backwards you need a very low hardware error rate - which is correspondingly expen...
This post may be interesting to some LWers.
In summary: it looks like our universe can support reversible computers which don't create entropy. Reversible computers can simulate irreversible computers, with pretty mild time and space blowup. So if moral value comes from computation, negentropy probably won't be such an important resource for distant future folks, and if the universe lasts a long time we may be able to simulate astronomically long-lived civilizations (easily 10^(10^25) clock cycles, using current estimates and neglecting other obstructions).
Has this been discussed before, and/or is there some reason that it doesn't work or isn't relevant? I suspect that this consideration won't matter in the long run, but it is at least interesting and seems to significantly deflate (long-run) concerns about entropy.