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 expensive in terms of heat dissipation.
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.
I am not clear how having a TOE will help with cosmic rays and thermal noise. Of course you can deal with thermal noise using a fridge—but then your fridge needs a power supply...
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).
Propagating a “reverse” signal through a large system and getting the components to synchronously reverse course is not exactly trivial. It’s also hard to do reversibly, since the “reverse” signal itself tends to dissipate at the edges of the system. - though as you say, that’s a one-off cost.
You proposed “tripling of the number of gates”. Plus the machine has twice the runtime because of the “running backwards” business. Reversibility has some costs, it seems...
I am pretty sceptical about the idea that the future will see reversible computers that people will bother to run backwards. Instead, I expect that we will see more of the strategies mentioned in my article.
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?
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?
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 expensive in terms of heat dissipation.
I am not clear how having a TOE will help with cosmic rays and thermal noise. Of course you can deal with thermal noise using a fridge—but then your fridge needs a power supply...
Propagating a “reverse” signal through a large system and getting the components to synchronously reverse course is not exactly trivial. It’s also hard to do reversibly, since the “reverse” signal itself tends to dissipate at the edges of the system. - though as you say, that’s a one-off cost.
You proposed “tripling of the number of gates”. Plus the machine has twice the runtime because of the “running backwards” business. Reversibility has some costs, it seems...
I am pretty sceptical about the idea that the future will see reversible computers that people will bother to run backwards. Instead, I expect that we will see more of the strategies mentioned in my article.