This sounds right to me. Reversible computing means you can get closer to the energy limit set by Landauer’s principle, but you still don’t drive the negentropy cost per bit to zero.
You get the energy limit set by Landauer’s principle without reversible computing. Reversible computing completely circumvents Landauer’s principle (although there may be other limitations).
I don’t follow. You still have to pay for erasing and changing your bits, regardless of whether you use reversible computing and do the erasure at the end, or whether you do it during the computation as in irreversible computing.
You generally uncompute intermediate results in reversible computation, rather than erasing them: if you produced some garbage by starting from a low entropy state and running the computation C forward, you can get rid of the garbage by just running C backwards (perhaps first copying whatever output you care about, so that it doesn’t get destroyed).
perhaps first copying whatever output you care about, so that it doesn’t get destroyed
Well, yeah. You’re going to use up negentropy for that—where are you copying to? Reversible computing just means you spend less negentropy. (Feel like I’ve said this before.)
Yes, you just produce less entropy. But you produce a lot less entropy and it is completely unrelated to Landauer’s principle.
Suppose I want to calculate a 1TB document created by a googol person civilization running for a googol googol years. I only have to produce a TB of entropy, not more than a googol googol googol bits (as I would have to if I used irreversible computing naively).
Very nice! So you don’t just cheaply compute-and-uncompute a lot of independent worlds, you can allow them to leave an arbitrarily-difficult-to-produce trace on the future worlds. Given how much entropy we really have, sufficiently small persons for example can be spared from uncomputation.
In particular, a person can live in an incrementally computed-and-uncomputed virtual world that is being regularly reversed to its initial state, with the effect that only the person consumes entropy, and the whole arbitrarily complicated world has zero entropic footprint. The world could also be optimized game-save-style over person-time, starting from an initial state, but going forward, so that some version of the person does all the updating, and so carries the excess entropy. Alternatively, improvements to the world could be carried out by discarded copies. Think of software downloaded from the distant future…
Or more generally, this is just time travel, where you can transport sufficiently small things (and people) to the past (or between timelines) and change things the next time over. You travel forwards in time by computing, backwards by uncomputing, you can take some luggage with you, you can climb up a different timeline by observing without interference, and you can intervene and change a timeline any time you want. The network of people navigating baseline network of virtual worlds builds up a second level (implementing meta-time), which itself can be navigated and intervened-in by other observers, and so forth. Sounds like a Greg Egan novel (that wasn’t written yet).
This sounds right to me. Reversible computing means you can get closer to the energy limit set by Landauer’s principle, but you still don’t drive the negentropy cost per bit to zero.
You get the energy limit set by Landauer’s principle without reversible computing. Reversible computing completely circumvents Landauer’s principle (although there may be other limitations).
I don’t follow. You still have to pay for erasing and changing your bits, regardless of whether you use reversible computing and do the erasure at the end, or whether you do it during the computation as in irreversible computing.
You generally uncompute intermediate results in reversible computation, rather than erasing them: if you produced some garbage by starting from a low entropy state and running the computation C forward, you can get rid of the garbage by just running C backwards (perhaps first copying whatever output you care about, so that it doesn’t get destroyed).
Well, yeah. You’re going to use up negentropy for that—where are you copying to? Reversible computing just means you spend less negentropy. (Feel like I’ve said this before.)
Yes, you just produce less entropy. But you produce a lot less entropy and it is completely unrelated to Landauer’s principle.
Suppose I want to calculate a 1TB document created by a googol person civilization running for a googol googol years. I only have to produce a TB of entropy, not more than a googol googol googol bits (as I would have to if I used irreversible computing naively).
Very nice! So you don’t just cheaply compute-and-uncompute a lot of independent worlds, you can allow them to leave an arbitrarily-difficult-to-produce trace on the future worlds. Given how much entropy we really have, sufficiently small persons for example can be spared from uncomputation.
In particular, a person can live in an incrementally computed-and-uncomputed virtual world that is being regularly reversed to its initial state, with the effect that only the person consumes entropy, and the whole arbitrarily complicated world has zero entropic footprint. The world could also be optimized game-save-style over person-time, starting from an initial state, but going forward, so that some version of the person does all the updating, and so carries the excess entropy. Alternatively, improvements to the world could be carried out by discarded copies. Think of software downloaded from the distant future…
Or more generally, this is just time travel, where you can transport sufficiently small things (and people) to the past (or between timelines) and change things the next time over. You travel forwards in time by computing, backwards by uncomputing, you can take some luggage with you, you can climb up a different timeline by observing without interference, and you can intervene and change a timeline any time you want. The network of people navigating baseline network of virtual worlds builds up a second level (implementing meta-time), which itself can be navigated and intervened-in by other observers, and so forth. Sounds like a Greg Egan novel (that wasn’t written yet).