michael_nielsen

Reversible computing can be used to do computing with (in principle) zero energy consumption.

The proof is usually attributed to a 1973 paper by Bennett, but I particularly like the explanation of Fredkin and Toffoli and their billiard ball model of computing: http://fab.cba.mit.edu/classes/862.16/notes/computation/Fredkin-2002.pdf …

Yes, my bad. @anderssandberg and @robinhanson have pointed this out to me, and I should have remembered. I definitely don't understand the physics yet, but thanks for pointing to the paper - it looks like it might be a clearer explanation than I've yet seen. But the point stands:https://twitter.com/davidmanheim/status/1083645585784299520 …

Section 3.6 has a model which is just billiard balls rolling round a table, doing universal computing. No energy consumption at all. It's a very nice model, albeit not physically practical.

In general, there will be noise in models like this, and some energy dissipation is required to remove it through error-correction. But there's no in principle lower bound to that noise.

Horizon radiation actually gives a lower bound to temperature. Still, Robin pointed out one can cool further with absurd insulation; am still working out the total thermodynamic cost and optimizing. Would love to see time bounds too - reversible is *slow*.

What horizon are you referring to? If you simply mean there's ambient noise due to the rest of the universe, the standard approach is to isolate the system. I'm not sure what the fundamental physical limits will be to that, but (a) they'll be incredibly low; and (b) they seem

I am thinking of the de Sitter radiation, ~10^-30 K - due to cosmological constant, so in a sense fundamental. Seems unavoidable, but can be isolated at the price of literal parsecs of foil insulators. (thickness due to need to avoid near field couplings)