On Wed, May 14, 2014 at 07:31:26PM -0400, Robert J. Hansen wrote: > On 5/14/2014 6:11 PM, Leo Gaspard wrote: [snip] > > * You state it is a lower bound on the energy consumed/generated by > > bruteforcing. Having a closer look at the Wikipedia page, I just > > found this sentence: "If no information is erased, computation may > > in principle be achieved which is thermodynamically reversible, and > > require no release of heat." > > Yeah, adiabatic computing. Give me a call as soon as we have an > adiabatic computer: I'll be deeply fascinated. Right now that's even > more theoretical than quantum computing -- we've actually observed > quantum computation in the lab on a small scale, while adiabatic > computing is so far a complete no-go, AFAIK. > > (Then again, it's been a few years since I've dived into the literature > on it -- if you can find a paper demonstrating real-world adiabatic, > energy- and entropy-free computing, I will be deeply fascinated. I > wasn't kidding about that.) > > > information on each flipped bit. Actually, IIUC, flipping a bit is a > > reversible operation, and so the landauer principle does not apply. > > Look! A bit of information: ___ > > That's what it was before. Of course, it's now carrying the value '1'. > So, tell me: you say bit flips are reversible, so what was the value > before it was 1? I promise, I generated these two bits with a fair coin > (heads = 0, tails = 1). > > "Reversible" means "we can recover previous state without guessing." > Current computing systems are not reversible.
I notice that the Wikipedia article refers here to "thermodynamically reversible" which is perhaps not the same thing as computationally reversible. So I looked up "thermodynamically reversible" and found http://www.brighthubengineering.com/thermodynamics/4616-what-are-reversible-and-irreversible-processes/ which gives the interesting summary: thermodynamically reversible processes are theoretical and don't occur in the real world. These seem to live in the same realm with 100% frictionless surfaces and insulation with infinite R-factor. That article seems confused as to whether a reversible process must be infinitely slow or infinitely fast, but Wikipedia says the former: http://en.wikipedia.org/wiki/Reversible_process_%28thermodynamics%29 But I'm way, way out of my depth here so I'll shut up. -- Mark H. Wood, Lead System Programmer mw...@iupui.edu Machines should not be friendly. Machines should be obedient.
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