Feynman’s lectures on computing also has a reply to this. He ask, what’s really the difference between saying that you could build a reversible computer if you can just keep track of the junk bits it produces, and saying that you could build a reversible car engine if you could just keep track of the heat it produces?

Well, the difference is that with the computer you do have a way to keep track of them! You build two copies of the computer, and run one of them (wait for it) in reverse on the output! This will unscramble all the random-looking bits. You need to nonreversibly make a copy of the answer you are interested in, but all the intermediate calculations can be undone without assuming a heat sink.

]]>A turing machine is so much simpler than the commodity PC hardware that we all use. It’s almost trivial to write a working program that simulates a limited form of turing machine that has a fixed-length tape. So PC hardware is not really equivalent to a turing machine in computational power because it only has a finite amount of storage, while the turing machine has an infinite amount of storage. But of course no one has ever built a turing machine that has infinite tape either.

Here’s my favorite definition for entropy: entropy is hidden information

The best example of this definition is in a black hole where the information inside really is hidden. But the holographic principle says that a non-spinning neutrally-charged black hole’s surface area is proportional to the amount of hidden information. This places an upper limit on information density in the universe. Also, the information content of any region of space can be considered to be encoded on its surface. A black hole just has a surface with maximum possible information content.

]]>The second part sounds a bit infinite-monkeys-infinite-typewriters to me. Not saying it’s wrong, it just sounds sort of equivalent.

]]>Re-read it, I confounded a few things. The Feynman referred work (Bennett) gives a lower-bound on work per step on non-reversable computation, and allows for free computation using reversible machines if you are willing to essentially wait forever.

]]>Regarding the artificial intelligence scam:

have look here

http://www.semanticsystem.com/en/company.html

Thanks John; I’ll have to look into that paper one of these days when I’m in a library.

It was the Feynman lectures which originally caused me to think of this about 6 years ago. For some reason I hadn’t read the chapter on reversible computing in his book; the bookmark was right before that place. Had read some in the literature about it though. His explanations make more intuitive sense than the arguments I have read about adiabatic reversible computing, but I guess I shouldn’t be surprised.

Weird, ain’t it?

]]>Did you just link heat and computation? Holy shit! /mental worlds collide/

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