Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!decvax!decwrl!greipa!pesnta!hplabs!sri-unix!KATZ@USC-ISIF.ARPA From: KATZ@USC-ISIF.ARPA Newsgroups: net.physics Subject: reversible computing Message-ID: <329@sri-arpa.ARPA> Date: Thu, 27-Jun-85 17:05:44 EDT Article-I.D.: sri-arpa.329 Posted: Thu Jun 27 17:05:44 1985 Date-Received: Mon, 1-Jul-85 07:09:48 EDT Lines: 32 From: Alan R. KatzIn response to the question of tunneling: I have not read all of the Sci Amer article, so I can't answer about this particular point. However, Richard Feynman (and others) have constructed Quantum Mechanical models of reverisble computers, so such machines are realizable in the Quantum world. The trouble is that these models consists of constructing a wierd Hamiltonian for a theoretical system. They do not really say how an actual system could be contructed. The one(ones) mentioned in Sci Amer may have problems, but in principle, you could do it. For those who have not read the article, it is about making computers that generate essentially no heat, and use essentially no energy. The key is using reversible microscopic processes (such as billiard ball collisions) as gates in the computer. Because the computation is reversible, any noise will just "undo" the computation a little, rather than mess it up. Since energy is conserved, it takes no (very little) energy to do the actual computation. You get strange things like the number of bits being conserved (so when you generate an answer, you also must generate its complement). After discussing various ways to do these computations based on billiard ball type collisions, you must take quantum mechanics into account. Feynman and others have done this and it still all works, at least theoretically. Alan -------