Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!njin!uupsi!sunic!dkuug!freja.diku.dk!rimfaxe.diku.dk!sestoft
From: sestoft@rimfaxe.diku.dk (Peter Sestoft)
Newsgroups: comp.theory
Subject: Re: Entropy and time in computers
Message-ID: <1990Jul4.071139.296@diku.dk>
Date: 4 Jul 90 07:11:39 GMT
References: <1990Jul3.152829.741@kth.se>
Sender: news@diku.dk (The Netnews System)
Organization: Department Of Computer Science, University Of Copenhagen
Lines: 40

d88-cwe@nada.kth.se writes:

>I would like some input on some thoughts of mine;

>Could one say that to gain information in computers, you would
>have to lose time. This is some kind of parallell to the second
>thermodynamic law.

>The reasoning:

>Since, for a given kind, brand etc of computer, a certain energy-
>consumption per timeunit is consumed, energyresources is limited.
>(I disregard secondary storagemedia etc, I only mean memory and
>cpu in this context)

>Then, to gain information, or lower entropy in other words, on would
>have to convert a certain amount of ordered energy to a less ordered
>form, eg heat.

>Thus, for a given brand there should exist a theoretical limit on
>entropy-gain per time unit.

Do you know the following reference:

Rolf Landauer: Dissipation and noise immunity in computation and
communication.  Nature vol. 335 (27 October 1988) 779-784.

Excerpt from the abstract: "Reversible computers which carry out each
step without discarding information can, in principle, dissipate
arbitrarily small amounts of energy per step if the computation is
carried out sufficiently slowly. ..."

This to me sounds quite relevant to your suggestion.  Not being a
physicist, I am not in a position to say whether the arguments put
forward in the article make sense.  (But the referees of Nature must
think so).

Peter Sestoft  *  sestoft@diku.dk   *   DIKU,  Department of Computer Science
University of Copenhagen, Universitetsparken 1, DK-2100 Copenhagen O, Denmark
Tel: +45 31 39 64 66  *  Direct: +45 31 39 33 11/406  *  Fax: +45 31 39 02 21