Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!mephisto!rutgers!aramis.rutgers.edu!athos.rutgers.edu!nanotech From: hcobb@walt.cc.utexas.edu (Henry J. Cobb) Newsgroups: sci.nanotech Subject: Re: A practical nano-mechanical perpetual motion machine (?) Keywords: thermodynamics, perpetual motion machine of the second type Message-ID: Date: 6 Mar 90 06:05:24 GMT Sender: nanotech@athos.rutgers.edu Organization: The University of Texas at Austin Lines: 57 Approved: nanotech@aramis.rutgers.edu This was covered in an article in Sci American. (Gee I'm making a habit of quoting SA to deflate the highly inflated schemes posted to this newsgroup ;-). The problem is that the control device must expend as much energy to forget the state of the device as it could posibly recive from the impact of the 'gas' molecule. The moral (for RAM designers ;-) is: 'Tis easy to remember, but forgetting takes effort'. Henry J. Cobb hcobb@ccwf.cc.utexas.edu "And may all your nanobots die of frozen rot." --- Me. [Boy, that's putting it concisely. Let me try to elaborate: The first thing that needs saying here is that we are really talking about entropy and not energy. Entropy is information; the entropy of a system in any given macrostate (ie, knowing things like volume, temperature, pressure) can be thought of as the number of bits necessary to tell which microstate (ie, tell me the position and velocity of each atom) that macrostate could represent. Now, we all know that at the macro level, useful work can be obtained in a closed system at the expense of raising its entropy--if we have two containers, one with a hot fluid in it, the other cold, we can run a heat engine off the "flow of heat" until they equalize (which macrostate has a higher entropy than the initial one). All of the Maxwell's demon schemes basically come down to an attempt to build a device that "eats" entropy, so that one can balance it with an entropy-raising device and produce endless work. (Usually such schemes actually obey energy conservation; that is why they're so seemingly paradoxical!) The great problem is that the laws of physics are reversible at the micro level. That means that I cannot have a system which starts in either micro-state A or B and ends up in microstate C either way; running it back from C I'd get (say) A only, and the process B-C wouldn't be reversible. Thus I can't build a machine that *truly* destroys a bit of information. I could always in theory, run it backward, and recover the bit. So if if I have a machine which appears to destroy a bit (such as a Maxwell's demon which cuts the possible positions of an atom in half) the resulting MACROstate must have a "hidden" bit of entropy separating those microstates that came from destroying a 1 and those from destroying a 0. So entropy has increased after all. (All this having been said, I should point out that there really exist entropy-eating objects: black holes. You can build machines that are impossible under the second law if they include a black hole--although a purist might debate whether they were "closed systems"!) --JoSH]