🚀 go-pugleaf

	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]