Entropy

[Robert Holmes]

I'm enjoying Roger's papers on Maxwell's Demon, though I can't say I
undestand them yet. A particular point I'm having issues with occurs early
in report 0311023 when the simplified model is being described: "We assume
there is no energy or entropy flow associated with the particle measurement
or in sending the signal to the trapdoor." My question is: just how robust
is this assumption?

I've only read the first chapter in Leff and Rex (the comprehensive review
to which the paper refers) but they have this to say about the issue of
detection:

        "The clever mechanical detector (Bennett, 1987) proposed. suggests
that, in principle, the presence of a molecule can be detected with
arbitrarily little work and dissipation. Bennett's scheme is compelling, but
is limited to a one-molecule gas. The general question of whether
measurement in a many-particle gas must be irreversible lacks a
correspondingly compelling answer. Maxwell's original temperature demon must
distinguish between molecular velocities among numerous molecules, a more
complex task than detecting the presence of a single molecule. To our
knowledge no specific device that can operate with arbitrarily little work
and dissipation has been proposed for such velocity measurements. Given this
void, the possibility of measurement without entropy generation in a
macrosopic system is not universally accepted." (pp. 8-9)

I know Roger's paper deals with differently labelled particles rather than
particles of different velocity; but has anyone any ideas on specifically
how one would distinguish between A and B particles in a manner that is free
from energy and entropy flows?
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[Roger D. Jones]

I do not mean anything deep by this assumption. I only intend that the
transmission lines between the system and the computer are lossless in
both an information sense and a physical sense. And also that the
particles are so heavy that light bouncing off them has negligible
effect on them (we are neglecting quantum effects). The light can be
used to identify the particles.

Roger

 

 

-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On
Behalf Of Robert Holmes
Sent: Sunday, November 23, 2003 10:42 AM
To: FRIAM
Subject: [FRIAM] Entropy

 

I'm enjoying Roger's papers on Maxwell's Demon, though I can't say I
undestand them yet. A particular point I'm having issues with occurs
early in report 0311023 when the simplified model is being described:
"We assume there is no energy or entropy flow associated with the
particle measurement or in sending the signal to the trapdoor." My
question is: just how robust is this assumption?

I've only read the first chapter in Leff and Rex (the comprehensive
review to which the paper refers) but they have this to say about the
issue of detection:

"The clever mechanical detector (Bennett, 1987) proposed. suggests that,
in principle, the presence of a molecule can be detected with
arbitrarily little work and dissipation. Bennett's scheme is compelling,
but is limited to a one-molecule gas. The general question of whether
measurement in a many-particle gas must be irreversible lacks a
correspondingly compelling answer. Maxwell's original temperature demon
must distinguish between molecular velocities among numerous molecules,
a more complex task than detecting the presence of a single molecule. To
our knowledge no specific device that can operate with arbitrarily
little work and dissipation has been proposed for such velocity
measurements. Given this void, the possibility of measurement without
entropy generation in a macrosopic system is not universally accepted."
(pp. 8-9)

I know Roger's paper deals with differently labelled particles rather
than particles of different velocity; but has anyone any ideas on
specifically how one would distinguish between A and B particles in a
manner that is free from energy and entropy flows?

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