Quantum Perpetual Motion

From: Stirling Westrup (sti@cam.org)
Date: Thu Jul 20 2000 - 14:37:46 MDT

It seems like there is now theoretical support for a kind of perpetual
motion machine. If I understand the article below, it looks like you could
get arbitrary amounts of work out of a thermodynamic bath by letting it
'warm-up' to the 4K background temperature whenever it got too cold.

The American Institute of Physics Bulletin of Physics News
Number 494 July 17, 2000 by Phillip F. Schewe and Ben Stein

flow might have begun in the 19th century, as a way of maximizing
engine efficiency, but thermodynamics is still a forefront discipline,
especially when it attempts to maximize information flow in
computers. Typical of the new work is a pair of articles which arrive
at some surprising conclusions. In the first paper, Armen
Allahverdyan of, CEA Saclay (France)/University of Amsterdam
(Netherlands)/Yerevan Physics Institute (Armenia),
aarmen@spht.saclay.cea.fr, and Theo Nieuwenhuizen of the
University of Amsterdam (nieuwenh@wins.uva.nl, 011-31-20-525-
6332) state that there may be a new, previously overlooked work-
producing process in classical heat engines. All heat engines are
driven by a temperature difference between two reservoirs or "baths."
Usually the hot and cold reservoirs are isolated from each other--they
interact via an intermediary, namely the "working substance" such as
a gas which receives heat from the hot bath, pushes a piston, and
sends unused heat to the cold bath. Now, the researchers suggest
that putting the hot and cold baths in direct contact for relatively short
amounts of time may result in useful work, if the baths have very
different relaxation times, the amount of time that each takes
individually to come to thermal equilibrium. One could accomplish
this, for example, by putting together a very large bath and a very
small bath. Traditional treatments of thermodynamics assume that
direct interactions between the hot and cold reservoirs result in the
dissipation of energy and nothing else. But these treatments make the
simplifying assumption that the baths interact very feebly or for
infinitely long times.
   The authors show that allowing the two reservoirs to interact for
short windows of time (relative to the time in which two reservoirs
arrive at an equal temperature) may bring about a transfer of energy
that can be converted to work, in cases where the final common
temperature of the baths is lowered by their direct interaction. If
verified experimentally, this proposal may lead to new engine designs
and perhaps revise estimates of the maximum efficiency of a heat
engine. (Physical Review Letters, 10 July 2000.)
     In the second paper, the same research team suggests that a
quantum particle (such as an electron) interacting strongly with a
reservoir of particles may violate the Clausius inequality--one
formulation of the second law of thermodynamics, which states that it
is impossible to do work without losing heat. What the researchers
term "appalling behavior" can be traced to the quantum mechanical
property of entanglement, in which a quantum particle (such as an
electron) is so strongly interlinked with another particle or group of
particles that the resulting behavior cannot be treated by standard
thermodynamic approaches. In this paper, the Amsterdam scientists
study the entanglement of a particle with a "quantum thermal bath," a
reservoir of particles with which the first particle can exchange
energy and momentum. According to the researchers, entanglement
prevents the quantum bath from observing the normal requirements
for a heat bath. Therefore, thermodynamics simply cannot say
anything useful about the system.
     Standard thermodynamics dictates that the bath be in thermal
equilibrium and not interact strongly with an external object. To the
contrary, the bath strongly interacts with something external to it (the
entangled particle) and it cannot reach equilibrium, since it constantly
exchanges energy and momentum with the particle. At low
temperatures where entanglement could be easily preserved, the
researchers state that this system can apparently violate the Clausius
inequality--in which the heat gained by the particle must be less than
or equal to the temperature multiplied by the change in its entropy (or
disorder). Near absolute zero temperatures, a situation which would
ordinarily require the particle to lose heat, the researchers show that
the particle could gain heat, by the Clausius relation. According to
this scenario, applying a cyclic parameter such a periodically varying
external magnetic field can cause the entangled particle to extract
work from the bath--something forbidden in a classical system.
Further, the researchers say that this phenomenon could be said to
constitute a perpetual motion machine of the second kind. However,
they are quick to point out that the particle can only extract
reasonable--but not limitless--amounts of work, as the bath must
maintain a minimum ground-state energy rather than be completely
exhausted. (Second paper: Phys. Rev. Lett. 7 August 2000; Select

 Stirling Westrup  |  Use of the Internet by this poster
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