1990h
The following is an item by Harold Aspden which appeared in the
journal 'Nature' on September 6, 1990 at p. 25 of vol.
347.
Breaching the Second law of Thermodynamics
SIR - Maddox
(Editor of Nature), in his discussion of the possibility of breaching the
second law of thermodynamics (Nature 345, 109; 1990) raises an issue which
deserves more than cursory attention. If Maxwell's demon could act effortlessly
to allow more energetic particles to pass from one cavity A to another B, while
restricting flow from B to A to less energetic particles, then one could
transfer heat up a temperature gradient to cause B to be hotter than A. The
question is important because our search for a pollution-free energy source
would soon be solved for ever if an army of Maxwell's demons could be put to
such use on a commercial scale.
Because we regard photons as carriers of
energy and see them as a kind of particle travelling at the speed of light, we
hardly need rely on the intelligence of Maxwell's demons to open and close a
shutter across a hole between the two cavities. Instead it is sufficient to
place a convex mirror located in cavity A and positioned away from but facing
the hole. The focusing action of this mirror will ensure that those photons
transfer heat from A to B until the walls of cavity B are at a higher
temperature than the walls of cavity A.
This should suffice as an
apparatus which will breach the second law of thermodynamics. Textbook teaching
declares that "In nature heat is never found to proceed up a temperature
gradient of its own accord". From this, the textbooks advance to a statement of
the law according to which it is impossible for any machine to abstract heat
from the coldest body of its surroundings and convert this into useful work
surplus to that needed to power the machine. Maxwell's demon can affect that
self-accord of the heat transfer and this causes one to wonder if that demon
does really do any work in controlling the shutter. But with the mirror
discharging this physical task, it is assuredly not doing work itself and those
photons proceed up that temperature gradient by their own accord as they bounce
from the mirror surface. In principle, therefore, there just has to be a failure
of the second law of thermodynamics.
Maddox, in his editorial on
mechanical engines driven by light (Nature 342, 13; 1989), suggested that "it
would be more than just fun if somebody were to build one". The practical
implications are enormous but they highlight the need to develop a miniature
thermoelectric power converter which could be incorporated between cavities A
and B in a stratified system containing a large superficial heat radiating
surface with numerous cavities and numerous mirror focusing elements so that a
significant net power per unit volume can be generated.
What is so
fascinating about this proposal is that such an energy device would not be
subject to the Carnot efficiency limit. All the heat fed into the system to
sustain the temperature of cavity A would emerge as electricity even though it
might cycle several times between cavity A and cavity B, going one way thanks to
the focusing power of the mirror and the other way as heat 'loss' through the
therrnoelectric converters. The efficiency of the heat-to-electricity conversion
of the thermoelectric power converter is not a factor limiting what has just
been said. It is just that the greater this efficiency, the smaller the volume
of the structure needed for a given power output and so the smaller the capital
expense incurred.
H. ASPDEN
Department of Electrical
Engineering,
University of Southampton,
Southampton SO9 5HN, UK
* Note: The above address is no longer applicable, the author having now
retired. Mail to the author should be sent c/o Sabberton Publications, P.O. Box
35, Southampton SO16 7RB, England.
