Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Confusion about Szilard's Engine



On 11/22/2010 06:35 PM, Derek McKenzie wrote:

How is
our knowledge of the particle's location essential here? To see what
I mean, replace the asymmetric piston assembly by a movable disk that
we can slide into the center of the cylinder at will, thereby
creating a partition. Now surely it doesn't matter in which half the
particle is located, the disk will move away from the region
containing the particle thereby transferring energy from the particle
to the movable disk. Well, I guess that's a question ;-)

I've got a feeling I'm missing the whole point of the Szilard engine,

A big part of the answer to the question comes from considering
_what comes next_. Sure, the disk has moved to one side or the
other. The problem is, you need to know which way it moved, in
order to grab it and reset the mechanism for the next cycle.

In general, fundamental thermodynamic arguments only work when
applied to complete cycles.

Also quite generally, there is a wide class of quantum measuring
devices where the crucially entropic step involves _initializing_
(or re-initializing) the device for the next use. This is related
to a famous result in the field of reversible computation, namely
that writing on blank tape is reversible and therefore "free";
the expensive irreversible step is erasing the tape.

Don't be surprised if it takes a while to get your arms around
this set of concepts. It is worth the effort.