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Maxwell's Demon, help!

  1. Jan 28, 2009 #1
    With my little reading on the subject (after listen to the argument in Stat.Mech. class a year ago), I am asking for help on clarifying the following points:

    The arguments against such a system violating the 2nd law focus on the two processes
    that would increase the entropy of the system; these two processes are
    - measurement (observing the atom or determining its velocity)
    - erasing memory
    those two are based on two different approaches by Szilard and Bennett.

    my questions are

    1. Is my description correct? (loosely speaking)

    2. I don't understand the second point. What if it's an intelligent creature who is moving the partition?

    3. If we are talking about the actual 'demon' (or any other intelligent creature with consciousness) in the original thought experiment proposed by Maxwell, what exactly is the process that would increase the entropy? Is it the interaction between the creature and the environment, like observing the atoms? Or is it the processing of information (thinking) that helps deciding weather to close or remove the partition?

    4. What if there's a kind of creature that does not require any energy and does not deposit any entropy in its thinking process, would the thought experiment be possible? Can it actually violate the 2nd law? or would this kind of creature be able to violate the second law in any other circumstances?
  2. jcsd
  3. Jan 28, 2009 #2
    1. There's also the gate.

    The demon-box system does not violate the second law of thermodynamics because the demon must do two things:
    * observe the particles (recording/measuring)
    * operate the gate

    Both presumably generate entropy overbalancing any decrease. If recording (measuring) the particle is thermodynamically reversible there is no increase in entropy, but this means that recorded measurements must not be erased.

    2. But the intelligent creature, if finite, must eventually reuse memory.

    3. No real thinking need be going on. The demon could be a machine. Any thinking itself increases entropy of the system, which would include the demon's brain.

    4. A creature that requires no energy to think? You would have to be more specific. Is it a ghostlike prime-mover, injecting energy into the universe? Or is it physical, perhaps with a brain, but thinks magically? Either way, it's not a physically plausible concept.

    Java applet where you are the demon.
  4. Jan 28, 2009 #3


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    Science Advisor

    A demon with infinite memory can violate the 2nd law. A demon with finite memory must eventually erase some information, at which point entropy increases.
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