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Next: Conclusions Up: The Computing Tunnel Demon Previous: Adding the Demon

The Stochastic Gas Model

Instead of calculating exact state of the gas at every time the demon observes, one may be able to make a more efficient, but less direct, model of a gas which uses statistical probability to determine whether or not the there are molecules in the tunnel when the demon looks. In this way the effects of the demon waiting for very long times between observations may be determined without calculating the state of the gas at any time between the observations.

If the demon's door is on the right side of the tunnel, and the gas is at equilibrium, the each molecule on the left of the door has a probability of being in the tunnel:


where tex2html_wrap_inline369 is the area of the tunnel and tex2html_wrap_inline371 is the area of the left chamber.

Likewise, if the door is on the left, then each molecule has a chance of being in the tunnel:


Our program proceeds as follows.

  1. Initialize volumes, energies, and numbers of particles
  2. Update system for desired number of demon observations

    1. For each molecule, determine whether or not it is in the tunnel
    2. Determine the energies of any particles in the tunnel using the equilibrium energy distribution
    3. Determine the results of the demon's observations, and shift the tunnel doors appropriately
    4. Update the energy and number of particles in each chamber
    5. Determine entropy
  3. Record desired information

This program has the advantages of being considerably faster and able to allow the system to come to equilibrium between each demon observation. In fact, this is simply assumed to be the case. This is at the expense of a more direct correspondence to physical gases.

The program was run with initial conditions approximating those of the tunnel demon. The results of this program had many of the same features of the hard disk model. Again, the three steps in the transfer of a molecule were visible on the graph. The memory used by the demon again increases linearly, as the entropy of the gas decreased irregularly. At later time steps, when the gas was further from equilibrium, the drops in entropy were large when molecules were transferred from the one chamber to the other, but these drops occurred less frequently. See Figure 15.

There is a great deal of variation between runs. There are some sections of some runs during the demon actually seems to maintain the system at a near-constant entropy while reducing the entropy of the gas, the best performance allowed by the second law.

next up previous
Next: Conclusions Up: The Computing Tunnel Demon Previous: Adding the Demon

Eric H. Neilsen
Mon Jun 16 13:53:44 EDT 1997