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Gas dynamics

  1. Mar 1, 2006 #1
    Someone asked me this question and I was wondering how you guys would answer it......

    Consider a container with some gas in it. It can be a sealed living room filled with ordinary air at typical pressure.

    The air molecules are in motion - colliding with each other and bouncing off the walls.

    Question: If this random motion of molecules around the room is allowed to proceed long enough (say, a billion years raised to the billionth power), will it at some point happen that all of the gas molecules are in one half of the room, leaving the other half in zero pressure?

    Is that situation forbidden by the principles of gas dynamnics, or are gas dynamics just statements about what is statistically likely to be the case at any one time as gas molecules knock each other around? Will those principles be wildly violated on statistically rare occasions?
  2. jcsd
  3. Mar 1, 2006 #2


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    Short answer: "Yes." You're a little short on the time scale necessary to be able to witness it, but, "sooner or later," the system passes through that state.
  4. Mar 1, 2006 #3

    Cool......the person who originated the question was allowing for as much time as theoretically necessary for the event to occur.

    My answer, with reservation, was that yes I did think such a state would occur given enough time.

    But Im an electrical engineer who is not an expert in fluid dynamics or statistical mechanics so I wasnt sure if my intial answer waas correct or an oversimplification.

    Do you know of any online resources that I could direct him to for a deeper explanation of why this could occur and how much time might be necessary?


  5. Mar 1, 2006 #4


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  6. Mar 1, 2006 #5
  7. Mar 3, 2006 #6


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    You should also search for "Poincaré cycle"...

    There's a theorem proved by Poincaré that allows this possibility.

  8. Mar 4, 2006 #7

    Will do.

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