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Ehrenfest Theorem

  1. Aug 24, 2012 #1
    Since it's true in quantum mechanics that expectation values obey the classical equations of motion, then if you have a whole bunch of particles, would most of them behave classically?

    I thought to get classical mechanics you needed big particles, but Ehrenfest's theorem says lots of small particles would do.
  2. jcsd
  3. Aug 24, 2012 #2


    Staff: Mentor


    All you are talking about is the law of large numbers in that the statistical average tends towards the average as the number increases. Each individual particle is still fully QM weird.

  4. Sep 6, 2012 #3
    "would most of them behave classically?"

    Ofcourse not, each is treated Quantum mechanically and will be so.

    Finally under a statistical average, under many many measurements everything will tend obey the classical theory.

    Bhobba please comment
  5. Sep 7, 2012 #4


    Staff: Mentor

    That looks about right - provided you mean by everything will tend to obey the classical theory you mean the averages do - not individual measurements.

  6. Sep 7, 2012 #5
  7. Sep 7, 2012 #6


    User Avatar
    Science Advisor

    If you have a lot of particles, then their centre of mass will move nearly classically.
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