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Question about applying the Pauli Exclusion Principle

  1. Sep 2, 2011 #1
    The principle states that no two identical fermions in a system can be in the same quantum state, but what I don't fully understand is how you define a "system". For example when you apply statistical thermodynamics to a gas of non-interacting fermions you say that a maximum of one can occupy each single particle state. Maybe I'm confused or forgetting something important but I don't see why you couldn't consider all fermions of a particular kind in the universe as a system of non-interacting particles in the same way and conclude that none of them can share a single particle state.
  2. jcsd
  3. Sep 2, 2011 #2


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    Of course, you may consider a whole Universe as a system. Pauli's 'system' was introduced just to abstract of spatial relations. Or to reduce those spatial relations to something feasible for calculations (like atomic orbitals)
  4. Sep 3, 2011 #3


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    In the abstract Dirac notation no spatial relations are required. Writing down a fermionic quantum state satisfies the Pauli principle by construction (or by the formalism). And of course it applies to the universe as a whole.
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