# What prevents bosons from occupying the same location?

The Pauli exclusion principle states that no two fermions can share identical quantum states. Bosons, one the other hand, face no such prohibition. This allows multiple bosons to essentially occupy the same space, a phenomenon that has been theorized responsible for superconductivity. Bosons do not, however, occupy exactly the same space as can be readily observed by the fact that a Bose-Einstein condensate does not collapse into a singularity.

Both of the rather unusual examples cited above are inherent to low-energy systems. A large collection of $^{12}$C (e.g. in a diamond) does not exhibit particularly unusual behavior. This leads me to hypothesize that the energy distribution of the system is largely responsible for keeping bosons apart. Given the rather basic nature of the question, however, I figured someone here would likely know the "correct" answer. So,

what keeps bosons from occupying the same location?

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Er ... nothing prevents this. That's what a Bose-Einstein condensate is: lots of bosons in the same place and quantum state.

You are observing that the sate is not perfectly localized, but that is a consequence of the state not being exactly zero momentum. Ultimately the Heisenberg principle puts a lower limit on how localized they could be.

If the bosons are composite objects (like Helium atoms, say) then you can write the state in terms of their constituent parts and the Fermionic bits have to obey the Pauli principle.

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So I was partly on the right track with the energy hypothesis. I thought about the constituent nature of the systems in question as well, but couldn't come up with sufficiently succinct wording to include it in the question. Thanks. –  AdamRedwine Apr 3 '13 at 17:43

This is really just a comment to dmckee's answer, but it got a bit long for a comment.