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Suppose we have a Hamiltonian that depends on various real parameters. When tuning the values of these parameters, the energy eigenvalues will often avoid crossing each other. Why?

Is there a physically intuitive justification for level repulsion and avoided crossings? It would be nice to see a general argument.

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Questions are expected to be clear and to show some research effort. Downvoting. – Ben Crowell Apr 8 '13 at 3:20
For a mathematically inclined person the exposition on this link might be intuitive… – anna v Apr 8 '13 at 4:35
@BenCrowell, I think the question is quite clear and direct. I don't see why it should necessarily demonstrate research effort. – Siva Apr 8 '13 at 6:08
The question (v1) is essentially a duplicate of – Qmechanic Apr 8 '13 at 8:45
@Qmechanic Actually, the question you linked is what inspired my question. Perhaps I should have added more detail in my question, but I was thinking Adiabatic Theorem, not perturbation theory. I will edit my question correspondingly. – ChickenGod Apr 8 '13 at 10:59

Consider what happens if there is a crossing. A crossing would imply a degeneracy in the system. A degeneracy would imply a symmetry. It would be unnatural for a perturbation to introduce a symmetry into a system, and so the eigenvalues cannot cross generically, but can under special circumstances.

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