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$\renewcommand{ket}[1]{|#1\rangle}$ If we have a particle and we know the initial state $|\psi\rangle$ of everything that is relevant, and we know the full Hamiltonian $H$, then we should be able to show that if we measure the position of the particle then we will find it in a position eigenstate, or if not a position eigenstate, at least a superposition of eigenstates in which the observer observes the particle to be in a position eigenstate. From this it would seem as though the Born rule might be able to be derived. Does this reasoning work? If it doesn't work, why not?

Edit: To be clear, it doesn't necessarily have to follow from unitary time evolution by itself as touched in this other question. Suppose $\ket{\psi(0)} = \ket{E}\ket{M}$ where $\ket{E}$ is an energy eigenstate describing the particle and $\ket{M}$ is the state of the measuring device or the environment or whatever is relevant. If one were to now measure the energy of the particle, then knowing the Hamiltonian $H$ (or maybe it is time dependent $H(t)$) it seems that it should be implied, by Schrodinger's equation, that $\ket{\psi(t)}=\ket{E}\ket{M(t)}$, and so the particle is still in the same energy eigenstate. If this is the case then it seems like the Born rule may also be a consequence of the other postulates of quantum mechanics. Is there something not right about what I've said?

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    possible duplicate of Born rule and unitary evolution – John Rennie Jul 24 '15 at 06:32
  • I'm a bit surprised this has attracted two downvotes. It's a perfectly reasonable question, and although it's a duplicate the duplicate wasn't that easy to find. – John Rennie Jul 24 '15 at 09:39
  • @JohnRennie Not sure about who downvoted or why, but just searching "Born Rule" finds the duplicate. – Omry Jul 24 '15 at 11:32
  • @JohnRennie Actually I did see the other link and I thought that either my question had a subtle difference or that it was unresolved. I am not interested in a derivation of the Born rule that follows necessarily from unitary time evolution, only that it follows from something. Prathyush's comment on the original post describes more or less how I am feeling about it. – JLA Jul 24 '15 at 16:13
  • How does Akhmeteli's answer not address your concerns? – Kyle Kanos Jul 24 '15 at 18:05
  • Have you heard of decoherence? – DanielSank Jul 26 '15 at 17:14
  • "From this it would seem as though the Born rule might be able to be derived. Does this reasoning work? If it doesn't work, why not?" Probability is a basic notion that has its own axiomatization; it cannot be derived from non-probabilistic theory. All one can hope for is to find a deterministic theory that predicts results with frequencies well described by the Born rule. – Ján Lalinský Jul 26 '15 at 23:27

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