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In quantum mechanics, it is postulated that the physical states of a system correspond to the points on the unit sphere in the associated Hilbert space.

And yet, people freely talk about e.g. scattering of plane waves by a potential? What does [itex]e^{\mathrm{i} \vec{k}\cdot\vec{x}}[/itex] correspond to physically?

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# Plane waves not physical states?

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