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The FAQ is a good answer, but it leaves me wondering about something.

From what I read, the jist is that a short enough measurement sees no difference between an energy eigenstate and a quantum superposition which mostly comprises of that energy eigenstate but also has a few other components from other eigenstates in there.

But when we make a measurement and get, say an off-centre energy E + dE, what does that mean? Why is the molecule not choosing between all the possible discrete energies based on the probability factors c1, c2..., and returning one of these? The fact that we actually measure an energy very close to but not equal to the energy eigenvalue implies (to me) that somehow we'reaveragingthe energies of all the states with the weighting factors. I don't understand why an average is being taken, if we make an energy measurement then the molecule is forced into an energy eigenstate, and we get an exact energy.

The uncertainty about the initial state is then removed, and our measurement is an energy distribution, but the distribution is discrete amongst the energy eigenstates, not just a smearing around the state with the largest weighting.

Thanks for any help

Mike

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# Time-energy uncertainty (question after reading FAQ!)

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