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When a quantum system is acted on by time dependent perturbation, the initial state evolves according to the new time-dependent Hamiltonian and grows to some superposition of states. During the time perturbation is on the evolution is governed by this time dependent Hamiltonian and when the perturbation is turned off time-evolution is governed by the unperturbed Hamiltonian. Mathematically the coefficients of the different states in the superposition change over time.

My question is what makes the system collapse to one of the states. Is the source of the perturbation being turned off equivalent to a measurement process happening?

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  • $\begingroup$ do you ask why each measure doesn't give ( potentially ) a superimposed result ? $\endgroup$
    – user46925
    Commented Dec 20, 2015 at 4:58
  • $\begingroup$ I m asking why is there absorption or emission at all $\endgroup$
    – user35122
    Commented Dec 20, 2015 at 5:03
  • $\begingroup$ that is the 1st question ... because an improbable underlying process occurred instead the most probable ones which loop on 'no change' $\endgroup$
    – user46925
    Commented Dec 20, 2015 at 5:12
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    $\begingroup$ With time depedenent perturbation theory you answer to this question: If I start in a unperturbed energy-eigenstate, and I let the perturbation act, what is the probability of finding a the system in another unperturberd energy-eigenstate? So, the system is not "jumping" by its own, rather it is you that are measuring the system hence forcing it to jump. $\endgroup$
    – giulio bullsaver
    Commented Dec 20, 2015 at 12:41
  • $\begingroup$ @giuliobullsaver : about 'forcing it to jump' : with passive measures too ( like counting the beta decay bips) ? $\endgroup$
    – user46925
    Commented Dec 22, 2015 at 6:13

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