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Very simple question, and I think it doesn't have an answer since CI is inherently incomplete. But when a particle is collapsed after being measured, what happens then? Does it remain a particle forever? Does it only instantaneously become a particle? What happens to the wave function? Does it change?

CI seems so obviously wrong, right? I know that the Copenhagen Interpretation has been endlessly pragmatic, its interpretation works for predicting the world. So is there any specification of the affects of a measurement to the wave function or what happens to particles upon/after measurements build in to the CI model? Or does that never become relevant in experiment/engineering?

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  • $\begingroup$ Copenhagen doesn't contain a "collapse". It has a formula for the behavior of the ensemble of the isolated system (the Schroedinger equation) and then it has a formula for the probability of the transfer of a quantum of energy from the isolated system to a second system called "the measurement system" (Born rule). After the quantum of energy has been transferred, the original quantum system is literally destroyed. It doesn't have any more energy to give (at least in the single quantum case). What people call "the collapse" is just this loss of energy by the quantum system. $\endgroup$
    – FlatterMann
    Commented Jul 29 at 0:08
  • $\begingroup$ This might help - Veritasium goes over the Copenhagen interpretation and other things. Parallel Worlds Probably Exist. Here’s Why $\endgroup$
    – mmesser314
    Commented Jul 29 at 2:49

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After a collapse, the wave function evolves unitarily according to the Schrodinger equation, just as it did before.

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  • $\begingroup$ +1 for being concise, but it might be helpful to clarify that collapse just means that a quantum state with a specific value for the observable is attained, it doesn't become a classical system all of a sudden. $\endgroup$
    – Vercassivelaunos
    Commented Jul 29 at 9:47
  • $\begingroup$ so the wave function continues as it would wether or not we perform a measurement? so measuring the particle does not affect the wave equation, it just gives us a collapsed value. but what about double slit experiments where measuring the particle before does change the outcome. are we not “collapsing the superposition into a position” and it persists as a particle until it hits the sensor? $\endgroup$
    – BENG
    Commented Aug 15 at 5:35
  • $\begingroup$ @beng : Except at the instant of measurement, the wave function evolves according to the schrodinger equation. At the instant of measurement, it jumps to an eigenstate of the observable being measured, and then proceeds unitarily from there. $\endgroup$
    – WillO
    Commented Aug 15 at 18:02
  • $\begingroup$ I see, and the measurement problem comes from delineating what is the mechanism that causes a random jump into an eigenstate, correct? $\endgroup$
    – BENG
    Commented Aug 16 at 1:17
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The OP seems to think that 'collapse' means that a wave function becomes a particle.

What it means instead is that a quantum state (the 'wave function') may proposes the validity of several outcomes to a specific measurement, but when the measurement is actually performed, only one of these outcomes is found. So one could say that the set of potentialities has 'collapsed' to a single one.

It can also be said that the quantum state itself has 'collapsed' in the sense that it is now deprecated by the measurement; a new state (a new wave function, taking into account the measurement result) is needed to predict the further evolution of the system, which will be unitary as per Schrodinger until the next measurement.

Note that the above is not an interpretation, just the plain application of the rules of quantum mechanics. Unless one considers the mere choice of the words used to describe the maths as an interpretation in itself, which is seemingly the curse attached to the word 'collapse'.

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