# Why does a wavefunction collapse when observation takes place?

Why does a wavefunction collapse when observation takes place? Can this question be explained in non mathematical terms? I have tried finding the answer but couldn't find a clear explanation.

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I'd like to know what people consider to be an actual example of wave function collapse. I've looked for a "top ten" list of examples and I don't think there is one on the internet. –  Marty Green Jan 4 at 22:51
I did not mean to say in that way, I meant an explanation in a non mathematical terms, as I'm a learner –  APARAJITA Jan 21 at 11:13

The wave-function collapse (in the Copenhagen interpretation) is by definition what the measurement does. There is no answer to the "why" question, at least not in the standard interpretation. A more modern interpretation is that the system decoheres by interaction with the environment and approximately reproduces the postulated wave-function collapse including the Born rule.

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+1 Do you have a reference for your last sentence: i.e. for the modern interpretation? My impression is that this is not anywhere near like fully worked out and would like to hear if I'm wrong. One process I know of that has been mooted along these lines is Einselection –  WetSavannaAnimal aka Rod Vance Jan 5 at 8:05
I have a reference... somewhere. Will try to find it. –  WIMP Jan 5 at 10:35
This one I found pretty useful arxiv.org/abs/quant-ph/0505070 –  WIMP Jan 5 at 10:37
Thank you WIMP, it was helpful –  APARAJITA Feb 8 at 14:34

Wave function collapse is just one interpretation of what happens when a "measurement" (whatever that means) of a quantum system is performed. The idea is that the wave function of the system "collapses" into a single eigenstate of one of its observables.

In other words, the system goes from having no definite value of position/momentum/energy/what-have-you to having a single, definite value of whatever you measured.

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A probability function, both classically and quantum mechanically, is a function of some variables .

Classical example : take the probability of death as a function of age (fig.1 in link). The probability at each age is small until one reaches venerable ages. At age 97 the probability is about 30% that the venerable one will die that year. The measurement is two valued , either dead or alive. Once death occurs, the probability "collapses" to certainty to one of the probable values. If one measures/checks for that year a specific venerable person of 97, and he/she is alive at the end of the year, that is also a measurement that has collapsed to certainty. Except this terminology of collapsing is not used for classical probabilities.

The wavefunction is a probability distribution for getting a specific measurement value for the variables it contains, say for a particle to hit (x,y,z) on a screen. If one sees a hit on a screen from a particle, a measurement has been made, the probability has collapsed to certainty that the particle was at (x,y,z).

Both for classical and quantum mechanical probability distribution functions a large number of measurements have to be made to validate the probability distribution, if it is one predicted by a theory. Each individual measurement is a "collapse" of the probability distribution, i.e. one item of the statistically described behavior has been measured.

Collapse is a confusing and fuzzy terminology, in my opinion of course.

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