Editing Copenhagen interpretation (section)

=== Collapse ===
{{Main|Wave function collapse}}
The concept of wave function collapse postulates that the wave function of a system can change suddenly and discontinuously upon measurement. Prior to a measurement, a wave function involves the various probabilities for the different potential outcomes of that measurement. But when the apparatus registers one of those outcomes, no traces of the others linger. Since Bohr did not view the wavefunction as something physical, he never talks about "collapse". Nevertheless, many physicists and philosophers associate collapse with the Copenhagen interpretation.<ref name="Faye-Stanford" /><ref name="Howard 2004" />

Heisenberg spoke of the wave function as representing available knowledge of a system, and did not use the term "collapse", but instead termed it "reduction" of the wave function to a new state representing the change in available knowledge which occurs once a particular phenomenon is registered by the apparatus.<ref>W. Heisenberg "Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik," ''Zeitschrift für Physik'', Volume 43, 172–198 (1927), as translated by John Wheeler and Wojciech Zurek, in ''Quantum Theory and Measurement'' (1983), p. 74.  ("[The] determination of the position selects a definite "''q''" from the totality of possibilities and limits the options for all subsequent measurements.  ... [T]he results of later measurements can only be calculated when one again ascribes to the electron a "smaller" wavepacket of extension λ (wavelength of the light used in the observation).  Thus, every position determination reduces the wavepacket back to its original extension λ.")</ref>