In the physical interpretation of wave mechanics, measurement plays an essential role. It is this procedure that provides us with new information, changes the state of our knowledge about the particle or system under investigation, and abruptly alters the form of the function ψ representing this knowledge. If, for example, the measurement is a more or less precise determination of position, then
... [Show full abstract] the wave train representing ψ before the measurement will be “reduced” to a wave train that is less extended and perhaps, when the measurement is very precise, nearly pointlike; which explains the name “reduction of the probability packet” given by Heisenberg to this abrupt modification of ψ. If, by contrast, the measurement is designed to determine the components of linear momentum, it is in momentum space, rather than coordinate space, where the sudden reduction of the wave train will take place.