wavefunction collapse

Wavefunction collapse is the name for the abrupt change a wavefunction seems to undergo at the instant of measurement. Before you look, ψ may be spread across many possible outcomes in superposition; the moment a measurement registers a definite result, the standard recipe says ψ jumps to the single state matching what was seen, with all the other possibilities suddenly gone. A particle whose cloud filled a whole region is, after detection, found at one spot.

This sudden jump sits awkwardly beside the rest of the theory. Between measurements, ψ glides along smoothly and predictably under the Schrödinger equation, conserving probability. Collapse is different: it is discontinuous, random in its outcome, and it picks out the act of measurement as somehow special. The probabilities of the various jumps follow the Born rule, but the jump itself is not described by the smooth equation at all.

It is important to be honest that collapse is interpretation-dependent, not an established physical mechanism. In the textbook Copenhagen view it is taken as a basic rule of how measurement works. Other interpretations deny that any real collapse occurs — in many-worlds the branches all persist; in pilot-wave theory the particle always had a position; decoherence explains why interference becomes unobservable without a true jump. What everyone agrees on is the practical recipe and its predictions; what 'really happens' remains genuinely open.