In quantum mechanics, the state of a physical system is described by a wave function. Before measurement, the system does not have definite values for every measurable property. Instead, the wave function gives probabilities for the different possible outcomes that could be observed.
After a measurement is made, the system has a definite value for the measured property. This requires a sudden change in the wave function: before measurement, it describes many possible outcomes, but after measurement, only one outcome remains with a 100% probability while all others become 0%.
If these other possibilities are not removed from the wave function, the predictions for future measurements of the same system would be incorrect. This transition from many possible outcomes to one actual outcome is called wave-function collapse.
The problem is that collapse is not described by the Schrödinger equation, which describes how quantum systems evolve over time. The Schrödinger equation only describes the continuous evolution of the wave function; it does not explain when or why collapse happens.
Because collapse is not produced by the Schrödinger equation, its cause cannot be found within the physical processes described by that equation. The main problem in quantum physics is explaining how a system goes from an undefined state containing multiple possibilities to a state with one definite outcome. Since all known physical interactions are already described by quantum mechanics, the cause of collapse cannot simply be another physical interaction within the same framework.
After a century of debate, collapse remains one of the central problems of quantum mechanics. On one hand, collapse seems to contradict the Schrödinger equation. On the other hand, it seems necessary because without collapse there would be no single physical event. The wave function contains possibilities, but an actual event requires one possibility to become real while the others are eliminated.
Quantum mechanics therefore challenges the idea that reality exists as a continuous physical world with definite properties at every moment. Instead, it describes reality as a series of events where definite properties appear only when they are measured. Before collapse, the properties of the system are not determined.
This does not mean that a quantum particle is literally in many places at once. A non-collapsed wave function does not describe one
Post too long. Click here to view the full text.