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 particle existing in multiple locations simultaneously; it describes multiple possible outcomes. If a system has no definite properties, then those properties do not exist in the classical sense.
Photons, electrons, and other quantum particles can therefore be understood as names we give to mathematical descriptions of possible events. Collapse represents the transition from possible outcomes to an actual event.
Quantum mechanics appears incompatible with traditional realism, because realism assumes that physical objects exist continuously and have properties independent of observation. Since collapse cannot be explained by the Schrödinger equation, it must involve something outside the equation.
A measuring device cannot fully explain collapse, because the device itself is made of quantum particles and follows quantum laws. Therefore, the collapse cannot simply be caused by another physical interaction already described by quantum mechanics.
The only known phenomenon that cannot be reduced to the Schrödinger equation is consciousness. Therefore, actual events may require the involvement of consciousness.
However, this does not mean that a person's mind directly creates reality. The argument is not that human observers magically force particles into existence. Instead, it argues that there is a connection between conscious observation and the appearance of definite events. A correlation does not necessarily mean direct causation.
From this perspective, idealism provides a solution to the measurement problem. Instead of assuming an external physical world independent of the mind, idealism argues that reality exists as experiences within consciousness.
In the Berkeleyan view, the universe exists as ideas in the mind of God. God directly sustains and creates the experiences we perceive. The universe is not a physical object existing independently from minds, but a consistent order of experiences created by God.
Under this interpretation, wave-function collapse is not a physical process that needs a mechanism. It is simply the mathematical description of how possible experiences become actual experiences.
This view is similar to the philosophy of George Berkeley, where God is not only the Creator but also the continual Sustainer of reality.
Idealism argues that it is unnecessary to assume a hidden physical reality existing independently of the mind. What we directly know are our experiences, and mathematics describes the patterns within those experiences. The laws of physics are therefore understood as the rational structure God uses to produce an ordered universe.
This explains both why conscious experience exists, and why mathematics is so effective at predicting nature.
The brain and mental experiences are correlated, but the brain does not necessarily create consciousness; instead, physical processes may represent mental processes.
The claim that decoherence solves the collapse problem is disputed. Decoherence explains why quantum systems appear classical when they interact with their environment, but it does not explain why only one possible outcome becomes actual.
Even after decoherence, the wave function still contains multiple possible outcomes. If one outcome is not selected and the others are not removed, then the predictions for later measurements remain problematic.
The laws of classical physics can be derived from quantum mechanics, but this does not solve the measurement problem. The same wave function can still correspond to many possible classical outcomes. Collapse is still required to explain why one possibility becomes a real event.
The many-worlds interpretation does not solve this problem because it simply denies that collapse happens. It keeps the Schrödinger equation as the fundamental law, but then must explain why observers experience only one outcome.
If two measurements are performed on the same system, standard quantum mechanics requires updating the wave function after the first measurement. Without this update, the predictions for the second measurement would be incorrect.
Many-worlds avoids collapse by claiming that all possible outcomes continue to exist in different branches of reality. However, critics argue that this introduces an unnecessary and untestable idea of countless universes without actually explaining why we experience one outcome.
The concept of "branching" remains controversial because it is difficult to explain exactly when and how these branches form.
Furthermore, many-worlds assumes that consciousness arises from physical processes described by quantum mechanics. Critics argue that consciousness cannot be explained by the Schrödinger equation alone.
Finally, the wave function itself is a mathematical concept, not a physical object. Since many-worlds treats the wave function as describing many physical universes, critics argue that it relies on abstract mathematical entities and the minds capable of understanding those abstractions.
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