The concept of wave function collapse in quantum mechanics is a subject of ongoing debate and interpretation. The question you raise is commonly referred to as the measurement problem. Different interpretations of quantum mechanics provide varying explanations for the process of wave function collapse. Here are a few perspectives:
Copenhagen Interpretation: According to the Copenhagen interpretation, which is the most widely known interpretation of quantum mechanics, wave function collapse occurs when a measurement is made. In this view, the act of measurement causes the system to "choose" a particular state, and the wave function collapses to a single eigenstate corresponding to the observed measurement outcome. However, the underlying mechanism or the nature of the collapse is not specified within this interpretation.
Many-Worlds Interpretation: The Many-Worlds interpretation suggests that the wave function does not collapse but instead branches into multiple parallel universes, each representing a different possible measurement outcome. According to this view, all possible outcomes of a measurement occur simultaneously in different branches of reality.
Objective Collapse Theories: Some alternative interpretations propose that wave function collapse happens spontaneously due to fundamental physical processes. These theories posit that there are certain thresholds or conditions that cause the wave function to collapse, regardless of an observer's involvement. These theories introduce additional elements, such as stochastic or non-linear dynamics, to explain the collapse.
Bohmian Mechanics: Bohmian mechanics, also known as the pilot-wave theory, suggests that particles possess both wave-like and particle-like properties. In this interpretation, the wave function is not considered to collapse, but instead, the particles' positions become definite during the measurement process, guided by a hidden deterministic pilot wave.
It's important to note that the issue of wave function collapse and its interpretation is still an active area of research and philosophical debate in quantum mechanics. Experimentally, the collapse of the wave function is inferred from the statistical behavior of measurements conducted on quantum systems. The probabilities of different outcomes align with the predictions of quantum mechanics, lending support to the concept of collapse even though the exact nature of the collapse is not yet fully understood or agreed upon.