In quantum mechanics, both "wave function collapse" and "measurement" are related concepts, but they refer to slightly different aspects of the quantum mechanical framework.
Wave Function Collapse: When a quantum system is in a superposition of multiple possible states, described by its wave function, the act of measurement or observation causes the wave function to collapse or "decohere" into a single state corresponding to the observed value. This collapse is a sudden transition from a state of superposition to a definite state, determined by the measurement outcome. The collapse is often described as a probabilistic process, where the probabilities of different outcomes are given by the squared magnitudes of the amplitudes in the wave function.
Measurement: Measurement in quantum mechanics refers to the process of extracting information or obtaining a value associated with a specific observable property of a quantum system. Observables can include quantities like position, momentum, energy, spin, or other properties that can be measured. When a measurement is performed on a quantum system, it involves an interaction between the system and the measuring apparatus, which is typically a classical macroscopic device.
During a measurement, the quantum system interacts with the measuring apparatus, leading to the entanglement of the system and the apparatus. As a result, the original superposition of the quantum system becomes entangled with the macroscopic degrees of freedom of the measuring apparatus, leading to the appearance of a definite measurement outcome.
The process of measurement causes the wave function collapse, where the superposition state collapses into a definite eigenstate of the measured observable. The measurement outcome is probabilistic, and the probability of obtaining a particular result is determined by the squared magnitude of the corresponding coefficient in the wave function.
It's important to note that the precise nature of wave function collapse and the interpretation of measurement in quantum mechanics are subjects of ongoing debate and various interpretations. The collapse interpretation discussed here is known as the Copenhagen interpretation, which is widely taught and used in many practical applications of quantum mechanics. However, other interpretations, such as the many-worlds interpretation or the consistent histories interpretation, propose different explanations for the role of measurement and wave function collapse in quantum mechanics.