In quantum mechanics, collapse refers to the sudden and irreversible change in the state of a quantum system when a measurement is made. When a measurement is performed on a quantum system, such as the position or momentum of a particle, the system is said to "collapse" into one of the possible eigenstates corresponding to the measured observable.
The concept of collapse arises from the wave-particle duality in quantum mechanics. Prior to measurement, a quantum system is described by a superposition of multiple possible states, where each state is associated with a certain probability amplitude. These probability amplitudes represent the likelihood of observing the system in a particular state.
However, upon measurement, the system is observed to be in a specific state corresponding to one of the possible outcomes of the measurement. The wave function "collapses" or "reduces" to a single eigenstate with a probability given by the squared magnitude of the associated probability amplitude. This collapse is a fundamental feature of quantum mechanics and distinguishes it from classical physics, where measurements reveal preexisting properties without changing the system's state.
The collapse postulate, also known as the projection postulate or wave function collapse, is a fundamental concept in the Copenhagen interpretation of quantum mechanics, which is one of the widely accepted interpretations of the theory. According to this interpretation, the act of measurement disturbs the quantum system, causing it to transition from a superposition of states to a definite state upon observation.
It's important to note that the mechanism underlying the collapse of the wave function is still an area of debate and interpretation in quantum mechanics. Different interpretations propose different explanations, such as the role of consciousness, decoherence, or multiple worlds. However, regardless of the interpretation, the collapse phenomenon is a crucial aspect of quantum mechanics that describes the sudden transition from a superposition of possibilities to a single, definite measurement outcome.