Interpretations of quantum mechanics

Based on the views of Bohr and Heisenberg in the 1920s, the Copenhagen interpretation is the most widely taught. Quantum theory gives the probabilities of measurement results rather than what is observed "in itself". A measurement is made with an apparatus described classically and yields a single result. According to the principle of complementarity, wave and particle descriptions are mutually exclusive yet jointly necessary perspectives. The interpretation has no single, definitive form.

According to this theory, put forward by de Broglie in 1927 and developed by Bohm in 1952, particles always have definite positions and are guided by a wave obeying the Schrödinger equation. The theory is deterministic; probabilities come only from our ignorance of initial positions. It gives the same predictions as standard quantum mechanics but is explicitly non-local: the motion of a distant particle depends instantly on what happens elsewhere.

In 1957 Hugh Everett proposed that the wave function never collapses. The Schrödinger equation always holds and every possible outcome of a measurement occurs. Everything, including the observer, splits into "branches" in which different outcomes happen and which do not interact. The interpretation is defended for adding no collapse postulate to the theory; critics object to its acceptance of countless unobservable worlds and question how probability is to be understood.

According to these theories the wave function collapses spontaneously through a physical process independent of any observer. For a single particle such collapse is very rare, but in large objects made of many particles it happens almost instantly, which explains why everyday objects are always found in one place. Unlike other interpretations they predict small deviations from standard quantum mechanics, so they can be tested by experiment.

According to QBism the wave function expresses not the world itself but an agent's personal beliefs about their own future experiences. A measurement is an action the agent takes on the world and the new experience received in return; "collapse" is just the updating of belief with new information. The interpretation reads quantum probabilities within Bayesian probability theory and is one example of epistemic approaches that treat the wave function as information.