QBism

QBism, short for Quantum Bayesianism, is a relatively recent interpretation built around a radical reading of the quantum state: the wavefunction is not a piece of the external world at all, but a catalogue of one particular agent's personal beliefs about what they will experience if they act on a system. Different well-informed agents can legitimately assign different states, just as two careful gamblers can hold different odds on the same horse.

On this view the Born rule is not a law of nature describing objective frequencies but a rule of rational gambling — a refined, quantum-flavoured cousin of ordinary probability theory that tells an agent how to keep their bets consistent. A measurement is an action the agent takes, and the so-called collapse is simply that agent updating their beliefs in light of the experience the action brings, exactly as you update a probability when you learn something new.

QBism's chief attraction is that it dissolves several classic puzzles. There is no measurement problem and no spooky collapse spreading across space, because the state was never an objective thing that had to collapse; it lived in the agent's head all along. The honest cost is that it places the experiencing agent at the centre and stays deliberately quiet about what the world is like in itself — a feature its admirers call humility and its critics call a retreat from describing reality.