QBism
Explanation
QBism (Quantum Bayesianism) is an interpretation of quantum mechanics developed mainly by Christopher Fuchs, Rüdiger Schack and David Mermin from the 2000s onwards. Its fundamental turn is conceptual: the wave function does not describe objective properties of a physical system, but the degrees of belief (in the Bayesian sense) of an agent about possible results of his future experiences with that system. Quantum mechanics is, on this reading, a theory of decision-making under uncertainty, not a snapshot of the world.
The technical core is the Born rule understood in personalist terms. Before measuring, an agent assigns probabilities to outcomes according to her knowledge, coherently with quantum laws. After measuring, the agent updates her beliefs with the observed result. That collapse is not a physical event happening to the world, but an epistemic update of the agent. Different agents may legitimately have different wave functions for the same system, just as they have different probabilities about a die that none of them has seen fall.
For QBism, the physical world is real and exists independently, but it is not made of wave functions. It is made of agents' experiences and the relations between them. Each measurement is a new interaction between an agent and the world, producing a new experience. Experience is primary; the quantum formalism is a calculation tool to navigate that experience coherently. This brings QBism close to a pragmatist tradition, with echoes of William James and John Dewey.
Regarding consciousness, QBism is not a theory of mind, but its framework gives the agent's subjectivity a structural role. It is not a matter of consciousness collapsing wave functions in Wigner's style, but that the quantum description is, from its origin, indexed to a subject. Fundamental physics is then not a description of the world as it is in itself, but of how cognitive agents deal with it. Mind, so to speak, comes in through the front door.
Among its philosophical advantages, QBism dissolves several classical paradoxes. Wigner's friend ceases to be mysterious: different agents have different updates, and that does not entail inconsistency or a privileged mind. Quantum non-locality is softened: it is not a matter of ghostly actions at a distance, but of personal updates after comparing results. Measurement ceases to be the problem because, for QBism, it was always an act of the agent, never an extra physical process of the world.
Critics charge that QBism may seem antirealist (quantum mechanics only speaks of our beliefs), that it does not say much about the structure of the physical world beyond the agent, and that it has difficulties accounting for intersubjective regularities without introducing some kind of objective structure. Despite this, QBism is today one of the liveliest interpretations of quantum mechanics and one of the most explicit in placing the subject, knowledge and experience at the heart of fundamental physics.
Strengths
- Resolves the measurement problem.
- Coherent with philosophical pragmatism.
- Takes the agent as a node of the theory.
- Mathematically articulated.
Main critiques
- Accused of radical subjectivism.
- Difficult to account for intersubjective regularity.
- Does not explain what consciousness is, only puts it in its place.
- Polemic with realists about the wave function.