QBism

In plain terms

Many physicists hold that the quantum state isn't a physical thing but a summary of somebody's information, and if you say that you owe an answer to the obvious question of what it's information about. QBism pushes the idea to its logical end and gives a bracing answer, which is that it's about nothing beyond the agent's own future experiences. Quantum states are personal degrees of belief, probabilities are subjective bets, and a measurement outcome is an experience of the agent who makes the measurement.

The measurement problem, the puzzle of how a smoothly evolving state can jump to one result, evaporates, because there was never a physical state out there to collapse. Wigner's friend, the puzzle of one observer describing another observer's measurement, the non-locality puzzles, and most of the paradox literature all dissolve, because nothing observer-independent was ever claimed.

The cost is the point of contention. Physics stops describing a world and starts describing each agent's gambling position, and proponents call that honesty while critics call it surrender.

In CSD

The agreement runs deep and the split is clean, which makes this the most clarifying comparison on the list. Both positions hold that the quantum state is epistemic, a matter of knowledge rather than a thing in the world, so both treat its update after a measurement as learning and not physics, and both walk away from the measurement problem in its standard form.

Then comes the floor. CSD's constraint surface exists whether or not anyone is looking. Probabilities are ratios of volumes on it and not credences, which are degrees of belief, and the measure that defines those volumes is fixed by a uniqueness theorem, so two rational agents can't disagree about it, while in QBism they legitimately can, because probability is personal. The slogan version, fair to both sides, is epistemic states over an ontic substrate versus epistemic states over nothing. This is a positional entry, and no theorem states it.

Mathematically

There's no formal content about the rival.

The formal contrast is that CSD's outcome weights are ratios of an invariant measure, agent-independent and uniqueness-backed, where QBist probabilities are Bayesian credences constrained by coherence but owned by an agent.

Related entries

• Copenhagen interpretationCopenhagen is the oldest position on quantum measurement and the one that declines the question, as measurement is where the theory stops and not something it explains.
• Psi-epistemicThe quantum state is what you know and not what exists, and the hard part was always saying what it's knowledge of, so CSD's whole architecture is an answer to that.
• Ontic vs epistemicWhat exists is one question and what a measurement context lets you say is another, and most quantum paradoxes are category errors between the two.
• Is the wavefunction real in CSD?The wavefunction is a genuine but incomplete coordinate of a real underlying world, so it can be read off the world's actual configuration, yet it leaves out the part of that configuration which decides what a measurement will show.
• Spontaneous collapse (GRW)Spontaneous collapse theories change the equations of quantum mechanics and can be killed by experiment, and CSD, so far, can't be, and its own documentation says so.