Many-Worlds interpretation

In plain terms

Here is the problem Everett solved, at a price still being argued about. Take quantum evolution seriously as the whole story -- no collapse, no exceptions -- and after a measurement the apparatus and the observer are themselves in superposition, one branch per outcome. Something has to be said about that.

Everett's move: say nothing is wrong. Every outcome happens. The branches decohere, stop interfering, and each contains an observer certain their result was the only one. Nothing is added to the formalism -- no collapse, no hidden variables -- which is the position's austere appeal.

The cost surfaces at probability. If everything happens, what can it mean that one outcome was more probable than another? Counting branches gives the wrong answer. The decision-theoretic patches are ingenious and contested. That is the famous open wound of the interpretation.

In CSD

CSD is single-trajectory, and this is a settled commitment recorded in the programme's non-goals -- not an open question to be revisited. One microstate, one history, one outcome. The alternatives are regions of the epistemic space the trajectory did not enter; nowhere is there a branch where they occurred.

The probability contrast is the instructive one. Everett's weight problem -- why observed frequencies match branch amplitudes -- does not arise here in that form, because there is always a definite state and probability is ignorance about it. The question becomes which measure weights the regions, and that has a proved answer: the unique symmetry-invariant one. Where the Everettian literature argues decision theory, this programme points at a uniqueness theorem. Positional entry; no theorem asserts it.

Mathematically

No formal content. The formal contrast: outcome weights here are volume ratios on one constraint surface, so the measure problem is identifying a measure on a single space -- discharged by uniqueness under symmetry -- rather than justifying a weighting across many worlds, which is where Everettian effort concentrates.