The Copenhagen-Everett bridge

Copenhagen and Everett have been treated as rivals for seventy years. The disagreement is usually staged as a clash of worlds: does the wavefunction collapse, or do all outcomes happen? Staged that way, the argument is unresolvable, because it is an argument about ontology and the experiments cannot see ontology. There is a narrower way to stage it, and in the narrow version the rivalry is one number.

The wedge is the Born rule

Copenhagen needs probabilities that are actually probabilities: real chances, realised by collapse, taken at face value by observers. Everett has no collapse and only amplitudes, so it owes an account of why an observer inside the branching should weight outcomes by squared amplitude. That account exists, through decision-theoretic arguments, and it is contested. The entire distance between the two interpretations is concentrated in that one weighting. Collapse mechanics versus world-splitting mechanics is atmosphere. The Born weight is the substance.

Call the Born weight the wedge. If you could show that both interpretations are forced to the same weighting by a shared constraint, the wedge thins to nothing, and what remains is a disagreement about what carries the weight, which no experiment will ever price.

What conservation does to the wedge

The bridge claim is that the Information Conservation Principle is that shared constraint. Both interpretations agree that measurement neither creates nor destroys information. Copenhagen says the missing information goes into the collapse, an accounting move available to observers inside a single history. Everett says it goes into the other branches, an accounting move available to the description as a whole. Once conservation is assumed, these are two placements of the same conserved quantity, and both placements force the same weighting on outcomes, because the previous post’s uniqueness argument does not care which placement you use.

That is the bridge. Not that both interpretations are right in a friendly, everyone-gets-a-trophy way, but that their operational content is identical under conservation, and the residual disagreement is about the ontology of the conserved quantity itself. That disagreement may be permanent. It is also, at this point, not physics.

I should flag the honest conditionals. The bridge is conditional on the ICP derivation, and if that falls, the wedge reopens at full width. It is also a convergence result, not an arbitration: it does not tell you whether collapse happens. It tells you that the part of the dispute that experiments could in principle price is empty.

Why the multiverse programme needs this

The framework on this site needs branch counting that an observer inside one branch can trust. The observer’s statistics are the only data anyone will ever have, and if those statistics said nothing about the structure holding the branches, the whole research programme would be sealed off from evidence. The bridge is what connects them: an inside observer’s frequencies carry Born weights, Born weights are forced by conservation, and conservation holds at the substrate level too. One conserved quantity, two vantage points, one set of numbers.

This is the argument of the Copenhagen-Everett bridge paper, the preprint linked from the announcement on this blog. The working version lives in the programme as Paper 2’s companion. What I would add here, beyond the paper, is the discipline point: the bridge was almost written as a resolution of the interpretation debate, which would have been an overclaim, and the status labels exist partly to catch that. It is a reduction of a disagreement, and reductions are the modest, load-bearing results.

That disagreement may be permanent. It is also, at this point, not physics. The last foundations post in this sequence turns to the quantity the entire programme is actually about — novelty flux density, one rate that every other claim reduces to.