Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S28 / 5 / Solution

The absence of visibly strange superposed records is an observational constraint, but it does not by itself uniquely confirm a physical collapse law. A macroscopic superposition in the global state need not mean that an observer sees two contradictory records simultaneously. Decoherence can suppress interference between record branches, and interpretations with additional configurations or branch-relative observers can reproduce definite experiences without stochastic collapse of the universal wavefunction.

Conversely, simply invoking decoherence does not prove that one global outcome has been selected. A reduced density matrix diagonal in a pointer basis gives the same local statistics as a classical mixture but can arise from an entangled pure state. That distinction is central to the measurement problem. The adequacy of a no-collapse account requires its own explanation of observers, records and their probabilities, rather than the assertion that small off-diagonal terms settle everything.

To distinguish dynamical collapse empirically one seeks consequences beyond the common prediction of stable records, such as a specified loss of interference with mass or separation, anomalous heating or spontaneous radiation. Those predictions depend on collapse parameters and how the law couples to matter. Experiments can constrain those models while ordinary everyday definiteness alone underdetermines the alternative laws.

Thus familiar outcomes are evidence that any acceptable theory must explain. They support collapse over a naive theory that literally predicts an experienced unresolved superposition without a record account, but not over every developed no-collapse interpretation with the same outcome statistics. A fair comparison tests the full alternatives and their likelihoods, rather than treating interpretive disagreement as an observation of collapse itself.^[ref]