Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S28 / 5

A dynamical collapse law specifies stochastic changes of the wavefunction, including a rate, localization scale and probabilities. Such changes can suppress macroscopic superpositions, but a law for a mathematical state does not automatically identify the stuff composing a macroscopic table. If the wavefunction is on configuration space, one must still explain how it represents a three-dimensional distribution of matter and records.

One option defines mass density in ordinary space by $m(\mathbf{x},t)={\sum }_i{m}_i\int dq|\psi (q,t){|}^2\delta (\mathbf{x}-{\mathbf{q}}_i)$. A localization then changes this density so that a single dominant macroscopic pattern remains. The formula is additional interpretive structure, even if it introduces no independent evolving variable beyond the wavefunction. Another option takes discrete spacetime localization events as the primitive ontology. These two proposals can accompany closely related wavefunction dynamics while telling different stories about what matter is.

This is why mass density in particular is not logically mandatory, but some account of the relation between collapse and ordinary physical objects is required if the theory is offered as a complete ontology. A wavefunction-only view may argue that no further primitive stuff is necessary, yet it owes an explanation of spatial localization and records on that basis. Merely asserting that interference terms become small is not that explanation.

Additional posits can also have costs: small wavefunction tails survive finite localization, and the ontology must say what their tiny matter components or corresponding events mean. Choosing primitive ontology and handling such tails are distinct from specifying the stochastic dynamical law. A defensible collapse interpretation therefore makes both commitments explicit rather than suggesting that adding randomness alone solves every part of the measurement problem.^[ref][ref]