Spontaneous collapse (GRW)

In plain terms

Quantum mechanics lets a system be in a superposition, a combination of several possibilities at once, and yet a measurement always shows one result. The textbook handles the transition with an abrupt jump, collapse, that isn't part of the smooth equations. Ghirardi, Rimini and Weber took a direct line, which is that if collapse is real you put it in the equations, so their theory, GRW, modifies quantum mechanics so that each particle's state spontaneously localises at random, snapping to a definite small region, extraordinarily rarely, about once per hundred million years per particle.

The trick is amplification. A cat contains a trillion trillion particles, and in a superposition their states are correlated, so one localisation anywhere drags the whole superposition down with it. Microscopic physics is untouched, macroscopic superpositions die in microseconds, and no observer is ever mentioned.

Because the modification has parameters, a rate and a length, it makes predictions that differ from standard quantum mechanics. Interferometry with large molecules, which tests how big an object can be while still showing wave behaviour, and ultra- quiet mechanical sensors have been squeezing the allowed range of those parameters for decades, so GRW might be refuted next year. That's a compliment, as it's a theory and not an interpretation.

In CSD

The comparison is uncomfortable in a direction the corpus states plainly, as GRW is falsifiable today and CSD isn't. CSD reproduces standard quantum mechanics exactly at leading order, and its candidate departures, finite-sample statistics and corrections the papers haven't yet pinned down, are recorded as owed rather than claimed. One theory risks its neck, and the other, so far, matches the textbook everywhere.

Structurally the two could hardly differ more. GRW adds random jumps to the dynamics, while CSD's substrate is deterministic and volume-preserving, meaning that its evolution never compresses or expands the size of any set of states, and the corpus proves that no volume-preserving process can implement collapse. So the very mechanism GRW adds is one CSD is formally barred from borrowing, and definite outcomes here come from records instead.

Mathematically

There's no formal content about the rival.

The formal boundary on CSD's side is that no measure-preserving map implements exact collapse, so a GRW-style mechanism is unavailable to this substrate as a matter of theorem and not taste.

Related entries

• CollapseIn CSD the quantum state never jumps, and that's a theorem about volume-preserving maps rather than a preference among interpretations.
• 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.
• Many-Worlds interpretationMany-Worlds keeps the smooth quantum evolution and gives up the single outcome, and CSD makes the opposite choice, one trajectory and one world, as a settled commitment.
• QBismQBism is epistemic all the way down, as the quantum state is one agent's betting book and there's nothing underneath, and CSD agrees about the state and insists on the underneath.
• Does CSD make new predictions?Not yet, and the programme doesn't pretend otherwise, as the present claim is that CSD reproduces the predictions of quantum mechanics from fewer postulates, with the reproduction machine-checked, while candidate departures are recorded as owed future work rather than asserted.