Collapse

In plain terms

The textbook says that when a measurement happens the quantum state jumps discontinuously to match the outcome, and you can ask whether that jump is a physical event or an update to a description. The dispute is as old as the theory, and the stakes are real, as a physical jump would be abrupt, irreversible, unexplained by any equation in the theory, and apparently instantaneous across any distance.

An update, on the other hand, needs no physics at all. When you learn which box the ball is in your probabilities collapse and nobody is alarmed, so the whole question is whether quantum collapse is more like the ball.

CSD's answer is the ball, with a twist that makes the answer checkable rather than rhetorical. On a volume-preserving substrate, one whose motion carries every region of the underlying state space to a region of the same size, a physical collapse isn't merely unnecessary, as it's impossible.

In CSD

The corpus proves that no measure-preserving map implements exact collapse, where a measure-preserving map is one that keeps the size of every region unchanged, which is exactly what the motion of a deterministic, volume-conserving substrate does. So such a substrate couldn't contain a collapse process even if its designer wanted one, and the stance becomes a constraint, with CSD on record as unable to cheat.

What replaces the jump is a record. System and apparatus evolve together under the one dynamics, the record fixes which region of the space of quantum states the trajectory realises, and the update rule the textbook postulates separately falls out of the record construction. The thing that changes discontinuously is the summary, and the world underneath does what it was always doing.

Mathematically

No measure-preserving map on the arena implements exact collapse. Approximate collapse exists but is priced, as accuracy is bounded against ready-state improbability, with the exchange rate explicit.

The post-measurement update is then derived rather than postulated, and it appears as the block projection realised on the pointer arena, an output of the record construction.

Related entries

• Measurement problemQuantum mechanics contains two incompatible rules of change and no instruction for when to switch between them, and CSD replaces the switch with theorems about what no dynamics can do.
• De-isolationNothing collapses, as a measured system stops being isolated, and measurement is what de-isolation dynamics looks like from inside.
• Luders ruleThe textbook's second rule, which says how a state changes when it's measured, isn't a second rule in CSD, as it's what the record layer does.
• Record layerA measurement is over when the world itself carries which outcome happened, and the record layer is where that fact lives, on the arena and not in anyone's notebook.
• Spontaneous collapse (GRW)Spontaneous collapse theories change the equations of quantum mechanics and can be killed by experiment, and CSD, so far, can't be, and its own documentation says so.