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>> When does "collapse" happen in principle? There's no clear answer. It does not happen at all. There is no measurable distinction between a particle whose wa

>> When does "collapse" happen in principle? There's no clear answer. It does not happen at all. There is no measurable distinction between a particle whose wave function has collapsed and one that hasn't. If there were a discernable difference the "spooky action at a distance" when an entangled pair is collapsed could be used for faster than light communication. >> When one system measures another, there is no collapse, only entanglement. This is the first time I've seen this explanation. To me it feels like an attempt at resolving the issue, but I think not. The right answer AFAICT is that we must embrace non-locality. But that's still another sort of punt.

monktastic1 5y ago
I put "collapse" in quotes for a reason. There is an experimentally verifiable distinction between a particle in superposition (w.r.t a particular basis) and one that is not. Both theory and experiment give us every reason to believe that prior to your observation, you can exhibit the superposition through an interference experiment (modulo decoherence), while afterward you cannot (even if "God" still can). Also, this surprises me (assuming you studied QM formally): > This is the first time I've seen this explanation. Entanglement is basically the definition of measurement (in this context).
- kgwgk 5y ago
  I’m not sure of what is your context but that unitary interaction is just the first step (also called pre-measurement) in von Neumann’s model of measurement.
  - monktastic1 5y ago
    Indeed. The context here is: > As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human intervention and that this can be shown. When pop sci treatments explain that any physical system interacting with the apparatus "measures" it, they are just talking about what you call "pre-measurement." This causes a lot of confusion for students.
    - kgwgk 5y ago
      I agree, I think. The observed “collapse” is not completely explained by decoherence (and even less satisfactory by many worlds).
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monktastic1 5y ago

I put "collapse" in quotes for a reason. There is an experimentally verifiable distinction between a particle in superposition (w.r.t a particular basis) and one that is not. Both theory and experiment give us every reason to believe that prior to your observation, you can exhibit the superposition through an interference experiment (modulo decoherence), while afterward you cannot (even if "God" still can). Also, this surprises me (assuming you studied QM formally): > This is the first time I've seen this explanation. Entanglement is basically the definition of measurement (in this context).

kgwgk 5y ago
I’m not sure of what is your context but that unitary interaction is just the first step (also called pre-measurement) in von Neumann’s model of measurement.
- monktastic1 5y ago
  Indeed. The context here is: > As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human intervention and that this can be shown. When pop sci treatments explain that any physical system interacting with the apparatus "measures" it, they are just talking about what you call "pre-measurement." This causes a lot of confusion for students.
  - kgwgk 5y ago
    I agree, I think. The observed “collapse” is not completely explained by decoherence (and even less satisfactory by many worlds).

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