4 ms·

When one system measures another, there is no collapse, only entanglement. The superposition just grows. As trillions of environmental particles entangle with t

When one system measures another, there is no collapse, only entanglement. The superposition just grows. As trillions of environmental particles entangle with the system, that superposition becomes in practice -- but not in principle -- impossible to detect or exploit. This is called decoherence, and it is often wrongly used to explain away the issue. When does "collapse" happen in principle? There's no clear answer. Yet, when you observe a system yourself, you are free to say it has one result (at least, in "your world / branch" if we use MWI lingo). So there is (possibly) a sense in which you are the only observer -- at least, w.r.t. the place you think of as "the world." (And of course when we say "you observe," we mean "you experience". If only your toe measures it, you may treat your toe as an external measuring device. This is how "consciousness" enters the picture, though nobody agrees on what exactly it means.)

phkahler 5y ago
>> 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).
- deleted 5y ago
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rkagerer 5y ago
If only your toe measures it, you may treat your toe as an external measuring device. But from the viewpoint of the toe would it have collapsed? Extrapolating that, why isn't your consciousness just another measuring device that's become entangled with the system?
- monktastic1 5y ago
  Yes, if you assume the toe has "a viewpoint," then from its viewpoint, things have collapsed. But as you are not your toe, this has no observable consequence for you. It's pure philosophy. Similarly, your consciousness could be considered just another measuring device from the perspective of someone outside the system. But by virtue of being "outside the system," their results have no bearing on you either. You are the point at which "the magic happens" in your world -- which is just the place you normally call "the" world. It is the point at which neither you nor anyone else you can communicate with (i.e., no one else in "the world") can demonstrate interference, even in principle.

phkahler 5y ago

>> 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).
deleted 5y ago
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rkagerer 5y ago

If only your toe measures it, you may treat your toe as an external measuring device. But from the viewpoint of the toe would it have collapsed? Extrapolating that, why isn't your consciousness just another measuring device that's become entangled with the system?

monktastic1 5y ago
Yes, if you assume the toe has "a viewpoint," then from its viewpoint, things have collapsed. But as you are not your toe, this has no observable consequence for you. It's pure philosophy. Similarly, your consciousness could be considered just another measuring device from the perspective of someone outside the system. But by virtue of being "outside the system," their results have no bearing on you either. You are the point at which "the magic happens" in your world -- which is just the place you normally call "the" world. It is the point at which neither you nor anyone else you can communicate with (i.e., no one else in "the world") can demonstrate interference, even in principle.