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I find it strange that they claim that QM lacks any explanation for measurement. There are explanations for measurement. Not everyone agrees with any single on

I find it strange that they claim that QM lacks any explanation for measurement. There are explanations for measurement. Not everyone agrees with any single one of them, but they definitely exist. Superdeterminism is one, but it's not the only one. It is really only the Copenhagen Interpretation of QM that has a measurement problem. Personally, I am on board with the "measurement is just entanglement" solution (it seems intuitively obvious to me, but then I'm sure superdeterminism seems intuitively obvious to those who believe in it as well), which makes superdeterminism unnecessary.

knzhou 7y ago
While I lean the same way, the measurement problem is definitely harder than you say. If it's just entanglement with an external system, you still have to answer how that superposition ends up with you personally observing only one result.
- MrEldritch 7y ago
  you are a large, warm object, and as such you decohere basically instantaneously. (In any case, far faster than the milliseconds required for you to perceive anything.) As such, the state-in-which-you-perceive-result-A is completely unable to causally influence the state-in-which-you-perceive-result-B, and vice versa.
- gliese1337 7y ago
  Because you are part of the entangled system. You can only know that a measurement apparatus is in a definite state because you interacted with it, so your own mental state is partially dependent on it. You, however, being a large, warm thing decohere rather rapidly, even if the formal measurement apparatus can be made not to. Your consciousness is a classical phenomenon, so you can only perceive one possible classical outcome. If you believe in Many Worlds (which is the most natural interpretation to pair with this resolution of the measurement problem, but is not absolutely forced), then there are other versions of you out there in different parts of state-space which perceive different outcomes, and may even be observable as elements of a superposed-you in principle if not ever in practice, but there is no means of transferring information between any of these different versions of you. EDIT: To whomever downvoted the parent... why? It is, after all, a perfectly reasonable question, especially (although I do not presume that knzhou personally holds this view) in light of the reasonably common assumption that consciousness is somehow quantum in nature itself.
repsilat 7y ago
I'm also a Many-Worldser, but I don't quite know how to answer the question "How does the Born Rule work?" That is, if "worlds" branch off with some amplitudes, and there's a "me" on this branch and another on that branch, how does it make sense to talk about the probability of "me me" being this one or that one? Empirically we know how to calculate it from the amplitude, but why is that more reasonable than any other answer (if any answer is reasonable)?
- gliese1337 7y ago
  Well, yeah, there is that... I suppose one could argue that lacking an a-priori explanation for the way probability is allocated over state-space is equivalent to lacking an explanation for measurement, but that seems to me like claiming that we have no answer to how thermal emission of light works because we can't explain the measurement rule in QM, which I expect most physicists would balk at. Yea, there is still work to be done at a lower level, and there are other competing explanations at this level, but that doesn't mean that we don't have an explanation at this level. "How does the Born Rule work?" is a lower-level question than "how does measurement work?"
  - repsilat 7y ago
    The "measurement problem" is really a Copenhagen-specific thing -- "measurement" is the fundamental and identifying mechanism of the interpretation, and it's problematic. And In the Copenhagen interpretation the Born Rule is inextricably tied to measurement outcomes. I guess you could say that MWI removes the wishy-nastiness around what constitutes measurement, when it happens etc, and that could constitute "solving the problem", but I think - Some of "the problem" has just been moved outside those borders. (And fine -- the MWI doesn't need to solve every problem to be an improvement. And I do think it's an improvement.) - The Born Rule is weirder* in the MWI. In Copenhagen it's axiomatic: We explicitly roll the dice, a nondeterministic outcome happens, that's the mechanism. In MWI it's an afterthought: The wavefunction evolves deterministically, and oh-by-the-way you should weight the likelihood of finding yourself here or there by the squared amplitude. Maybe that's unfair though?
- Strilanc 7y ago
edna314 7y ago
> I am on board with the "measurement is just entanglement" solution Then what is "entanglement"? I sure would agree that 'measurement' and 'entanglement' have a similar nature. I just think this argument doesn't answer the original question, but it is trying to hide it.
- lmm 7y ago
  Entanglement is a well-understood part of QM, and even Copenhagenists agree on how it applies to elemental particles. (They just think something strange and different happens to macroscopic objects).
  - edna314 7y ago
    If it is well understood, could you tell me how it works? I know the mathematics of entanglement just as I know the the mathematics of a collapse of a wave function, so I wouldn't take any mathematical argument as a valid answer.
    - gliese1337 7y ago
      The math is the description of how it works. If you are not willing to accept any mathematical argument as a valid answer, you better explain what kind of answer you would accept as an alternative, 'cause it's not at all obvious that any other explanation could even exist.
      - edna314 7y ago
        I can argue in the same way in favor of Copenhagen. The math of Copenhagen is clearly defined, as the collapse of a wave function is the projection on eigenfunctions of the measuring device.
        gliese1337 7y ago
        No, it isn't. The mathematics that describe the results of measurement are clearly defined, but what constitutes a measurement--i.e., when you need to apply that process--is not. There is no mathematics to back it up--it's purely ad-hoc. That is what "measurement is just entanglement" provides.

knzhou 7y ago

While I lean the same way, the measurement problem is definitely harder than you say. If it's just entanglement with an external system, you still have to answer how that superposition ends up with you personally observing only one result.

MrEldritch 7y ago
you are a large, warm object, and as such you decohere basically instantaneously. (In any case, far faster than the milliseconds required for you to perceive anything.) As such, the state-in-which-you-perceive-result-A is completely unable to causally influence the state-in-which-you-perceive-result-B, and vice versa.
gliese1337 7y ago
Because you are part of the entangled system. You can only know that a measurement apparatus is in a definite state because you interacted with it, so your own mental state is partially dependent on it. You, however, being a large, warm thing decohere rather rapidly, even if the formal measurement apparatus can be made not to. Your consciousness is a classical phenomenon, so you can only perceive one possible classical outcome. If you believe in Many Worlds (which is the most natural interpretation to pair with this resolution of the measurement problem, but is not absolutely forced), then there are other versions of you out there in different parts of state-space which perceive different outcomes, and may even be observable as elements of a superposed-you in principle if not ever in practice, but there is no means of transferring information between any of these different versions of you. EDIT: To whomever downvoted the parent... why? It is, after all, a perfectly reasonable question, especially (although I do not presume that knzhou personally holds this view) in light of the reasonably common assumption that consciousness is somehow quantum in nature itself.

repsilat 7y ago

I'm also a Many-Worldser, but I don't quite know how to answer the question "How does the Born Rule work?" That is, if "worlds" branch off with some amplitudes, and there's a "me" on this branch and another on that branch, how does it make sense to talk about the probability of "me me" being this one or that one? Empirically we know how to calculate it from the amplitude, but why is that more reasonable than any other answer (if any answer is reasonable)?

gliese1337 7y ago
Well, yeah, there is that... I suppose one could argue that lacking an a-priori explanation for the way probability is allocated over state-space is equivalent to lacking an explanation for measurement, but that seems to me like claiming that we have no answer to how thermal emission of light works because we can't explain the measurement rule in QM, which I expect most physicists would balk at. Yea, there is still work to be done at a lower level, and there are other competing explanations at this level, but that doesn't mean that we don't have an explanation at this level. "How does the Born Rule work?" is a lower-level question than "how does measurement work?"
- repsilat 7y ago
  The "measurement problem" is really a Copenhagen-specific thing -- "measurement" is the fundamental and identifying mechanism of the interpretation, and it's problematic. And In the Copenhagen interpretation the Born Rule is inextricably tied to measurement outcomes. I guess you could say that MWI removes the wishy-nastiness around what constitutes measurement, when it happens etc, and that could constitute "solving the problem", but I think - Some of "the problem" has just been moved outside those borders. (And fine -- the MWI doesn't need to solve every problem to be an improvement. And I do think it's an improvement.) - The Born Rule is weirder* in the MWI. In Copenhagen it's axiomatic: We explicitly roll the dice, a nondeterministic outcome happens, that's the mechanism. In MWI it's an afterthought: The wavefunction evolves deterministically, and oh-by-the-way you should weight the likelihood of finding yourself here or there by the squared amplitude. Maybe that's unfair though?
Strilanc 7y ago

edna314 7y ago

> I am on board with the "measurement is just entanglement" solution Then what is "entanglement"? I sure would agree that 'measurement' and 'entanglement' have a similar nature. I just think this argument doesn't answer the original question, but it is trying to hide it.

lmm 7y ago
Entanglement is a well-understood part of QM, and even Copenhagenists agree on how it applies to elemental particles. (They just think something strange and different happens to macroscopic objects).
- edna314 7y ago
  If it is well understood, could you tell me how it works? I know the mathematics of entanglement just as I know the the mathematics of a collapse of a wave function, so I wouldn't take any mathematical argument as a valid answer.
  - gliese1337 7y ago
    The math is the description of how it works. If you are not willing to accept any mathematical argument as a valid answer, you better explain what kind of answer you would accept as an alternative, 'cause it's not at all obvious that any other explanation could even exist.
    - edna314 7y ago
      I can argue in the same way in favor of Copenhagen. The math of Copenhagen is clearly defined, as the collapse of a wave function is the projection on eigenfunctions of the measuring device.
      - gliese1337 7y ago
        No, it isn't. The mathematics that describe the results of measurement are clearly defined, but what constitutes a measurement--i.e., when you need to apply that process--is not. There is no mathematics to back it up--it's purely ad-hoc. That is what "measurement is just entanglement" provides.