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I thought "coherence" was the model by which quantum systems spread entanglement to other systems; the larger the system the 1st system into contact with, the b

by platz 10y ago

I thought "coherence" was the model by which quantum systems spread entanglement to other systems; the larger the system the 1st system into contact with, the bigger the effect of coherence loss in the 1st system and the larger the "measurement"

monktastic1 10y ago
It is generally agreed (except, perhaps, by the strongest champions of the decoherence program) that decoherence does not completely solve the measurement problem. Some good references here: http://physics.stackexchange.com/questions/295527/decoherence-does-it-solve-the-measurement-problem-is-it-discontinuous-when-do http://physics.stackexchange.com/questions/295527/decoherenc... It helps explain the loss of interference, but it does not resolve the question of why and how we see one particular outcome.
- platz 10y ago
  > it does not resolve the question of why and how we see one particular outcome. agreed. dechoherence doesn't explain particular outcomes. It explains the scale & magnitude of mixing quantum states from different systems. I thought "what kinds of physical interactions qualify as measurements" was referring to a different part of understanding QM. Decoherence doesn't explain which outcome, it does explains "part" of (or place constraints upon) the mechanism of measurement process.
  - adrianratnapala 10y ago
    The problem here is that very much of what we mean by "scale and mangitude" boils down to the frequency of particular outcomes when situations are repeated. So Decoherence only explains those things after you already have the Born rule (P ∝ |Ψ|^2). But that's the very thing we are trying to explain!
    - platz 10y ago
      Maybe you are trying to explain the Born rule, but I was not claiming to explain that; I've already stated decoherence doesn't explain why we get certain outcomes instead of others.
      - akvadrako 10y ago
        It explains what we experience perfectly well. In the simple case of a 50/50 coin flip, there will be two equally real future versions of you. One will see heads, one tails. You don't need to dive into quantum mechanics to understand this concept; instead, consider being cloned twice while you are asleep, then killing the original. Your current self knows what will happen; you'll fall asleep and wake up as either one or the other, becoming both but never being both.
- Xcelerate 10y ago
  > that decoherence does not completely solve the measurement problem It's kind of funny how the problem keeps getting pushed to higher levels of "meta": If you consider the experimenter and his system, measurements of (non-eigenstate) quantum systems appear indeterministic to him. However, the state of [experimenter + system] is governed by an entirely deterministic equation that follows a reversible, unitary path through time. Great! But the problem is that you then have another experimenter who measures that composite system, and the outcomes he sees likewise appear indeterministic. So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem.
  - empath75 10y ago
    I've long thought that the measurement problem is a problem with the interface between consciousness and reality and that it's more of a psychology problem than a physics one.
    - platz 10y ago
      ok, but I don't think that is what Xcelerate is quite saying here
  - foldr 10y ago
    >So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem. At the risk of sending things off an a huge tangent, it's interesting to see physicists recognizing that an infinite regress is, at least sometimes, unsatisfactory (even though there is of course nothing incoherent per se about the concept of an infinite sequence). Physicists usually tend to give short shrift to metaphysical arguments that rule out certain states of affairs on the grounds that they would involve an infinite regress of a problematic kind. But the logic you're using to argue against decoherence as a solution to the measurement problem is very similar to e.g. Aristotle/Aquinas's argument that the causal hierarchy must have a terminus. I'm not saying that in an attempt to start an argument about God. It's just interesting to see similar reasoning used in such different domains. (And of course in neither domain is it entirely clear that the reasoning works.)
monktastic1 10y ago
Also see this piece by Steven Weinberg: http://www.nybooks.com/articles/2017/01/19/trouble-with-quantum-mechanics/ http://www.nybooks.com/articles/2017/01/19/trouble-with-quan... "One common answer is that, in a measurement, the spin (or whatever else is measured) is put in an interaction with a macroscopic environment that jitters in an unpredictable way. For example, the environment might be the shower of photons in a beam of light that is used to observe the system, as unpredictable in practice as a shower of raindrops. Such an environment causes the superposition of different states in the wave function to break down, leading to an unpredictable result of the measurement. (This is called decoherence.) It is as if a noisy background somehow unpredictably left only one of the notes of a chord audible. But this begs the question. If the deterministic Schrödinger equation governs the changes through time not only of the spin but also of the measuring apparatus and the physicist using it, then the results of measurement should not in principle be unpredictable. So we still have to ask, how do probabilities get into quantum mechanics?"
- naasking 10y ago
  Doesn't the probability basically follow from the uncertainty of your own eigenstate, ie. the system performing the measurement? This contextuality is why deterministic interpretations of QM also entail probabilistic measurements.

by platz 10y ago

monktastic1 10y ago

It is generally agreed (except, perhaps, by the strongest champions of the decoherence program) that decoherence does not completely solve the measurement problem. Some good references here: http://physics.stackexchange.com/questions/295527/decoherence-does-it-solve-the-measurement-problem-is-it-discontinuous-when-do http://physics.stackexchange.com/questions/295527/decoherenc... It helps explain the loss of interference, but it does not resolve the question of why and how we see one particular outcome.

platz 10y ago
> it does not resolve the question of why and how we see one particular outcome. agreed. dechoherence doesn't explain particular outcomes. It explains the scale & magnitude of mixing quantum states from different systems. I thought "what kinds of physical interactions qualify as measurements" was referring to a different part of understanding QM. Decoherence doesn't explain which outcome, it does explains "part" of (or place constraints upon) the mechanism of measurement process.
- adrianratnapala 10y ago
  The problem here is that very much of what we mean by "scale and mangitude" boils down to the frequency of particular outcomes when situations are repeated. So Decoherence only explains those things after you already have the Born rule (P ∝ |Ψ|^2). But that's the very thing we are trying to explain!
  - platz 10y ago
    Maybe you are trying to explain the Born rule, but I was not claiming to explain that; I've already stated decoherence doesn't explain why we get certain outcomes instead of others.
    - akvadrako 10y ago
      It explains what we experience perfectly well. In the simple case of a 50/50 coin flip, there will be two equally real future versions of you. One will see heads, one tails. You don't need to dive into quantum mechanics to understand this concept; instead, consider being cloned twice while you are asleep, then killing the original. Your current self knows what will happen; you'll fall asleep and wake up as either one or the other, becoming both but never being both.
Xcelerate 10y ago
> that decoherence does not completely solve the measurement problem It's kind of funny how the problem keeps getting pushed to higher levels of "meta": If you consider the experimenter and his system, measurements of (non-eigenstate) quantum systems appear indeterministic to him. However, the state of [experimenter + system] is governed by an entirely deterministic equation that follows a reversible, unitary path through time. Great! But the problem is that you then have another experimenter who measures that composite system, and the outcomes he sees likewise appear indeterministic. So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem.
- empath75 10y ago
  I've long thought that the measurement problem is a problem with the interface between consciousness and reality and that it's more of a psychology problem than a physics one.
  - platz 10y ago
    ok, but I don't think that is what Xcelerate is quite saying here
- foldr 10y ago
  >So now you consider the system of [experimenter 2 + [experimenter 1 + system]], and we've got infinite regress — a.k.a. the measurement problem. At the risk of sending things off an a huge tangent, it's interesting to see physicists recognizing that an infinite regress is, at least sometimes, unsatisfactory (even though there is of course nothing incoherent per se about the concept of an infinite sequence). Physicists usually tend to give short shrift to metaphysical arguments that rule out certain states of affairs on the grounds that they would involve an infinite regress of a problematic kind. But the logic you're using to argue against decoherence as a solution to the measurement problem is very similar to e.g. Aristotle/Aquinas's argument that the causal hierarchy must have a terminus. I'm not saying that in an attempt to start an argument about God. It's just interesting to see similar reasoning used in such different domains. (And of course in neither domain is it entirely clear that the reasoning works.)

monktastic1 10y ago

Also see this piece by Steven Weinberg: http://www.nybooks.com/articles/2017/01/19/trouble-with-quantum-mechanics/ http://www.nybooks.com/articles/2017/01/19/trouble-with-quan... "One common answer is that, in a measurement, the spin (or whatever else is measured) is put in an interaction with a macroscopic environment that jitters in an unpredictable way. For example, the environment might be the shower of photons in a beam of light that is used to observe the system, as unpredictable in practice as a shower of raindrops. Such an environment causes the superposition of different states in the wave function to break down, leading to an unpredictable result of the measurement. (This is called decoherence.) It is as if a noisy background somehow unpredictably left only one of the notes of a chord audible. But this begs the question. If the deterministic Schrödinger equation governs the changes through time not only of the spin but also of the measuring apparatus and the physicist using it, then the results of measurement should not in principle be unpredictable. So we still have to ask, how do probabilities get into quantum mechanics?"

naasking 10y ago
Doesn't the probability basically follow from the uncertainty of your own eigenstate, ie. the system performing the measurement? This contextuality is why deterministic interpretations of QM also entail probabilistic measurements.