7 ms·

Peter Woit in his blog "Not Even Wrong" [0] sometimes deals with the Many Worlds interpretation. He doesn't seem to be a fan. He had a recent post [1] about Ev

Peter Woit in his blog "Not Even Wrong" [0] sometimes deals with the Many Worlds interpretation. He doesn't seem to be a fan. He had a recent post [1] about Everett that's interesting. [0] https://www.math.columbia.edu/~woit/wordpress/ https://www.math.columbia.edu/~woit/wordpress/ [1] http://www.math.columbia.edu/~woit/wordpress/?p=11202 http://www.math.columbia.edu/~woit/wordpress/?p=11202 [2] https://www.math.columbia.edu/~woit/wordpress/?p=10522 https://www.math.columbia.edu/~woit/wordpress/?p=10522

platz 7y ago
That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen. > What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.
- lisper 7y ago
  The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-worlds.html http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...
  - doubleunplussed 7y ago
    It should be noted though that few interpretations have an explanation for the born rule.
    - lisper 7y ago
      Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.
      - lmm 7y ago
        MWI is not incompatible with postulating the Born rule if you want to. It just doesn't elevate it to an axiom of the theory. It's like doing constructive mathematics: you're not denying the law of excluded middle, you can postulate it and it will have the usual consequences, but the core theory doesn't rely on it, and it's possible to hope for a more elegant alternative.
        lisper 7y ago
        Yes, you can just add the Born rule as an additional axiom, but that undermines the whole point of adopting the MWI in the first place, which is to remain mathematically "clean" with no ad-hoc assumptions.
        lmm 7y ago
        Yes and no. I'd argue that the Born rule as a rule for subjective probabilities in a MWI context is much cleaner than a full Copenhagen-style collapse operation.
        lisper 7y ago
        You are, of course, free to argue that. But most advocates of the MWI will disagree with you. That's why they put so much effort into trying to derive the Born rule.
        lmm 7y ago
        There's a significant gap between "cleaner than a Copenhagen-style collapse" and "clean enough to be a good part of our fundamental theory of nature" :).
        akvadrako 7y ago
        Not that it really matters but I disagree that MWI advocates like it because of the born rule derivations. Actually most born rule derivations apply equally to all interpretations. I’m with Vaidman on this, who is one of the best known supporters of MW, which is that the clearest solution is just to postulate the connection between psi magnitude and subjective experience. It’s an empirical discovery that works.
    - akvadrako 7y ago
      Actually most interpretations have the same explanations for the Born rule, it's just that people who study many worlds are more inclined to consider them. People who believe in Copenhagen don't care about this stuff. For a recent high-quality example: https://www.nature.com/articles/s41467-019-09348-x https://www.nature.com/articles/s41467-019-09348-x
  - platz 7y ago
    > The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenges against MWI that needs to be answered. Carroll offers "self-location uncertainty" as a way to recover the probabilities predicted by the Born Rule despite the determinism of the Schrodinger Equation > But even if both people know the wave function of the universe, there is now something they don’t know: which branch of the wave function they are on. There will inevitably be a period of time after branching occurs but before the observers find out what outcome was obtained on their branch. They don’t know where they are in the wave function. That’s self-locating uncertainty, as first emphasized in the quantum context by the physicist Lev Vaidman. (https://news.ycombinator.com/item?id=20927410 https://news.ycombinator.com/item?id=20927410) * edit - OP's post was edited but i'm leaving this post as-is. > MWI insists that experiments have multiple outcomes in point of metaphysical fact. That doesn't seem right. under MWI there is only one Wafefunction and one big quantum state. There are not additional 'worlds'
    - lisper 7y ago
      > OP's post was edited For the record, the original just said (as quoted in the above response) "The main difference is that the MWI has trouble explaining the Born rule." Upon reflection I decided that wasn't really the main difference, that the main difference is the difference in ontology. But the fact that the MWI has to recover the Born rule from the linear dynamics is a major consequence of this difference in ontology. The jury is still out about whether or not self-location uncertainty actually allows the Born rule to be recovered without begging the question. If it works, it would be a major breakthrough. Personally, I'll give long odds against. [UPDATE]: > There are not additional 'worlds' Um, what do you think that the "MW" in "MWI" stands for?
      - platz 7y ago
        > the main difference is the difference in ontology Sure, I'm not sure how much Science has to say about ontology but I'm interested in the outcome.
  - lmm 7y ago
    Conversely, Copenhagen-like explanations need to explain (or postulate) a decoherence/collapse operation that violates all the usual rules of QM: it would be non-time-symmetric, non-unitary, likely non-local (propagating faster than light), ...
    - TheOtherHobbes 7y ago
      Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling. MWI doesn't really solve either the locality or the FTL problem. If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction. That's a rather larger and more serious signalling and information transfer problem than the much smaller-scale non-locality required to explain entanglement.
      - asfarley 7y ago
        I don't think MWI claims, exactly, that an entire universe 'appears instantaneously'. Couldn't MWI represented with a probability cloud? Maybe the many-worlds exists as a fixed-volume probability on all states in the universe. From this perspective, MWI would be similar to a multi-universal annealing system, where different quantum outcomes result in fractures within the probability cloud (as opposed to an expansion of the tree-graph).
        platz 7y ago
        Carroll postulates that all the necessary degrees of freedom are already just sitting there in hilbert space - it's only when the states become orthogonal to each other (e.g. "branching") that we no longer have access to them. https://www.youtube.com/watch?v=jHLfMXvQqX8 https://www.youtube.com/watch?v=jHLfMXvQqX8
      - lmm 7y ago
        > Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling. My complaint is exactly that the Copenhagen interpretation introduces that unnecessary FTL signalling requirement. Under the MWI, nothing weird is going on in the EPR thought experiment. But if you believe that a collapse occurs when we measure system A, then that collapse has to somehow occur non-locally and/or be FTL-transmitted between A and B. > If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction. You don't; to talk about it in those terms is putting the cart before the horse. You have to believe that a wavefunction evolves in the usual way and represents the thing that it usually represents. You have to believe that when a pure system interacts with a system in superposition then the result is two systems that are entangled and in superposition - something you presumably already believe if you believe in any kind of QM at all. You only have to believe that a new universe appears instantaneously if your interpretation of putting a particle into superposition is that a new particle appears instantaneously, which is not an approach I've ever heard advocated.

platz 7y ago

That characterization of Woit does not appear quite accurate. Woit's claim may even be bolder, in that he claims that there is no functional difference between MWI and Copenhagen. > What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all.

lisper 7y ago
The main difference is one of rhetoric: Copenhagen insists that experiments have single outcomes. MWI insists that experiments have multiple outcomes in point of metaphysical fact. Another difference is that the MWI has to explain the Born rule (which is problematic), whereas Copenhagen simply takes it as an axiom. http://blog.rongarret.info/2019/07/the-trouble-with-many-worlds.html http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...
- doubleunplussed 7y ago
  It should be noted though that few interpretations have an explanation for the born rule.
  - lisper 7y ago
    Right, most of them just take it as an axiom. But the MWI explicitly rejects the Born rule as an axiom, insisting that the Schrödinger equation is all the actual physics there is.
    - lmm 7y ago
      MWI is not incompatible with postulating the Born rule if you want to. It just doesn't elevate it to an axiom of the theory. It's like doing constructive mathematics: you're not denying the law of excluded middle, you can postulate it and it will have the usual consequences, but the core theory doesn't rely on it, and it's possible to hope for a more elegant alternative.
      - lisper 7y ago
        Yes, you can just add the Born rule as an additional axiom, but that undermines the whole point of adopting the MWI in the first place, which is to remain mathematically "clean" with no ad-hoc assumptions.
        lmm 7y ago
        Yes and no. I'd argue that the Born rule as a rule for subjective probabilities in a MWI context is much cleaner than a full Copenhagen-style collapse operation.
        lisper 7y ago
        You are, of course, free to argue that. But most advocates of the MWI will disagree with you. That's why they put so much effort into trying to derive the Born rule.
        lmm 7y ago
        There's a significant gap between "cleaner than a Copenhagen-style collapse" and "clean enough to be a good part of our fundamental theory of nature" :).
        akvadrako 7y ago
        Not that it really matters but I disagree that MWI advocates like it because of the born rule derivations. Actually most born rule derivations apply equally to all interpretations. I’m with Vaidman on this, who is one of the best known supporters of MW, which is that the clearest solution is just to postulate the connection between psi magnitude and subjective experience. It’s an empirical discovery that works.
  - akvadrako 7y ago
    Actually most interpretations have the same explanations for the Born rule, it's just that people who study many worlds are more inclined to consider them. People who believe in Copenhagen don't care about this stuff. For a recent high-quality example: https://www.nature.com/articles/s41467-019-09348-x https://www.nature.com/articles/s41467-019-09348-x
- platz 7y ago
  > The main difference is that the MWI has trouble explaining the Born rule. To re-phrase, the claim in that article is that once MWI has done away with the Born rule, it has to deal with how the probabilities that we find in experiment are calculated under MWI. .. Which is not obvious at first glance, because MWI seems to suggest everything happens with probability = 1. Carroll acknowledges it is one of the challenges against MWI that needs to be answered. Carroll offers "self-location uncertainty" as a way to recover the probabilities predicted by the Born Rule despite the determinism of the Schrodinger Equation > But even if both people know the wave function of the universe, there is now something they don’t know: which branch of the wave function they are on. There will inevitably be a period of time after branching occurs but before the observers find out what outcome was obtained on their branch. They don’t know where they are in the wave function. That’s self-locating uncertainty, as first emphasized in the quantum context by the physicist Lev Vaidman. (https://news.ycombinator.com/item?id=20927410 https://news.ycombinator.com/item?id=20927410) * edit - OP's post was edited but i'm leaving this post as-is. > MWI insists that experiments have multiple outcomes in point of metaphysical fact. That doesn't seem right. under MWI there is only one Wafefunction and one big quantum state. There are not additional 'worlds'
  - lisper 7y ago
    > OP's post was edited For the record, the original just said (as quoted in the above response) "The main difference is that the MWI has trouble explaining the Born rule." Upon reflection I decided that wasn't really the main difference, that the main difference is the difference in ontology. But the fact that the MWI has to recover the Born rule from the linear dynamics is a major consequence of this difference in ontology. The jury is still out about whether or not self-location uncertainty actually allows the Born rule to be recovered without begging the question. If it works, it would be a major breakthrough. Personally, I'll give long odds against. [UPDATE]: > There are not additional 'worlds' Um, what do you think that the "MW" in "MWI" stands for?
    - platz 7y ago
      > the main difference is the difference in ontology Sure, I'm not sure how much Science has to say about ontology but I'm interested in the outcome.
- lmm 7y ago
  Conversely, Copenhagen-like explanations need to explain (or postulate) a decoherence/collapse operation that violates all the usual rules of QM: it would be non-time-symmetric, non-unitary, likely non-local (propagating faster than light), ...
  - TheOtherHobbes 7y ago
    Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling. MWI doesn't really solve either the locality or the FTL problem. If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction. That's a rather larger and more serious signalling and information transfer problem than the much smaller-scale non-locality required to explain entanglement.
    - asfarley 7y ago
      I don't think MWI claims, exactly, that an entire universe 'appears instantaneously'. Couldn't MWI represented with a probability cloud? Maybe the many-worlds exists as a fixed-volume probability on all states in the universe. From this perspective, MWI would be similar to a multi-universal annealing system, where different quantum outcomes result in fractures within the probability cloud (as opposed to an expansion of the tree-graph).
      - platz 7y ago
        Carroll postulates that all the necessary degrees of freedom are already just sitting there in hilbert space - it's only when the states become orthogonal to each other (e.g. "branching") that we no longer have access to them. https://www.youtube.com/watch?v=jHLfMXvQqX8 https://www.youtube.com/watch?v=jHLfMXvQqX8
    - lmm 7y ago
      > Non-locality is practically axiomatic if you understand Bell's Inequalities, and doesn't require true FTL signalling. My complaint is exactly that the Copenhagen interpretation introduces that unnecessary FTL signalling requirement. Under the MWI, nothing weird is going on in the EPR thought experiment. But if you believe that a collapse occurs when we measure system A, then that collapse has to somehow occur non-locally and/or be FTL-transmitted between A and B. > If you're going to believe in MWI you have to explain how an entire universe appears instantaneously, identical in every way to another universe, except for one almost insignificant difference in exactly one quantum interaction. You don't; to talk about it in those terms is putting the cart before the horse. You have to believe that a wavefunction evolves in the usual way and represents the thing that it usually represents. You have to believe that when a pure system interacts with a system in superposition then the result is two systems that are entangled and in superposition - something you presumably already believe if you believe in any kind of QM at all. You only have to believe that a new universe appears instantaneously if your interpretation of putting a particle into superposition is that a new particle appears instantaneously, which is not an approach I've ever heard advocated.