4 ms·

Can you explain how MWI is local? In the period of time before the observer is 'entangled' into the same system as the observed particles, the different possib

Can you explain how MWI is local? In the period of time before the observer is 'entangled' into the same system as the observed particles, the different possibilities can interfere with each other. If this can happen even if the entangled particles are separated by a lot of space, surely that is the same as nonlocality. Maybe my confusion is that I'm not sure a which point the universe splits. Suppose I entangle two particles, then separate them by a large amount, my universe hasn't split yet, because before I make the measurement, I haven't become entangled with the state of the particles, but when I do, surely the spreading entanglement of myself with the possibilities centers at both entangled particles (i.e. it is nonlocal)?

kybernetikos 14y ago
Let me try to explain myself better: Two particles are entangled and separated by a vast distance. At some point, I conduct a measurement on the first particle, causing me to split into two different versions of myself that have measured two different states. Now, the effect of that measurement spreads outwards from the site of the measurement, so I may affect other things based on the measurement, etc. My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. I imagine this as a wave of universe splitting spreading out from the site of my measurement and the entangled particle despite the separation. Now, perhaps this is a nontechnical use of the word nonlocal, but I'd describe that as a nonlocal phenomenon.
- nessus42 14y ago
  > My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. Imagine that your friend Joe writes the same word on two pieces of paper, puts them in envelopes and hands one envelope to you and mails the other envelope to Paris. At some point in time, you open up your envelope and read the word "popcorn". At some other point in time, Pierre open up his envelope and reads the same word. The word that Pierre reads is not "dependent on your measurement". Nothing nonlocal occurred.
  - kybernetikos 14y ago
    That's a hidden variable theory.
    - nessus42 14y ago
      No, it's an analogy, not a theory. There are no hidden variables in the Many Worlds Interpretation, but it will appear to us as if there are. The Bohm Interpretation is the experimentally indistinguishable dual of MWI that proposes hidden variables rather than the absence of wave collapse. Despite the hidden variables in the Bohm Interpretation, there is nothing wrong with it, modulo Occam's razor.
      - kybernetikos 14y ago
        Sure, I used the word 'theory' incorrectly, I should have said 'explanation'. As I understand it, hidden variables are fine if you're happy to throw away locality, which is exactly what Bohm interpretation does and exactly what we're talking about here. Since your analogy is purportedly showing me how MWI is local, showing me that something that is significantly different (i.e. is a hidden variable explanation) is also local doesn't help a lot.
        nessus42 14y ago
        Here's a more thorough explanation: http://plato.stanford.edu/entries/qm-manyworlds/ http://plato.stanford.edu/entries/qm-manyworlds/
- Symmetry 14y ago
  then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. I think that's your trouble there. Those are totally independent of your measurement, the waveform splits at the measurement of the other particle the exact same way, regardless of what happened to you locally.
  - kybernetikos 14y ago
    except I can never be in a universe where it split at my measurement site in one way and it split at the other measurement site the other way. That means that I am entangled with a measurement that happens nonlocally to me and everything that it interacts with.
    - Symmetry 14y ago
      You're not entangled with the measurement, you're entangled with the entire wave despite the inconceivable number of measurements that happen within it. The same way my friends are still my friends despite us being in different places. I won't know instantly if they die or something, but I react differently when I hear about what happens to them than I do with ordinary people. If one of them gets into a car accident outside my light-cone and I'm more sad when I hear about it than about the car accident of a stranger, that doesn't mean that I have a non-local interaction with the car accident.
Symmetry 14y ago
The splitting is a quantitative rather than a qualitative process. You can observe quantum effects with 100 atom molecules the same way as with single particles, it's just they require far finer observations to detect. If no collapse occurs, the the time entangled complexes reach visible scales there's no way you could ever detect the quantum interference. It isn't that the universe has split, it's that one section of the universe's wavefunction has split, or more exactly become a bimodal distribution. At both locations there are bunches of the wavefunction corresponding to both possible observations - but both locations are entangled since the two particles that touched this off were. That means that when you observe the two experiments you don't "split" into four, but only into two since they are already entangled and you only get entangled with this complex once. The information of "What's entangled with what" spread at strictly sub-light speeds, and the information of "Did it pass through the polarizing filter or not" also traveled at sub-light speeds.
- kybernetikos 14y ago
  Yes, but this complex is growing the whole time, as it entangles with other parts of the universe around it in both locations. That means that I become entangled with it in one location, and simultaneously become entangled with some other part of the universe that is not remotely local to me. That's why I find the description of it as 'local' somewhat weird, but I accept that I'm probably using the term incorrectly.
nessus42 14y ago
> Maybe my confusion is that I'm not sure a which point the universe splits. The answer to that is that the universe never really splits. It remains one big very complicated universe, with very very very messy superpositions of state in it. But different components of these superpositions lose "coherence", which gives the appearance to us of wave collapse, even though there actually is no wave collapse. Lookup "decoherence" for more info on this.

kybernetikos 14y ago

Let me try to explain myself better: Two particles are entangled and separated by a vast distance. At some point, I conduct a measurement on the first particle, causing me to split into two different versions of myself that have measured two different states. Now, the effect of that measurement spreads outwards from the site of the measurement, so I may affect other things based on the measurement, etc. My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. I imagine this as a wave of universe splitting spreading out from the site of my measurement and the entangled particle despite the separation. Now, perhaps this is a nontechnical use of the word nonlocal, but I'd describe that as a nonlocal phenomenon.

nessus42 14y ago
> My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. Imagine that your friend Joe writes the same word on two pieces of paper, puts them in envelopes and hands one envelope to you and mails the other envelope to Paris. At some point in time, you open up your envelope and read the word "popcorn". At some other point in time, Pierre open up his envelope and reads the same word. The word that Pierre reads is not "dependent on your measurement". Nothing nonlocal occurred.
- kybernetikos 14y ago
  That's a hidden variable theory.
  - nessus42 14y ago
    No, it's an analogy, not a theory. There are no hidden variables in the Many Worlds Interpretation, but it will appear to us as if there are. The Bohm Interpretation is the experimentally indistinguishable dual of MWI that proposes hidden variables rather than the absence of wave collapse. Despite the hidden variables in the Bohm Interpretation, there is nothing wrong with it, modulo Occam's razor.
    - kybernetikos 14y ago
      Sure, I used the word 'theory' incorrectly, I should have said 'explanation'. As I understand it, hidden variables are fine if you're happy to throw away locality, which is exactly what Bohm interpretation does and exactly what we're talking about here. Since your analogy is purportedly showing me how MWI is local, showing me that something that is significantly different (i.e. is a hidden variable explanation) is also local doesn't help a lot.
      - nessus42 14y ago
        Here's a more thorough explanation: http://plato.stanford.edu/entries/qm-manyworlds/ http://plato.stanford.edu/entries/qm-manyworlds/
Symmetry 14y ago
then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement. I think that's your trouble there. Those are totally independent of your measurement, the waveform splits at the measurement of the other particle the exact same way, regardless of what happened to you locally.
- kybernetikos 14y ago
  except I can never be in a universe where it split at my measurement site in one way and it split at the other measurement site the other way. That means that I am entangled with a measurement that happens nonlocally to me and everything that it interacts with.
  - Symmetry 14y ago
    You're not entangled with the measurement, you're entangled with the entire wave despite the inconceivable number of measurements that happen within it. The same way my friends are still my friends despite us being in different places. I won't know instantly if they die or something, but I react differently when I hear about what happens to them than I do with ordinary people. If one of them gets into a car accident outside my light-cone and I'm more sad when I hear about it than about the car accident of a stranger, that doesn't mean that I have a non-local interaction with the car accident.

Symmetry 14y ago

The splitting is a quantitative rather than a qualitative process. You can observe quantum effects with 100 atom molecules the same way as with single particles, it's just they require far finer observations to detect. If no collapse occurs, the the time entangled complexes reach visible scales there's no way you could ever detect the quantum interference. It isn't that the universe has split, it's that one section of the universe's wavefunction has split, or more exactly become a bimodal distribution. At both locations there are bunches of the wavefunction corresponding to both possible observations - but both locations are entangled since the two particles that touched this off were. That means that when you observe the two experiments you don't "split" into four, but only into two since they are already entangled and you only get entangled with this complex once. The information of "What's entangled with what" spread at strictly sub-light speeds, and the information of "Did it pass through the polarizing filter or not" also traveled at sub-light speeds.

kybernetikos 14y ago
Yes, but this complex is growing the whole time, as it entangles with other parts of the universe around it in both locations. That means that I become entangled with it in one location, and simultaneously become entangled with some other part of the universe that is not remotely local to me. That's why I find the description of it as 'local' somewhat weird, but I accept that I'm probably using the term incorrectly.

nessus42 14y ago

> Maybe my confusion is that I'm not sure a which point the universe splits. The answer to that is that the universe never really splits. It remains one big very complicated universe, with very very very messy superpositions of state in it. But different components of these superpositions lose "coherence", which gives the appearance to us of wave collapse, even though there actually is no wave collapse. Lookup "decoherence" for more info on this.