4 ms·

Measuring spin up of particle A is NOT a cause of measuring spin down of its entangled particle B.

ethanbond 4y ago
Isn’t waveform collapse at Location B caused by waveform collapse at Location A? (Maybe a stupid question, I’ve only a passing interest in this stuff)
- jinwoo68 4y ago
  My understanding is that when particles are entangled with each other, they must be described together with one wave function rather than one wave function for each of them. So once you measure particle A, the wave function collapses both for A and B.
  - ethanbond 4y ago
    Right. That… seems like causality to me.
- Filligree 4y ago
  Location B will measure both up and down, but will only be able to communicate with the version of location A that measured the opposite value. There’s no need for FTL if you accept MWI.
- codethief 4y ago
  No, if you change reference frames (in the special relativistic sense), the direction of causality might be opposite, i.e. collapse at B might suddenly cause the collapse at A.
  - ethanbond 4y ago
    Well sure, but that’s still causality.
    - chrisfosterelli 4y ago
      We are mixing up correlation and causation in this thread. In quantum entanglement you have a correlation, not causation. For example, if you go on a trip and bring your suitcase with you. You discover on opening it that you packed only one blue sock. You then gain the information that your other blue sock must be at home. You didn't "cause" your sock to appear at home at that moment, you simply gained that information from a correlation that you know must be true. The cause occurred when you were packing. In this case the correlation is entirely local though: you always had that information with you in your suitcase, you just didn't look at it until now. In quantum entanglement the cause and effect were the entanglement itself. When one of the particles is later observed, you're using correlation to decide what the other end is. The difference is that in quantum theory this correlation can be demonstrated to be non-local -- the information wasn't with you the whole time and was only determined at the moment of observation. But you can't take any action based on it and the other side can't either as you have no way to know who caused the collapse. You can't cause any meaningful effect on the other end with the information you get from your observation or the act of your observation. If we are using "cause" in the colloquial sense then yes I see what you're saying, it "causes" it. But it's important to be specific with domain terms here because entanglement does not violate causality, which has a specific definition.
      - ethanbond 4y ago
        Yeah this points to why I think information is actually the salient feature. Thanks for taking the time to comment. How could I go about learning more about the specific definition of causality you’re referring to here?
        chrisfosterelli 4y ago
        The wikipedia page on causality is a good place to start for a clear overview, specifically the section on causality as a physical concept: https://en.m.wikipedia.org/wiki/Causality_(physics) https://en.m.wikipedia.org/wiki/Causality_(physics) As the wikipedia article touches on, there's not an agreed global definition and it can be used differently in multiple contexts (hence why it's important to be specific). This stack exchange answer gives a good list of terminology: https://physics.stackexchange.com/a/34675 https://physics.stackexchange.com/a/34675 And this article puts a far more technical description on terms: https://www.mprl-series.mpg.de/media/proceedings/3/9/Proc3ch8.pdf https://www.mprl-series.mpg.de/media/proceedings/3/9/Proc3ch...

Isn’t waveform collapse at Location B caused by waveform collapse at Location A? (Maybe a stupid question, I’ve only a passing interest in this stuff)

jinwoo68 4y ago
My understanding is that when particles are entangled with each other, they must be described together with one wave function rather than one wave function for each of them. So once you measure particle A, the wave function collapses both for A and B.
- ethanbond 4y ago
  Right. That… seems like causality to me.
Filligree 4y ago
Location B will measure both up and down, but will only be able to communicate with the version of location A that measured the opposite value. There’s no need for FTL if you accept MWI.
codethief 4y ago
No, if you change reference frames (in the special relativistic sense), the direction of causality might be opposite, i.e. collapse at B might suddenly cause the collapse at A.
- ethanbond 4y ago
  Well sure, but that’s still causality.
  - chrisfosterelli 4y ago
    We are mixing up correlation and causation in this thread. In quantum entanglement you have a correlation, not causation. For example, if you go on a trip and bring your suitcase with you. You discover on opening it that you packed only one blue sock. You then gain the information that your other blue sock must be at home. You didn't "cause" your sock to appear at home at that moment, you simply gained that information from a correlation that you know must be true. The cause occurred when you were packing. In this case the correlation is entirely local though: you always had that information with you in your suitcase, you just didn't look at it until now. In quantum entanglement the cause and effect were the entanglement itself. When one of the particles is later observed, you're using correlation to decide what the other end is. The difference is that in quantum theory this correlation can be demonstrated to be non-local -- the information wasn't with you the whole time and was only determined at the moment of observation. But you can't take any action based on it and the other side can't either as you have no way to know who caused the collapse. You can't cause any meaningful effect on the other end with the information you get from your observation or the act of your observation. If we are using "cause" in the colloquial sense then yes I see what you're saying, it "causes" it. But it's important to be specific with domain terms here because entanglement does not violate causality, which has a specific definition.
    - ethanbond 4y ago
      Yeah this points to why I think information is actually the salient feature. Thanks for taking the time to comment. How could I go about learning more about the specific definition of causality you’re referring to here?
      - chrisfosterelli 4y ago
        The wikipedia page on causality is a good place to start for a clear overview, specifically the section on causality as a physical concept: https://en.m.wikipedia.org/wiki/Causality_(physics) https://en.m.wikipedia.org/wiki/Causality_(physics) As the wikipedia article touches on, there's not an agreed global definition and it can be used differently in multiple contexts (hence why it's important to be specific). This stack exchange answer gives a good list of terminology: https://physics.stackexchange.com/a/34675 https://physics.stackexchange.com/a/34675 And this article puts a far more technical description on terms: https://www.mprl-series.mpg.de/media/proceedings/3/9/Proc3ch8.pdf https://www.mprl-series.mpg.de/media/proceedings/3/9/Proc3ch...