7 ms·

Just out of curiosity, what is weird about non-locality? From my super naive perspective that's just saying that things don't necessarily work underneath the h

Just out of curiosity, what is weird about non-locality? From my super naive perspective that's just saying that things don't necessarily work underneath the hood the way they appear to work. For me (super naive, remember ;-) ), that seems completely reasonable even if it might be very inconvenient. What am I missing?

Erlich_Bachman 7y ago
I can't see what's so hard about it either. Nor what would be the problem with something like hidden states/variables. Why would it be so hard to assume that there could be hidden states in partcles which we simply can't measure (maybe not yet)? Why does the world has to be directly measurable? Who told people that they ought to be able to measure every single variable directly (like hidden state of a quantum particle), why are they assuming that?
- mrmonkeyman 7y ago
  Those hidden variables have to violate known physics in order to exist. So that's a pickle.
  - marcus_holmes 7y ago
    well, that's the point isn't it? They're saying that "known physics" is wrong
    - brain5ide 7y ago
      incomplete
- tsimionescu 7y ago
  You should look into Bell's theorem. It is mathematical proof that (discounting superdeterminism) there is no way to explain QM observations with local hidden variables. You could have hidden variables, but only if they produce effects at infinite speed. The big problem with infinite speeds is that, somewhat like superdeterminism, they mean that you can't do fully controlled experiments. If effects can propagate at infinite speeds, the whe universe has an impact on any experiment, including the state of your measuring apparatus and so on. That doesn't make them impossible, but it explains why they are disliked in theories.
  - Erlich_Bachman 7y ago
    I know that they cannot be local. My point of view is "just let them be global, build theories from there", global hidden variables don't interfere with any intuitions about the world for some reason.
    - tsimionescu 7y ago
      The problem is you can't really build theories from global hidden variables. If the details of any experiment depend significantly\* on the state of the entire universe, until we can account for the entire universe in our measurements, we might as well stop measuring. \* even with Newtonian physics, the universal attraction of any object does have non-0 values everywhere, but we know that the influence is negligible. However, with global hidden variables, the speed a billiard ball will take when I hit it may depend on the size of a planet in a different galactic cluster.
      - snarfy 7y ago
        I also have a hard time disbelieving in global variables. We have a lot of evidence validating quantum field theories, and the fields in QFT are global.
        vincent-toups 7y ago
        Only in a trivial sense. Quantum Field theories are explicitly local theories, constructed from Langrangians which purposefully have and express Lorentz invariance, exactly to maintain locality. In any case, quantum field theories are good at predicting stuff but almost certainly not descriptions of the true fundamental dynamics of the universe, given their known and relatively well understood divergences.
      - naasking 7y ago
        > If the details of any experiment depend significantly\* on the state of the entire universe, until we can account for the entire universe in our measurements, we might as well stop measuring. Every experiment does depend on the entire state of the universe, even in QM, but those influences are typically small due to symmetries. At the quantum level, many of these symmetries no longer apply.
GoblinSlayer 7y ago
You're missing special theory of relativity. Nature is local. When you add nonlocality, contradictions arise, you can try to just ignore them or patch them with ad hoc hypotheses, such things were tried before and turned out to be failures indicating that the premise is wrong.
- mikekchar 7y ago
  Interesting. If you have some pointers for something to read that discusses why special relativity requires locality, I'd love to read it. I have no real idea where to start searching. Edit: Just to be clear, I'm aware that Bell's theorem says that QM must either break locality or realism, but I don't really understand why it can't break locality. While incredibly inconvenient, wouldn't that solve the problem? Again, I realise I'm naive, so I don't actually suppose my line of reasoning is correct ;-)
  - GoblinSlayer 7y ago
    Contradictions are inconsistency in the theory, i.e. the theory can give different results depending on how you compute. To evade this you need to apply abstract reasoning outside of theory to decide how to compute in every situation. This means theory doesn't work by itself, i.e. it's not an objective theory. Also by realism Bell means hidden variables, not realism at large.
  - tsimionescu 7y ago
    Special relativity is essentially an explanation of why the speed of light is a constant regardless of how you measure it. That is, if you're in a train moving at half the speed of light relative to the ground, and someone fires a laser in the same direction as the train from the last station you passed through, that laser beam will move towards you at at the speed of light. If you fire a laser back, it will reach the station at the same time as the laser from the station reaches you (as seen by an observer in the station). This makes no sense unless the speed of light is a fundamental physical constant, so that motion in general depends on the speed of light, which is what special relativity postulates. Now there are ways to have a special kind of non-locality that do not violate special relativity - you can have phenomena that happen at infinite speed, but only if they do not carry mass or energy or any information at all. The common interpretation of wave-function collapse is an example of such a phenomenon. I'd also note that the famous E=mc^2 is also a limit on speed, since kinetic energy (mv^2/2) is part of the total energy of an object.

Erlich_Bachman 7y ago

I can't see what's so hard about it either. Nor what would be the problem with something like hidden states/variables. Why would it be so hard to assume that there could be hidden states in partcles which we simply can't measure (maybe not yet)? Why does the world has to be directly measurable? Who told people that they ought to be able to measure every single variable directly (like hidden state of a quantum particle), why are they assuming that?

mrmonkeyman 7y ago
Those hidden variables have to violate known physics in order to exist. So that's a pickle.
- marcus_holmes 7y ago
  well, that's the point isn't it? They're saying that "known physics" is wrong
  - brain5ide 7y ago
    incomplete
tsimionescu 7y ago
You should look into Bell's theorem. It is mathematical proof that (discounting superdeterminism) there is no way to explain QM observations with local hidden variables. You could have hidden variables, but only if they produce effects at infinite speed. The big problem with infinite speeds is that, somewhat like superdeterminism, they mean that you can't do fully controlled experiments. If effects can propagate at infinite speeds, the whe universe has an impact on any experiment, including the state of your measuring apparatus and so on. That doesn't make them impossible, but it explains why they are disliked in theories.
- Erlich_Bachman 7y ago
  I know that they cannot be local. My point of view is "just let them be global, build theories from there", global hidden variables don't interfere with any intuitions about the world for some reason.
  - tsimionescu 7y ago
    The problem is you can't really build theories from global hidden variables. If the details of any experiment depend significantly\* on the state of the entire universe, until we can account for the entire universe in our measurements, we might as well stop measuring. \* even with Newtonian physics, the universal attraction of any object does have non-0 values everywhere, but we know that the influence is negligible. However, with global hidden variables, the speed a billiard ball will take when I hit it may depend on the size of a planet in a different galactic cluster.
    - snarfy 7y ago
      I also have a hard time disbelieving in global variables. We have a lot of evidence validating quantum field theories, and the fields in QFT are global.
      - vincent-toups 7y ago
        Only in a trivial sense. Quantum Field theories are explicitly local theories, constructed from Langrangians which purposefully have and express Lorentz invariance, exactly to maintain locality. In any case, quantum field theories are good at predicting stuff but almost certainly not descriptions of the true fundamental dynamics of the universe, given their known and relatively well understood divergences.
    - naasking 7y ago
      > If the details of any experiment depend significantly\* on the state of the entire universe, until we can account for the entire universe in our measurements, we might as well stop measuring. Every experiment does depend on the entire state of the universe, even in QM, but those influences are typically small due to symmetries. At the quantum level, many of these symmetries no longer apply.

GoblinSlayer 7y ago

You're missing special theory of relativity. Nature is local. When you add nonlocality, contradictions arise, you can try to just ignore them or patch them with ad hoc hypotheses, such things were tried before and turned out to be failures indicating that the premise is wrong.

mikekchar 7y ago
Interesting. If you have some pointers for something to read that discusses why special relativity requires locality, I'd love to read it. I have no real idea where to start searching. Edit: Just to be clear, I'm aware that Bell's theorem says that QM must either break locality or realism, but I don't really understand why it can't break locality. While incredibly inconvenient, wouldn't that solve the problem? Again, I realise I'm naive, so I don't actually suppose my line of reasoning is correct ;-)
- GoblinSlayer 7y ago
  Contradictions are inconsistency in the theory, i.e. the theory can give different results depending on how you compute. To evade this you need to apply abstract reasoning outside of theory to decide how to compute in every situation. This means theory doesn't work by itself, i.e. it's not an objective theory. Also by realism Bell means hidden variables, not realism at large.
- tsimionescu 7y ago
  Special relativity is essentially an explanation of why the speed of light is a constant regardless of how you measure it. That is, if you're in a train moving at half the speed of light relative to the ground, and someone fires a laser in the same direction as the train from the last station you passed through, that laser beam will move towards you at at the speed of light. If you fire a laser back, it will reach the station at the same time as the laser from the station reaches you (as seen by an observer in the station). This makes no sense unless the speed of light is a fundamental physical constant, so that motion in general depends on the speed of light, which is what special relativity postulates. Now there are ways to have a special kind of non-locality that do not violate special relativity - you can have phenomena that happen at infinite speed, but only if they do not carry mass or energy or any information at all. The common interpretation of wave-function collapse is an example of such a phenomenon. I'd also note that the famous E=mc^2 is also a limit on speed, since kinetic energy (mv^2/2) is part of the total energy of an object.