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I never understood why non-locality is a problem to begin with. I mean, are we assuming that the universe is implemented on an infinite grid of computers, or wh

I never understood why non-locality is a problem to begin with. I mean, are we assuming that the universe is implemented on an infinite grid of computers, or what? (...which, before you get any ideas, I don't think would work with relativity.) Is there any reason to filter out nonlocal theories beyond vague notions of aesthetics?

kolpa 8y ago
Einstein's theory of relativity is in large part a simple statement that the Universe has a speed limit (the speed of light), and all the complex and amazing facts that entails. If they Universe doesn't have a speed limit after all, how is that everywhere we look there appears to be a speed limit, except in certain corners? How does the speed limit not hold in non-local action, and what prevents non-local action from making every thing else avoid the speed limit?
ben_w 8y ago
If I understand correctly (a big if), then non-locality can always be made to work as time travel. Because locality is the same as saying causation is limited to speed of light, and causality outside a light cone is always time travel from someone’s frame of reference (I’ve never seen a space-time globe demonstration of that last part, but it gets said often enough). Is that what you had in mind, or?
- ahelwer 8y ago
  > Locality is the same as saying causation is limited to speed of light Nope. When two qbits are entangled, operations on one affect the other instantaneously. This breaks locality. However, it does not break causality - we cannot get any classical information from one qbit's frame to the other. See the No-communication theorem: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem You can have non-locality without time travel, or breaking causality. If I entangle two qbits, give you one, and we both go to opposite ends of the universe, then when I measure my qbit I'll know yours instantly collapsed to the same value. However, all we've done is created a shared random number generator. I can't communicate anything to you. One might be tempted to think - well, why do we believe entanglement is a non-local phenomenon? Surely the qbits just decided at the time of entanglement how they would collapse, then remembered that? No. That is called hidden variable theory, which has been disproved. Entanglement breaks locality, but not causality.
  - philipov 8y ago
    It seems that it is more sensible to say that entanglement is locality. The notion of two points being close to each other appears to be a course-grained approximation of high amounts of entanglement in the vacuum. If you divide the space along a boundary and break the entanglement between regions on both sides, the effect is to increase the distance between them. So perhaps it is better to think of gravity, locality and dynamical spacetime, as the "hydrodynamics of entanglement" [0] and [1] are a public lecture, [2] is colloquium [0]: https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw [1]: https://www.youtube.com/watch?v=uiG_EtVQu5o https://www.youtube.com/watch?v=uiG_EtVQu5o [2]: https://www.youtube.com/watch?v=PwAKr-h6kAI https://www.youtube.com/watch?v=PwAKr-h6kAI
Koshkin 8y ago
Of course - non-locality in its various forms is a fundamental feature of the quantum world. Nobody argues that the slits (in the two-slit experiment) are separated, or that a particle, when it has a definite momentum, can be found anywhere in the Universe...

kolpa 8y ago

Einstein's theory of relativity is in large part a simple statement that the Universe has a speed limit (the speed of light), and all the complex and amazing facts that entails. If they Universe doesn't have a speed limit after all, how is that everywhere we look there appears to be a speed limit, except in certain corners? How does the speed limit not hold in non-local action, and what prevents non-local action from making every thing else avoid the speed limit?

ben_w 8y ago

If I understand correctly (a big if), then non-locality can always be made to work as time travel. Because locality is the same as saying causation is limited to speed of light, and causality outside a light cone is always time travel from someone’s frame of reference (I’ve never seen a space-time globe demonstration of that last part, but it gets said often enough). Is that what you had in mind, or?

ahelwer 8y ago
> Locality is the same as saying causation is limited to speed of light Nope. When two qbits are entangled, operations on one affect the other instantaneously. This breaks locality. However, it does not break causality - we cannot get any classical information from one qbit's frame to the other. See the No-communication theorem: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem You can have non-locality without time travel, or breaking causality. If I entangle two qbits, give you one, and we both go to opposite ends of the universe, then when I measure my qbit I'll know yours instantly collapsed to the same value. However, all we've done is created a shared random number generator. I can't communicate anything to you. One might be tempted to think - well, why do we believe entanglement is a non-local phenomenon? Surely the qbits just decided at the time of entanglement how they would collapse, then remembered that? No. That is called hidden variable theory, which has been disproved. Entanglement breaks locality, but not causality.
- philipov 8y ago
  It seems that it is more sensible to say that entanglement is locality. The notion of two points being close to each other appears to be a course-grained approximation of high amounts of entanglement in the vacuum. If you divide the space along a boundary and break the entanglement between regions on both sides, the effect is to increase the distance between them. So perhaps it is better to think of gravity, locality and dynamical spacetime, as the "hydrodynamics of entanglement" [0] and [1] are a public lecture, [2] is colloquium [0]: https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw [1]: https://www.youtube.com/watch?v=uiG_EtVQu5o https://www.youtube.com/watch?v=uiG_EtVQu5o [2]: https://www.youtube.com/watch?v=PwAKr-h6kAI https://www.youtube.com/watch?v=PwAKr-h6kAI

Koshkin 8y ago

Of course - non-locality in its various forms is a fundamental feature of the quantum world. Nobody argues that the slits (in the two-slit experiment) are separated, or that a particle, when it has a definite momentum, can be found anywhere in the Universe...