4 ms·

Quantum physicist here. If two particles are entangled, the very value of the entangled properties does not exist locally (if it did it would be a hidden variab

Quantum physicist here. If two particles are entangled, the very value of the entangled properties does not exist locally (if it did it would be a hidden variable, with crazy consequences, but there are even cases where pre-existing values are forbidden algebraically, like for the GHZ state). Entanglement only gives us a promise of joint behaviour like "if the particles are in the Bell singlet state and we measure them with the same measurement settings, the results are going to be opposite" (notice that the promise is only about the values being opposite, not about what they will be locally). I believe that what Einstein meant by spooky action is exactly about this: that a particle seems to know which settings have been used for the measurement of the other, because if the settings are different then the promise can be broken.

pmoriarty 5y ago
"the very value of the entangled properties does not exist locally" Does physics have a definition for existence?
- ziofill 5y ago
  Yes! (and good question) In this case we use Einstein's definition of local hidden variable (which he called "element of reality"): "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity."
  - pmoriarty 5y ago
    If I may infer that this statement has been interpreted as not a conditional as it is stated verbatim (in the form A -> C where A is the antecedent and C is the consequent), but instead is a biconditional (in the form A <-> C), then it seems to be stating that to exist is to be certainly predictable. That's a curious definition of existence. And now that I think about it a bit more, it seems like it's not really talking about what existence is, but rather the consequences of existence and the consequences of certain prediction. That's not quite the same as a definition of existence, is it?
    - ziofill 5y ago
      What the statement says is that if a property is certainly predictable, then it exists. It doesn't say that all existing things are certainly predictable.
      - pmoriarty 5y ago
        But that doesn't say what it means by existence. It leaves that part unsaid. To go a bit deeper in to this, it's not just saying certainly predictable properties are certainly predictable. It wants to say something else about them: that they exist. What does it mean by that? The statement as it stands is more about certainly predictable properties than it is about existence itself.
        ziofill 5y ago
        I think I understand your point: we don't define existence per se. But here we only need to define the existence of the value of a property. And we define it by saying that if the we can predict the value before measuring it, then the value "exists". Take it as an axiom. When I said "the very value of the entangled properties does not exist locally" in my first post I meant that there is no way to predict the value (or of consistently assigning a value to the property) before performing a measurement.
- javajosh 5y ago
  The EPR paradox is about precisely this: https://en.wikipedia.org/wiki/EPR_paradox https://en.wikipedia.org/wiki/EPR_paradox
  - Dracophoenix 5y ago
    Beat me to it!
jgrowl 5y ago
I'll be that guy and say for completeness that while local hidden variables are ruled out, global variables are not via something like Bohmian mechanics.
- ziofill 5y ago
  True. However each alternative comes with a lot of baggage, and there is no easy way out: 1) it could be that locality doesn't hold (as in Bohmian mechanics), but then causality takes a big hit because we must accept that it can also work backward in time. 2) it could be that realism doesn't hold (which is the more commonly accepted evil) i.e. that there is no cause for the particular value emerging from the measurement of an entangled property. True randomness. (And yet, it satisfies the required correlations). After years of losing sleep over it, I like to think that entangled properties are somehow 'part of the same whole', which is independent of space-time, and that space-time is a construct that emerges from the particular way decoherence seems to choose position and momentum as preferred bases.

pmoriarty 5y ago

"the very value of the entangled properties does not exist locally" Does physics have a definition for existence?

ziofill 5y ago
Yes! (and good question) In this case we use Einstein's definition of local hidden variable (which he called "element of reality"): "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity."
- pmoriarty 5y ago
  If I may infer that this statement has been interpreted as not a conditional as it is stated verbatim (in the form A -> C where A is the antecedent and C is the consequent), but instead is a biconditional (in the form A <-> C), then it seems to be stating that to exist is to be certainly predictable. That's a curious definition of existence. And now that I think about it a bit more, it seems like it's not really talking about what existence is, but rather the consequences of existence and the consequences of certain prediction. That's not quite the same as a definition of existence, is it?
  - ziofill 5y ago
    What the statement says is that if a property is certainly predictable, then it exists. It doesn't say that all existing things are certainly predictable.
    - pmoriarty 5y ago
      But that doesn't say what it means by existence. It leaves that part unsaid. To go a bit deeper in to this, it's not just saying certainly predictable properties are certainly predictable. It wants to say something else about them: that they exist. What does it mean by that? The statement as it stands is more about certainly predictable properties than it is about existence itself.
      - ziofill 5y ago
        I think I understand your point: we don't define existence per se. But here we only need to define the existence of the value of a property. And we define it by saying that if the we can predict the value before measuring it, then the value "exists". Take it as an axiom. When I said "the very value of the entangled properties does not exist locally" in my first post I meant that there is no way to predict the value (or of consistently assigning a value to the property) before performing a measurement.
javajosh 5y ago
The EPR paradox is about precisely this: https://en.wikipedia.org/wiki/EPR_paradox https://en.wikipedia.org/wiki/EPR_paradox
- Dracophoenix 5y ago
  Beat me to it!

jgrowl 5y ago

I'll be that guy and say for completeness that while local hidden variables are ruled out, global variables are not via something like Bohmian mechanics.

ziofill 5y ago
True. However each alternative comes with a lot of baggage, and there is no easy way out: 1) it could be that locality doesn't hold (as in Bohmian mechanics), but then causality takes a big hit because we must accept that it can also work backward in time. 2) it could be that realism doesn't hold (which is the more commonly accepted evil) i.e. that there is no cause for the particular value emerging from the measurement of an entangled property. True randomness. (And yet, it satisfies the required correlations). After years of losing sleep over it, I like to think that entangled properties are somehow 'part of the same whole', which is independent of space-time, and that space-time is a construct that emerges from the particular way decoherence seems to choose position and momentum as preferred bases.