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I don't understand why physics is spooked out by entanglement. It is just a correlation-- and if we want to have any interesting patterns in the universe at all

by tbabb 9y ago

I don't understand why physics is spooked out by entanglement. It is just a correlation-- and if we want to have any interesting patterns in the universe at all, we are going to have correlations. All this seems to be saying is that there can be an entanglement in the present with something in the past. At to me, that seems equivalent to saying that the past and present are correlated, which seems boringly trivial. Am I missing something?

resource0x 9y ago
If I read it correctly, the implication is that the present affects the past. If photon 1 died a billion years ago (without being 'measured' by sentient observer) - e.g. by just hitting a wall, its polarization remained undefined until we measured photon 4 today. This is not very intuitive. Polarization of photon 1 could, in principle, cause a galactic catastrophe, and by measuring photon 4 today you either trigger this catastrophe (a billion years ago!) or your don't - depending on the results of your measurement.
- solipsism 9y ago
  You didn't read it correctly. They measure photon 1 before photon 3 or 4 even exist.
  - resource0x 9y ago
    They measured photon 1 because they wanted to prove correlation (it would be impossible to do without measuring it). If you believe they proved it, it works both ways. Hadn't they measured photon 1, our measurement of photon 4 would determine polarization of photon 1 a billion years ago. Entanglement works both ways, according to the theory.
    - boyce 9y ago
      Not just in theory! Isn't this what Alain Aspect tested?
- GolDDranks 9y ago
  Sentient observers have got nothing to do with that – interaction with any macroscopic system will destroy the entanglement. If that macroscopic system (a galaxy that might have a catastrophe triggered or not) is causally connected to us, then it's destiny doesn't depend on the measurement, because it's state have already been "decohered" with us. (That is; the information about the other photon has already indirectly entangled with us.) If it isn't, then it's still entangled, and it indeed does depend on the measurement – it's a Schrödinger's galaxy.
  - resource0x 9y ago
    > interaction with any macroscopic system will destroy the entanglement. How do you know it? I'm not aware of any proof of this statement.
    - darawk 9y ago
      Are you aware of some proof that it is specific to sentient entities? If not, Occams razor implies we assume it is any macroscopic entity.
      - TheOtherHobbes 9y ago
        Can you present a proof that hasn’t been created by a sentient entity? Edit: of course you can, but only in the formal systems sense, which is relativel trivial. What we think of as “proof” in physics is a combination of “this belief creates predictable experiences” and “this abstraction of belief is logically consistent.” The first part is subjective - our beliefs about physics create predictable experiences for us, but we have no idea how large the entire domain of possible experiences is. The second part is a tar pit full of bones, haunted by undefinability theorems. https://en.m.wikipedia.org/wiki/Tarski%27s_undefinability_theorem https://en.m.wikipedia.org/wiki/Tarski%27s_undefinability_th... So any suggestion that physics efficiently reveals the secret mechanisms of the universe is a conceit. Physics reveals what we’re able to deduce about the universe given the biases built into our experience of sentience. When weird things fall out of quantum theory, this makes it hard to know if the weirdness is truly out there, or if it’s an artifact of our limited sentience.
eoinmurray92 9y ago
It not correlation - bell inequalities show that local variables don't explain the correlations: ie no correlated variable exists because states are decided at measurement (or wavefunction collapse) time. This is why entanglement is interesting.
- tbabb 9y ago
  Sure, "no hidden variables" is interesting/weird. And interference is interesting/weird. And both of those things happen independently from entanglement. But entanglement itself seems very not-weird to me once you buy into those other things.
  - whatshisface 9y ago
    What you're on to is that all of the big popular examples of "quantum weirdness" (uncertianty, entanglement, tunneling) emerge from a very small and elegant set of axiomatic postulates. That's the beauty of physics. One such group of postulates that chemists like to use can be found at http://vergil.chemistry.gatech.edu/notes/quantrev/node20.html http://vergil.chemistry.gatech.edu/notes/quantrev/node20.htm... . You can probably beat the elegance of those if you're willing to step further from what's actually used in calculations, but that's not what chemists like to do. Here's a better shot at being axiomatic and elegant, but it's a lot less clear (from a lay-perspective) how they relate to reality: https://en.wikipedia.org/wiki/Dirac%E2%80%93von_Neumann_axioms https://en.wikipedia.org/wiki/Dirac%E2%80%93von_Neumann_axio...
    - mikhailfranco 9y ago
      I find Lucien Hardy's discussion enlightening: Quantum Theory From Five Reasonable Axioms https://arxiv.org/abs/quant-ph/0101012 https://arxiv.org/abs/quant-ph/0101012
- h2g3yhogw8 9y ago
  As you're the only person so far on this thread who seems to know about this, is my understanding correct that non-locality of entanglement is because all the entangled entities are described by a single hamiltonian which means the energy expressed in each of their degrees of freedom must sum to zero, so only certain combinations of measurements are possible of the system as a whole
solipsism 9y ago
Yes you're missing something. The idea that an event in one place can influence an event in another place, even though it's not possible information traveled from the first place to the second, is spooky. And the idea that this can even be the case even if the associated objects never coexisted in time is anything but "boringly trivial".
- ComputerGuru 9y ago
  Everyone keeps coming back to this point, but I maintain that just because there’s no way that we know of (or that makes sense in the framework of our understanding) for information to travel between the two, it absolutely does not mean information _did not_ travel between the two. It’s the simpler explanation, if you don’t let hubris get in the way.
  - lowpro 9y ago
    It's more than that. It can be shown that information could not travel between 2 points provably, so it's not like we're missing some communication method. We can show there is no intermediate communication method. See the MIT introduction lecture on Quantum Physics: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/ https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-s... I believe the experiment where this is shown is lecture 1, possibly 2. It's been awhile since I watched this but a great source of information.
    - kirrent 9y ago
      To further clarify, any communication would have to be superluminal. Sure there are people who claim that there's some unobservable superluminal background to the universe that makes non-local hidden variable theories possible but I don't know that I'd call that the simpler explanation.
      - ComputerGuru 9y ago
        I was thinking more along the lines of undetected hyper surfaces or wormholes.

by tbabb 9y ago

resource0x 9y ago

If I read it correctly, the implication is that the present affects the past. If photon 1 died a billion years ago (without being 'measured' by sentient observer) - e.g. by just hitting a wall, its polarization remained undefined until we measured photon 4 today. This is not very intuitive. Polarization of photon 1 could, in principle, cause a galactic catastrophe, and by measuring photon 4 today you either trigger this catastrophe (a billion years ago!) or your don't - depending on the results of your measurement.

solipsism 9y ago
You didn't read it correctly. They measure photon 1 before photon 3 or 4 even exist.
- resource0x 9y ago
  They measured photon 1 because they wanted to prove correlation (it would be impossible to do without measuring it). If you believe they proved it, it works both ways. Hadn't they measured photon 1, our measurement of photon 4 would determine polarization of photon 1 a billion years ago. Entanglement works both ways, according to the theory.
  - boyce 9y ago
    Not just in theory! Isn't this what Alain Aspect tested?
GolDDranks 9y ago
Sentient observers have got nothing to do with that – interaction with any macroscopic system will destroy the entanglement. If that macroscopic system (a galaxy that might have a catastrophe triggered or not) is causally connected to us, then it's destiny doesn't depend on the measurement, because it's state have already been "decohered" with us. (That is; the information about the other photon has already indirectly entangled with us.) If it isn't, then it's still entangled, and it indeed does depend on the measurement – it's a Schrödinger's galaxy.
- resource0x 9y ago
  > interaction with any macroscopic system will destroy the entanglement. How do you know it? I'm not aware of any proof of this statement.
  - darawk 9y ago
    Are you aware of some proof that it is specific to sentient entities? If not, Occams razor implies we assume it is any macroscopic entity.
    - TheOtherHobbes 9y ago
      Can you present a proof that hasn’t been created by a sentient entity? Edit: of course you can, but only in the formal systems sense, which is relativel trivial. What we think of as “proof” in physics is a combination of “this belief creates predictable experiences” and “this abstraction of belief is logically consistent.” The first part is subjective - our beliefs about physics create predictable experiences for us, but we have no idea how large the entire domain of possible experiences is. The second part is a tar pit full of bones, haunted by undefinability theorems. https://en.m.wikipedia.org/wiki/Tarski%27s_undefinability_theorem https://en.m.wikipedia.org/wiki/Tarski%27s_undefinability_th... So any suggestion that physics efficiently reveals the secret mechanisms of the universe is a conceit. Physics reveals what we’re able to deduce about the universe given the biases built into our experience of sentience. When weird things fall out of quantum theory, this makes it hard to know if the weirdness is truly out there, or if it’s an artifact of our limited sentience.

eoinmurray92 9y ago

It not correlation - bell inequalities show that local variables don't explain the correlations: ie no correlated variable exists because states are decided at measurement (or wavefunction collapse) time. This is why entanglement is interesting.

tbabb 9y ago
Sure, "no hidden variables" is interesting/weird. And interference is interesting/weird. And both of those things happen independently from entanglement. But entanglement itself seems very not-weird to me once you buy into those other things.
- whatshisface 9y ago
  What you're on to is that all of the big popular examples of "quantum weirdness" (uncertianty, entanglement, tunneling) emerge from a very small and elegant set of axiomatic postulates. That's the beauty of physics. One such group of postulates that chemists like to use can be found at http://vergil.chemistry.gatech.edu/notes/quantrev/node20.html http://vergil.chemistry.gatech.edu/notes/quantrev/node20.htm... . You can probably beat the elegance of those if you're willing to step further from what's actually used in calculations, but that's not what chemists like to do. Here's a better shot at being axiomatic and elegant, but it's a lot less clear (from a lay-perspective) how they relate to reality: https://en.wikipedia.org/wiki/Dirac%E2%80%93von_Neumann_axioms https://en.wikipedia.org/wiki/Dirac%E2%80%93von_Neumann_axio...
  - mikhailfranco 9y ago
    I find Lucien Hardy's discussion enlightening: Quantum Theory From Five Reasonable Axioms https://arxiv.org/abs/quant-ph/0101012 https://arxiv.org/abs/quant-ph/0101012
h2g3yhogw8 9y ago
As you're the only person so far on this thread who seems to know about this, is my understanding correct that non-locality of entanglement is because all the entangled entities are described by a single hamiltonian which means the energy expressed in each of their degrees of freedom must sum to zero, so only certain combinations of measurements are possible of the system as a whole

solipsism 9y ago

Yes you're missing something. The idea that an event in one place can influence an event in another place, even though it's not possible information traveled from the first place to the second, is spooky. And the idea that this can even be the case even if the associated objects never coexisted in time is anything but "boringly trivial".

ComputerGuru 9y ago
Everyone keeps coming back to this point, but I maintain that just because there’s no way that we know of (or that makes sense in the framework of our understanding) for information to travel between the two, it absolutely does not mean information _did not_ travel between the two. It’s the simpler explanation, if you don’t let hubris get in the way.
- lowpro 9y ago
  It's more than that. It can be shown that information could not travel between 2 points provably, so it's not like we're missing some communication method. We can show there is no intermediate communication method. See the MIT introduction lecture on Quantum Physics: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/ https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-s... I believe the experiment where this is shown is lecture 1, possibly 2. It's been awhile since I watched this but a great source of information.
  - kirrent 9y ago
    To further clarify, any communication would have to be superluminal. Sure there are people who claim that there's some unobservable superluminal background to the universe that makes non-local hidden variable theories possible but I don't know that I'd call that the simpler explanation.
    - ComputerGuru 9y ago
      I was thinking more along the lines of undetected hyper surfaces or wormholes.