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Bell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me

Bell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me at least) of just how it works.

sciolizer 6y ago
Yudkowsky's explanation[1] is the first one that worked for me. I later found Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing. 1: https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-theorem-no-epr-reality https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-the... 2: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...
- ShamelessC 6y ago
  I started to read the first one but his insistence that Many Worlds is true was too frustrating. Many Worlds Theorem seems specifically useful at saying "the variables aren't hidden because everything before wavefunction collapses actually plays out in different worlds. But, we specifically have no way of proving that theory. So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?
  - kubanczyk 6y ago
    Well, I'd recommend to read the whole series. It's not so bad as it sounds. There are so many steps from where you are to appreciating the utter weirdness of Bell's experimental result. Not the weirdness of any theory (or an interpretation, which Many Worlds actually is) but of the basic experimental result. If you are properly amazed by it, rejecting MWI or any crazy-ish borderline-conspiracy theory seems suddenly a lot harder. I feel the whole Yudkowsky's QM series in fact served to deliver that one post.
    - ShamelessC 6y ago
      To be clear, I don't reject Many Worlds at all and in fact consider it a promising candidate due to it sort of "falling out" of the Schrodinger's equations taken literally unless you add complexity. But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails.
      - kubanczyk 6y ago
        If experimenters disprove Many Worlds, they've also disproved Copenhagen. These are exactly the same equations after all. Theoreticians choose very different mindsets about the same equations, which (they say) somehow create them grounds to form various new hypotheses. As far as I know neither approach was very fruitful so far in terms of new science, so people try multitude of others. What I've meant to say above, I have much trouble using Copenhagen to understand Bell's experiment. MWI fits the bill here for me.
      - AgentME 6y ago
        >But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails. Is that not like trying to get a better intuition for planetary movement by using an epicycle-based model? The fact that the interpretation is conveniently shaped in a way that a paradox isn't an issue is not a coincidental thing that should be overlooked in the spirit of fairness to alternative interpretations. Regardless, I think my post below is useful for answering your want. >So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables? The world is only deterministic under Many-Worlds, and it's deterministic in the sense of "each outcome happens (mostly) separately". It doesn't make any sense to try to make sense of the "deterministic" part separately from MWI. MWI is the only deterministic QM theory (unless you're going to consider "superdeterminism", but there's nothing concrete to that interpretation besides "what if there existed a way that we had QM+determinism but not MWI". There's no basis to it, besides a yearning from people that like the abstract idea of determinism and don't like the abstract idea of MWI). EPR doesn't tell us that the world is deterministic. It tells us that local hidden variable interpretations (where experiments have a single outcome) of QM can't work, because it shows that a measurement on a particle can appear to you to affect the measurement made by someone else on a distant particle. The Copenhagen interpretation response to this is that the wave function collapse must be faster than light. Therefore, the Copenhagen interpretation is not a "local" theory. (The Copenhagen interpretation doesn't give us any answer for who we should expect to trigger this wave function collapse first when two measurements are taken simultaneously at a distance though.)
- kubanczyk 6y ago
  I trust Yudkowsky on many things, but not on that explanation. It's still quite complicated, and a couple of times I miserably failed to reconstruct it over a beer or two. A red flag. Plus, I'd rather expect at least one professional (QED) physicist exists able to explain it and he isn't one. Mermin is, but the explanation is decidedly less clear. BTW I came here to say Bell's inequality as well. For me it's as baffling as science could ever be.
wizeman 6y ago
Perhaps you can try the following article, as it was the simplest and clearest explanation I've found anywhere: https://www.physics.wisc.edu/undergrads/courses/spring2016/407/experiments/bell/Bell's%20Theorem%20Background%20Papers/Mermin_quantum_mysteries-Am.J.P.49.940.pdf https://www.physics.wisc.edu/undergrads/courses/spring2016/4... AFAICS it was published in the American Journal of Physics in 1981 but it's addressed to the general reader. It requires no knowledge of quantum physics.
monocasa 6y ago
Minute Physics and 3brown1blue did a two part collab that simplifies some of it down to a counting problem that'd be suitable for grade school. https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=MzRCDLre1b4 https://www.youtube.com/watch?v=MzRCDLre1b4
- lpellis 6y ago
  I have seen the example with the polarized lenses in a few places, but they dont explain why (imo) the simplest explanation does not apply. Namely that the lens itself might disturb the phase of the light, which would then mean it can pass through the next lens.
  - wwarner 6y ago
    The idea is that polarization is only one of many places where the effect is observed.
  - sooheon 6y ago
    But if you take away the third lens, there is no light of any polarization. How is it that by adding a filter, you create light where there was none?
    - lpellis 6y ago
      Only if you take away the middle lens, not the third. Here's what I would have thought happens: After the first lens, you get polarized light, 90deg offset from the last lens, so no light passes. Then you introduce a 3rd lens in the middle, 45deg offset. This could alter the polarization (maybe it widens the band, or introduce some greater variance, shifts it who knows), and this is why now some light will pass through number 3. No need to create any light
      - sooheon 6y ago
        If it is true that placing the 45 degree lens third or first does not show the same effect, it is much less astonishing.
lisper 6y ago
I recommend this book: https://www.amazon.com/Quantum-Non-Locality-Relativity-Metaphysical-Intimations/dp/1444331272 https://www.amazon.com/Quantum-Non-Locality-Relativity-Metap... If you don't want to read a whole book then I recommend this article: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/... but the book will give you a much deeper understanding.
abdullahkhalids 6y ago
The short answer is this. Suppose thread OP gave you and I a box each. Each box has a some knobs to choose some settings, and if one chooses a setting and presses the GO button, a number pops up on the screen. We go far away from each other so that, due to the finite speed of light, it takes quite a while for any information to travel between the boxes. We do this, because we don't want the boxes to communicate live, even though thread OP might have recorded the same data into the memory drive of both boxes. Then we choose some settings and press GO and record whatever number pops up. We do this many times so we each have a nice frequency chart. Now Bell proved that if you live in a local hidden variable universe, the correlations between these numbers is upper bounded, no matter how you choose settings on the boxes. Then, he also gave a prescription for choosing the settings, such that if you live a in quantum universe, the correlations between these numbers will be higher than the upper bound. The rest is mathematics, which cannot really be simplified without leaving the reader unsatisfied.
- eranation 6y ago
  So what I don't understand is, if both of us take an identical Gemalto token / Yuvikey, we can be light years apart and get the same sequences, no? Is the explanation that these will have one type of distribution vs if they had real "spooky movement at a distance" they have a clear different distribution? EDIT: what about this one: https://arxiv.org/abs/quant-ph/0301059 https://arxiv.org/abs/quant-ph/0301059
  - wwarner 6y ago
    The distance is part of the proof, but not really part of the mystery. I'm going to throw out an analogy that gets at what's observed and why it's surprising, but doesn't relate to the physics of spin, momentum, position or anything that's actually under observation in these experiments. It's as if we have a pair of dice, and I throw my die and you throw your die many times. In a classical world, if I throw a three, it has no influence on what you throw; you're equally likely to throw 1-6. But in the quantum world it's as if when I throw a one, your die still has the expected uniform distribution, but when I happen to throw a three, you're a little bit more likely to throw a three. Your die is fair if I happen to roll a one, but it's weighted if I happen to throw a three. Back in the real world, this is the strange behavior that is observed in experiment. Schroedinger's equation predicts the probabilities perfectly. But Bell shows that it's far from intuitive.
wwarner 6y ago
I can recommend Ball's _Beyond Weird_ for the best explanation targeting a lay audience that I've read.

Bell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me at least) of just how it works.

sciolizer 6y ago

Yudkowsky's explanation[1] is the first one that worked for me. I later found Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing. 1: https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-theorem-no-epr-reality https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-the... 2: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...

ShamelessC 6y ago
I started to read the first one but his insistence that Many Worlds is true was too frustrating. Many Worlds Theorem seems specifically useful at saying "the variables aren't hidden because everything before wavefunction collapses actually plays out in different worlds. But, we specifically have no way of proving that theory. So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?
- kubanczyk 6y ago
  Well, I'd recommend to read the whole series. It's not so bad as it sounds. There are so many steps from where you are to appreciating the utter weirdness of Bell's experimental result. Not the weirdness of any theory (or an interpretation, which Many Worlds actually is) but of the basic experimental result. If you are properly amazed by it, rejecting MWI or any crazy-ish borderline-conspiracy theory seems suddenly a lot harder. I feel the whole Yudkowsky's QM series in fact served to deliver that one post.
  - ShamelessC 6y ago
    To be clear, I don't reject Many Worlds at all and in fact consider it a promising candidate due to it sort of "falling out" of the Schrodinger's equations taken literally unless you add complexity. But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails.
    - kubanczyk 6y ago
      If experimenters disprove Many Worlds, they've also disproved Copenhagen. These are exactly the same equations after all. Theoreticians choose very different mindsets about the same equations, which (they say) somehow create them grounds to form various new hypotheses. As far as I know neither approach was very fruitful so far in terms of new science, so people try multitude of others. What I've meant to say above, I have much trouble using Copenhagen to understand Bell's experiment. MWI fits the bill here for me.
    - AgentME 6y ago
      >But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails. Is that not like trying to get a better intuition for planetary movement by using an epicycle-based model? The fact that the interpretation is conveniently shaped in a way that a paradox isn't an issue is not a coincidental thing that should be overlooked in the spirit of fairness to alternative interpretations. Regardless, I think my post below is useful for answering your want. >So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables? The world is only deterministic under Many-Worlds, and it's deterministic in the sense of "each outcome happens (mostly) separately". It doesn't make any sense to try to make sense of the "deterministic" part separately from MWI. MWI is the only deterministic QM theory (unless you're going to consider "superdeterminism", but there's nothing concrete to that interpretation besides "what if there existed a way that we had QM+determinism but not MWI". There's no basis to it, besides a yearning from people that like the abstract idea of determinism and don't like the abstract idea of MWI). EPR doesn't tell us that the world is deterministic. It tells us that local hidden variable interpretations (where experiments have a single outcome) of QM can't work, because it shows that a measurement on a particle can appear to you to affect the measurement made by someone else on a distant particle. The Copenhagen interpretation response to this is that the wave function collapse must be faster than light. Therefore, the Copenhagen interpretation is not a "local" theory. (The Copenhagen interpretation doesn't give us any answer for who we should expect to trigger this wave function collapse first when two measurements are taken simultaneously at a distance though.)
kubanczyk 6y ago
I trust Yudkowsky on many things, but not on that explanation. It's still quite complicated, and a couple of times I miserably failed to reconstruct it over a beer or two. A red flag. Plus, I'd rather expect at least one professional (QED) physicist exists able to explain it and he isn't one. Mermin is, but the explanation is decidedly less clear. BTW I came here to say Bell's inequality as well. For me it's as baffling as science could ever be.

wizeman 6y ago

Perhaps you can try the following article, as it was the simplest and clearest explanation I've found anywhere: https://www.physics.wisc.edu/undergrads/courses/spring2016/407/experiments/bell/Bell's%20Theorem%20Background%20Papers/Mermin_quantum_mysteries-Am.J.P.49.940.pdf https://www.physics.wisc.edu/undergrads/courses/spring2016/4... AFAICS it was published in the American Journal of Physics in 1981 but it's addressed to the general reader. It requires no knowledge of quantum physics.

monocasa 6y ago

Minute Physics and 3brown1blue did a two part collab that simplifies some of it down to a counting problem that'd be suitable for grade school. https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=MzRCDLre1b4 https://www.youtube.com/watch?v=MzRCDLre1b4

lpellis 6y ago
I have seen the example with the polarized lenses in a few places, but they dont explain why (imo) the simplest explanation does not apply. Namely that the lens itself might disturb the phase of the light, which would then mean it can pass through the next lens.
- wwarner 6y ago
  The idea is that polarization is only one of many places where the effect is observed.
- sooheon 6y ago
  But if you take away the third lens, there is no light of any polarization. How is it that by adding a filter, you create light where there was none?
  - lpellis 6y ago
    Only if you take away the middle lens, not the third. Here's what I would have thought happens: After the first lens, you get polarized light, 90deg offset from the last lens, so no light passes. Then you introduce a 3rd lens in the middle, 45deg offset. This could alter the polarization (maybe it widens the band, or introduce some greater variance, shifts it who knows), and this is why now some light will pass through number 3. No need to create any light
    - sooheon 6y ago
      If it is true that placing the 45 degree lens third or first does not show the same effect, it is much less astonishing.

lisper 6y ago

I recommend this book: https://www.amazon.com/Quantum-Non-Locality-Relativity-Metaphysical-Intimations/dp/1444331272 https://www.amazon.com/Quantum-Non-Locality-Relativity-Metap... If you don't want to read a whole book then I recommend this article: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/... but the book will give you a much deeper understanding.

abdullahkhalids 6y ago

The short answer is this. Suppose thread OP gave you and I a box each. Each box has a some knobs to choose some settings, and if one chooses a setting and presses the GO button, a number pops up on the screen. We go far away from each other so that, due to the finite speed of light, it takes quite a while for any information to travel between the boxes. We do this, because we don't want the boxes to communicate live, even though thread OP might have recorded the same data into the memory drive of both boxes. Then we choose some settings and press GO and record whatever number pops up. We do this many times so we each have a nice frequency chart. Now Bell proved that if you live in a local hidden variable universe, the correlations between these numbers is upper bounded, no matter how you choose settings on the boxes. Then, he also gave a prescription for choosing the settings, such that if you live a in quantum universe, the correlations between these numbers will be higher than the upper bound. The rest is mathematics, which cannot really be simplified without leaving the reader unsatisfied.

eranation 6y ago
So what I don't understand is, if both of us take an identical Gemalto token / Yuvikey, we can be light years apart and get the same sequences, no? Is the explanation that these will have one type of distribution vs if they had real "spooky movement at a distance" they have a clear different distribution? EDIT: what about this one: https://arxiv.org/abs/quant-ph/0301059 https://arxiv.org/abs/quant-ph/0301059
- wwarner 6y ago
  The distance is part of the proof, but not really part of the mystery. I'm going to throw out an analogy that gets at what's observed and why it's surprising, but doesn't relate to the physics of spin, momentum, position or anything that's actually under observation in these experiments. It's as if we have a pair of dice, and I throw my die and you throw your die many times. In a classical world, if I throw a three, it has no influence on what you throw; you're equally likely to throw 1-6. But in the quantum world it's as if when I throw a one, your die still has the expected uniform distribution, but when I happen to throw a three, you're a little bit more likely to throw a three. Your die is fair if I happen to roll a one, but it's weighted if I happen to throw a three. Back in the real world, this is the strange behavior that is observed in experiment. Schroedinger's equation predicts the probabilities perfectly. But Bell shows that it's far from intuitive.

wwarner 6y ago

I can recommend Ball's _Beyond Weird_ for the best explanation targeting a lay audience that I've read.