6 ms·

Bell inequality violation finally done right

109 points by rvern 11y ago

fitzwatermellow 11y ago
Money quote: "if people had just understood and believed Bohr and Heisenberg back in 1925, there would’ve been no need for this whole tiresome discussion" Prof. Aaronson is teaching a grad seminar this Fall at MIT on Computation and Physics. Here's hoping he makes the lecture notes public ;)
weinzierl 11y ago
Worth reading because it has an interesting, new (to me, at least) and approachable explanation of local realism. Money quote: Perhaps the best way to explain local realism is that it’s the thing you believe in, if you believe all the physicists babbling about “quantum entanglement” just missed something completely obvious.
- hasenj 11y ago
  Forget the realism part. Can we at least get the locality part? In my mind, all the quantum observables are not real properties, therefor it's obvious/trivial that they don't exist until measured. That is, I have no problem accepting they are not "real". What gets to me is the locality aspect of it. Of course I'm not a physicist (read: I am layman).
  - sciolizer 11y ago
    Not a physicist either, but I think "local realism" is a conjunction of "local" and "realism". Bell inequality experiments show that "local realism" is false, and so we have to reject either "local" or "realism", but we don't necessarily have to reject both.
    - digi_owl 11y ago
      Best i can tell local realism basically means that if something blows up at the other side of the universe, it should not affect whatever is happening on this side until we can observe said event via the reception of energy/particles from said event.
      - norea-armozel 11y ago
        Well that's the problem with local realism. All sorts of things do affect local conditions at a distance. If entanglement between particle pairs is not affected by any of the physical forces as described then whatever happens between them is just a matter of fact, but no where have I seen this mean we'll ever get ansibles or teleportation. It just means the universe as a substructure we're not fully able to grasp and that it may be just one of those things that connects everything despite the appearance of separation of objects and states.
- trhway 11y ago
  that quote,and pretty much whole blog post, is among the best illustrations of orthodoxy. Speaking about obvious - the paper contains S>2 for entangled case. For some, probably obvious reason, the paper doesn't contain S for non-entangled case. We know that is should be <=2. Why not to show it? Their setup is exactly one very suitable to run without entanglement. From there i stand it actually would be >2 in their setup even for non-entangled case.
  - ajkjk 11y ago
    What problem do you have with the orthodoxy?
ryandamm 11y ago
This is a really good explanation... but I can personally only square most of this entanglement non-locality with a many-worlds interpretation. Which is itself hugely problematic.
- gizmo686 11y ago
  What is problematic about the many-worlds interpretation. Even without non-locality, it solves the non-determinism problem, which seems far more problematic to me.
  - ploxiln 11y ago
    Well, the only one of the many-worlds that actually matters is this particular one, right here. What can we say/predict about this exact one. The others don't matter to me in the slightest... EDIT: well that's not a great explanation of why I dislike the many-worlds hypothesis, let me try again. Like some other ideas, including the existence of God, it's not (yet?) falsifiable. Which doesn't make it false, of course. But it's a really extraordinary idea. It's not just many worlds, it's a fuck-ton. And what can we do with all of them? Are they for nothing? Does nothing that happens matter because it happened differently in a separate but equally real "world"? I guess, I can believe that "god plays dice with the universe", but I can not believe that "god makes infinite copies of the whole universe and tries all possible quantum outcomes".
    - lisper 11y ago
      > it's a fuck-ton Or zero. (But it's definitely not one.) Personally I find the zero-worlds view to be the most intuitively satisfying: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf (Or the movie version: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc) David Mermin's Ithaca interpretation is essentially the same thing: http://arxiv.org/abs/quant-ph/9609013 http://arxiv.org/abs/quant-ph/9609013 The slogan is that "we are correlations without correlata".
    - gizmo686 11y ago
      The interpretation we use does not have any effect on our ability to make predictions; the equations are the same. EDIT: response to parent edit Almost by definition interpretations are not falsifiable. The many world interpenetration is literally equivalent to the other interpretations, because they all describe how we understand the meaning of the same mathematical model; and it is that model that is making predictions. In my opinion (as a mathematician, not a physicist), the many worlds interpretation is the more natural way to read quantum theory. That is to say, if I were tasked to design a universe given quantum physics as the specification, my naive implementation would correspond to the many worlds interpretation. An equally valid interpretation is that we are a computer simulation, and quantum physics describes the rounding behavior of said simulation. As far as I am concerned the only point of having an interpretation is to help build an intuition for the models.
amelius 11y ago
Locality (as opposed to non-locality) is a strange concept if you think about it. Because how would different points in space "know" to run the same set of equations? Hence all points have something in common, giving rise to the possibility (or non-weirdness) of non-locality. But of course, this is completely non-scientific :)
- renox 11y ago
  > Locality (as opposed to non-locality) is a strange concept if you think about it. Because how would different points in space "know" to run the same set of equations? I'm sorry but I don't understand your point, can you explain it further? Also one weirdness of QM is not only that it is non-local, it is that it is non-local allowing instantaneous remote 'interaction' BUT without allowing to send information instantaneously (not faster than light).
  - amelius 11y ago
    Well, there isn't much to explain really. Physicists have long had an "intuition" that says that non-locality is somehow a strange concept. However, intuition is often misleading. So I'm flipping the "argument" around and I say that it is more intuitive to think of locality as a strange concept, and to think of non-locality as something which is more natural.
    - aroberge 11y ago
      Physicist's "intuition" (as you put it) is educated by codifying our observations into mathematical equations, using those equations to make predictions, and performing experiments to verify those predictions. Simply 'flipping around the "argument"' is playing with words and has no usefulness from a scientific point of view; it is definitely not "more intuitive" (as you put it) since it has nothing to do with science.
      - TeMPOraL 11y ago
        It's an interesting thought exercise IMO. Keep in mind that QM is widely regarded as "counterintuitive" even by scientists, and as far as we can tell, it's the intuitions that are wrong. I.e. it's not QM that's crazy. QM is normal. We are crazy.
renox 11y ago
Thanks for this article, the previous description of the experiment I read omitted the 'discard the result unless a measurement show that the entanglement was successful' step, leaving me very confused.. I didn't know that this was possible, that said I still don't have any clue about HOW this is possible.
Ono-Sendai 11y ago
Einstein et. al.'s original paper on the EPR paradox is very clear and understandable, I recommend it: http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777 http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777
snarfy 11y ago
I don't have a problem with locality being violated. According to relativity, distance is relative and depends on the reference frame. There will always been some reference frame where their is no distance between points.
- Manishearth 11y ago
  Noooooooooooooooo. No. "distance is relative and depends on the reference frame." does not imply " There will always been some reference frame where their is no distance between points.". It just means that distances may be different. Special relativity is a linear transformation, so a nonzero distance cannot become zero. Relativity doesn't help locality violation, far from it. Relativity has issues (EPR) with locality violation.
  - snarfy 11y ago
    What's the distance between two points when measured from a third reference frame moving at the speed C?
    - ryandamm 11y ago
      I don't think you can have a reference point moving at speed C, the universe throws a 'divide by zero' exception.
      - snarfy 11y ago
        While true, I thought in QM you had to evaluate all probabilities, even those where the speed is > C.
        Manishearth 11y ago
        Nope. In QM you have probabilities where energy conservation is violated for a little bit of time, or where a particle appeared to jump in a distance longer than allowed. This doesn't mean you can have reference frames moving at the speed of light.

109 points by rvern 11y ago

Money quote: "if people had just understood and believed Bohr and Heisenberg back in 1925, there would’ve been no need for this whole tiresome discussion" Prof. Aaronson is teaching a grad seminar this Fall at MIT on Computation and Physics. Here's hoping he makes the lecture notes public ;)

weinzierl 11y ago

Worth reading because it has an interesting, new (to me, at least) and approachable explanation of local realism. Money quote: Perhaps the best way to explain local realism is that it’s the thing you believe in, if you believe all the physicists babbling about “quantum entanglement” just missed something completely obvious.

hasenj 11y ago
Forget the realism part. Can we at least get the locality part? In my mind, all the quantum observables are not real properties, therefor it's obvious/trivial that they don't exist until measured. That is, I have no problem accepting they are not "real". What gets to me is the locality aspect of it. Of course I'm not a physicist (read: I am layman).
- sciolizer 11y ago
  Not a physicist either, but I think "local realism" is a conjunction of "local" and "realism". Bell inequality experiments show that "local realism" is false, and so we have to reject either "local" or "realism", but we don't necessarily have to reject both.
  - digi_owl 11y ago
    Best i can tell local realism basically means that if something blows up at the other side of the universe, it should not affect whatever is happening on this side until we can observe said event via the reception of energy/particles from said event.
    - norea-armozel 11y ago
      Well that's the problem with local realism. All sorts of things do affect local conditions at a distance. If entanglement between particle pairs is not affected by any of the physical forces as described then whatever happens between them is just a matter of fact, but no where have I seen this mean we'll ever get ansibles or teleportation. It just means the universe as a substructure we're not fully able to grasp and that it may be just one of those things that connects everything despite the appearance of separation of objects and states.
trhway 11y ago
that quote,and pretty much whole blog post, is among the best illustrations of orthodoxy. Speaking about obvious - the paper contains S>2 for entangled case. For some, probably obvious reason, the paper doesn't contain S for non-entangled case. We know that is should be <=2. Why not to show it? Their setup is exactly one very suitable to run without entanglement. From there i stand it actually would be >2 in their setup even for non-entangled case.
- ajkjk 11y ago
  What problem do you have with the orthodoxy?

ryandamm 11y ago

This is a really good explanation... but I can personally only square most of this entanglement non-locality with a many-worlds interpretation. Which is itself hugely problematic.

gizmo686 11y ago
What is problematic about the many-worlds interpretation. Even without non-locality, it solves the non-determinism problem, which seems far more problematic to me.
- ploxiln 11y ago
  Well, the only one of the many-worlds that actually matters is this particular one, right here. What can we say/predict about this exact one. The others don't matter to me in the slightest... EDIT: well that's not a great explanation of why I dislike the many-worlds hypothesis, let me try again. Like some other ideas, including the existence of God, it's not (yet?) falsifiable. Which doesn't make it false, of course. But it's a really extraordinary idea. It's not just many worlds, it's a fuck-ton. And what can we do with all of them? Are they for nothing? Does nothing that happens matter because it happened differently in a separate but equally real "world"? I guess, I can believe that "god plays dice with the universe", but I can not believe that "god makes infinite copies of the whole universe and tries all possible quantum outcomes".
  - lisper 11y ago
    > it's a fuck-ton Or zero. (But it's definitely not one.) Personally I find the zero-worlds view to be the most intuitively satisfying: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf (Or the movie version: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc) David Mermin's Ithaca interpretation is essentially the same thing: http://arxiv.org/abs/quant-ph/9609013 http://arxiv.org/abs/quant-ph/9609013 The slogan is that "we are correlations without correlata".
  - gizmo686 11y ago
    The interpretation we use does not have any effect on our ability to make predictions; the equations are the same. EDIT: response to parent edit Almost by definition interpretations are not falsifiable. The many world interpenetration is literally equivalent to the other interpretations, because they all describe how we understand the meaning of the same mathematical model; and it is that model that is making predictions. In my opinion (as a mathematician, not a physicist), the many worlds interpretation is the more natural way to read quantum theory. That is to say, if I were tasked to design a universe given quantum physics as the specification, my naive implementation would correspond to the many worlds interpretation. An equally valid interpretation is that we are a computer simulation, and quantum physics describes the rounding behavior of said simulation. As far as I am concerned the only point of having an interpretation is to help build an intuition for the models.

amelius 11y ago

Locality (as opposed to non-locality) is a strange concept if you think about it. Because how would different points in space "know" to run the same set of equations? Hence all points have something in common, giving rise to the possibility (or non-weirdness) of non-locality. But of course, this is completely non-scientific :)

renox 11y ago
> Locality (as opposed to non-locality) is a strange concept if you think about it. Because how would different points in space "know" to run the same set of equations? I'm sorry but I don't understand your point, can you explain it further? Also one weirdness of QM is not only that it is non-local, it is that it is non-local allowing instantaneous remote 'interaction' BUT without allowing to send information instantaneously (not faster than light).
- amelius 11y ago
  Well, there isn't much to explain really. Physicists have long had an "intuition" that says that non-locality is somehow a strange concept. However, intuition is often misleading. So I'm flipping the "argument" around and I say that it is more intuitive to think of locality as a strange concept, and to think of non-locality as something which is more natural.
  - aroberge 11y ago
    Physicist's "intuition" (as you put it) is educated by codifying our observations into mathematical equations, using those equations to make predictions, and performing experiments to verify those predictions. Simply 'flipping around the "argument"' is playing with words and has no usefulness from a scientific point of view; it is definitely not "more intuitive" (as you put it) since it has nothing to do with science.
    - TeMPOraL 11y ago
      It's an interesting thought exercise IMO. Keep in mind that QM is widely regarded as "counterintuitive" even by scientists, and as far as we can tell, it's the intuitions that are wrong. I.e. it's not QM that's crazy. QM is normal. We are crazy.

renox 11y ago

Thanks for this article, the previous description of the experiment I read omitted the 'discard the result unless a measurement show that the entanglement was successful' step, leaving me very confused.. I didn't know that this was possible, that said I still don't have any clue about HOW this is possible.

Ono-Sendai 11y ago

Einstein et. al.'s original paper on the EPR paradox is very clear and understandable, I recommend it: http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777 http://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777

snarfy 11y ago

I don't have a problem with locality being violated. According to relativity, distance is relative and depends on the reference frame. There will always been some reference frame where their is no distance between points.

Manishearth 11y ago
Noooooooooooooooo. No. "distance is relative and depends on the reference frame." does not imply " There will always been some reference frame where their is no distance between points.". It just means that distances may be different. Special relativity is a linear transformation, so a nonzero distance cannot become zero. Relativity doesn't help locality violation, far from it. Relativity has issues (EPR) with locality violation.
- snarfy 11y ago
  What's the distance between two points when measured from a third reference frame moving at the speed C?
  - ryandamm 11y ago
    I don't think you can have a reference point moving at speed C, the universe throws a 'divide by zero' exception.
    - snarfy 11y ago
      While true, I thought in QM you had to evaluate all probabilities, even those where the speed is > C.
      - Manishearth 11y ago
        Nope. In QM you have probabilities where energy conservation is violated for a little bit of time, or where a particle appeared to jump in a distance longer than allowed. This doesn't mean you can have reference frames moving at the speed of light.