6 ms·

Quantum magic: A skeptical perspective (2011)

62 points by lainon 9y ago

Strilanc 9y ago
> We argue that many popular "quantum paradoxes" stem from a confusion between mathematical formalism and physics [...] most "conceptual puzzles" of QM are not much different from the well-known paradoxes from probability theory This is really really true. Particularly for any experiment with "delayed choice" in the name, where pop-science explanations follow correlations backwards instead of forwards then confuse correlation for causation and start talking about time travel (sigh).
- mlevental 9y ago
  i would love to see a translation of quantum paradoxes to probability paradoxes - e.g. quantum tunneling is the same as basically measure 0 events.
  - whatshisface 9y ago
    Tunneling isn't really a paradox, it's just a direct contradiction with classical mechanics.
    - kurthr 9y ago
      Agreed... although a probabilistic energy distribution (e.g. thermal) could cause electron/hole current exponential with voltage, it would be unlike tunneling across a potential barrier that is too high. Thermal current would not be exponentially dependent on the separation gap.
  - k__ 9y ago
    Or what about wave particle dualities? How can one particle interfere with itself?
    - deleted 9y ago
      [deleted]
    - tbabb 9y ago
      Personally I think the concept of "particle" needs to go away completely. I think really what "particle" means is that "all measurements are quantized". But the probabilities of any given measurement happening flows around as a wave. For example, "there is a quantity of energy `e` at location `x`" is an observation you could make. The answer to that question has a discrete yes/no answer. The probability of the answer being "yes" flows around like a wave between all the different possible `x` as time progresses.
      - k__ 9y ago
        Really? You mean all particles are virtual? Whaz is the alternative?
        tbabb 9y ago
        No, I mean that there are no particles. Only waves. Only that if you try to measure something about the wave, you will always get a discrete "yes/no" answer.
        tedsanders 9y ago
        Particles are a really useful fiction. I'm having a tough time thinking of how to teach and explain chemistry without the concept of valence electrons. Do you have any ideas? (Related tangent: a super important but occasionally missed nuance of quantum mechanics is that the waves are not waves over 3D real space. They are waves over configuration space, with way more than 3 dimensions.)
        k__ 9y ago
        Then what are they in 3D space?
  - Strilanc 9y ago
    Well, for example, the no cloning theorem has a classical analogy. I prepare a coin so that it is heads with probability p. I give you the coin. Your job is to prepare two coins so that they are independently heads with probability p. You aren't told p, you only get the coin to work with. We are going to repeat this many times, and with various values of p, and someone will test whether the coins you are producing follow the right distribution. It's obviously not possible for you to do particularly well at this task. See [1] I'm not aware of a probability analogy for tunneling. 1: https://marozols.wordpress.com/2012/05/21/three-myths-about-quantum-computing-part-2/ https://marozols.wordpress.com/2012/05/21/three-myths-about-...
  - justinpombrio 9y ago
    > e.g. quantum tunneling is the same as basically measure 0 events. It is not. Quantum tunneling is just an event with low probability, and when you notice that quantum tunneling doesn't happen (on a macro scale), you're just noticing that events with sufficiently low probability don't happen. The probabilistic analogy is flipping a billion coins and having them all come up heads. Flipping a billion coins and having them all come up heads does not have measure zero. It has a positive measure. Contrast this to measure zero events. Flipping infinitely many coins and having them all come up heads has measure zero. You need measure theory to talk usefully about measure zero events. You don't need measure theory to talk about quantum tunneling. Just ordinary probability.
    - mlevental 9y ago
      > The probabilistic analogy is flipping a billion coins and having them all come up heads. Flipping a billion coins and having them all come up heads does not have measure zero. It has a positive measure. wut? 1. in each instance it's 1MM conditionally independent trials so i don't see the difference 2. any realization of any continuous random variable has measure zero >Contrast this to measure zero events. Flipping infinitely many coins and having them all come up heads has measure zero. you're playing fast and loose with the difference between a counting measure (no measure zero events) and a continuous/discrete measure. anyway my point was exactly that quantum tunneling is counterintuitive exactly for the same reason that measure zero events are counterintuitive
pjs_ 9y ago
The paper is way more nuanced than the abstract suggests. There are huge numbers of papers on the arXiv which try to say, "quantum mechanics is normal and boring, these fools are trying to dress it up as something special because they don't understand probabilities". This paper is more interesting. Here's a quote: > In fact, QM is so weird and surprising that even the very esoteric interpretations, such as the many universes one, can not capture the subtlety normally associated with quantum paradoxes. For example, theorems such as the Kochen-Specter theorem [23] (essentially, a generalization of EPR with three particles) imply that during its quantum evolution no ”classical” variables can fully describe the probabilities of a quantum system.
- dbranes 9y ago
  The following of course is a comment on the quote not on your comment, but no Kochen Specker is not a generalization of EPR to 3 particles. It's easy to come up with a generalization of EPR in 3 particles - just exhibit a 3-party entangled state! (e.g GHZ state). K-S theorem exhibits something a lot more subtle than entanglement known as contextuality. There are examples of systems which have the K-S property but doesn't have any entanglement.
- n4r9 9y ago
  I also found the paper to have more depth than I expected. I was going to remark that it gives the EPR paradox too short a shrift, but I realised that it is not arguing against the actual paradox, but rather the paradox as it is most commonly presented/understood and "mystically" explained. If anyone is interested, the original EPR paper is readable and profound in that timeless way that characterises much of what Einstein touched: http://www.drchinese.com/David/EPR.pdf http://www.drchinese.com/David/EPR.pdf
- naasking 9y ago
  > For example, theorems such as the Kochen-Specter theorem [23] (essentially, a generalization of EPR with three particles) imply that during its quantum evolution no ”classical” variables can fully describe the probabilities of a quantum system [...] while preserving these other properties we assume must hold, but don't really know for certain do actually hold.
tbabb 9y ago
A few thoughts: The Monty Hall comparison is about shifting human knowledge about an underlying system with a definite state. What, then, is the source of the uncertainty in QM in this author's view? It is not our knowledge about the "true state" of things, because of course there is no single true state. The same could be said about any appeal to "perturbations" from the measurements we make-- in order for a "push" from our measurement instruments be the source of uncertainty, we must explain it in terms of an unknown disturbance making a precise value imprecise (otherwise, the use of "our instruments" as an explanation for "uncertainty" can't be the final tortoise upon which the world stands). Because of the impossibility of a "precise physical value" this explanation doesn't seem satisfactory to me. It seems the article (like many interpretations) does no better than to say "don't ask" about what lies underneath those probabilities. At best it says "our knowledge is uncertain" but then stops before it says precisely what we are uncertain about. His points about analogues with classical probability are well taken, but I don't think they suffice to explain or dismiss quantum "weirdness", though it's quite possible I missed the point he was trying to make. In particular, the article sweeps many-worlds away too quickly, IMO. Many worlds is what we get if we assume that observers are ensembles of particles which can exist in a state of superposition, just like any other ensemble of particles in the universe. "Wavefunction collapse" is what such an observer would expect to see upon becoming entangled with another system, without any additional a priori assumptions. Fundamentally it means that, after measurement, we cannot treat the system and the environment (and the observer therein) as separate-- another way of stating the phenomenon of decoherence. In other words, I think it is not unparsimonious to assume that "multiple universes" exist, because we already know that ensembles of particles exist in a multitude of states. We simply note that the universe, too, is an ensemble of particles and draw a straightforward conclusion from well-established facts. To me, it seems almost more ontologically bold to reject this. It is also not unparsimonious to assume that "multiple universes" exist, even after measurement, because delayed-choice eraser demonstrates that wavefunctions can be "un-collapsed"; that is to say re-combined with the other superpositions that Copenhagen supposes "disappear" after measurement. It fact it seems to me a fairly clear refutation of (common presentations of) Copenhagen-- But perhaps the author is on my side about that one. Either way, this article seems to be a quite thoughtful overview, and I quite agree there's a lot of confusion and woo both within and outside the field.

62 points by lainon 9y ago

> We argue that many popular "quantum paradoxes" stem from a confusion between mathematical formalism and physics [...] most "conceptual puzzles" of QM are not much different from the well-known paradoxes from probability theory This is really really true. Particularly for any experiment with "delayed choice" in the name, where pop-science explanations follow correlations backwards instead of forwards then confuse correlation for causation and start talking about time travel (sigh).

mlevental 9y ago
i would love to see a translation of quantum paradoxes to probability paradoxes - e.g. quantum tunneling is the same as basically measure 0 events.
- whatshisface 9y ago
  Tunneling isn't really a paradox, it's just a direct contradiction with classical mechanics.
  - kurthr 9y ago
    Agreed... although a probabilistic energy distribution (e.g. thermal) could cause electron/hole current exponential with voltage, it would be unlike tunneling across a potential barrier that is too high. Thermal current would not be exponentially dependent on the separation gap.
- k__ 9y ago
  Or what about wave particle dualities? How can one particle interfere with itself?
  - deleted 9y ago
    [deleted]
  - tbabb 9y ago
    Personally I think the concept of "particle" needs to go away completely. I think really what "particle" means is that "all measurements are quantized". But the probabilities of any given measurement happening flows around as a wave. For example, "there is a quantity of energy `e` at location `x`" is an observation you could make. The answer to that question has a discrete yes/no answer. The probability of the answer being "yes" flows around like a wave between all the different possible `x` as time progresses.
    - k__ 9y ago
      Really? You mean all particles are virtual? Whaz is the alternative?
      - tbabb 9y ago
        No, I mean that there are no particles. Only waves. Only that if you try to measure something about the wave, you will always get a discrete "yes/no" answer.
        tedsanders 9y ago
        Particles are a really useful fiction. I'm having a tough time thinking of how to teach and explain chemistry without the concept of valence electrons. Do you have any ideas? (Related tangent: a super important but occasionally missed nuance of quantum mechanics is that the waves are not waves over 3D real space. They are waves over configuration space, with way more than 3 dimensions.)
        k__ 9y ago
        Then what are they in 3D space?
- Strilanc 9y ago
  Well, for example, the no cloning theorem has a classical analogy. I prepare a coin so that it is heads with probability p. I give you the coin. Your job is to prepare two coins so that they are independently heads with probability p. You aren't told p, you only get the coin to work with. We are going to repeat this many times, and with various values of p, and someone will test whether the coins you are producing follow the right distribution. It's obviously not possible for you to do particularly well at this task. See [1] I'm not aware of a probability analogy for tunneling. 1: https://marozols.wordpress.com/2012/05/21/three-myths-about-quantum-computing-part-2/ https://marozols.wordpress.com/2012/05/21/three-myths-about-...
- justinpombrio 9y ago
  > e.g. quantum tunneling is the same as basically measure 0 events. It is not. Quantum tunneling is just an event with low probability, and when you notice that quantum tunneling doesn't happen (on a macro scale), you're just noticing that events with sufficiently low probability don't happen. The probabilistic analogy is flipping a billion coins and having them all come up heads. Flipping a billion coins and having them all come up heads does not have measure zero. It has a positive measure. Contrast this to measure zero events. Flipping infinitely many coins and having them all come up heads has measure zero. You need measure theory to talk usefully about measure zero events. You don't need measure theory to talk about quantum tunneling. Just ordinary probability.
  - mlevental 9y ago
    > The probabilistic analogy is flipping a billion coins and having them all come up heads. Flipping a billion coins and having them all come up heads does not have measure zero. It has a positive measure. wut? 1. in each instance it's 1MM conditionally independent trials so i don't see the difference 2. any realization of any continuous random variable has measure zero >Contrast this to measure zero events. Flipping infinitely many coins and having them all come up heads has measure zero. you're playing fast and loose with the difference between a counting measure (no measure zero events) and a continuous/discrete measure. anyway my point was exactly that quantum tunneling is counterintuitive exactly for the same reason that measure zero events are counterintuitive

pjs_ 9y ago

The paper is way more nuanced than the abstract suggests. There are huge numbers of papers on the arXiv which try to say, "quantum mechanics is normal and boring, these fools are trying to dress it up as something special because they don't understand probabilities". This paper is more interesting. Here's a quote: > In fact, QM is so weird and surprising that even the very esoteric interpretations, such as the many universes one, can not capture the subtlety normally associated with quantum paradoxes. For example, theorems such as the Kochen-Specter theorem [23] (essentially, a generalization of EPR with three particles) imply that during its quantum evolution no ”classical” variables can fully describe the probabilities of a quantum system.

dbranes 9y ago
The following of course is a comment on the quote not on your comment, but no Kochen Specker is not a generalization of EPR to 3 particles. It's easy to come up with a generalization of EPR in 3 particles - just exhibit a 3-party entangled state! (e.g GHZ state). K-S theorem exhibits something a lot more subtle than entanglement known as contextuality. There are examples of systems which have the K-S property but doesn't have any entanglement.
n4r9 9y ago
I also found the paper to have more depth than I expected. I was going to remark that it gives the EPR paradox too short a shrift, but I realised that it is not arguing against the actual paradox, but rather the paradox as it is most commonly presented/understood and "mystically" explained. If anyone is interested, the original EPR paper is readable and profound in that timeless way that characterises much of what Einstein touched: http://www.drchinese.com/David/EPR.pdf http://www.drchinese.com/David/EPR.pdf
naasking 9y ago
> For example, theorems such as the Kochen-Specter theorem [23] (essentially, a generalization of EPR with three particles) imply that during its quantum evolution no ”classical” variables can fully describe the probabilities of a quantum system [...] while preserving these other properties we assume must hold, but don't really know for certain do actually hold.

tbabb 9y ago

A few thoughts: The Monty Hall comparison is about shifting human knowledge about an underlying system with a definite state. What, then, is the source of the uncertainty in QM in this author's view? It is not our knowledge about the "true state" of things, because of course there is no single true state. The same could be said about any appeal to "perturbations" from the measurements we make-- in order for a "push" from our measurement instruments be the source of uncertainty, we must explain it in terms of an unknown disturbance making a precise value imprecise (otherwise, the use of "our instruments" as an explanation for "uncertainty" can't be the final tortoise upon which the world stands). Because of the impossibility of a "precise physical value" this explanation doesn't seem satisfactory to me. It seems the article (like many interpretations) does no better than to say "don't ask" about what lies underneath those probabilities. At best it says "our knowledge is uncertain" but then stops before it says precisely what we are uncertain about. His points about analogues with classical probability are well taken, but I don't think they suffice to explain or dismiss quantum "weirdness", though it's quite possible I missed the point he was trying to make. In particular, the article sweeps many-worlds away too quickly, IMO. Many worlds is what we get if we assume that observers are ensembles of particles which can exist in a state of superposition, just like any other ensemble of particles in the universe. "Wavefunction collapse" is what such an observer would expect to see upon becoming entangled with another system, without any additional a priori assumptions. Fundamentally it means that, after measurement, we cannot treat the system and the environment (and the observer therein) as separate-- another way of stating the phenomenon of decoherence. In other words, I think it is not unparsimonious to assume that "multiple universes" exist, because we already know that ensembles of particles exist in a multitude of states. We simply note that the universe, too, is an ensemble of particles and draw a straightforward conclusion from well-established facts. To me, it seems almost more ontologically bold to reject this. It is also not unparsimonious to assume that "multiple universes" exist, even after measurement, because delayed-choice eraser demonstrates that wavefunctions can be "un-collapsed"; that is to say re-combined with the other superpositions that Copenhagen supposes "disappear" after measurement. It fact it seems to me a fairly clear refutation of (common presentations of) Copenhagen-- But perhaps the author is on my side about that one. Either way, this article seems to be a quite thoughtful overview, and I quite agree there's a lot of confusion and woo both within and outside the field.