5 ms·

Steven Weinberg: What Is Quantum Field Theory, and What Did We Think It Is? (1997)

38 points by nabla9 5y ago

nabla9 5y ago
https://arxiv.org/abs/hep-th/9702027 https://arxiv.org/abs/hep-th/9702027
- tpfour 5y ago
  @dang the article link is broken, the above works for me.
  - dang 5y ago
    Fixed now. Thanks!
deleted 5y ago
[deleted]
deleted 5y ago
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yourenotsmart 5y ago
As I'm not deep into this matter, can someone share how this theory solves the "action from distance" problem of entanglement? The collapse is described as either non-local, or non-existent (many worlds intepretation). I'm not sure adding fields about every particle solves this, but again, I'm ignorant on the details.
- deleted 5y ago
  [deleted]
- leephillips 5y ago
  There is no action at a distance problem, because no influence travels between the particles.
  - yourenotsmart 5y ago
    The whole point of entanglement is that measuring one of the pair affects the outcome for the other one. Whether the influence "travels" between the particles is subject to debate, but there is influence.
    - leephillips 5y ago
      Sorry, no. There is no debate on this point. The confusion here may be merely linguistic, but there can be no instantaneous influence, in the sense of a causal effect or transmission of information, from one particle to a distant entangled particle. If there were, that would violate everything we know is true about causation, and therefore reality. Nonlocal entanglement does not, and can not, involve an instantaneous influence.
      - yourenotsmart 5y ago
        You're arguing about something I'm not. The fact the waveform collapse appears instant doesn't violate causality at all. To break causality the collapse should be usable for communication, but it isn't. Yet it does appear instant, from all data we have. And that is not explained at all by our current understanding of quantum physics. It's "explained" as in we observe it, and can calculate how it works. But it's not explained as in how come that collapse happens instantly regardless of the distance. Therefore spacetime is either non-local, or we need another interpretation like many worlds that avoids the collapse and replaced it with neverending waveform, or something else. To say there's nothing here left to explain to me says you don't understand the problem in the first place. Either that, or you know something that leading physicists don't know right now.
        codethief 5y ago
        > Therefore spacetime is either non-local You might be interested in Travis Norsen's book "Foundations of Quantum Mechanics - An Exploration of the Physical Meaning of Quantum Theory" (2017) where he argues that quantum mechanics (in the orthodox Copenhagen interpretation) is not even non-local because the word doesn't even make sense in quantum mechanics.
- phscguy 5y ago
  It doesn't solve it. The action at a distance is not a problem with quantum theory. This is because it both is predicted by quantum theory and also observed in experiment. It is a fact of nature and must be accepted. Entanglement does not violate special relativity, and QFT happily merges both special relativity and quantum mechanics into one theory without issue.
  - yourenotsmart 5y ago
    "It's a fact of nature and must be expected" is the Copenhagen interpretation. Which is well-known to be incomplete and basically referred to as the "shut up and calculate" interpretation. The issue still remains.
    - vadansky 5y ago
      My high school science teacher explained it well with something a long the lines of "You can't grog it since you're big and stupid". Technically even normal measurement is action at a distance since we're collapsing a wave function that covers an area into a small point instantaneously. In entanglement the wave function is just spread out even further, but if we already accept wave functions over areas for particles it's not that big a stretch. Problem is that it obviously doesn't jive with what we have experience with everyday as big stupid people.
      - yourenotsmart 5y ago
        The issue of waveform collapse is fundamentally different than "it collapses". The fact it collapses is the easy part. The hard part is why it collapses simultaneously for two particles which can't communicate at the same(ish) moment due to light speed limitations.
    - boublepop 5y ago
      No, there is no issue. And the Copenhagen interpretation isn’t just “shut up and calculate” even though people may humorously refer to it as such. Basically what we have is that if you had a model of reality where small invisible angels tug at the feet of people and thereby cause gravity, if I come along with a Newtonian model of gravity, then you can’t dismiss my theory because “It doesn’t explain the angels!” Classically you want to assume that there are small local particles traveling along always in one specific position with one specific speed. QM instead postulates that this is not the case, but that reality can accurately be described based on probability wave distributions that when we do the math accurately predict your particle observations. But does so without explaining “the angels” and without bridging what you perceive as a gap between localized classical particles being the ground truth and what QM postulates. But the incorrect notion of localized particles behaving classically was never a universal truth that we have to include. It was just an incorrect model that seems to give us correct predictions in some but not all cases. QM now gives us both the same predictions when they were correct, but also correct predictions in all the cases where it fails.
- auntienomen 5y ago
  The 'action at a distance' referred to here it not the one that crops up in entanglement. In entanglement, the nature of our knowledge changes instantaneously when we make a measurement. The action at a distance problem Weinberg refers to relates to the speed at which one object influences a distant one. For example, in Newtonian gravity, with its V = GM1M2/r potential, the speed of propagation of gravity is effectively infinite.
- yann2 5y ago
  Heres a better intro - https://m.youtube.com/watch?v=zNVQfWC_evg https://m.youtube.com/watch?v=zNVQfWC_evg
auntienomen 5y ago
This article came out about the same time as the first volume of Weinberg's textbook The Quantum Theory of Fields. It's a nice semi-technical distillation of the core argument in his textbook: We see a world made of particles, but Poincare invariance and cluster decomposition (the fact that widely separated systems behave approximately independently) force us to organize these particles into fields.

38 points by nabla9 5y ago

nabla9 5y ago

https://arxiv.org/abs/hep-th/9702027 https://arxiv.org/abs/hep-th/9702027

tpfour 5y ago
@dang the article link is broken, the above works for me.
- dang 5y ago
  Fixed now. Thanks!

deleted 5y ago

[deleted]

deleted 5y ago

[deleted]

yourenotsmart 5y ago

As I'm not deep into this matter, can someone share how this theory solves the "action from distance" problem of entanglement? The collapse is described as either non-local, or non-existent (many worlds intepretation). I'm not sure adding fields about every particle solves this, but again, I'm ignorant on the details.

deleted 5y ago
[deleted]
leephillips 5y ago
There is no action at a distance problem, because no influence travels between the particles.
- yourenotsmart 5y ago
  The whole point of entanglement is that measuring one of the pair affects the outcome for the other one. Whether the influence "travels" between the particles is subject to debate, but there is influence.
  - leephillips 5y ago
    Sorry, no. There is no debate on this point. The confusion here may be merely linguistic, but there can be no instantaneous influence, in the sense of a causal effect or transmission of information, from one particle to a distant entangled particle. If there were, that would violate everything we know is true about causation, and therefore reality. Nonlocal entanglement does not, and can not, involve an instantaneous influence.
    - yourenotsmart 5y ago
      You're arguing about something I'm not. The fact the waveform collapse appears instant doesn't violate causality at all. To break causality the collapse should be usable for communication, but it isn't. Yet it does appear instant, from all data we have. And that is not explained at all by our current understanding of quantum physics. It's "explained" as in we observe it, and can calculate how it works. But it's not explained as in how come that collapse happens instantly regardless of the distance. Therefore spacetime is either non-local, or we need another interpretation like many worlds that avoids the collapse and replaced it with neverending waveform, or something else. To say there's nothing here left to explain to me says you don't understand the problem in the first place. Either that, or you know something that leading physicists don't know right now.
      - codethief 5y ago
        > Therefore spacetime is either non-local You might be interested in Travis Norsen's book "Foundations of Quantum Mechanics - An Exploration of the Physical Meaning of Quantum Theory" (2017) where he argues that quantum mechanics (in the orthodox Copenhagen interpretation) is not even non-local because the word doesn't even make sense in quantum mechanics.
phscguy 5y ago
It doesn't solve it. The action at a distance is not a problem with quantum theory. This is because it both is predicted by quantum theory and also observed in experiment. It is a fact of nature and must be accepted. Entanglement does not violate special relativity, and QFT happily merges both special relativity and quantum mechanics into one theory without issue.
- yourenotsmart 5y ago
  "It's a fact of nature and must be expected" is the Copenhagen interpretation. Which is well-known to be incomplete and basically referred to as the "shut up and calculate" interpretation. The issue still remains.
  - vadansky 5y ago
    My high school science teacher explained it well with something a long the lines of "You can't grog it since you're big and stupid". Technically even normal measurement is action at a distance since we're collapsing a wave function that covers an area into a small point instantaneously. In entanglement the wave function is just spread out even further, but if we already accept wave functions over areas for particles it's not that big a stretch. Problem is that it obviously doesn't jive with what we have experience with everyday as big stupid people.
    - yourenotsmart 5y ago
      The issue of waveform collapse is fundamentally different than "it collapses". The fact it collapses is the easy part. The hard part is why it collapses simultaneously for two particles which can't communicate at the same(ish) moment due to light speed limitations.
  - boublepop 5y ago
    No, there is no issue. And the Copenhagen interpretation isn’t just “shut up and calculate” even though people may humorously refer to it as such. Basically what we have is that if you had a model of reality where small invisible angels tug at the feet of people and thereby cause gravity, if I come along with a Newtonian model of gravity, then you can’t dismiss my theory because “It doesn’t explain the angels!” Classically you want to assume that there are small local particles traveling along always in one specific position with one specific speed. QM instead postulates that this is not the case, but that reality can accurately be described based on probability wave distributions that when we do the math accurately predict your particle observations. But does so without explaining “the angels” and without bridging what you perceive as a gap between localized classical particles being the ground truth and what QM postulates. But the incorrect notion of localized particles behaving classically was never a universal truth that we have to include. It was just an incorrect model that seems to give us correct predictions in some but not all cases. QM now gives us both the same predictions when they were correct, but also correct predictions in all the cases where it fails.
auntienomen 5y ago
The 'action at a distance' referred to here it not the one that crops up in entanglement. In entanglement, the nature of our knowledge changes instantaneously when we make a measurement. The action at a distance problem Weinberg refers to relates to the speed at which one object influences a distant one. For example, in Newtonian gravity, with its V = GM1M2/r potential, the speed of propagation of gravity is effectively infinite.
yann2 5y ago
Heres a better intro - https://m.youtube.com/watch?v=zNVQfWC_evg https://m.youtube.com/watch?v=zNVQfWC_evg

auntienomen 5y ago

This article came out about the same time as the first volume of Weinberg's textbook The Quantum Theory of Fields. It's a nice semi-technical distillation of the core argument in his textbook: We see a world made of particles, but Poincare invariance and cluster decomposition (the fact that widely separated systems behave approximately independently) force us to organize these particles into fields.