4 ms·

According to modern QFT, there are no particles except as an approximation. There are no fields except as mathematical formalisms. There's no locality. There is

by nine_k 2y ago

According to modern QFT, there are no particles except as an approximation. There are no fields except as mathematical formalisms. There's no locality. There is instead some kind of interaction of graph nodes, representing quantum interactions, via "entanglement" and "decoherence". In this model, there are no "split particle" paradoxes, because there are no entities that resemble the behavior of macroscopic bodies, with our intuitions about them. Imagine a Fortran program, with some neat index-based FOR loops, and some per-element computations on a bunch of big arrays. When you look at its compiled form, you notice that the neat loops are now something weird, produced by automatic vectorization. If you try to find out how it runs, you notice that the CPU not only has several cores that run parts of the loop in parallel, but the very instructions in one core run out of order, while still preserving the data dependency invariants. "But did the computation of X(I) run before or after the computation of X(I+1)?!", you ask in desperation. You cannot tell. It depends. The result is correct though, your program has no bugs and computes what it should. It's counter-intuitive, but the underlying hardware reality is counter-intuitive. It's not illogical or paradoxical though.

soup10 2y ago
just because QFT follows an internal logic, doesn't mean the jump from macro physics to quantum physics itself is logical. In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.
- perlgeek 2y ago
  The Universe is fundamentally quantum in nature; if anything, we'd need a model that explains why classical physics works so well most of the time.
  - soup10 2y ago
    i started writing a response about how the human brain is designed to operate in an environment where classical physics is the norm, so we need to bridge the deviations from that if we are to really understand the world. But I don't know how much that's really true if you consider neural biology and I won't claim to know where quantum stops and classical begins as it relates to brain function.
    - kybernetikos 2y ago
      I do think there's something to this approach though - our sensory organs and processing ability are not abstract powers of understanding the universe - they developed exactly to give us enhanced survival chances. We should not expect to even be able to detect (let alone intuitively understand) aspects of reality that can't be used for survival.
      - hnlmorg 2y ago
        I do understand the point you’re making but my counter argument to that would be that physics hasn’t relied on our sensory input for a hundred years or more. It’s been almost entirely based on maths and careful measurements from machined instruments purpose built for observing phenomena. So at this point you’d hope the limitations of our biological senses would have been long surpassed.
        anon84873628 2y ago
        It's impossible to surpass the physical limits of our biological senses. Can a person blind from birth form visual mental images? We can try to retrain and reuse the sense for other purposes though. I'm reminded of that film "The Zero Theorem".
      - Xmd5a 2y ago
        >our [...] processing ability are not abstract powers of understanding the universe Neural nets are called universal approximators for a reason. If what you guys are discussing is true, then a neural net would not be able to learn from a dataset about quantum experiments. I doubt this is the case. Also there is quantum cognition, and by that I mean the fact some researchers figured out a lot of puzzling results from experimental cognitive science seem to make more sense once analyzed from a quantum perspective.
        GoblinSlayer 2y ago
        Humans are affected by prejudice, so learning straight from a dataset is easier said than done, but possible, yes.
  - pinkmuffinere 2y ago
    As a naïve fool with no understanding of quantum physics, I want to take a stab at this! Here’s my hypothesis: Consider a world in which everything is “very quantum”, and there are no easy approximations which can generally be relied on. In such a world, our human pattern-matching behavior would be really useless, and “human intelligence” in the form we’re familiar with will have no evolutionary advantage. So the only setting in which we evolve to be confused by this phenomena is one where simple approximations do work for the scales we occupy. Sincerely, I don’t think this argument is super good. But it’s fun to propose, and maybe slightly valid.
    - groestl 2y ago
      > have no understanding of quantum physics But you know about the Anthropic Principle :)
    - movpasd 2y ago
      This is called the anthropic principle. I personally have objections to it, specifically that due to emergence it is hard to make definitive statements about what complex phenomena may emerge in alternate universes. However, it's taken seriously by many philosophers of physics and certainly has merit.
      - GoblinSlayer 2y ago
        Isn't that argument from ignorance? You can consider a class of physics similar enough to our physics, it should give enough space for research.
        movpasd 2y ago
        My point is that it isn't possible to determine the emergent behaviour of a complex system from first principles. So arguments of the type "these physics don't result in atoms being produced, so life can't emerge" doesn't imply that other complex structures _like_ life don't emerge.
        2y ago
  - xtacy 2y ago
    We do have a model. That’s statistical physics. Any standard course goes over various derivations of classical physics laws (Newtonian dynamics) from quantum mechanics.
    - perlgeek 2y ago
      I did study physics, and our statistical physics lecture only derived thermodynamic laws. We also had a somewhat shoddy derivation of Newton's Laws from the Schrödinger equation, but wasn't really satisfactory either, because it doesn't really answer the question when I can treat things classically. What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant). (Another thing that drove me nuts in our QM classes where that "observations" where introduced as: a classical system couples to a quantum system. Which presupposes the existence of classical systems, without properly defining or delineating them. And here QM was supposed to be the more fundamental theory).
      - GoblinSlayer 2y ago
        The error is usually taken as ratio of wavelength to your desired precision, but in general depends on your use case, sometimes you have full precision all the way down, sometimes you have insufficient precision on astronomic scale. Quantum physics doesn't have an absolute scale cutoff.
      - vonneumannstan 2y ago
        >What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant). There are plenty of ways to do this and things like Wigner functions literally calculate quantum corrections to classical systems. But generally if you can't even measure a system before it's quantum state decoheres then it's quantum status is pretty irrelevant. I.e. the time it takes for a 1 micrometer wide piece of dust to decohere is ~10^-31 s and it takes a photon ~10^12s to cross it's diameter. So it decoheres 10 billion billion times faster that a photon could even cross it.
- vonneumannstan 2y ago
  >In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics. I think you have this backwards. QM IS the law of the universe and Classical Physics is just a high mass low energy approximation of it. In any case there doesn't need to be a logical explanation at all, the laws of physics are as they are. Why is the value of the fine structure constant what it is?
cjfd 2y ago
This is incorrect. There are particles. They are excitations in the field. There still is the 'split particle paradox' because QFT does not solve the measurement problem. The 'some kind of interaction of graph nodes' by which I am guessing you are referring to Feynman diagrams are not of a fundamental nature. They are an approximation known as 'perturbation theory'.
- movpasd 2y ago
  I think what they must be referring to is the fact that particles are only rigorously defined in the free theory. When coupling is introduced, how the free theory relates to the coupled theory depends on heuristic/formal assumptions. We're leaving my area of understanding, but I believe Haag's theorem shows that the naïve approach, where the interacting and free theories share a Hilbert space, completely fails -- even stronger than that, _no_ Hilbert space could even support an interacting QFT (in the ways required by scattering theory). This is a pretty strong argument against the existence of particles except as asymptotic approximations. Since we don't have consensus on a well-defined, non-perturbative gauge theory, mathematically speaking it's difficult to make any firm statements about what states "exist" in absolute. (I'm certain that people working on the various flavours of non-perturbative (but still heuristic) QFT -- like lattice QFT -- would have more insights about the internal structure of non-asymptotic interactions.)
  - codethief 2y ago
    > This is a pretty strong argument against the existence of particles except as asymptotic approximations. I think it's also a pretty strong argument against the mathematical well-definedness of typical (interacting) QFTs in the first place.
  - westurner 2y ago
    Though it doesn't resolve whether a "quanta" is a particle or a measurable convergence of waves, Electrons and Photons are observed with high speed imaging. "Quantum microscopy study makes electrons visible in slow motion" https://news.ycombinator.com/item?id=40981054 https://news.ycombinator.com/item?id=40981054 There exist single photon emitters and single photon detectors. Qualify that there are single photons if there are single photon emitters: Single-photon source: https://en.wikipedia.org/wiki/Single-photon_source https://en.wikipedia.org/wiki/Single-photon_source QFT is not yet reconciled with (n-body) [quantum] gravity, which it has 100% error in oredicting. random chance. TOD IIRC, QFT cannot explain why superfluid helium walks up the sides of a container against gravity, given the mass of each particle/wave of the superfluid and of the beaker and the earth, sun, and moon; though we say that gravity at any given point is the net sum of directional vectors acting upon said given point, or actually gravitational waves with phase and amplitude. You said "gauge theory", "Topological gauge theory of vortices in type-III superconductors" https://news.ycombinator.com/item?id=41803662 https://news.ycombinator.com/item?id=41803662 From https://news.ycombinator.com/context?id=43081303 https://news.ycombinator.com/context?id=43081303 .. https://news.ycombinator.com/item?id=43310933 https://news.ycombinator.com/item?id=43310933 : > Probably not gauge symmetry there, then.
GoblinSlayer 2y ago
>There's no locality. How so? QFT is Lorentz invariant. Even has such a thing as the norm flux.
- Ygg2 2y ago
  > Lorentz invariance is also violated in QFT assuming non-zero temperature. Source: https://en.wikipedia.org/wiki/Lorentz_covariance https://en.wikipedia.org/wiki/Lorentz_covariance
  - r0uv3n 2y ago
    If you couple your system to a heat path that is at rest wrt a specific Lorentz frame, you of course lose Lorentz incariance. On the other hand the lagrangian of the standard model itself is to my knowledge fully Lorentz invariant.
  - GoblinSlayer 2y ago
    I don't know what they talk about there, but it sounds like some kind of thermodynamic approximation is involved there. Does thermodynamics survive Lorentz transformation?
- nine_k 2y ago
  My bad; QFT actually postulates locality. I was thinking about the casual set theory which strives to solve some of the QFT's difficulties, and where locality is an emergent / statistical phenomenon rather than a postulated condition.
WhitneyLand 2y ago
False in multiple ways. QFT doesn’t discard local fields and replace them with only nonlocal graph nodes. Maybe this is coming from some speculative quantum gravity ideas.
pklausler 2y ago
s/FOR/DO/

by nine_k 2y ago

soup10 2y ago

just because QFT follows an internal logic, doesn't mean the jump from macro physics to quantum physics itself is logical. In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.

perlgeek 2y ago
The Universe is fundamentally quantum in nature; if anything, we'd need a model that explains why classical physics works so well most of the time.
- soup10 2y ago
  i started writing a response about how the human brain is designed to operate in an environment where classical physics is the norm, so we need to bridge the deviations from that if we are to really understand the world. But I don't know how much that's really true if you consider neural biology and I won't claim to know where quantum stops and classical begins as it relates to brain function.
  - kybernetikos 2y ago
    I do think there's something to this approach though - our sensory organs and processing ability are not abstract powers of understanding the universe - they developed exactly to give us enhanced survival chances. We should not expect to even be able to detect (let alone intuitively understand) aspects of reality that can't be used for survival.
    - hnlmorg 2y ago
      I do understand the point you’re making but my counter argument to that would be that physics hasn’t relied on our sensory input for a hundred years or more. It’s been almost entirely based on maths and careful measurements from machined instruments purpose built for observing phenomena. So at this point you’d hope the limitations of our biological senses would have been long surpassed.
      - anon84873628 2y ago
        It's impossible to surpass the physical limits of our biological senses. Can a person blind from birth form visual mental images? We can try to retrain and reuse the sense for other purposes though. I'm reminded of that film "The Zero Theorem".
    - Xmd5a 2y ago
      >our [...] processing ability are not abstract powers of understanding the universe Neural nets are called universal approximators for a reason. If what you guys are discussing is true, then a neural net would not be able to learn from a dataset about quantum experiments. I doubt this is the case. Also there is quantum cognition, and by that I mean the fact some researchers figured out a lot of puzzling results from experimental cognitive science seem to make more sense once analyzed from a quantum perspective.
      - GoblinSlayer 2y ago
        Humans are affected by prejudice, so learning straight from a dataset is easier said than done, but possible, yes.
- pinkmuffinere 2y ago
  As a naïve fool with no understanding of quantum physics, I want to take a stab at this! Here’s my hypothesis: Consider a world in which everything is “very quantum”, and there are no easy approximations which can generally be relied on. In such a world, our human pattern-matching behavior would be really useless, and “human intelligence” in the form we’re familiar with will have no evolutionary advantage. So the only setting in which we evolve to be confused by this phenomena is one where simple approximations do work for the scales we occupy. Sincerely, I don’t think this argument is super good. But it’s fun to propose, and maybe slightly valid.
  - groestl 2y ago
    > have no understanding of quantum physics But you know about the Anthropic Principle :)
  - movpasd 2y ago
    This is called the anthropic principle. I personally have objections to it, specifically that due to emergence it is hard to make definitive statements about what complex phenomena may emerge in alternate universes. However, it's taken seriously by many philosophers of physics and certainly has merit.
    - GoblinSlayer 2y ago
      Isn't that argument from ignorance? You can consider a class of physics similar enough to our physics, it should give enough space for research.
      - movpasd 2y ago
        My point is that it isn't possible to determine the emergent behaviour of a complex system from first principles. So arguments of the type "these physics don't result in atoms being produced, so life can't emerge" doesn't imply that other complex structures _like_ life don't emerge.
        2y ago
- xtacy 2y ago
  We do have a model. That’s statistical physics. Any standard course goes over various derivations of classical physics laws (Newtonian dynamics) from quantum mechanics.
  - perlgeek 2y ago
    I did study physics, and our statistical physics lecture only derived thermodynamic laws. We also had a somewhat shoddy derivation of Newton's Laws from the Schrödinger equation, but wasn't really satisfactory either, because it doesn't really answer the question when I can treat things classically. What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant). (Another thing that drove me nuts in our QM classes where that "observations" where introduced as: a classical system couples to a quantum system. Which presupposes the existence of classical systems, without properly defining or delineating them. And here QM was supposed to be the more fundamental theory).
    - GoblinSlayer 2y ago
      The error is usually taken as ratio of wavelength to your desired precision, but in general depends on your use case, sometimes you have full precision all the way down, sometimes you have insufficient precision on astronomic scale. Quantum physics doesn't have an absolute scale cutoff.
    - vonneumannstan 2y ago
      >What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant). There are plenty of ways to do this and things like Wigner functions literally calculate quantum corrections to classical systems. But generally if you can't even measure a system before it's quantum state decoheres then it's quantum status is pretty irrelevant. I.e. the time it takes for a 1 micrometer wide piece of dust to decohere is ~10^-31 s and it takes a photon ~10^12s to cross it's diameter. So it decoheres 10 billion billion times faster that a photon could even cross it.
vonneumannstan 2y ago
>In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics. I think you have this backwards. QM IS the law of the universe and Classical Physics is just a high mass low energy approximation of it. In any case there doesn't need to be a logical explanation at all, the laws of physics are as they are. Why is the value of the fine structure constant what it is?

cjfd 2y ago

This is incorrect. There are particles. They are excitations in the field. There still is the 'split particle paradox' because QFT does not solve the measurement problem. The 'some kind of interaction of graph nodes' by which I am guessing you are referring to Feynman diagrams are not of a fundamental nature. They are an approximation known as 'perturbation theory'.

movpasd 2y ago
I think what they must be referring to is the fact that particles are only rigorously defined in the free theory. When coupling is introduced, how the free theory relates to the coupled theory depends on heuristic/formal assumptions. We're leaving my area of understanding, but I believe Haag's theorem shows that the naïve approach, where the interacting and free theories share a Hilbert space, completely fails -- even stronger than that, _no_ Hilbert space could even support an interacting QFT (in the ways required by scattering theory). This is a pretty strong argument against the existence of particles except as asymptotic approximations. Since we don't have consensus on a well-defined, non-perturbative gauge theory, mathematically speaking it's difficult to make any firm statements about what states "exist" in absolute. (I'm certain that people working on the various flavours of non-perturbative (but still heuristic) QFT -- like lattice QFT -- would have more insights about the internal structure of non-asymptotic interactions.)
- codethief 2y ago
  > This is a pretty strong argument against the existence of particles except as asymptotic approximations. I think it's also a pretty strong argument against the mathematical well-definedness of typical (interacting) QFTs in the first place.
- westurner 2y ago
  Though it doesn't resolve whether a "quanta" is a particle or a measurable convergence of waves, Electrons and Photons are observed with high speed imaging. "Quantum microscopy study makes electrons visible in slow motion" https://news.ycombinator.com/item?id=40981054 https://news.ycombinator.com/item?id=40981054 There exist single photon emitters and single photon detectors. Qualify that there are single photons if there are single photon emitters: Single-photon source: https://en.wikipedia.org/wiki/Single-photon_source https://en.wikipedia.org/wiki/Single-photon_source QFT is not yet reconciled with (n-body) [quantum] gravity, which it has 100% error in oredicting. random chance. TOD IIRC, QFT cannot explain why superfluid helium walks up the sides of a container against gravity, given the mass of each particle/wave of the superfluid and of the beaker and the earth, sun, and moon; though we say that gravity at any given point is the net sum of directional vectors acting upon said given point, or actually gravitational waves with phase and amplitude. You said "gauge theory", "Topological gauge theory of vortices in type-III superconductors" https://news.ycombinator.com/item?id=41803662 https://news.ycombinator.com/item?id=41803662 From https://news.ycombinator.com/context?id=43081303 https://news.ycombinator.com/context?id=43081303 .. https://news.ycombinator.com/item?id=43310933 https://news.ycombinator.com/item?id=43310933 : > Probably not gauge symmetry there, then.

GoblinSlayer 2y ago

>There's no locality. How so? QFT is Lorentz invariant. Even has such a thing as the norm flux.

Ygg2 2y ago
> Lorentz invariance is also violated in QFT assuming non-zero temperature. Source: https://en.wikipedia.org/wiki/Lorentz_covariance https://en.wikipedia.org/wiki/Lorentz_covariance
- r0uv3n 2y ago
  If you couple your system to a heat path that is at rest wrt a specific Lorentz frame, you of course lose Lorentz incariance. On the other hand the lagrangian of the standard model itself is to my knowledge fully Lorentz invariant.
- GoblinSlayer 2y ago
  I don't know what they talk about there, but it sounds like some kind of thermodynamic approximation is involved there. Does thermodynamics survive Lorentz transformation?
nine_k 2y ago
My bad; QFT actually postulates locality. I was thinking about the casual set theory which strives to solve some of the QFT's difficulties, and where locality is an emergent / statistical phenomenon rather than a postulated condition.

WhitneyLand 2y ago

False in multiple ways. QFT doesn’t discard local fields and replace them with only nonlocal graph nodes. Maybe this is coming from some speculative quantum gravity ideas.

pklausler 2y ago

s/FOR/DO/