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We think we see classical physics, but this micro/macro divide is not that clear-cut. It's more like a convenient textbook classification. For instance you can

by mnl 8y ago

We think we see classical physics, but this micro/macro divide is not that clear-cut. It's more like a convenient textbook classification. For instance you can build a very macro magnet which is not classical physics at all, you can't explain ferromagnetism without QM. Why don't we fall through the floor? that's not classical. Chemistry makes no sense without QM either. A classical world, albeit nice and easy to mirror with mental images involving differential equations of continuous stuff would fall to pieces. I like Bohm's book on Quantum Physics, but the pilot wave theory... well it doesn't explain anything new and the only problems it might solve are related to prejudices. Besides, the nature of that duplicity is never explained (and very ugly), so you're trading the ignorance you don't like for the one you like. In this kind of situations just go for the more simple and consistent theory as the only thing that matters is measurements. (There's also the problem of getting to some form of QFT with it. Bell has written about this elsewhere, it's not pretty but I'm sure somebody has arrived to a working model by now.)

naasking 8y ago
I think you might be using an overly simplistic meaning for "classical". There is nothing preventing a classical description of ferromagnetism, for instance [1]. Bohmian simulations of atoms has established stability despite the orbits, and would similarly entail the diamagnetism needed for this phenomenon, so classical theories can in fact explain it. > well it doesn't explain anything new and the only problems it might solve are related to prejudices. Correct, it actually explains everything old which prior interpretations like Copenhagen could not. I assume by "prejudices", you're referring to classical prejudices, but I think those prejudices are perfectly warranted. > In this kind of situations just go for the more simple and consistent theory as the only thing that matters is measurements. I disagree! John Bell formed the first no-go theorem because he was a fan of Bohmian mechanics. Without this focus on interpretations and quantum fundamentals, we may never have seen that landmark result or the series of subsequent no-go theorems which have all proven critical to our scientific understanding. Science and physics does not consist solely of measurements; considerations of ontology are at least equally important because they inform the thought experiments which really drive scientific revolutions. > Bell has written about this elsewhere, it's not pretty but I'm sure somebody has arrived to a working model by now There are a few workable models, but Bell was correct to say that non-locality was the unsolved problem of quantum mechanics. Other interpretations let you "solve" it by sweeping it under the rug, but it always rears its ugly head elsewhere. [1] https://physics.stackexchange.com/a/365970/43861 https://physics.stackexchange.com/a/365970/43861
- mnl 8y ago
  No, they can't explain ferromagnetism. Sometimes you use a semiclassical approximation, but that's a technical term. Don't look into the words used in Physics, this is not a social science, words in themselves don't carry an inherent meaning, they're just labels. You need spin to describe ferromagnetism, there's no room for that in a classical model of charges. I'm not opposed to alternative formulations of QM, but I'd rather prefer people would try to come up with more fundamental theories than QM (if possible) instead of trying to get the same results with something else because for some ideological reason they don't like standard QM. And it really is all about the measurements: Physics is an experimental science. You can think all you want and do crazy math and lots of gedanken, but we need at least a number and see what Nature thinks about it.
  - naasking 8y ago
    Did you even read the link I provided? I mean, you've basically said that the physics textbooks are wrong and used a bunch of words, which you yourself said I shouldn't trust, to dismiss them without evidence. > You need spin to describe ferromagnetism, there's no room for that in a classical model of charges. I suggest you actually read the link I provided. A classical theory has certain characteristics that distinguish it from something like a quantum theory. As I suspected in my initial reply to you, you have a bizarre notion of what constitutes a classical theory. Rest assured, a classical theory of spin and ferromagnetism is entirely possible. As for measurements, physics is not solely an experimental science. Yes, theory must match experiment because empirical knowledge is what we're after, but measurements alone do not advance this agenda.
    - mnl 8y ago
      Yes, I've checked it and understood what it said. If you take spin, which can't exist by itself in classical physics and use it in a semiclassical approximation you get a good enough approach for your problem that has workable maths. People do this all the time in quantum optics, you consider for instance quantum atoms and classical EM fields, because the whole machinery would be too much trouble. But that's not classical at all, it's just model building. With classical physics alone you wouldn't be able to calculate anything because it wouldn't provide the basic phenomenology. Try to build a classical theory of the photoelectric effect, it's impossible yet it opens the door of your elevator.
westoncb 8y ago
You're statements attempt to negate the GP's claim that the macro is classical by saying that the micro is not classical. But that's the whole larger question we're trying to answer here, so asserting an answer in one direction (micro is not classical) as a reply to some sub-point ends the possibility of a reasonable discussion. If there is a formulation of quantum mechanics which retains classical determinism, then just because it coheres so well with the behavior of everything we have directly observed (insofar as that's possible through the intermediaries of our senses) so far, it should be given a good amount of attention. > but the pilot wave theory... well it doesn't explain anything new and the only problems it might solve are related to prejudices. It can only be considered a prejudice if you're already pre-decided that non-classical physical descriptions are necessary before attempting to evaluate pilot wave theory. You're saying the prejudice is against non-classical behavior, right? Just put yourself in the shoes for a moment of a person judging (the modern formulations of) these theories, but at a point in time before any of them had a reputation (in an alternate history I mean). Would it be a prejudice to select from among essentially equivalent (predictive power) theories the one which doesn't force us to leave the then reigning classical paradigm? If Pilot Wave theory and the more established quantum theories have comparable predictive power, but pilot wave theory can do the same thing without introducing new classes of behavior never observed at the macro level, then that's a point in its favor via Occam's Razor if nothing else.
- mnl 8y ago
  No, I'm actually saying that everything is QM, and yet for some phenomena you can use instead of that some effective theories called classical physics, that have these nice properties of needing just calculus and vector analysis and being easy to draw. That, and they having been discovered before more explanatory theories make them very popular, but it doesn't mean the world is actually separated in domains. Usually physics is taught in terms of micro and macro descriptions, the macro applies to those situations likely to be adequately characterized by classical physics, but that's because Planck's constant is really small, not because there's such a thing as a classical world. Hence for macroscopical everyday stuff classical physics usually works reasonably well, but even there, right in front of you there are very obvious phenomena that can't be explained with classical physics. About the pilot wave theory, well it came after the standard formulation of QM, so people didn't choose between them, and it's quite a patchwork that brings its own complications. Apparently as it includes classical trajectories some people love it, ignoring that you put by hand the actual QM wavefunction to make it work, then you fix things to get the old stuff back. I find all that weirder than standard QM.

by mnl 8y ago

naasking 8y ago

I think you might be using an overly simplistic meaning for "classical". There is nothing preventing a classical description of ferromagnetism, for instance [1]. Bohmian simulations of atoms has established stability despite the orbits, and would similarly entail the diamagnetism needed for this phenomenon, so classical theories can in fact explain it. > well it doesn't explain anything new and the only problems it might solve are related to prejudices. Correct, it actually explains everything old which prior interpretations like Copenhagen could not. I assume by "prejudices", you're referring to classical prejudices, but I think those prejudices are perfectly warranted. > In this kind of situations just go for the more simple and consistent theory as the only thing that matters is measurements. I disagree! John Bell formed the first no-go theorem because he was a fan of Bohmian mechanics. Without this focus on interpretations and quantum fundamentals, we may never have seen that landmark result or the series of subsequent no-go theorems which have all proven critical to our scientific understanding. Science and physics does not consist solely of measurements; considerations of ontology are at least equally important because they inform the thought experiments which really drive scientific revolutions. > Bell has written about this elsewhere, it's not pretty but I'm sure somebody has arrived to a working model by now There are a few workable models, but Bell was correct to say that non-locality was the unsolved problem of quantum mechanics. Other interpretations let you "solve" it by sweeping it under the rug, but it always rears its ugly head elsewhere. [1] https://physics.stackexchange.com/a/365970/43861 https://physics.stackexchange.com/a/365970/43861

mnl 8y ago
No, they can't explain ferromagnetism. Sometimes you use a semiclassical approximation, but that's a technical term. Don't look into the words used in Physics, this is not a social science, words in themselves don't carry an inherent meaning, they're just labels. You need spin to describe ferromagnetism, there's no room for that in a classical model of charges. I'm not opposed to alternative formulations of QM, but I'd rather prefer people would try to come up with more fundamental theories than QM (if possible) instead of trying to get the same results with something else because for some ideological reason they don't like standard QM. And it really is all about the measurements: Physics is an experimental science. You can think all you want and do crazy math and lots of gedanken, but we need at least a number and see what Nature thinks about it.
- naasking 8y ago
  Did you even read the link I provided? I mean, you've basically said that the physics textbooks are wrong and used a bunch of words, which you yourself said I shouldn't trust, to dismiss them without evidence. > You need spin to describe ferromagnetism, there's no room for that in a classical model of charges. I suggest you actually read the link I provided. A classical theory has certain characteristics that distinguish it from something like a quantum theory. As I suspected in my initial reply to you, you have a bizarre notion of what constitutes a classical theory. Rest assured, a classical theory of spin and ferromagnetism is entirely possible. As for measurements, physics is not solely an experimental science. Yes, theory must match experiment because empirical knowledge is what we're after, but measurements alone do not advance this agenda.
  - mnl 8y ago
    Yes, I've checked it and understood what it said. If you take spin, which can't exist by itself in classical physics and use it in a semiclassical approximation you get a good enough approach for your problem that has workable maths. People do this all the time in quantum optics, you consider for instance quantum atoms and classical EM fields, because the whole machinery would be too much trouble. But that's not classical at all, it's just model building. With classical physics alone you wouldn't be able to calculate anything because it wouldn't provide the basic phenomenology. Try to build a classical theory of the photoelectric effect, it's impossible yet it opens the door of your elevator.

westoncb 8y ago

You're statements attempt to negate the GP's claim that the macro is classical by saying that the micro is not classical. But that's the whole larger question we're trying to answer here, so asserting an answer in one direction (micro is not classical) as a reply to some sub-point ends the possibility of a reasonable discussion. If there is a formulation of quantum mechanics which retains classical determinism, then just because it coheres so well with the behavior of everything we have directly observed (insofar as that's possible through the intermediaries of our senses) so far, it should be given a good amount of attention. > but the pilot wave theory... well it doesn't explain anything new and the only problems it might solve are related to prejudices. It can only be considered a prejudice if you're already pre-decided that non-classical physical descriptions are necessary before attempting to evaluate pilot wave theory. You're saying the prejudice is against non-classical behavior, right? Just put yourself in the shoes for a moment of a person judging (the modern formulations of) these theories, but at a point in time before any of them had a reputation (in an alternate history I mean). Would it be a prejudice to select from among essentially equivalent (predictive power) theories the one which doesn't force us to leave the then reigning classical paradigm? If Pilot Wave theory and the more established quantum theories have comparable predictive power, but pilot wave theory can do the same thing without introducing new classes of behavior never observed at the macro level, then that's a point in its favor via Occam's Razor if nothing else.

mnl 8y ago
No, I'm actually saying that everything is QM, and yet for some phenomena you can use instead of that some effective theories called classical physics, that have these nice properties of needing just calculus and vector analysis and being easy to draw. That, and they having been discovered before more explanatory theories make them very popular, but it doesn't mean the world is actually separated in domains. Usually physics is taught in terms of micro and macro descriptions, the macro applies to those situations likely to be adequately characterized by classical physics, but that's because Planck's constant is really small, not because there's such a thing as a classical world. Hence for macroscopical everyday stuff classical physics usually works reasonably well, but even there, right in front of you there are very obvious phenomena that can't be explained with classical physics. About the pilot wave theory, well it came after the standard formulation of QM, so people didn't choose between them, and it's quite a patchwork that brings its own complications. Apparently as it includes classical trajectories some people love it, ignoring that you put by hand the actual QM wavefunction to make it work, then you fix things to get the old stuff back. I find all that weirder than standard QM.