3 ms·

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

by 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.

naasking 8y ago
> If you take spin, which can't exist by itself in classical physics There are literally dozens of papers you can readily find that reproduce spin predictions using classical statistical mechanics. You denounce this as "model building", but every physical theory was model building. > Try to build a classical theory of the photoelectric effect, it's impossible yet it opens the door of your elevator. http://adsabs.harvard.edu/abs/2009APS..TSS.C1003E http://adsabs.harvard.edu/abs/2009APS..TSS.C1003E
- mnl 8y ago
  No, a Texas section APS meeting presentation about some fun academic exercise is simply some fun academic exercise for a Texas section APS meeting. If you're not familiar with these things, well this is a profession, it takes more than googling stuff. This is exactly what I was talking about, you consider that your atom has energy levels and then you plug in classical EM fields, sometimes that's good enough to get somewhere. But it can never be classical because there's no way to have energy levels in a classical atom. Just take one positive charge and one negative charge (let's call electron to this one), then use Maxwell equations (you need the Liénard–Wiechert potentials) and try to calculate the energy of the electron, now it has a trajectory and everything. It's not for the faint-hearted (you have to learn classical electrodynamics first -whose consistency problem BTW has only been solved recently, or at least that's what Rohrlich thinks about it-). You won't get discrete energy levels, you won't even get a stable atom. Precisely this was the starting point of QM: Bohr coming up with a quantization rule for angular momentum and magical orbits where the accelerated classical electron does not radiate. Check "old quantum theory". These problems were solved almost a century ago, the thing is people don't study enough or apparently have forgotten about them. They simply consider the QM postulates and then wonder why all that happened. Well, don't start the house by the roof, study what you should know already, the answers are there.
  - naasking 8y ago
    > This is exactly what I was talking about, you consider that your atom has energy levels and then you plug in classical EM fields, sometimes that's good enough to get somewhere. But it can never be classical because there's no way to have energy levels in a classical atom. And this is what I'm talking about: this conversation is becoming an ever-shrinking pocket of you claiming, "well that's not classical", or "that can't be done in a classical theory", and me demonstrating that it has been done, toy theory or not. I have no problem with discrete theories, I have a problem with people claiming something can't be done when it most assuredly can. As for your derivation of old QM, I don't see how this is relevant. You seem stuck in the mindset that "classical theory" means building off of some known classical theory. As I've been saying, this isn't necessarily true. My discussion of classical theories are about the class of theories with various properties that differentiate it from something like QM. People working on Bohmian mechanics are perfectly familiar with the history of quantum mechanics, see [1]. This doesn't somehow negate the fact that Bohmian mechanics can be viewed as a classical theory that nevertheless reproduces the predictions of QM. And it solves the radiating electron orbit problem just fine. Again, you're stuck thinking about this like classical mechanics rather than as a different classical theory that reduces to classical mechanics in the limit. [1] https://arxiv.org/pdf/1206.1084.pdf https://arxiv.org/pdf/1206.1084.pdf

by mnl 8y ago

naasking 8y ago

> If you take spin, which can't exist by itself in classical physics There are literally dozens of papers you can readily find that reproduce spin predictions using classical statistical mechanics. You denounce this as "model building", but every physical theory was model building. > Try to build a classical theory of the photoelectric effect, it's impossible yet it opens the door of your elevator. http://adsabs.harvard.edu/abs/2009APS..TSS.C1003E http://adsabs.harvard.edu/abs/2009APS..TSS.C1003E

mnl 8y ago
No, a Texas section APS meeting presentation about some fun academic exercise is simply some fun academic exercise for a Texas section APS meeting. If you're not familiar with these things, well this is a profession, it takes more than googling stuff. This is exactly what I was talking about, you consider that your atom has energy levels and then you plug in classical EM fields, sometimes that's good enough to get somewhere. But it can never be classical because there's no way to have energy levels in a classical atom. Just take one positive charge and one negative charge (let's call electron to this one), then use Maxwell equations (you need the Liénard–Wiechert potentials) and try to calculate the energy of the electron, now it has a trajectory and everything. It's not for the faint-hearted (you have to learn classical electrodynamics first -whose consistency problem BTW has only been solved recently, or at least that's what Rohrlich thinks about it-). You won't get discrete energy levels, you won't even get a stable atom. Precisely this was the starting point of QM: Bohr coming up with a quantization rule for angular momentum and magical orbits where the accelerated classical electron does not radiate. Check "old quantum theory". These problems were solved almost a century ago, the thing is people don't study enough or apparently have forgotten about them. They simply consider the QM postulates and then wonder why all that happened. Well, don't start the house by the roof, study what you should know already, the answers are there.
- naasking 8y ago
  > This is exactly what I was talking about, you consider that your atom has energy levels and then you plug in classical EM fields, sometimes that's good enough to get somewhere. But it can never be classical because there's no way to have energy levels in a classical atom. And this is what I'm talking about: this conversation is becoming an ever-shrinking pocket of you claiming, "well that's not classical", or "that can't be done in a classical theory", and me demonstrating that it has been done, toy theory or not. I have no problem with discrete theories, I have a problem with people claiming something can't be done when it most assuredly can. As for your derivation of old QM, I don't see how this is relevant. You seem stuck in the mindset that "classical theory" means building off of some known classical theory. As I've been saying, this isn't necessarily true. My discussion of classical theories are about the class of theories with various properties that differentiate it from something like QM. People working on Bohmian mechanics are perfectly familiar with the history of quantum mechanics, see [1]. This doesn't somehow negate the fact that Bohmian mechanics can be viewed as a classical theory that nevertheless reproduces the predictions of QM. And it solves the radiating electron orbit problem just fine. Again, you're stuck thinking about this like classical mechanics rather than as a different classical theory that reduces to classical mechanics in the limit. [1] https://arxiv.org/pdf/1206.1084.pdf https://arxiv.org/pdf/1206.1084.pdf