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Is quantum theory really necessary?

  1. Apr 11, 2008 #1
    I often encounter statements to the effect that classical physics cannot describe processes at the atomic and subatomic level. I also understand fully well that no one ever has to date successfully described these quantum processes, even the most basic ones, using classical physics. But that something has never been done is not a proof of impossibility, obviously. So, what are the best arguments for the universe being essentially non-classical? I would like to find out if there are some I don't know, and whether I can sustain an argument that the position that quantum theory is unique and essential is no more than an observation that there is no classical description that works.

    To start it off I will offer the problem of radiative decay of the classical atom. This is one of several arguments for essentialness of quantum theory I can think of off the top of my head. If we consider the Rutherford (i.e., planetary) model of hydrogen where the electron orbits the proton under Coulomb attraction, in Keplerian orbits, it is commonly claimed that this system must decay due to radiation damping. For highly-excited states, such as in Rydberg atom experiments, this type of radiative decay is a good description of observations. Yet obviously atoms do not decay beyond a ground state. You are probably wondering how I could possibly rationally propose that classical physics could nonetheless result in a stable and nonradiative atom. Yet not only do I think it plausible, I even think there is a fair amount of progress in constructing just such a model.

    The reason I believe it's rash to conclude quantum theory is unassailably fundamental to describing the atom, is that even the simplest atom, hydrogen (or even simpler still, positronium) has not been solved in classical electrodynamics. This is not disputed in the literature. The electromagnetic two-body problem has not been solved and people are publishing papers on it in Physical Review E and Jour Math Phys and on arxiv.org as recently as a few weeks ago. The problem is rigorously handling the effect of propagation delay, as well as the radiation damping force. The former leads to functional differential equations of motion rather than ODEs, while the latter leads to third-order-in-time equations and run-away solutions. The latter problem is in Jackson (see the last chapter, the Abraham-Lorentz equation, or for that matter Abraham-Lorentz force on wikipedia). Raju is openly suggesting in a 2004 Foundations of Physics paper (here as #4 with his new one as #3: [I had to remove the links due to not enough posts yet but search c k raju on arxiv dot org]) that the origin of quantum behavior is potentially due to that the proper equations of motion of the EM two-body problem are functional differential equations, not ordinary ones. De Luca has done a series expansion of the full classical electrodynamic Lagrangian for hydrogen and kept only the linear terms and solved it to find stable (i.e. non-radiative) orbits at approximately the proper energy levels, and an explanation for monochromaticity of transition radiation, and all without introducing Planck's constant. This was in Phys Rev E in 2006, also on arxiv here: [search Jayme De Luca on arxiv dot org]. What De Luca did seems to me is essentially what was proposed by Hestenes at least as early as the 90s, that zitterbewegung resonances could explain both atomic stability and monochromaticity of transition radiation (these links are tougher although Hestenes has two recent posts to arxiv I intended to post links to some papers elsewhere. The first is "Zitterbewegung in Radiative Processes" which I did link to in my blog entry #2. The other is a page with about 10 different quantum-theory and zitterbewegung related papers on Hestenes' personal cite, which I found from the wikipedia article on him.)

    So, seems to me, the facts that no one has done it, and that until recently no one had any ideas for how to do it, isn't a safe basis for claiming that classical physics can't yield up quantum behavior. Personally I suspect it can and will but I am interested to know if there are some convincing arguments I am overlooking. I look forward to your responses.
     
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  3. Apr 11, 2008 #2

    Mentz114

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    The two-slit interference is difficult to explain classically. So are the results of the Aspect experiments.

    I know QM can be difficult to understand, but it gives very close agreement with experiment.

    What I cannot, and never will understand, is why classical mechanics is put on a pedestal, and QM disbelieved in principle, by cranks like De Luca.
     
  4. Apr 11, 2008 #3

    reilly

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    Almost solving for the hydrogen atom's characteristics by classical theory is interesting, but totally unconvincing. Consider that the basic phenomena of atomic spectra and the periodic table, diffraction of electrons, photoelectric effect, then later spin and spin-orbit corrections, nuclear structure, and on and on. These were all "solved" by quantum theory in a period of about 30 years. After almost one hundred years, if the best that a classical approach can give us is a partial theory of the hydrogen atom, then we are talking virtually no progress. So, when will the classical QMers get to Fermi-Thomas or Hartree Fock techniques? -- sorta' useful in atomic physics. How about quark containment?

    I suppose it might be possible to do some QM via classical approaches. But, crudely speaking, why bother? --unless we run into situations that defy QM. Remember, in order to displace QM, you'll have to reproduce all of QM's applications and theory, and bring new things to the table. QM works admirably; classical theory in the quantum domain does not.
    Regards,
    Reilly Atkinson
     
  5. Apr 11, 2008 #4
    You illustrate my point. These things are difficult to explain classically. No one has explained them to date with classical physics. Therefore they cannot be explained classically? Is that your position?

    I don't disbelieve quantum theory "in principle" or in practice, and I don't discount that it's very accurate and useful and important. I merely question that it is essential and fundamental to the description of certain systems.

    I challenge you to support your contention that QM is "disbelieved in principle" by Professor De Luca, from his published work. I believe this contention is false, and I know he is no crank. Why would you resent that he would attempt to solve an important unsolved problem of physics, the electromagnetic two-body problem? He is continuing effort that engaged many great physicists including Abraham, Lorentz, Dirac, Eliezer, Feynman and many more.
     
  6. Apr 11, 2008 #5

    Mentz114

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    DaveLush,
    I'm willing to give De Luca the benefit of some doubt, and I withdraw my categorization of 'crank'.

    The electromagnetic two-body problem can be approached with equal validity either classically or with quantised fields, so it is irrelevant in any argument about QM being necessary.

    You must admit that the title of your thread is invites controversy, especially in view of the list of problems that classical mechanics cannot describe in reilly's response.

    M
     
  7. Apr 11, 2008 #6

    ZapperZ

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    If you can use classical mechanics to describe superconductivity, field emission, and the fractional quantum hall effect phenomena, then I'd say there's something to pay attention to. Till then I don't think there any kind of evidence that what you have claimed is valid.

    Zz.
     
  8. Apr 11, 2008 #7
    I don't expect that QM is going to disappear, if it is discovered there's a classical basis for it. It will probably remain formally the most efficient way to solve complicated systems. All that has to be done is to provide a new basis for it. I think it's worth the bother because there are problems with it as it stands. There's little consensus on what is the interpretation of the wavefunction, for example. Wouldn't it be nice if there were an explanation so compelling there could be a general consensus? What if it was a new interpretation? How can a deeper understanding not be worthwhile? You can say you don't think it will happen or that you don't want to work on it personally, but I don't understand why somebody would begrudge the trying.

    I also don't expect what's been done by De Luca or Hestenes or others to be found convincing as yet. There're problems with all of it. Big problems it's true. But, I think it's provocative, and flies in the face of what at least I was taught as the justification for QM as an essential theory. This thread isn't about convincing everybody to jump on the classical physics bandwagon, though, it's about finding convincing arguments that classical physics can't provide a basis for quantum behavior. So far I'm still not hearing anything other than that nobody has done it, therefore it can't be done. I think there are better ones than that, at the deeper levels like string theory or loop quantum gravity, and that ultimately the universe is probably a discrete (i.e. not continuous) mathematical structure. Planck's constant probably *is* fundamental. Maybe De Luca getting it back from other constants that aren't thought to involve it means that they do involve it and that would be something I would find interesting. Plausibly though it has been carried a little too far such as in assertions that angular momentum cannot even exist except in multiples of h-bar. I continue to think it would be worthwhile to know if such as we have been taught is not the case.

    Hestenes argues explicitly that the uncertainty principle is a consequence of the spin, and that the converse cannot be true. I am thinking of some old papers but you can read about it in his papers posted on the arxiv a month ago. If this is true, isn't it worth knowing? And, if the spin is simply the consequence of relativistic electrodynamics, then we have a whole new world view to consider. Quantum theory would be explained, not refuted.
     
  9. Apr 11, 2008 #8
    No hard feelings, Mentz114, and I don't mind if you call me a crank, although I think I'm better described as a crackpot.

    I agree one can approach the EM 2-body problem with equal validity either way, but I think that not even in the QED picture is the result perfectly satisfying. But please don't call me on that because I'm speaking out of my depth there. About the classical approach, there is no solution as yet and I don't think it's known that there is one. Seems like it would be important to know that there isn't one, if there isn't. That would be an argument that QM truly is fundamental, in itself.

    Perhaps I didn't title thre thread too well. Maybe I should have said "fundamental" instead of "necessary".
     
  10. Apr 12, 2008 #9
    I Believe it is all B----s---- Ok! So yes, quantum mechanics is a mind blowing experience, and there is no denying it! However, right at the moment of inception, there was already a huge misunderstanding. QM formulated its theory of electricity and electrical conduction, much before photons were even discovered. Naturally the whole view point is skewed, how can you have a theory of electricity or electrical conduction without any reference being made to photons. It is a lunatic scenario!
     
  11. Apr 12, 2008 #10
    As a matter of fact everything and I mean absolutely everything can be explained by classical physics ( with suitable insights into what QM has to offer). Look take FEL's as an example. Who would ever have dreamt of it! It is forbidden by QM, yet here you are with FEL's, which is something near what I had predicted. RIGHT!! So does this get formulated into a law or a principle, you tell me?
     
  12. Apr 12, 2008 #11

    reilly

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    DaveLush -- In fact, when physicists do actual computations, they all use the same basic "truncated" or practical Copenhagen approach -- even David Bohm worked that way in his important work on the physics of the electron gas. It's basically the Schrodinger EQ. +Born, and, in fact, this approach is so commonly used it is virtually never mentioned -- everybody knows what's going on. Many of us have no problems whatever with the interpretation of QM as it's done today.

    If folks want to try to give a classical substrate to QM, fine; why not? But many physicists will say "Show me some real results --say like computing the electron's magnetic moment to 8 -- instead of 13 -- decimal places.

    You write:
    Hestenes argues explicitly that the uncertainty principle is a consequence of the spin, and that the converse cannot be true. I am thinking of some old papers but you can read about it in his papers posted on the arxiv a month ago. If this is true, isn't it worth knowing? And, if the spin is simply the consequence of relativistic electrodynamics, then we have a whole new world view to consider. Quantum theory would be explained, not refuted.

    ----------
    What about spinless particles? And, spin is a consequence of Lorentz transformations; specifically, as Weinberg in his Field Theory shows, spin emerges from the representations of the Lorentz group -- little groups and all that stuff. I'll say directly that no way is spin the consequence of electrodynamics.

    How about trying classically to do Compton Scattering or photoproduction of pions, or derive the Goldberger-Treiman relation, or explain the temporal interference patterns of neutral K-mesons, ...Nobody is going to pay much attention to classically supported QM unless the approach yields new physics, or unless it can reproduce a big chunk of QM -- all of atomic physics, all of many body theory, the supers--superfluidity and superconductivity.

    McQween --You say everything can be explained classically; show us -- do it, don't talk about it.

    Regards,
    Reilly Atkinson
     
  13. Apr 12, 2008 #12

    ZapperZ

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    I happen to know quite a bit of FEL since I work on the photocathode sources for them, thank you very much. FEL, and almost all of particle accelerator and beam physics, do not use QM because these free electrons do not have any substantial overlap between their wavefunctions. That's why classical physics truly works in this case. How it is "forbidden" by QM, I haven't the foggiest idea. That's like saying accelerated charges producing EM wave is "forbidden" by QM.

    Your "explanation" to me that everything can be explained by classical mechanics rings hollow, because you have shown no derivation of those three phenomena that I asked for. Just because you said so doesn't mean anything. There have been ZERO published papers on the description of those phenomena based classical physics. We only have people like you to say that can be done, but don't show how.

    Zz.
     
  14. Apr 12, 2008 #13
    I think, most of the time the interpretation does not really even come up. It's rare that there's an experiment where, say, the statistical (Einsteinian, let's call them) interpretations differ from a Born interpretation. Seems like the single-particle experiments are typically or at least often motivated by an interpretational question. Why are all the papers written and experiments done based on a Bell inequality if there is no issue here?

    Yes I agree totally the classicalists need to put up something that's thought to be in the exclusive domain of QM, to expect to be taken seriously as having a replacement for quantum theory. I think they know this. I don't think they are out there claiming they can replace QM, generally, either. I think the electromagnetic 2-body problem is interesting and worth working on its own right. De Luca is arguably simply working that problem and if he happens to get a solution that looks like a quantum theory of the H atom, that's just how it came out. However I think it's "raising the bar" to say they have to do everything, or must completely replace quantum theory. Quantum theory works because it's either right, or mostly right. I want it to be understood better why it works. If it's simple enough, maybe I'll even get to be one of the people who understands. Anyhow I don't think it'll be that there will be a classical replacement for QM. I think the belief is that there will never be closed-form solutions, only numerical ones, and the timescale of integration is very small. Hence it's not going to be practical to solve many-body electrodynamic systems exactly accounting for delay and self-force. QM will remain the only way to handle things in most applications.

    First the classicalists will derive the Dirac electron theory and from there it's not implausible, seems to me, that everything except maybe quantum gravity might be understood in a new light. Actually just to derive the spin from classical electrodynamics probably immediately gives you the Dirac theory. Hestenes argues that the Dirac theory can be derived from the existence of the spin, I think it would be fair to say. If not exactly that, at least something close. He reformulated it over 30 years ago to be all real and the wavefunction phase related directly to the spin, as the zitterbewegung phase.

    I am laboring under the belief that all fundamental charged particles possess spin. Are not all spin-0 massive particles composite particles? I am not too up on this so I will appreciate being enlightened, if this is not so.

    I fail to see how just because we have one explanation some people are satisfied with, having some other explanation that fits in with it is ruled out. Also I fail to see how spin being due to (invariance under?) Lorentz transformations, makes it either a nonclassical phenomenon or exclusive of electrodynamics, which I thought had something to do with Lorentz transformations too.

    Back to my raising of the bar argument - would you say then that getting a classical explanation or equivalent for the Dirac theory would be of no consequence? Anyhow, isn't that a big chunk of QM right there?

    I read an interesting old paper by Corben (Am. J. Phys 61(6) (1993) p551) the other day, that says that the electron g factor has to be exactly 2 by angular momentum conservation and classical electrodynamics. He does this without allowing for radiation but it would not be that hard to include it, seems to me, and including it would seem to get some value slightly greater than 2. I have put this on my list of things to try to do. Also, Hestenes in his 2003 Am J Phys paper (v71(7), July, p691) says he has an explanation for why g has to be 2, but I have yet to understand it.

    I agree with you here, Reilly. I am not claiming classical physics can explain everything, although I am optimistic that it can explain at least a lot. That's not this thread at least to claim it. This thread is to find out what are the best arguments if any for why it's impossible to have a classical theory that can replace, or give rise to, QM. I expect Bell's theorem is the best one there is, and I used to try to argue against that based on retrocausality, but now I can simply invoke Christian. I do not claim Christian is clearly right, merely that he has formulated the best prospective counter-argument that a local, deterministic model can reproduce the results of Bell's inequality-based experiments.
     
  15. Apr 12, 2008 #14
    ZapperZ I know your second post is not directed at me but I wonder if maybe there are some published papers that contend there is a classical explanation for tunneling, at least. Will that cover the field-emission one? I'll hunt you up the cites later but I am thinking of at least one paper by Hestenes where he argues that quantum tunneling is a natural result of the spin being the result of the zitterbegung motion. Foundations of Physics from 1979 I think.
     
  16. Apr 13, 2008 #15
    the direct answer to your question is imagine the scince now without quantum mechanics as a major part in it i think mosst of the physics scinece will disapear and every single phenomen we will say it is magic but by quantum mechanics we can explain everything with high level of reality and experiemently right>
     
  17. Apr 13, 2008 #16

    f95toli

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    I haven't read the paper in question, but as far as I can tell that makes no sense whatsoever. There are plenty of variables that can tunnel where spin isn't involved at all. E.g. tunnelling of macroscopic variables such as phase.
    One problem with most alternative theories is that they tend to focus on "exotic" physics such as particle physics etc, and tend to forget that most of the time QM is used for for more down-to-earth physics; an obvious example being the III-V quantum well structures that are used in the semiconductor industry to make e.g. lasers.
    QM is NOT something "exotic" that we should put on a pedestal; it it is the foundation of most modern physics and is used by thousands of scientists and engineers every day to understand everything from phenomena in cosmology to components in DVD-players.
    The "philosophical" implications are interesting, but they are in reality of little importance to how we actually use QM in our daily life.
     
  18. Apr 13, 2008 #17

    ZapperZ

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    Sure... if you can find me the reference, I'll certainly look into it and see if he could explain field emission, the tunneling spectroscopy we see in superconductors, etc. But as f95toli pointed out, this makes no sense because in many cases, the "spin" of a particle has no bearing on the tunneling properties. There ARE spin-dependent tunneling, but even in those cases, we are talking about a quantum spin, which has no analog in classical mechanics. Spintronics is entirely a QM phenomenon with no classical description.

    Zz.
     
  19. Apr 13, 2008 #18

    Fredrik

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    This is a lot like saying that maybe all medical conditions are caused by the flow of Qi in the body. No one has disproved that either.

    Experiments violating Bell Inequalities, delayed choice experiments, quantum eraser experiments,...

    And how about the fact that quantum mechanics combined with special relativity tells us what properties elementary particles can have, and the fact that all the particles that have been found are consistent with that?
     
  20. Apr 13, 2008 #19
    Hi everyone, I am new to the forum. I hope maybe being active on here will help me clarify my thoughts about QM.

    I was also going to mention the Bell inequalities and the various descendents. Are these not sufficient to prove that nature is non-classical?

    Wave particle duality?

    The appearance of quantum resonances?

    The discrete energy levels of atoms?(didn't that kind of start this whole game we play)
     
  21. Apr 13, 2008 #20

    Mentz114

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    DaveLush,
    One of the most difficult things to handle in CM is situations where particles are created and destroyed. If we have N particles we can do CM in a 6N dimensional configuration space. If N changes, the space now has a different number of dimensions. Has this been modelled classically ?
     
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