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If, as Richard Feyman insisted, nobody understands quantum mechanics

  1. Apr 19, 2007 #1
    ...why believe that there could be a theory of everything that unifies quantum theory with general relativity?

    So one of the reasons for developing a theory of quantum gravity is that it could explain the behaviour of quantum objects described in quantum mechanics that can't be explained given the present quantum theory alone.

    But then suppose the fundamental reason why a successful theory of quantum gravity can't be constructed in the first place is just that quantum theory can't explain how there is such quantum behaviour described in quantum mechanics as wave, spin and entanglement?
     
    Last edited: Apr 19, 2007
  2. jcsd
  3. Apr 19, 2007 #2
    I'm not even sure what your point is, or how "nobody understanding quantum mechanics" plays a role in unification theories. The reasons for spin are reasonably well understood as being a consequence of having a relativistically covariant theory of quantum mechanics, and entanglement is a result of conservation laws and our poor understanding of how measurement works (or at least, that's my somewhat naive view of it).

    Again, what you're saying doesn't really seem to make a lot of sense.
     
  4. Apr 19, 2007 #3
    But then 'having a relativistically covariant theory of quantum mechanics' isn't an explanation of how the spin behaviour occurs in the first place. How is it that quantum objects don't spin like tennis balls?

    Also, you don't need to 'understand how measurent works' to understand how gravity produces its effects. And the fact is that no known effects of the forces are like those of quantum entanglement, which have no measurable strength, have not been measured to vary in any way with increasing distance between objects and to occur at much faster than the speed of light.
     
    Last edited: Apr 19, 2007
  5. Apr 19, 2007 #4
    Physics is not about understanding.
    Understanding seems to be an anthropomorphic concept: relating facts to the archaïc human experience.

    Physics really is about relating facts together in theories able to predict experimental outcome on the basis of a minimal amount of information.
     
  6. Apr 19, 2007 #5
    Sure it is. If you want special relativity to apply to quantum mechanics, you find all manner of paradoxes if you only look at the wave function as a scalar quantity. In order to make it Lorentz covariant, then you end up with an additional effect, which is spin and anti-matter.

    Also, as for the tennis balls, our current theory treats the elementary particles (leptons are probably what we're going to focus on here) as point particles. No matter how fast they "spin like tennis balls", the angular momentum from this spinning will be zero. If you try to compute how fast an electron is spinning if its "radius" is smaller than our current resolution on measuring radii, you find that the electron is rotating at nigh-relativistic speeds or beyond. Tell me that's not a problem.
     
  7. Apr 19, 2007 #6
    Absolutely not. In all science other than in the standard model of quantum and partiicle physics, theory has been about the discovery of just that which already exists in the world as a cause or causes that can be described in enough clear detail to explain the observed findings. And so you can instist that this is essentially what a factual understanding is all about.

    Physicists have abandoned the idea of cause and effect as a means of understanding quantum mechanics only because, given all their previous theoretical methodology that explains the effects of the known forces, it seems as though quantum wave, spin and entanglement have no cause.

    Whereas what they should be doing is seeking a new understanding that is based upon a cause that is something like the quantum potential that is described in Bohmian mechanics. And only this could lead towards a genuine understanding of how matter exists and can persist in all its forms despite the action of the forces.
     
    Last edited: Apr 20, 2007
  8. Apr 19, 2007 #7

    JesseM

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    Where in any theory of physics do you see anything aside from a mathematical description of a relationship between various measurable quantities? For example, in Newtonian physics, can you name a situation where you see definite "cause and effect" (it's hard to see how, since the fundamental Newtonian laws are all time-symmetric), or an ultimate explanation for why systems obey the particular equations they do? In Feynman's book The Character of Physical Law, in the chapter "The Relation of Mathematics to Physics", Feynman actually argued we should not expect physics to be about anything more than mathematical relationships, using Newtonian gravity as an example:
    And what makes you say the potential is the "cause" even in Bohmian mechanics? There is the potential evolving over time and there are particles whose positions change over time, and there is a mathematical relationship between the two.
     
  9. Apr 20, 2007 #8
    Bob,

    If you tried something "like" physics, mathematics and logic, you would recognize easily that the Bohmian mechanics is totally and exactly equivalent to the Schrödinger equation. There is nothing new in Bohmian mechanics with respect to the work of Schrodinger. I like this derivation quite a lot for its pedagogical aspects and for it illustrates quite clearly where the differences are between CM and QM and when these differences vanish (see for example: http://en.wikipedia.org/wiki/Bohm_interpretation).

    However, in a sense the whole Bohmian mechanics has been built-in QM by Schrodinger: he derived his masterpiece on the foundations of classical mechanics, as surprising as that can be. The people who made that possible are from the classical times, to name a few think of Hamilton, Jacobi and Lagrange.

    Finally note that the Bohmian formulation is probably the best way to illustrate to non-causal aspects of QM. It is just highlighted in the quantum potential term. To transform that in a causal view would imply much more than a new interpretation. This would imply certainly new experimental data, without these the current QM would be more than sufficient. A naïve possibility that I could introduce, as a joke, would be that the quantum potential is related to cosmological entanglement, but more work is needed ... I would not engage the scientific community to invest all his time on this idea ... There are already more than enough unproductive speculative works these days, I think ...
     
    Last edited: Apr 20, 2007
  10. Apr 20, 2007 #9
    but isn't the mathematical description just the description of what has been observed?

    Without trying to understand the "cause and effect" of the dropping apple, would Newton not come up with his theories, equations, etc.?

    I consider math to be secondary to the original idea or reason. A mathematical description is the utilization of the understanding of the initial and correct (or accepted) observation and measurement put into terms that can be usable.

    Bob Eldritch: "...why believe that there could be a theory of everything that unifies quantum theory with general relativity?"

    I think that it's just a very hard thing to be creative in this area and it work with EVERYTHING that is known/accepted.

    To quote bush from one of the presidential debates: "IT'S HARD WORK" (squared).
     
  11. Apr 20, 2007 #10

    tpm

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    umm..my question is If:

    - 'NObody Understands Quantum Mechanics'

    -'Everybody Understands (or at least can) Classical Mechanics'

    Then What happens with Semiclassical WKB Quantum mechanics ??... :bigrin:
     
  12. Apr 20, 2007 #11

    Demystifier

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    Allegedly, nobody understands quantum mechanics (QM) and everybody understands classical mechanics (CM). Why is that? Because QM is conceptually very different from CM. But is it really? The Bohmian interpretation of QM mentioned above makes it conceptually very similar to CM, which makes QM much easier to comprehend.

    But there is also an opposite way, to reinterpret CM such that it looks more similar to QM:
    http://arxiv.org/abs/quant-ph/0505143
    With such an interpretation, I wonder if anybody understands CM. :biggrin:
     
  13. Apr 20, 2007 #12

    Demystifier

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    Nobody understands it either, but whenever you do not understand it, you can comfort yourself that you do not need to understand it as it is only an approximation anyway. :biggrin:
     
  14. Apr 20, 2007 #13
    I think that there are many fundamentals problems with quantum mechanics, that make it an original theory in Physics.

    Among these particularities, and may be the most important, is the "circularity" of the theory. That is, the fact that QM needs its limit theory, Classical Mechanics, for its own definition. This problem has been clearly exposed by Landau and Lifschitz in "Quantum Mechanics - non relativistic theory".

    Up to now, this problem has'nt been solved, even by Zurek.

    The other, and linked problem is about the nature of the wave function. The two interpretations, the realistic-objective, supported by Einstein, Schrödinger , etc..., and the subjective informational, supported by Bohr, Born, Heisenberg, etc ... both present some difficulties, and cannot explain all situations.

    Bertrand
     
  15. Apr 20, 2007 #14

    JesseM

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    I don't see any explanation of "cause and effect" in Newton's gravitational laws, and according to Feynman, neither did Newton:
    Do you see any significant difference between the following two statements?

    1. There is a mathematical relationship between the acceleration of a body (like an apple, or the moon), and its distance from some other massive body of mass M (like Earth), and although we have no idea why this relationship obtains, we can write down the equation as a = GM/r^2

    2. We can say that a body creates a gravitational "force field", and we can write down the relationship between the force on a body with mass m and its distance from another body of mass M as F = GMm/r^2, although we have no idea why this relationship obtains; then the acceleration of the body with mass m will be equal to a = F/m

    Is the second more of a "causal explanation" than the first? Isn't the second just a way of restating the first, since this entity called "force" has no measurable properties other than determining the acceleration?

    This is not to say that Newton's insight was purely a mathematical one--but his physical insight just consisted in the idea that the dropping of an apple and the orbiting of the moon could both be described by the same basic rule, he didn't have any causal explanation for why this rule holds true. Physical insight often consists of noticing symmetries or seeing how a single law might explain a variety of phenomena which were previously explained with different laws, but I don't think this is what you mean by "cause and effect".
     
  16. Apr 20, 2007 #15
    Out of curiosity, can you tell me the specific problems with those interpretations? I think I'm leaning towards the subjective informational side, but knowing problems with it would probably be a good thing!

    Also, at the moment I feel like I understand QM more than CM! But that probably just means I don't understand either... :(
     
  17. Apr 20, 2007 #16
    I'm afraid that's just not true at all.

    Bohm's 1952 paper was called 'A suggested interpretation of the quantum theory in terms of "hidden" variables." So the whole point of this account was show what the Copenhagenists insisted was not possible and John von Neumann was supposed to have proved impossible, which was that you could, after all, account for the results of experiments in terms of a mathematically systematic and determinate description of the hidden variable behaviour of quantum objects in motion.

    Whereas one problem with the Schrodinger equation just was how it could be interpreted and one way was Bohr's indeterminate account whereby it could be maintained that the Heisenberg/Schrodinger/Dirac quantum mechanics provided a complete theory of the quantum findings.

    And indeed, later it was shown with computer generated diagrams how both the quantum wave and the particle trajectories in a double slit experiments could be visualised using Bohmian mechanics. Something that you couldn't do with any other quantum mechanics or interpretation.

    While Bohm's theory is often called causal partly because the wave, which continually accompanies the travelling particle, causes the particle trajectory deflections that result in the interference pattern.

    http://www.math.rutgers.edu/~oldstein/quote.html#M

    http://www.math.rutgers.edu/~oldstein/

    http://www.math.rutgers.edu/~oldstein/papers/qts/node4.html

    http://plato.stanford.edu/entries/qm-bohm/


    For a very clear plain English account of Bohm's theory see also Jim Baggott's book Beyond Measure
     
    Last edited: Apr 21, 2007
  18. Apr 21, 2007 #17
    Problems linked withe subjective interpretation :

    1) It requires a "classical world" given a priori. In its essence, Born's priniciple requires a classical, non quantum, measurement apparatus. On the other hand, the classical theory is the limit of the quantum theory, through the correspondance principle. This means that in that case, quantum theory requires its limit theory for its own formulation. A unique case in physics. For instance, relativity doesn't need the non relativistic classical theory for its own formulation, and accepts it just as its limit theory.

    2) It is difficult to interpret everything just as a subjective information. Where does the reality of matter come from in this model ? The quantum theory is necessary to explain the chemical link, for instance. However, when to hydrogen atoms merge to for an hydrogen molecule, there is no observer of any kind in the process.

    3) It has difficulties to explain the non-locality : in non-local processes, there is "something" which is transmitted between the two entangled parts, and this "something" is not simply a subjective information acquired by one of the obsevre, on the distant part.

    4) If you consider the whole universe as a quantum object, where does classicality come from in the first place.

    Objective interprétations :

    They lead either to reltivity violation, or to the multi-world hypothesis, which is totally unacceptable.

    Bertrand
     
  19. Apr 21, 2007 #18
    Nonlocality only violates relativity if something travels instantaneously or at faster than light speed between quantum objects through four dimensional spacetime.

    But then if you were to describe a cause of the non-local effects of entanglement, because such effects do not vary with distance around objects, their cause could not be described as surrounding objects in 4D spacetime and so could be thought to act from one or more extra dimensions of space to the three of the world experienced, and to which relativity exclusively applies.
     
  20. Apr 21, 2007 #19
    Yes Bob,

    This is an interesting hypothesis.

    However, although in a non-local process there is "something" that moves instantly, this "something" cannot carry any information. It is necessary to have an information going through a classical channel, to let this "something" become efficient.

    So this element should although be taken into account in yout theory. Is it the case ?

    Bertrand
     
  21. Apr 21, 2007 #20
    Have you heard of Carlo Rovelli's relational interpretation? It says that all systems are quantum systems, even macroscopic measuring devices. I guess classical mechanics can come out of it through the correspondance principle.

    In RQM you don't need an outside observer, one hydrogen atom can act as an observer, so from the point of view of one H atom, it would want to combine with another H atom.

    I don't see why you need for something to be transmitted non locally. What is this something? If all you have is subjective information acquired by one observer, you still get consistency, if you allow the distant observer to exist in a superposition of states, ie. treat it quantum mechanically.

    There is no universal wavefunction because of locality, which splits up the universe into disconnected regions, which have to be treated separately. Classicality comes from these separate parts interacting so much that there is no chance for superpositions to build up.
     
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