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Difference between CM and QM

  1. Jan 22, 2008 #1
    what is the difference between classical mechanics and quantum mechanics??

    other than that classical mechanics dealing with macroscpic particles while quantum mechanics with microscopic particles
     
  2. jcsd
  3. Jan 22, 2008 #2

    vanesch

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    Quantum mechanics allows for the simultaneous existance of several classical states. It is the content of the superposition principle. If classically, a certain system can be in state A or in state B or in state C, then quantummechanically, the system can be in any combination of A, B and C, with complex coefficients to A, B and C.

    If you do a measurement on the system, to find out whether it is in the classical state A, the classical state B, or the classical state C, then you will find one of these states, with a probability proportional to the absolute square of those complex coefficients.
     
  4. Jan 22, 2008 #3
    Classical mechanics already exists. Physicists understand it.
    Quantum mechanics does not exist yet. Physicists do not understand it. But they may use it... like cave man may use cell phone. :smile:
     
  5. Jan 22, 2008 #4

    vanesch

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    It sounds a bit silly to say that an 80-year old theory doesn't exist (but we can nevertheless use it - even though its non-existence ?).

    You are probably referring to the interpretational problems of quantum theory. But let us not forget that we've given up interpreting classical physics, exactly because of the advent of quantum physics!
     
  6. Jan 22, 2008 #5
    Words in human language may have several meanings. We may use one meaning in one sentence and another meaning of the same word in another sentence.

    For example:
    (FINAL VERSION OF) "quantum mechanics" does not exist yet.
    Physicists may use (PRELIMINARY VERSION OF) "quantum mechanics"

    Not only to the interpretational problems, but to other not solved problems as well
     
    Last edited: Jan 22, 2008
  7. Jan 22, 2008 #6

    vanesch

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    But we do have a final version of classical physics ?
     
  8. Jan 22, 2008 #7
    We believe we do have a final version of CM, but actually we do not. Because tomorrow somebody may prove a new theorem about Lagrangian and we will get a new "final version" of CM. :smile:

    As for QM, we believe we don't have a final version of QM and we indeed do not have it.
     
  9. Jan 22, 2008 #8
    How do we define "final version" of a theory. How do we know that a theory is a "final version" of a previous theory.
     
  10. Jan 22, 2008 #9

    vanesch

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    I think you're confusing "final theory" and "final version of quantum theory".

    Classical theory has its problems, but it also has a rich body of "working" machinery, and we leave it at that. Probably the problems will never be solved entirely within its scope (like the radiation reaction and so on). That doesn't stop people to add stuff to the edifice.

    You can say exactly the same about quantum theory. I don't think that its fundamental problems will ever be solved within its own scope.

    Tomorrow, we might have a new theoretical paradigm. But there's no reason to call it "new quantum physics". We may also stay stuck with our present tools for a long time to come. Who knows ?

    So I don't see any fundamental difference between the state of current quantum theory, and the state of classical theory, except for the fact that, because as of now, we don't have as of yet an underlying, newer theory, we seem to make more fuzz about the problems quantum theory has than we are making of the problems classical theory has.

    But I'd say that quantum theory is "up and running" just as much as was classical physics, when it was still thought to be "fundamental".
     
  11. Jan 22, 2008 #10
    I don't think so.
    The "final theory" is about EVRYTHING (including gravitation, soul, God and many other undiscovered yet phenomena)
    The "final version of quantum theory" is about atomic, nuclear, particle etc. phenomena ONLY

    I understand your point of view... thanks...
    but my point of view is different...

    It is like a possibility to choose one or another coordinate frame in the theory of relativity... any would be good... :smile:
     
  12. Jan 22, 2008 #11

    vanesch

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    Mmm, but quantum physics as of today does a very good job at most of that already...

    Just like newtonian physics does a very good job at describing the orbit of the earth around the sun, and the tides, as well as falling apples.

    However, look at the tiny deviation of mercury's orbit: is general relativity then the "final version of Newtonian physics" because we now have a quasi perfect description of the solar system, which is "Newtonian physics" ?

    So if ever we discover new principles underlying atomic, nuclear etc... phenomena, which have nothing to do with the superposition principle or unitary operators, is that then "final quantum theory" ?

    I make a distinction between certain categories of phenomena (solar system, atoms, galaxies, nuclear physics, GeV-level elementary particle physics, lasers, ...) on one hand, and theoretical paradigms (Newtonian, general relativistic, quantum theory, ...) on the other.

    You seem to associate "quantum physics" to whatever paradigm is going to adress best atomic and nuclear phenomena. So, 120 years ago, "quantum physics" was Newtonian physics when it was the best tool available to describe microscopic physics ?

    Quantum physics to me is the application of the superposition principle, and the linearity of time evolution. Whether this paradigm works for atoms, quarks, molecules, planets or galaxies is a different matter.
     
  13. Jan 22, 2008 #12
    Yes, absolutely! It does a VERY good job, but NOT ALL job!

    No, GR is not the "final version of Newtonian physics" and GR is not even a "version of Newtonian physics".

    If only new principles , probably not yet.
    If ALL principles , probably yes.
    :smile:
     
    Last edited: Jan 22, 2008
  14. Jan 22, 2008 #13

    Q_Goest

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    I’d be interested in hearing what others think of this particular explanation, good, bad or otherwise.

    Any phenomena (ex: such as vortex shedding from an aircraft wingtip or even the coupling of fluids and solids that may create nonlinear phenomena) which are seen to be explainable at the classical mechanical level, can be both physically and analytically reduced to their individual parts such as is done using finite element analysis or computational fluid dynamics for example. This is a type of reductionism which allows one to consider what is occurring within some small volume of space, contingent only on the local affects of aggregates of molecules.

    In contrast, phenomena which are produced at a quantum mechanical level are not physically nor are they analytically reducible to small volumes of space which are contingent only on the local affects of particles. In quantum mechanics, molecules and molecular interactions must be considered in some holistic sense. Instead of examining local interactions, a quantum mechanical system must be described using a single wavefunction, “and this wavefunction must be a function of the different position coordinates of all the separate particles.” (Penrose)
     
  15. Jan 28, 2008 #14

    samalkhaiat

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    Last edited: Jan 28, 2008
  16. Jan 30, 2008 #15

    samalkhaiat

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    correction

     
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