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Clarification Needed

  1. Feb 23, 2010 #1
    Hi All,

    Please clarify my thinking on the basic philosophical difference between Special Relativity and Quantum Mechanics.

    Quantum Mechanics is not understood by any human being because humans are not capable of understanding it. To justify my thinking, I will quote Richard Feynmann who said "I can't expect my students to understand Quantum Mechanics because I don't understand it" or something like that.

    Special Relativity is different in the sense that although it is counterintuitive, with the appropriate stretching and bending of one's brain, it can truly be understood.

    Is this true?

    I am partly asking this question because I think that I am starting to understand Special Relativity!

    Also, if this is true, what is the story regarding General Relativity of which I know virtually nothing?

    Thank you,

    Bob
     
  2. jcsd
  3. Feb 23, 2010 #2

    JesseM

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    With quantum mechanics the conceptual difficulties revolve around the fact that the QM formalism doesn't seem to give you any clear "objective" description of how the universe evolves, or how a system evolves independently of interactions with anything outside it; in order to make sensible predictions you have to feed in facts about measurements made by outside observers which aren't part of the system. There are some alternate "interpretations" of QM which try to give a more objective description of reality though, like Bohmian mechanics and the many-worlds interpretation. With relativity (both special and general) there are no such problems, the theory gives an objective description of events in spacetime and how they relate to one another.
     
  4. Feb 23, 2010 #3

    Meir Achuz

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    RF liked to make outrageous statements that we have to live down.
    The only problems in QM come if you try to put it into classical terms.
    The same for SR, except that it is compelling and not counter-intuitive once you understand it, which I am glad you're starting to.
    I will leave GR to others.
     
  5. Feb 23, 2010 #4

    JesseM

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    I think it's deeper than that...the lack of ability to describe the evolution of a quantum system in a self-contained way that I mentioned above means there are problems with understanding what it would mean to have a quantum description of the universe as a whole ('quantum cosmology'), for example.
     
  6. Feb 23, 2010 #5

    Mentz114

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    QM and SR are both counter-intuitive, but for very different reasons. To misquote Douglas Adams,
    "When the universe was created a lot of people thought it was a bad idea and it made them very angry".

    You mentioned ph*l*s*phy. But I don't think you meant it so I won't shout at you.:biggrin:
     
  7. Feb 23, 2010 #6
    Hi All,

    I thought that I would add to this post the reason that I feel this way.

    It seems to me that Special Relativity can be understood with the "gedanken" or simple thought experiment from which everything seems to follow naturally. The simple thought experiment makes it all seem so obvious!

    Further, it also seems to me that simple thought experiments can be used to understand all classical physics.

    Can a simple thought experiment be devised to understand Quantum Mechanics in the same way? I think not!

    Thanks again,
    Bob
     
  8. Feb 23, 2010 #7
    Oh don't worry. I'm pretty thick skinned at this late stage of my life!:smile::smile:
     
  9. Feb 23, 2010 #8

    Mentz114

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    Bob,

    your point about thought experiments is good. SR is a sort of universal principle which yields to reason alone, but QM is a whole 'jump' away from classical mechanics. The key to understanding QM ( in a formal sort of way ) is to understand Lagrange/Hamiltonian theory. There are amazing correspondences, like the Poisson bracket ( classical) goes over to the commutator bracket (quantum), and they have the same meaning, but operating on different things.
     
  10. Feb 23, 2010 #9
    Maybe to you buddy. I find SR/GR and QM both unintuitive in ways. I am feeling OK about SR when I think in terms of math. I find it amusing that the math for GR is so complex that effectively no human has ever solved any case expect for a few toy problems.

    Of the two I find QM more intuitive. Especially in Feynman's terms of paths.
     
  11. Feb 23, 2010 #10

    bapowell

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    The main difference between relativity (both special and general) and quantum mechanics is that the former can be fully derived axiomatically (eg the postulates of SR, or the Equivalence Principle of GR), whereas the latter lacks any unifying, fundamental axiom.

    Otherwise, they are equally unfamiliar to humans that are slow moving, macroscopic objects.
     
  12. Feb 23, 2010 #11

    JesseM

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    I think physicists have come up with various sets of axioms that can be used to derive QM, although there doesn't seem to be as much agreement about the "best" axioms to start from, and maybe it's harder to come up with axioms for relativistic quantum field theory than for ordinary nonrelativistic QM. See this paper or this article for example.
     
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