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Phenomena only explicable via QM

  1. Oct 7, 2011 #1
    Can someone list some of the phenomena only explicable via QM?
     
  2. jcsd
  3. Oct 7, 2011 #2

    berkeman

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    What is the context of the question? Is this for schoolwork?
     
  4. Oct 7, 2011 #3
    No. Not at all. I just want to see if I can come up with a classical counterpart or where the classical counterpart fails.
     
  5. Oct 7, 2011 #4

    berkeman

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    How about tunneling? I'm not aware of any classical explanation for that.
     
  6. Oct 7, 2011 #5
    I am not sure about "classical explanation", but there seems to be at least a "classical counterpart" of tunneling. As I wrote earlier, "I suspect theoretically you can jump over a classical barrier having lesser kinetic energy than the potential energy your mass would have at the top of the barrier. In fact, in the course of a high jump you can bend over the barrier in such a way that your center of gravity will be outside of your body and pass under the barrier." According to Wikipedia (http://en.wikipedia.org/wiki/Fosbury_Flop ), high jumpers do achieve this, but I did not check the Wikipedia source.
     
  7. Oct 7, 2011 #6

    berkeman

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    That's not what QM tunneling is.
     
  8. Oct 7, 2011 #7
    If you don't think this is a counterpart of quantum tunneling, what are your arguments?
     
  9. Oct 7, 2011 #8

    Bill_K

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    1) The universe

    That's only partly in jest, because more seriously what cannot be explained without quantum mechanics:

    2) Atoms
    3) Electromagnetism
    which does not leave much.

    By (3) I mean more specifically black body radiation, which is finite only because of the Planck distribution.
    But that's Ok, because here's something else that could not exist:

    4) Thermodynamics. Without the correct counting of states as given by quantum mechanics, the Gibbs paradox would make a definition of entropy impossible.
     
  10. Oct 7, 2011 #9

    Drakkith

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    The "barrier" referred to in quantum tunneling isn't a physical barrier like a wall you can jump over. It refers to the amount of energy needed to overcome the strength of a repulsive or attractive force. For example, fusion in the sun could not occur in classical physics because the energy of the particles in the core is not sufficient to bring them together in range of the strong force to fuse. Quantum tunneling describes the mechanism that allows protons to overcome the repulsion and fuse if they have "almost" enough energy. Similarly this allows alpha particles to overcome the strong force in heavy elements like uranium or plutonium and is the mechanism behind fission.
     
  11. Oct 7, 2011 #10

    DaveC426913

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    akhmeteli, having a classical explanation for something does not simply mean you can come up with an analogy that might form the start of a plausible description, and then leave the details of the math to the eggheads.

    With a classical description of subatomic particles, there is no known way a particle can jump from one side of an energy barrier to another. The particles are essentially little balls - there is no "bending in the middle".

    To posit that particles can "bend in the middle" is to toss out classical mechanics and start proposing quantum mechanical explanations.
     
  12. Oct 7, 2011 #11
    Thanks all.
    I knew about tunneling, and double slit, and electron orbits. I was just wondering if there were a few more.
     
  13. Oct 7, 2011 #12
    Discrete emission spectra.
     
  14. Oct 8, 2011 #13
    I think this is still a "counterpart" of quantum tunneling. Indeed, what is "tunneling", in the first place? This is a way to pass through a high hill, and this is what is done (or theoretically can be done) in high jump without any "tunnel" under the bar. I agree that the analogy is not complete, as quantum tunneling can occur in one dimension, whereas high jump needs at least two dimensions. This does not seem very important though, as you can "rewrite" my example for one dimension for some elastic forces instead of gravity. The main point still stands: an extended object can pass through a high potential energy area, having kinetic energy below that required for the entire object to be at the top of the potential energy hill. The only problem is "esthetic": such an example would not be as graphic and natural as that of high jump, but the principle is the same.
     
  15. Oct 8, 2011 #14

    DaveC426913

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    The trouble, as explained before, is that, just because you describe something that is plausible does not make it possible. Classical phyiscs does not predict tunneling.

    In our classical description of particles they simply do not have the properties you ascribe to them. And you can't just patch it up by tacking on an extra variable to an equation here or there.
     
    Last edited: Oct 8, 2011
  16. Oct 8, 2011 #15
    Right now I just wanted to say that this was perhaps the fastest reply I've ever received at physicsforums:-)

    I'll try to answer your posts "in the order of their appearance".
     
  17. Oct 8, 2011 #16
    DaveC426913, I don't quite understand why my post seems problematic to you. The OP said "I just want to see if I can come up with a classical counterpart or where the classical counterpart fails." Another poster mentioned quantum tunneling, so I just offered a "classical counterpart." What's wrong with that? It was certainly relevant, could be interesting for some people, and did not challenge any principles of physics. So I just don't understand why I cannot "come up with an analogy".

    Furthermore, how do you know I "leave the details of the math to the eggheads"?

    I don't quite agree with everything you're saying (for example, why classical description cannot use anything but "little balls"?), but first things first: how is this relevant? Where did I "posit" anything? The OP wanted a classical analogy, and I just offered that.
     
  18. Oct 8, 2011 #17

    Drakkith

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    The OP wanted things which do NOT have classical counterparts. Quantum tunneling is one of them. Your analogy, while certainly applicable under other circumstances, is not correct here. If there was a classical counterpart we wouldn't have to use analogies in the first place!
     
  19. Oct 8, 2011 #18
    I believe my analogy is legitimate. What plausibility/possibility are you talking about and how is it relevant? If you think that in real life a high jumper cannot always have his/her center of gravity below the bar, maybe you're right, but, first, what are your arguments, and, second, is it really important for physics? If you are saying that quantum theory cannot be emulated by a classical theory, are you sure you want to open this can of worms in this thread? I am not sure OP’s questions or my humble analogy warrant that.
    As my analogy shows, in some sense it does.
    Did I mention any particles, let alone ascribe anything to them?
    Again, it seems to me that you are pushing a disproportionate expansion of the topic of this thread. Are you sure you really want that? Let me just say that once I started a thread dealing with these issues (https://www.physicsforums.com/showpost.php?p=2530114&postcount=1 ), and there are almost 750 posts there:-)
     
  20. Oct 8, 2011 #19
    Yes, but another poster mentioned quantum tunneling, and I just tried to say that, in my opinion, this is not what the OP wanted, as it does have a classical counterpart.

    As I believe I offered a classical counterpart, I have some doubts about your statement.

    Look, Drakkith, English is probably your native tongue, but not mine, so I had to look up “counterpart” in Webster. Some of the meanings I found there:

    “one remarkably similar to another”
    “one having the same function or characteristics as another”

    I believe my ”analogy” fits these meanings.
     
  21. Oct 8, 2011 #20

    Drakkith

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    Sorry, your example is just plain incorrect. A person jumping over a barrier and shifting their weight is using energy to move their body. ALSO, a person must have enough energy on the jump to get enough of their body over the pole in the first place, otherwise shifting their weight will not work. The key is that a particle does NOT have to have enough energy to move past the barrier while the person does.

    And no, your analogy does not fit those two definitions, as it is not remarkably similar nor does it have the same function or characteristics.
     
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