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The shape of an atom

  1. Aug 5, 2006 #1
    Hello,

    Is there any reason to believe an atom, say Hydrogen, is spherical via experiments, or is this merely a model for us to visualize with?

    Furthermore, can one prove that an atom is not a cube?

    Finally, is there any model in physics that can account for how a photon is emitted from a deexcited electron? Or do we still simply say that the photon is equal to the energy level drop and leave it at that?

    Thanks.
     
  2. jcsd
  3. Aug 6, 2006 #2

    HallsofIvy

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    First you will have to say exactly what you mean by "the shape of an atom" at the quantum level.
     
  4. Aug 6, 2006 #3
    The sphere is a perfect distribution. Anything otherwise would require a greater amount of energy to create, and would therefore deviate from equilibrium. :bugeye:
     
  5. Aug 6, 2006 #4

    ZapperZ

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    You may want to look at the geometry of the p,d,f, etc. orbitals before you say that.

    Zz.
     
  6. Aug 6, 2006 #5
    There's no reason to believe that it's spherical: even the ground state wavefunction, while spherically symmetric, is not spherical.

    Well, none of the energy eigenstates of the hydrogen atom are cubes, but if you chose the appropriate linear combination of states, you should be able to find a cube-shaped one.

    Well, from a logical standpoint, if you're only considering electromagnetic potentials, conservation of energy says that an electron falling to a lower energy state has to give its energy up somewhere. In this case, it goes to the EM field in the form of a photon. More formally, quantum electrodynamics (QED) describes the emission of a photon.
     
  7. Aug 6, 2006 #6

    So there is no physical model of geometry that can show how an electron's energy drop is transferred into a photon then?
     
  8. Aug 6, 2006 #7
    QED is the physical model, but it's not really a geometric one.
     
  9. Aug 6, 2006 #8
    I think he was looking for the formal description. I too was wondering the same thing: When we say "a human eats a sandwich", there are a lot of details that we don't include but it still makes sense to us. Saying, "an electron emits a photon", may make sense to the electron itself (if we personified it), but that understanding doesn't necessarily transfer to humans. This happened often for me when I took High School Physics, because it isn't advanced enough to provide common sense answers for these more complex areas (I use "common sense" loosely of course) - a lot of it just "magically happened". QED seems like the best bet, do you have any good resources on the subject Manchot (or anyone)?
     
  10. Aug 21, 2006 #9
    I just read Feynman's book 'QED the Theory of Light and MAtter' and there was no mention of how the photon is emitted. The book mostly covers feynman diagrams showing the probability amplitudes of a photon taking route A vs. B, etc. I have always wanted a physical geometric explanation, is that asking too much of physicists?
     
  11. Aug 21, 2006 #10
    I think you're missing something about the shape of an atom, Chaos. The thing is, if you say something is spherical, that would imply that it has a spherical surface. That then implies that it has a surface. But you know this isn't right if you think of a black hole. All that's really there is some singularity. It's like a point particle. Sure, the event horizon is spherical, but there's nothing at this event horizon, it's just a place where gravity exceeds a particular measure. And it doesn't make much sense to say the black hole is a sphere. Or any other shape.
     
  12. Aug 21, 2006 #11

    jtbell

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    At this point in time, yes, that is asking too much of physicists. QED is as far as we know how to go. Maybe someday physicists will agree on a "deeper" theory that underlies QED (more precisely the unified electroweak theory that includes it) and QCD (for quark-gluon interactions). It might be string theory. It might be something else.
     
  13. Aug 22, 2006 #12
    I cannot accept any theory as complete until it can be represented geometrically. I am frustrated that some people are posting scientific dogma without seriously contemplating my question. Is there no theory or white paper outlining how photons and electrons interact in the real, 'physical' terms of geometry?

    Are we still stuck at Feynman's 'Magic bag' analogy?
     
  14. Aug 22, 2006 #13
    What is real and physical. These are simply models that agree with experiment. No one has ever seen a photon!

    We cannot not say that the hydrogen atom is spherically symmetric, no one has or ever will observe a wavefunction. We may only say things about observables. The only observables are transition energies in the form of detected photons. This leads to certain conclusions about transition probabilities between the possible 'states' an electron may be in in a coulomb field. Observation supports the lowest state being one with zero angular momentum (when the inherent angular momentum of the elctron is taken into account).
     
  15. Aug 29, 2006 #14
    An accelerating electron produces EM radiation. In quantum mechanics, when an electron shifts energy levels it's moving to a lower (or higher) angular momentum, which is an acceleration.
     
  16. Aug 29, 2006 #15

    ZapperZ

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    Er... what you are saying are highly incompatible with each other.

    1. An accelerating electron produces EM radiation

    This is fine.

    2. In quantum mechanics, when an electron shifts energy levels it's moving to a lower (or higher) angular momentum, which is an acceleration.

    While this is also true, you cannot equate this with "accelerating electron produces EM radiation". This is because (i) an electron in an "orbital" is not accelerating; if it is, it would be radiating (ii) an electron making an energy transition is also not "accelerating" - there is no dynamics describing such a transition. So you or I cannot say that this electron is undergoing an acceleration.

    Note that I can also produce light without having to make any atomic transition. I can do that simply by having an electron in a conduction band recombine into a hole in the valence band. That is, primitively, what is going on when light is created in your LED's.

    Zz.
     
  17. Aug 30, 2006 #16
    ** an electron making an energy transition is also not "accelerating" - there is no dynamics describing such a transition. So you or I cannot say that this electron is undergoing an acceleration. **

    Huh ?? Of course there is a description of the transition. ``Just'' start with a stationary state, make a full non perturbative calculation within QED (or Barut self field - there will be agreement up to fifth order in alpha) with this state as initial data and the EM field in the vacuum state (say) and you can perfectly follow what is happening (if you interpret psi^2 as a density).

    In the same way you could study photon formation within Barut Self field (lets do that, it is easier, well defined and agrees up to high accuracy) during such transition. Of course this is a LOCAL phenomon, since QFT as well as BSF are local theories (the only non-local thing is this idiotic Von Neumann postulate). The problem is that this is hard to do and that the Feynman plane wave scattering diagrams are obscuring the local character of the dynamics.

    The same comments apply to your so called ``non local´´ phonons in lattice theory.

    Careful
     
  18. Aug 30, 2006 #17

    ZapperZ

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    There is no dynamics during the transition. Could you give me the equation of motion during the instant that the atom went from one energy state to another?

    Er... what "comment" about "non local phonons in lattice theory"? Are you back to your habit of putting words into people's mouths again?

    Zz.
     
  19. Aug 30, 2006 #18
    What do you mean by "physical terms of geometry"?

    What kind of explanation do you desire?
     
  20. Aug 30, 2006 #19
    **There is no dynamics during the transition. Could you give me the equation of motion during the instant that the atom went from one energy state to another? **

    There is no ``instant'' that the electron jumps from one energy level to another (otherwise you exclude the logical possibility of dynamics in the same way that dr Chinese proves the existence of entanglement); that is a simplified QM picture for kids emerging from better calculations in QFT. You could calculate the wave function and then set up a Bohmian interpretation, or you could be smarter and take a self interacting Dirac soliton-like solution (in which the coulomb wave functions would be solutions of the linear equation) and follow the ``center of mass trajectory´´ of the latter...

    **
    Er... what "comment" about "non local phonons in lattice theory"? Are you back to your habit of putting words into people's mouths again? **

    And you are in self denial again ? I and others have seen you make plenty of comments about phonons being non local lattice vibrations which is clearly nonsense (I know, they teach you that in the QM courses, but still ...).

    Careful
     
    Last edited: Aug 30, 2006
  21. Aug 30, 2006 #20

    ZapperZ

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    Again, you did not answer my question. Please show me the "dynamics" of the equation of motion of such a transition to allow one to conclude that, yes, the electron was accelerating during the transition.

    I'm not in self-denial. You are self-delusional. What phonons? I'm looking over this thread and I see no "phonons" being mentioned by me. Please point to me explicitly where I mentioned phonons in here till you brought it up. I only made two postings in messages #4 and #15 before you stepped in. Where did I mentioned anything about phonons?

    Zz.
     
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