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Empty spaces and virtual particles in atom

  1. Jul 26, 2011 #1
    as i know,bohr's discovery reveals that atom consist of central nucleus and electron orbit around them in empty spaces

    my question is-why aren't virtual particles being produced in those empty spaces?
     
  2. jcsd
  3. Jul 26, 2011 #2

    SpectraCat

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    1) That was Rutherford's discovery, not Bohr's.

    2) The Bohr model (i.e. electrons traveling in discrete orbits around the atom) was disproven long ago. Electrons in atoms are described probabilistically, not as discrete particles traveling in planetary orbits, which would violate the Heisenberg Uncertainty principle.

    3) Virtual particles are only a manifestation of a particular kind of mathematical treatment (i.e. perturbation theory). It is not clear that they have any independent "reality" .. many people would claim that they do not.

    However, virtual particles can be a useful mnemonic for qualitative rationalization of how the "communication" between the electron and nucleus obeys relativity. Namely, the electromagnetic force between electron and nucleus can be mathematically construed as an exchange of virtual photons which carry the momentum between the real particles. Thus if the nucleus suddenly disappeared instantaneously, there would be a slight delay before the electron was aware of it, because "virtual particles" emitted by the nucleus before it disappeared would still cause the electron to behave as if the nucleus were still there.

    Note that this is description is HIGHLY qualitative (verging on a pop-sci description). However, that is basically what the Feynman diagrams (which are a visual representation of the mathematical perturbation theory treatment) indicate happens. The reason this picture gained traction is because the QED description of the electron-nucleus interaction is very hard to solve exactly, and the perturbation theory approximation gave excellent agreement with experiment. In fact, QED successfully reproduced the relativistic shift of the H-atom energy levels called the "Lamb shift". This can be rationalized as follows: the virtual photons carrying the EM force between the electron and nucleus have a very small probability of becoming a positron-electron pair. These two cases would have different energies, and the Lamb shift is the manifestation of that effect ... QED calculations of the Lamb shift give incredibly good agreement with experiment.

    So, are there virtual particles in the empty space between electron and nucleus? Well, it kind of depends on your point of view. Personally, I find the QED description in terms of Feynman diagrams quite useful for qualitatively describing what is going on. It is also clearly responsible for providing incredibly accurate calculations in some cases. However, I also think it is important to recognize that this is still just based on a mathematical description ... the physical reality could be quite different. However, that last comment can be applied to pretty much any phenomenon in quantum mechanics and atomic physics.
     
    Last edited: Jul 26, 2011
  4. Jul 26, 2011 #3
    if i apply the heisenberg's principle at those empty spaces ,there must be existence of virtual particles unless field could exist without matter
     
  5. Jul 26, 2011 #4
    This is a common misconception promulgated by introductory science texts for the gee-whiz factor. The atom does not consist of mostly empty space. It consists of electrons http://en.wikipedia.org/wiki/File:HAtomOrbitals.png" [Broken] throughout it's entirety. Yes, Rutherford's gold foil experiment showed that most of the atomic mass is concentrated at the center, but not all of it is there. People erroneously think that the atom is mostly empty space, so we should be able to shrink atoms (and objects) by removing the empty space, but it just does not work that way.
     
    Last edited by a moderator: May 5, 2017
  6. Jul 27, 2011 #5
    what way it could work then?
     
    Last edited by a moderator: May 5, 2017
  7. Jul 27, 2011 #6
    The atom works by having an electron's wavefunction smeared through-out the volume of the atom. We can't shrink atoms by simply moving the electrons in closer because the ground state electron wavefunction around an atom is already the most compact arrangement possible. While we cannot shrink an atom, we may be able to shrink an object by making the atomic wavefunctions overlap, such as in a http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate" [Broken].
     
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  8. Jul 27, 2011 #7
    This is one of the reasons that physics is not scale invariant (i.e. you can't build a functioning grandfather clock on an arbitrarily small scale): atoms are the building blocks of all matter and atoms have a definite size that can't be changed much.
     
  9. Jul 27, 2011 #8
    what explanation would you give for rutherford conclusions,i mean penetration of bombarded alpha particles closer to nucleus
     
    Last edited by a moderator: May 5, 2017
  10. Jul 27, 2011 #9

    SpectraCat

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    The rutherford experiment was a scattering experiment. Think about momentum conservation, and the fact that an alpha particle is about 8000 times heavier than an electron, and then try to answer your own question.
     
  11. Jul 27, 2011 #10
    The mass density of an atom is very high at its nucleus, therefore an alpha particle that is fired very close to the nucleus will experience the most deflection. But just because the density is high in the center does not mean that it is zero everywhere else. Think of the atom as a bowling ball surrounded by several meters of tissue paper (this analogy has severe limitations, I know). If you fire hundreds of bullets at it, most will go right through the tissue paper, miss the hard bowling ball at the core, and come out the other side with little deflection. But a few bullets will hit the ball and experience a large deflection. The Rutherford experiment did not say much about the low-density electron cloud surrounding the nucleus, but it did say something about the nucleus (its density, effective size, etc.). http://www.chemistry.mcmaster.ca/esam/Chapter_3/section_2.html" [Broken]e for more.
     
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  12. Jul 29, 2011 #11
    but there is a scope for virtual particles to exist?
     
    Last edited by a moderator: May 5, 2017
  13. Jul 29, 2011 #12
    that's a bit of a simplification.

    true in everyday low gravitational fields, just not so in cosmology:

    In everyday atoms "empty space" is not such a bad description. But the wave description is a much more accurate way to think about atoms...quantum theory just gives better insights than classical at atomic and sub atomic levels.

    Yet Sirius B, the white dwarf star, has a density of about 60 tons per cc, due to electron degeneracy...far above anything on earth. That's where electrons are smooshed together by gravity, forced in toward the nucleus...electron clouds get smaller.

    And if a star is massive enough, that contraction results in an even denser neutron star where electrons are forced into the nucleus by gravity....they combine with protons to produce neutrons.....which may be followed by the creation of a black hole which further compresses particles and "space" where even matter and time is compressed beyond our theoretical ability to understand.
     
    Last edited: Jul 29, 2011
  14. Jul 29, 2011 #13
    they are.

    Why aren't they produced everywhere?

    It's not like any particle, say a neutron, is sitting there solid, immoble, preventing it...all particles have are bouncing around due to quantum jitters and have wave characteristics.

    Is there anywhere quantum jitters don't exist: it's present in even quantum ground state and even degenerate conditions.
     
  15. Jul 29, 2011 #14
    Yes, thank you for the clarification. I simply meant that the picture of small electrons tracing out orbits in empty space around nuclei is misleading. Electrons can be pushed closer by strong enough external interactions, but the effect is a quantum one of altering the wavefunctions. That is why Bose-Einstein condensation is so interesting: we are shrinking an object, but the effect is purely quantum.
     
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