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Neutral body radiation

  1. Jul 6, 2010 #1
    Hi there!
    An accelerated charge will emit an electromagnetic radiation. But what about a neutral body? If I accelerate a neutral body, I accelerate also the electrons and the protons it contains, so it radiates. Is this right?
     
  2. jcsd
  3. Jul 6, 2010 #2
    Outside of your hydrogenic "atom" the radiant field vectors point in opposite directions and effectively cancel (See Griffiths, "Introduction to Electrodynamics", Eq. 9.107). Between the "nucleus" and the "orbiting" negative charge, significant additional field energy accumulates as the atom accelerates.
     
  4. Jul 6, 2010 #3
    Rayleigh scattering of light off of neutral nitrogen atoms in the atmosphere is a form of dipole radiation from a neutral atom. In this case, the electron cloud is oscillating up and down with respect to the nitrogen nucleus. However, it will not radiate if the electron cloud is stationary with respect to the nucleus.

    Bob S
     
  5. Jul 7, 2010 #4
    This means that outside my atom there is no radiation, while inside the atom there is radiation?


    So if I have a non-oscillating dipole and I accelerate it, it doesn't radiate?
     
  6. Jul 7, 2010 #5
    These are very good questions. If the neutral atom has no electric dipole moment, meaning that the charge centers of the bound electrons and the nucleus are the same, there is no radiation. This answer is complicated by the fact that if we accelerate a neutral atom by pushing on the bound electrons (using atomic collisions or photons, for example), then the charged nucleus would be dragged by the electron field, producing a non-zero dipole moment.
    If you accelerate a neutral atom with a non-oscillating non-zero electric dipole moment, even though it is uncharged, it should radiate, probably with a higher order field profile, like quadrupole. Good questions.

    Bob S
     
  7. Jul 7, 2010 #6
    I'm speculating, but I would guess that inside the atom there are standing waves with nodes at the electron "wall". Outside the atom there "are" waves (heh heh), but their electric fields cancel at all external points. Of course this is all classical, and the actual phenomenon of accelerating atoms is properly a problem for quantum (QED?) theory. For example, the internal magnetic field energy (between the nucleus and the electrons) of an atom that has always moved with a constant velocity might alternatively be represented as virtual photons. I expect this reply will return feedback, as I'm truly out of my depth here.
     
  8. Jul 7, 2010 #7

    Andy Resnick

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    An accelerating neutral body does not radiate, AFAIK- otherwise satellites would need to deal with their own radiation. Alternatively, the radiation could be detected, and I don't think Gravity Probe B detected that effect.

    An accelerating body will observe black-body radiation, even if a inertial observer does not.

    http://en.wikipedia.org/wiki/Unruh_radiation
     
  9. Jul 7, 2010 #8

    alxm

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    Well, by and large 'accelerating an atom' means accelerating the nucleus. Since it's tens of thousands of times heaver than the electrons. The motion of the electrons and the nucleus is largely decoupled (Born-Oppenheimer approximation, Franck-Condon principle) due to this large difference in sizes and relative velocities. A popular analogy is to flies buzzing around an elephant. The elephant moves so much more slowly than them, that they hardly change their motion at all to compensate for it.

    So in general, you won't see emissions from an atom when another atom collides (e.g. two helium atoms) because the relative velocities of the nucleus and electrons does not change enough, fast enough, to cause electronic excitations. Remember, the electrons in an atom are moving at almost relativistic speeds. If you're talking about a polyatomic molecule, however, accelerating a single atom is just a vibrational excitation of sorts, which means some or all the kinetic energy can be re-emitted as radiation (selection rules apply, your results may vary).

    Given that the atom/molecule is neutral, you can't accelerate it with a field. (ignoring possible but largely negligible effects like using a magnetic field to accelerate a paramagnetic molecule) The only way this can occur is to slam another atom/molecule into it, in which case you may be able to get thermal radiation, especially if the molecule has some dipole moment.
     
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