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Photons have no charge. Light is a form of electromagnetic energy.

All spectroscopic effects (to my knowledge) are due to changes in electron state, induced either through an interior or exterior EM (Electro-Magnetic) field. EM forces may affect that state, which in turn affects the light emitted during the change of state, e.g. the Zeeman effect, etc.

However, if we exclude all such spectroscopic phenomena, and consider EM fields purely energetically on the one side, and light energy on the other, are there any know direct effects of EM fields on light itself?

(I'd really like to hear from physicists with hands on knowledge - and please keep all the Tags as is because I have reasons for selecting them, thx.)

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  • $\begingroup$ "Light" is either Electromagnetic radiation energy itself; photons or EM fields if you will, OR it is the psychophysical response of the human eye to such radiant energy generally in the wavelength range from about 400 to about 800 nm. The Commission on Colorimetry of the Optical Society of America, defines it as the latter; the psychophysical response, measured in lumens, candela, and other units distinct from Joules, Watts, or electron Volts, used for EM radiation. But in any case, "light" is a consequence of EM fields. $\endgroup$
    – user26165
    Oct 18 '13 at 19:08
  • $\begingroup$ thx for expanding on the details, @GeorgeE.Smith. Do you know of any phenomena in addition to the predictions of QED mentioned by ahkmeteli's answer? $\endgroup$ Oct 18 '13 at 19:14
  • $\begingroup$ Howard, I'm generally not fluent in QCD, QED, and real QM, so I leave that to those who are. BUT , I know that in the standard model, each of the four forces is moderated by an "exchange" particle. The EM force has the Photon, as its exchange particle. Two electrons approaching each other exchange a photon, back and forth, that creates their mutual repulsion. Two ice skaters, tossing a 12# bowling ball back and forth, will gradually push each other apart, and it gets more difficult the further apart they get. Same concept. $\endgroup$
    – user26165
    Oct 18 '13 at 20:16
  • $\begingroup$ @George - thx - nice simple answer! I'm uncertain if it addresses only part of the question. Since photon-photon interactions are also possible (vacuum charge fluctuations ala gamma-gamma physics). I am wondering if ultra-dense magnetic fields (or ultra-high charge fields) might do something similar. I'm not fluent in these areas either, and am learning, but, ignoring QED phenomena, I think it's clear that the standard model would say 'no' ... it'd be like the two skaters just waving at each other. (?) $\endgroup$ Oct 18 '13 at 20:30
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Quantum electrodynamics does predict scattering of light on light, but this effect is small and has not been observed experimentally, AFAIK. Scattering of electromagnetic radiation in the Coulomb field of nuclei was, however, observed (http://en.wikipedia.org/wiki/Delbr%C3%BCck_scattering )

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  • $\begingroup$ Thx - Delbruek scattering, though in the Coulomb field of (heavy) nuclei might not be considered a spectroscopic effect per se, but it seems similar as the effect is in the presence of an atom. Do you have any idea of how 'small' the QED effect would be? $\endgroup$ Oct 18 '13 at 19:06
  • $\begingroup$ Please see arxiv.org/abs/1106.0592 and arxiv.org/abs/1106.0465, although those authors offer a new formula, different from the formula obtained previously by others. I cannot be sure the new formula is correct, but they give estimates for both formulas. $\endgroup$
    – akhmeteli
    Oct 18 '13 at 19:45
  • $\begingroup$ Please see also physics.stackexchange.com/questions/1361/… - there seems to be a reference to (indirect) experimental observation of light-on-light inelastic scattering $\endgroup$
    – akhmeteli
    Oct 18 '13 at 19:50
  • $\begingroup$ thx for the excellent references which are well worth following up on! $\endgroup$ Oct 18 '13 at 21:31
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I have an example in mind that may satisfy your curiosity. It is a Nature paper you can find here:

It describes a scenario in which the EM intensity is so high that electron-positron pairs are created, whose subsequent annihilation generate additional photons. This translates in a non-linearity in the Maxwell equations which give rise to an interference pattern typical of double-slit experiments.

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