Why do accelerated charges emits a photon?

  • Thread starter Fisix
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  • #26
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OK, so you are insisting the electrically charged moon would not radiate. Nonetheless, do you agree that there would be a periodic variation in the electric field measured from earth? And from any distant star?
 
  • #27
Andrew Mason
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OK, so you are insisting the electrically charged moon would not radiate.
The principle of equivalence requires that an electrically charged moon not radiate.

Nonetheless, do you agree that there would be a periodic variation in the electric field measured from earth? And from any distant star?
Since the sun and the earth and moon are moving relative to a distant star, the field of a charged moon measured from a distant star would be constantly changing.

From the earth, the electric field would change direction periodically.

AM
 
  • #28
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The principle of equivalence requires that an electrically charged moon not radiate. [But] the field of a charged moon measured from a distant star would be constantly changing.

The principle of equivalence requires no such double-speak.
 
  • #29
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An electron whipping around a curved path radiates (synchrotron radiation). Microgravity experiments have shown that a falling electron does not [..] Emit a photon.

The article you cited does not appear to gives any evidence as to whether or not falling charges radiate. It discusses one particular apparatus for an Eotvos experiment (for charged particles), analyses several error sources, and proposes that it might obtain higher accuracy from in orbit.

As for the issue of emitting photons, one lesson from attempting to join QM with relativity is that the existence of particles is subjective (eg. Unruh radiation); I think genneth spoke for the classical equations correctly.

Back to the charged moon scenario, you agreed this produces an equivalent field as to a synchrotron. Classical electrodynamics will show both transmit energy to infinity, conservation implies the moon will be slowed, and ... I assume this deviation from geodesic motion can be attributed somehow to the electromagnetic self-force of a charge upon itself in curved geometry?
 
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  • #30
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I think one issue here is the mis-identification of classical fields with radiation of photons. In the classical situations I presented, none of the fields oscillate in an obvious wave fashion. The fields are simply a bit complicated, and non-zero, extending out to infinity. We don't say that the classical field due to a stationary electron correspond to emission of photons (maybe virtual photons, but I'd hesitate to interpret any QFT particles as ontology, especially in time-evolution processes). Thus, uniformly accelerated charges don't really *emit* a photon. The field they set up is a coherent state involving essentially infinite number of photons.
 
  • #31
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Can you recommend any literature that supports this approach? That the only electromagnetically accelerated charge emits electromagnetic radiation?
 
  • #32
vanhees71
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I just got a marvelous book on this very fascinating (and very complicated) issue of classical charged particles within the Maxwell theory. I highly recommend it since it's a very clear exposition of classical electromagnetics and on top solves (at least it claims so) the age-old problem of the selfconsistent dynamics of classical point charges and electromagnetic fields:

Rohrlich, F.: Classical charged particles, 3 edition, World Scientific Pub Co Inc, 2007
 
  • #33
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Didn't know about his book. He also has a http://www.sciencedirect.com/science/article/pii/S037596010100264X" [Broken] in which he describes his method. I found some threads in PF and some papers in arXiv. It seems that:
  • as long as the particle is not too small, the semi-classical approach can be used. This is called the Abraham-Lorentz-Dirac equation see e.g. http://arxiv.org/abs/gr-qc/9912045" [Broken].
  • otherwise you should use the QED approach which is called Abraham-Lorentz-Dirac-Lengevin (ALDL) equation.
I was also checking the movement of charge particle in the gravitational field. There are many papers trying to derive equations to determine whether and when the particle does and doesn't radiate. For example http://iopscience.iop.org/0264-9381/21/16/R01". But in none of the sources I could see any hint, that confirms the post #21 and #4 in this thread.

Any ideas?
 
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