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Is gravity more fundamental than EM?

  1. Jun 19, 2012 #1
    If photons emit virtual gravitons (but gravitons don't emit virtual photons) why isn't gravity considered to be a more fundamental force?
  2. jcsd
  3. Jun 19, 2012 #2
    Suppose a particle A 'emits' some other particle B (real or virtual). This is NOT to say that A is composed of or can be represented by B in any sense. Rather it means that there exists a coupling AAB (incoming A, outgoing A, outgoing B) in your theory. Say A is the photon and B is the graviton. AAB is an allowed coupling in your theory because it does not violate any of the symmetries that you might want. On the other hand, the coupling BBA is not allowed because B, the graviton, is a singlet under U(1)EM (i.e. it has no electric charge), so such a coupling would break U(1)EM gauge invariance, a symmetry of the theory related to charge conservation. In short, whether some A can emit some B depends on the representation of that particle in terms of the symmetry groups of the theory, and this has nothing to do with whether one is more 'fundamental' than the other.
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