Linearity of Maxwell's equations as a result of special relativity.

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SUMMARY

The discussion centers on the linearity of Maxwell's equations as a consequence of special relativity. Participants assert that the linearity of these equations allows for the principle of superposition in electromagnetism, which is crucial for understanding Lorentz forces. The conversation highlights that while special relativity provides a kinematic framework, it does not inherently dictate the dynamics of field theories, as evidenced by the existence of non-linear wave equations consistent with Lorentz invariance, such as the Yang-Mills equations. Ultimately, the linearity of Maxwell's equations is attributed to the abelian nature of the gauge group U(1) rather than the flatness of spacetime.

PREREQUISITES
  • Understanding of Maxwell's equations and their implications in electromagnetism.
  • Familiarity with special relativity and Lorentz transformations.
  • Knowledge of gauge theories, specifically the U(1) gauge symmetry.
  • Basic concepts of linear algebra as applied to physics, particularly in the context of affine spaces.
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  • Research the implications of the principle of superposition in electromagnetism.
  • Study the properties of the Yang-Mills equations and their relation to Lorentz invariance.
  • Explore the role of gauge symmetries in classical and quantum field theories.
  • Investigate the differences between linear and non-linear wave equations in the context of special relativity.
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Physicists, particularly those specializing in electromagnetism, theoretical physics students, and researchers interested in the interplay between special relativity and field theories.

  • #31
Thanks Tom.
I guess I interpreted the phrase "a gauge theory is linear if and only if its gauge group is Abelian" in a previous post as meaning "a gauge theory is non-linear if and only if its gauge group is non-Abelian" which are not exactly equivalent, the latter is wrong while the former is right.
 
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  • #32
TrickyDicky said:
However what I fail to see is how QED contains that non-linear term only with the U(1) abelian gauge symmetry. This might be getting offtopic and not related to relativity, so maybe I should open a new thread.

QED is not a pure gauge theory; it is coupled to fermions. The fermions provide the nonlinearity as Tom described above.

Generally, when we solve classical E&M problems, we consider the sources J to be non-dynamical. They are fixed, or are given by a predetermined forcing function (cf. antennas). In this case, the problem we are left to solve is actually pure E&M.

If you allow the sources to be dynamical--i.e., to allow the fields to move the charges in the way that they actually would in nature--then the theory as a whole is no longer linear. At least I think so. This fact may actually depend on the sources having a nonlinear coupling to the fields, as in the case with fermions.

In any case, we should be careful what we mean by "gauge theory". I've been referring to the pure gauge sector, possibly up to including external, non-dynamical sources. QED, QCD, etc. are "gauge theories coupled to matter".
 

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