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

Click For Summary

Discussion Overview

The discussion centers on the linearity of Maxwell's equations and its relationship to special relativity. Participants explore theoretical implications, mathematical properties, and the nature of electromagnetic fields in both flat and curved spacetime, as well as the broader context of field theories.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the linearity of Maxwell's equations allows for the principle of superposition in electromagnetic fields, which may be linked to the affine nature of spacetime.
  • Others argue that Maxwell's equations remain linear even in curved spacetime, suggesting that Lorentz symmetry does not inherently dictate linearity.
  • A later reply questions the assumption that special relativity implies linearity, citing examples of non-linear wave equations that are still consistent with Lorentz invariance, such as Yang-Mills equations.
  • Some participants discuss the implications of gauge groups, specifically the abelian nature of U(1), as a reason for the linearity of Maxwell's equations.
  • There is a suggestion to examine the dynamics of wave packets in Yang-Mills theory to gain further insight into the behavior of non-linear interactions.
  • One participant raises a hypothetical scenario involving point particles with intrinsic degrees of freedom, questioning whether special relativity truly specifies their dynamics.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between special relativity and the linearity of Maxwell's equations. While some assert a connection, others provide counterexamples that challenge this notion, indicating that the discussion remains unresolved.

Contextual Notes

Participants note that the discussion involves complex interactions between theoretical frameworks and specific field equations, highlighting the need for careful consideration of definitions and assumptions in the context of 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.
 
Physics news on Phys.org
  • #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".
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 5 ·
Replies
5
Views
1K
  • · Replies 10 ·
Replies
10
Views
1K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 9 ·
Replies
9
Views
1K
  • · Replies 0 ·
Replies
0
Views
1K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K