vanhees71 said:
The Galilean limit often can be derived as the limit ##\beta=v/c \rightarrow 0##, but also a bit depends on the theory you are looking at. E.g., for mechanics the limit ##\beta \rightarrow 0## is indeed usually getting you to the non-relativistic approximation.
For the Maxwell equations it's another business, and you have to distinguish between different "Galilean limits" ("electric" and "magnetic" ones):
M. LeBellac, J. M. Levy-Leblond, Galilean electromagnetism, Nuovo Cim. 14B, 217 (1973)
Generally, I also do not understand what
@PeroK is after.
I have read the paper
M. LeBellac, J. M. Levy-Leblond, Galilean electromagnetism, Nuovo Cim. 14B, 217 (1973)
in a first round now, but am struggling with some basic prerequisites, or maybe motivations for their undertaking (by the way, I am happy to spin this off to a separate thread, if generally preferred):
For example, the authors make a distinction between "mostly-timelike" and "mostly-spacelike" 4-vectors. This is the first statement I am struggling with: "mostly-timelike" suggests essentially the space components are extremely small, and vice-versa for "mostly-spacelike". But a relativistic 4-vector to start with is either timelike or spacelike, invariantly so, although of course by LT, the value of the space and time components each change. In a Galilean limit, so that we are looking at a Galilei spacetime, there is no causal structure, hence no "timelike" and "spacelike" at all!
OK, but let's move on. Accordingly, in eqs (2.1) and (2.2.) they define the 2 different Galilean limits. First of all, at this point, they are not Galilean limits at all, because ##c## is still in both expressions.
But then in §§2.2/2.3 they look at the electromagnetic 4-current ##j_\mu= (c\rho,\mathbf{j})## and look at the 2 cases, essentially, ##j_\mu## either "mostly timelike" or "mostly spacelike". But the 4-current is a timelike 4-vector to start with! How and by what limiting process will that get transmuted into a "mostly-spacelike" 4-vector, on order to justify §2.3?
The authors criticize on p.234 the result referenced from Landau-Lifshitz, which "coincides neither with (2.7) nor with (2.14)", and remark that this does not correspond to any kind of Galilean limit. I have not studied those parts of Landau-Lifshitz yet (they seem to refer to some old edition, where the section numbering was different -- §3.10 in the most current Landau-Lifshitz editions does not exist).
Although the calculations are quite scarce, the math seems to be right, as it is not overall complicated, but I am also beginning to fail to see the point in the overall investigation, and I am struggling with the assumptions.