sci.physics.electromag

On Sep 8, 5:26 pm, "Timo A. Nieminen" wrote:
> >> ...
> >> Present-time field theory is just an alternative formulation of a
> >> retarded-time AAAD theory ( ., Wheeler-Feynman). Is it just a
> >> mathematical convenience?
>
> > On Fri, 7 Sep 2007, maxwell wrote:
> > Present time theories result from assuming the existence of
> > instantaneous interactions across space (the consequence of the
> > incorrect Coulomb gauge used by Maxwell - only the Lorenz gauge is
> > real as this represents the real time difference between emission and
> > absorption!) This is just another example of mathematicians ignoring
> > physics so they can continue to use (single-time) calculus.
>
> Not necessarily. That is, you don't need to assume instantaneous
> interaction across space - that's the whole point of having a field. The
> field is a present-time device that includes the effect of all present and
> past charges/currents.
>
> This was my point. We can either track the present-time field (and it's
> first derivative wrt time), or we can track all present and past charges
> and currents. Sometimes the field is more convenient, sometimes the
> charges/currents.
>
Physics is not about convenience - that is a solid enfineering
approach; physics is about foundational concepts & whether these are
suitable for describing reality. Electrical charges (electrons) match
our criteria for real objects - fields do not.
The consequence of building fundamental theories in physics upon weak
metaphysical foundations is well-illustrated by the ongoing
controversy associated with Maxwell's "displacement current". As
recently as 1998, Roche published a paper in a leading physics journal
that reviewed the extended history of this dispute into the nature of
one of Maxwell's major theoretical innovations. This often-vehement
disagreement centers on the question of whether this concept
represents an electric current (in free space) in some real sense or
not. In Roche's view, this issue reflects the contemporary response
to unresolved interpretations of electromagnetism inherited directly
from the 19th Century. Only two years after publication of Maxwell's
famous EM paper "Dynamical Theory" (of the aether) in 1865, L. V.
Lorenz had published in The Philosophical Magazine ALL of Maxwell's EM
results without any use of aetherial concepts but this paper is so
little known (although briefly acknowledged by Maxwell in his 1873
Treatise) that in 1894 Poincare had to re-establish the fact (without
mentioning Lorenz) that calculations with retarded electric
potentials, propagating at light-speed, only need reference the real
conduction sources. He compared this with Maxwell's EM field theory
that used (incorrectly) the instantaneous (non-retarded) Coulomb
potential plus the 'displacement current' to calculate Maxwell's
vector potential. Unfortunately, neither Poincare nor Lorentz could
face up to the logic of their own analysis as they retained their
belief in the reality of the luminiferous aether with its time-varying
stresses, in other words, they believed in the reality of Maxwell's
'displacement current'. Although acknowledging a few EM theorists had
dismissed this aetherial concept, such as F. W. Warburton's 1954 paper
entitled "Displacement Current, a Useless Concept", Roche also could
not face up to the fury of Maxwell's vociferous defenders and support
the Newtonian view that Lorenz's action-at-a-distance EM theory of
retarded potentials had successfully replaced any need for mysterious,
all-pervasive fields as early as 1867. The survival of this
controversy to this day demonstrates that old ideas in physics die
hard, especially when they are propagated by generations of textbook
writers who continue to repeat what their own professors told them as
undergraduates, without any knowledge of the historical evolution of
their subject (Timo, this does not include you.)
Incidentally, readers should be warned that Timo's recommendation of
Post's EM book fails to mention that this book is an exercise purely
in applied mathematics as its title accurately describes ("Formal
Structure of Electromagnetics"); readers will find almost zero physics
& no insights into how Maxwell's Equations became the be-all & end-all
of classical field theories.