sci.physics.electromag

On Sun, 23 Sep 2007, maxwell wrote:

> 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.

Both present time fields or present+past source work for describing
reality. How can we tell which is foundational? If we want a present-time
meaning of conservation of energy, momentum, and angular momentum, these
must be somewhere, and fields must be real. How can something not real
have energy, momentum, and AM? If we give up this present-time picture,
then we can have something like Feynman-Wheeler, but, as "absorber theory"
suggests, there _must_ be something to absorb the radiation, else the
conservation laws again fail. If radiation depends on what happens in the
future, what then of causality? Both are deeply problematic, but the
present-time fields view is more aligned with our preconceptions, and is
naturally preferred in teaching and thinking by most.

[cut, but I note that Roche's paper (in European Journal of Physics,
which is about physics education, rather than physics research, so
contains useful and lucid papers) is very worthwhile reading.]

> 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

Originally apeared in Annalen der Physik, and is available online from the
French national library (in German). Also in his collected works (in
French translation), also out of copyright and available online. The
English translation in Phil Mag is a bit dodgy, but OK, and certainly
easier for Anglophones.

> 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.

I must disagree on some of this. Yes, there is no insight into the "how".
Given general covariance or Lorentz invariance as a requirement, what must
a field theory look like? What should the field equations look like? What
are the restrictions on the constitutive relations? Yes, this is at the
formal mathy end of physics, but I think there's more than "almost zero"
physics there.

--
Timo Nieminen - Home page: http://www. /people/nieminen/
E-prints: /view/person/Nieminen,_Timo_A..html
Shrine to Spirits: /timo_nieminen/