sci.physics.electromag

On Mon, 10 Mar 2008, Bill Miller wrote:

> "Timo A. Nieminen" wrote:
> > On Mon, 10 Mar 2008, Bill Miller wrote:
> >> "Timo A. Nieminen" wrote:
> >
> > Not so. Consider Maxwell's remarks on Faraday vs the German
> > electrodynamicists (I'd quote it if the appropriate google words came to
> > me, but they don't).
>
> Hmmm... I wish I could refer to non-referenced sources to prove MY points.
> Oh well.

The relevant quote is:
"saw lines of force traversing all space where the mathematicians saw
centers of force acting at a distance. Faraday saw a medium where they
saw nothing but distance: Faraday sought the seat of the phenomena in
real actions going on in the medium, they were satisfied that they had
found it in a power of action at a distance impressed on the electric
fluids."

My copy-and-paste source says this is from Treatise, 1861, which is
clearly wrong. A quick look through his 1861-2 papers didn't find this, so
perhaps it is from the Treatise.

Anyway, the point is that Maxwell assumed a mechanical medium, through
which effects must propagate at a less-than-infinite speed.

> >> Likewise, extracting the wave equation has nothing to do with
> >> retardation.
> >
> > It has everything to do with it. If you get the wave equation, with a
> > non-infinite speed of propagation of effects, then you have retardation.
>
> Yes, but Maxwell's Equations do not provide explicit information about the
> nature of the retardation.

Implicit, but included.

> In other words, Maxwell's equations -- of and by
> themselves -- do not define the nature of the starting points of E and H.
> See both Panofsky and Jefimenko, as well as McDonald, for an analysis of
> what actually *causes* E and H.
>
> Hint: E cannot cause H and H cannot cause E because they occur
> simultaneously. Causality requires that a cause must precede an event. It
> cannot be simultaneous with the event.
>
> Of course, if you wish to discard causality, then all your statements are
> correct. But many of them, like E causes H & vice versa seem to defy
> experimental verification.

That's not my statement. I'd say that it's pretty clear that
charge/current causes EM fields, although this isn't a necessary
conclusion from the Maxwell equations.

The usual relativistic view of E and B as components of the same rank-2
4-tensor means that neither can be the cause of the other.

That said, "changing E causes B" and vice versa is a common just-so story
in introductory textbooks. I wonder why? I also see statements such as
"the fields produced by the antenna propagate outwards at the speed of
light", which is essentially nonsense, as the _fields_ don't propagate -
the field exists everywhere already. _Changes_ in the field can propagate,
but that's different. For a simple analogy, a water wave is a change in
the height of the water surface, a deviation from the equilibrium height,
that propagates along the surface. It makes no sense to say that the
height of the water surface propagates, but this is just what "the field
propagates" would mean.

Oh well, that's textbooks and the usual textbook-myths for you.

> > The accounting for retardation in the usual version of classical
> > electrodynamics is implicit, not explicit.
>
> Herein lies a major problem. Something as important as retardation should
> _never_be implicit. Too many erroneous conclusions can arise if the implicit
> nature of something is not made explicit.

But it's convenient to make it implicit - a set of equations that only
includes the present-time values of charge and current density means that
we don't need to keep track of past values.

I don't think this is a big issue where applications are concerned, but it
is a pedagogical issue, so it's good to see it get some attention.

I recently realised that Lorenz electrodynamics is the simplest version,
with a seamless join between the tradition 3-space + time version and the
SR spacetime version. A pity this isn't more useful in practice. It's got
a definite place in particle physics, what with the Lienard-Wiechert
potentials being the relevant free-space Green function, but I wouldn't
want to do antenna problems or optics problems that way.

> >> It is interesting that "Displacement Current" provides the missing
> >> ingredient without which the wave equation does not exist. And THAT has
> >> led
> >> several generations of scientists and engineers to believe in the
> >> physical
> >> existence of Displacement Current as a *cause* of a magnetic field, even
> >> though over a century of measurements have not detected any such
> >> magnetism.
> >
> > A different story (but an interesting one).
>
> Yep. Except that it causes otherwise-brilliant engineers and physicists to
> dogmatically (there's that word again) hang onto concepts that should have
> been expunged from textbooks decades ago.

Probably even worse with the non-otherwise-brilliant ones.

For a nice story about dogma and preconceptions, see Hanbury Brown's
book ("Boffin") for his tale of the reception of the optical intensity
interferometer in the quantum optics community. (A worthwhile book in its
own right!)

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html