sci.physics.electromag

On Mon, 10 Mar 2008, Bill Miller wrote:

>
> "Timo A. Nieminen" wrote in message
> news:Pine.WNT.4.64.0803090613190.296@serene.st...
>> On Sat, 8 Mar 2008, Benj wrote:
>>
>>> On Mar 7, 7:17 pm, "Timo A. Nieminen" wrote:
>>>
>> No. If you use the Maxwell equations, you are taking retardation into
>> account. Notice that you can extract the wave equation from the Maxwell
>> equations.
>
> Sorry, Timo, but Maxwell's equations are a wonderful example of action at a
> distance. Retardation plays no part whatsoever in their constitution.

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).

> 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.

The accounting for retardation in the usual version of classical
electrodynamics is implicit, not explicit. The equivalence of the Coulomb
and Lorenz gauges shows that it is there. But it's there in Maxwell's work
too, in his assumption that there is a mechanical medium - the ether -
through which effects take a finite time to propagate.

> 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).

--
Timo