sci.physics.electromag

On Sat, 8 Mar 2008, Benj wrote:

> On Mar 8, 3:28 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.
>
> Um excuse me but the usual form of Maxwell's Equations are NOT causal
> which is an error right off the bat, PLUS the usual argument
> "extracting" a wave equation is usually based on an argument that an E
> field "creates" an H field and vice versa. That is a statement
> demonstrably incorrect!

Define "causal".

The usual form of the Maxwell equations takes retardation into account.
This is one of the big mathematical advantages of a field theory - you
only need the present-time charge and current distributions, the field has
all the information we need about past charge and current.

There is no "usual argument" for extracting a wave equation from the
Maxwell equations, it's a mathematical derivation, based on the
mathematics alone. No argument that E creates H or vice versa is needed.
That some people might use such language to explain the derivation in no
way makes the derivation invalid or dependent on such assumptions.

The usual form of the Maxwell equations (the Hertz-Heaviside equations) is
equivalent to electrodynamics done using potentials only, in the Lorenz
gauge, which explicitly takes retardation into account. Thus,
Maxwell-Hertz-Heaviside takes retardation into account, even if not
obviously.

>> No, _you_ are saying that physics is a religion. I don't see how a
>> reasonable person could interpret a statement that the only definitely
>> correct stance is agnosticism as "religion".
>
> Obviously agnosticism is a "religion" (system of belief) but that
> wasn't what I was talking about. Agnosticism is a valid position of
> science. (I don't know! However the rest of that "this can NEVER be
> known" is clearly NOT valid in science) My attack upon Fred was
> because of his overt insistence in validity of physics dogma (fields
> exist, Maxwell's equations are absolutely true in ALL cases and never
> fail, etc.). These statements are clearly matters of "faith" for him
> rather than a result of a careful examination of all experimental
> evidence. I call matters of "faith" religion. Don't you?

No. I call religion religion. You appeared to be denying the reality
fields, as a matter of dogma, without any justification.

You attack supposedly-dogmatic belief in the existence of fields as
"religious". You attack the agnostic statement that we don't know whether
fields are really "real" as "religious". Would you attack your own dogma
as "religious"?

You also mis-characterise Fred's claims. (Again, what's with the cheap
debating tricks?) I didn't see any claims that Maxwell's equations are
absolutely true in ALL cases and never fail, though I did see an
acknowledgment that Maxwellian fields are a classical average of quantum
behaviour.

A physicist's statement that a "theory is true" is very different from a
mathematician's statement that a "theorem has been proven". Both will use
"true", "proof" and "correct" in ways that significantly differ from the
everyday English usage.

If you want to discuss physics (or the history of physics, or the
philosophy of physics), do so. If all you want to do is make unsupported
dogmatic statements, attack your respondants, raise strawmen, and practice
your usual cheap debating tricks, then there isn't much point in replying
to you.

What does need to be brought is exactly what is meant by "exist". It is
absolutely certain that electromagnetic fields exist in the sense in which
they are needed in order to be used in the Maxwell equations - D and H are
functions of space and time related to charge and current densities by the
Maxwell source equations, and E and B are functions of space and time
giving the force that would be exerted on an idealised test
charge/current. But this is existence in a mathematical sense, not a
physical sense.

Which is why I think conservation laws are very important here. If the
fields possess energy, momentum, or angular momentum (or other fields
possess charge, mass, or whatever), then one can say they are physically
real. The current situation is ambiguous.

>> Physics isn't about "truth", because we don't know, and perhaps can't
>> know, what is "true". Physics is about what works, and fields work very
>> well. While your last sentence is OTT (there's no need for "constantly" in
>> practice), field theories of electromagnetism are pretty much tested
>> constantly against observation. And they work, and work well. Where they
>> fail (entering the quantum domain) is well-known.
>
> And your point is? Lessee is the earth flat or round? Oh hell, who
> can know? Who gives a damn? Our current navigation maps work pretty
> well around our known world so why bother with these stupid questions?
> EVERYBODY "knows" the earth is flat and "modern" navigation "proves"
> it every day! It's pretty much tested constantly every day by ships
> sailing the Mediterranean and where it doesn't work (off the map where
> the sea monsters be) the failure is well known.
>
> Do you realize that this is in essence what you are arguing?

Do you realise that this is, in essence, merely your strawman?

Roundness/flatness of the earth is easily determined, with science even,
and was well-known to the ancient Egyptians, Greeks, and Chinese (and
likely others).

Anyway, the point is that physics _can't_ be dogmatic, at least for now.
You complain that physics is too dogmatic, and now you complain that it
isn't dogmatic enough, that it doesn't proclaim truth?

Your point is?

--
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