Re: teeny atoms absorb huge EM waves

[William Beaty <billb@ESKIMO.COM>]

On Tue, 3 Aug 1999, Leigh Palmer wrote:

> > I don't quite understand.  Here's where my brain-fuzz is located:
>
> Stick to one problem at a time.

OK, now I see the difficulty.  My counterarguements are defined as having
no bearing on the current debate!  :)


> You said that you believed the energy
> was located in the static electric field; I pointed out that it could
> as well be considered to reside in the geometrical configuration of
> the charges. Refute my claim or reject the idea that the energy can
> be localized.

Perhaps my intent is not clear.  My arguement is based on the consistent
application of explanatory concepts across wide regions of physics.  I
refuse to make decisions about one problem without considering the
implications this has for other problems.

My technique for deciding the location of the energy is to pick one of the
options, then look at the consequences this has for other parts of
physics.  For example, if I decide that the energy is located in the
fields, then I obtain a wonderful insight.  I discover that I can explain
energy in capacitors, waveguides, DC energy transmission, laser pulses,
etc.  I've found a 'unification.' On the other hand, suppose I decide that
fields do not really exist, and they cannot carry energy.  If I insist
that the energy of a pattern of charges is in the charges themselves, and
if I apply this idea to other EM problems, then I run into serious
contradictions.  I would predict that the energy in a microwave oven is in
the metal walls, not in the empty chamber.  I would also predict that
light does not carry energy.  Since simple experiments show that microwave
ovens cook meat, and that light can deliver energy to an object, then my
second choice was incorrect, and the energy in an EM system is contained
in the empty space where the fields lie.

I see this energy-location issue as originating from artificially imposed
boundaries.  If high frequency EM is defined to be a different problem
than low frequency EM, and if light must be a different problem from
capacitors (or configurations of charges), then we have installed an
artificial boundary, and created "artificial confusion."  If there is no
genuine boundary between DC and light, and if the energy carried by a
laser pulse is located within the boundaries of that pulse, then I
conclude that the energy in a group of charges is located in their
surrounding fields.


However, my above arguments do not entirely convince me.  There is a
difference between the capacitor and the light-beam.  It involves relative
energy versus absolute energy.  If a handheld laser-pointer loses energy,
and then a small spot on the wall gains energy, I could claim that "light"
moved between the laser and the wall, and everyone agrees that "light" is
real.  The light beam involves a transfer of energy; it involves a
relative gain in one place and a loss in another.  If we wish to know the
quantity of energy within a closed surface, we can integrate the flow of
energy through that surface throughout its entire history, and this gives
us information about the amount of energy which has built up within it,
but there is a problem: it does not tell us the initial energy within the
surface.

Where is the energy in a capacitor?  I can start out with an "empty"
capacitor, and watch what happens as I "charge" it by rectifying some
received microwave energy.  I'll follow the Poynting flux, and I'll decide
that the energy has been stored in the fields between the capacitor
plates.  But if somebody hands me an assemblage of point-charges and asks
me to locate the energy, then my reasoning no longer works.  The particles
themselves contain energy even when the capacitor has been discharged.
Where is their energy located?  I don't know, because I don't know the
diameter of the particles, or even if "diameter" is a sensible concept.
When transfers of energy can be followed, energy seems to have location.
When absolute energy is investigated, the location is not so clear.

A related problem: how much energy is in a particular region of empty
space?  If a gamma ray photon should create a positron and an electron, we
have a situation which is fundamentally the same as a neutral capacitor
being charged.  How much energy was in the vacuum before the gamma photon
arrived and created the two opposite "capacitor plates"?  I don't know.
I can assume that it was zero, but I have no way of knowing if my
assumption is valid.


> Energy is a quantity that can be calculated *given a physical system
> and a frame of reference*. You calculate it by integrating over all
> space outside the charges; I calculate *the same number* by looking
> only at the geometrical relations of the charges on a small scale. We
> calculate in the same frame; we get the same number. The energy can't
> be localized.

Maybe I've misunderstood everything.  Are you saying that a contact-force
theory (where particles interact w/fields) is identical to an
action-at-a-distance theory, and therefor we cannot tell whether the
energy is in the geometry, or in the fields which connect the particles?

But if charged particles apply forces to each other by exchanged photons,
then *something* is connecting the charged particles.  If we look only at
force and distance, we miss an important part of the problem.  Ignoring
the exchanged photons does not force them out of existence.  If
photon-exchange in reality does not exist, and if charged particles by
definition create distant forces, then I do see your point.  If fields
do not exist, light is simply a "delayed distant action", and the energy
in a system does not necessarily flow along as the light beam propagates.


> > In a short pulse of laser light, energy is located in the propagating EM
> > radiation, OK?
>
> No. The energy cannot be localized. Your difficulties are associated
> with pushing a metaphor too far.

A beam of light does not carry energy?  I don't know how to respond to
that.  Or do I misunderstand?  If a laser pulse has diameter, length, and
intensity, then there is a certain amount of energy within the boundaries
of that pulse.  If this is an incorrect concept, then exactly where does
it fall apart?  It works well for real-world power meters.  Move the
photodetector out of the light, and the received energy falls to zero.



> If one calculates the Poynting flux into something simple, say a
> current carrying cylindrical wire of resistive conductor, one will
> correctly infer the joule heat ing due to that current. There is
> absolutely nothing wrong with that calculation because it gives
> the correct answer. It is a perfectly satisfactory description of
> the system in the real world. If you wish to view it as a flow of
> energy (or, perhaps, heat) then you are free to do so, but it is
> misleading in my view. I just don't like to think of this energy
> as flowing in from infinity!

Not from infinity.  (If the Poynting flux came in from infinity, then I
too would dislike using it.)  For example, plot the Poynting flux around a
charged capacitor which is connected to a single resistor and is heating
it.  The flux extends outwards from the space between the capacitor
plates, but does not fly out to infinity.  Instead it generally flows
parallel to the wires, and exists almost entirely in the region where the
fields are significant.  When it arrives at the region around the
resistor, it bends inwards.  Each "flux line" which touches the resistor's
surface can be traced back to the capacitor's dielectric.  My
high-frequency argument then comes into play: if this was a 10GHZ system
instead of a low-frequency system, the Poynting flux would still take the
same path.  But at 10GHZ, small objects can be placed in the field, and
they will be heated: the "stuff" that flows along the wires and dives into
the resistor resembles light in every way except frequency.  My
light-concept has been unified with my circuit-energy concept, and I don't
perceive any conceptual barriers between optical radiation and the
Poynting flux surrounding a circuit.

Whew, big message.  I'll certainly understand if you ignore it as being
far to large to deal with.



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