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Science Forum Index » Physics - Research Forum » Longitudinal electrical field in black-body radiation?...
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| Robin Whittle... |
 Posted: Sun Jun 01, 2008 3:53 am |
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Here are some questions which are perplexing me.
When a short monopole antenna is driven by a sinusoidal signal, the
resulting electromagnetic radiation (emr) has an electrical component
aligned in the direction of travel of the emr. At least this would be
true when the antenna is significantly shorter than the wavelength.
Another monopole antenna would be able to pick up the signal. This
situation is in principle the same as one plate of a capacitor
coupling a signal to the other plate.
A free charged particle such as an electron in vacuum would be moved
alternately towards and away from the transmit antenna, with all
movement parallel to the direction of travel of the emr.
The usual descriptions of black body radiation involve the electrical
field being perpendicular to the direction of travel - aligned along
one arbitrarily chosen direction perpendicular to the line of travel
and also along the other direction perpendicular to this and to the
direction of travel.
Since black body radiation results from random movements of charged
particles in three dimensions, wouldn't some component of the
resulting emr have its electrical field aligned in the direction of
travel?
A calcite crystal can separate black body light into two beams, each
of which is electrically polarised perpendicular to the direction of
travel. Those two beams seem to contain all the energy of the input
light.
Suppose we create some emr, for instance with a microwave transmitter
with an electrical field radiating antenna much shorter than the
wavelength - such as 1mm long with a 1GHz (300mm) signal. I would
expect essentially all of the emr's energy to be in the form of
electrical field oscillations where the field variations are parallel
to the line of travel. This means a charged particle in space would
be moved alternately towards and away from the antenna, but not side
to side. This also means that a short (say 10mm) dipole antenna with
one wire pointing towards the transmit antenna and the other away from
it would pick up essentially all the signal.
What would happen if we put that signal through the microwave
equivalent of a polaroid polarizer? No signal should pass, I would
think, because the polarizer only passes emr with its electrical field
aligned at one of the axes perpendicular to the line of travel.
Black body microwave emr from cool or warm objects is detectable
electronically. Would this be true if the dipole detector antenna was
aligned so its wires were as described above - parallel to the
direction of travel?
- Robin http://astroneu.com (Critique of the
Big Bang Theory) |
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| nuny at (no spam) bid.nes... |
| Posted: Sun Jun 01, 2008 5:41 pm |
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On Jun 1, 6:53 am, Robin Whittle <r... at (no spam) firstpr.com.au> wrote:
Quote: Here are some questions which are perplexing me.
When a short monopole antenna is driven by a sinusoidal signal, the
resulting electromagnetic radiation (emr) has an electrical component
aligned in the direction of travel of the emr. At least this would be
true when the antenna is significantly shorter than the wavelength.
There can be no such thing as a "monopole antenna".
What "when a short monopole antenna is driven by a sinusoidal
signal" physically
means is shoving electrons in and out of a conductive object. Whatever
method you use to shove the electrons into/out of the object
(alternately charge it negative/positive) will inevitably leave some
other object with a dearth/excess of electrons (alternately charge it
positive/negative), the charges will naturally spread themselves
across the surfaces of your intended monopole and that other object,
and an electric field will reach across empty space to connect them;
that's your basic dipole. That field will alternately intensify and
weaken as you shove charges back and forth reversing direction each
time; it's known as the "near field". Its electrical component is
indeed longitudinal but by definition does not radiate.
If the physical dimensions and separation of the charged objects fit
certain parameters, the time-varying near field can shed loops which
propagate away as "far field radiation" aka photons comprising
transverse electric and magnetic field components. (In free space
those fields are solely transverse; in dielectrics there can also be
longitudinal field components).
Quote: Another monopole antenna would be able to pick up the signal. This
situation is in principle the same as one plate of a capacitor
coupling a signal to the other plate.
A capacitor IS NOT a pair of monopoles. You need to understand the
difference between "near field" and "far field".
(Everything following snipped due to your garbled understanding.)
Mark L. Fergerson |
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| Boo... |
| Posted: Mon Jun 02, 2008 5:07 am |
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Quote: There can be no such thing as a "monopole antenna".
What "when a short monopole antenna is driven by a sinusoidal
signal" physically
means is shoving electrons in and out of a conductive object. Whatever
method you use to shove the electrons into/out of the object
(alternately charge it negative/positive) will inevitably leave some
other object with a dearth/excess of electrons (alternately charge it
positive/negative), the charges will naturally spread themselves
across the surfaces of your intended monopole and that other object,
and an electric field will reach across empty space to connect them;
What if the object was a white hole though ?
--
Boo |
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| Robin Whittle... |
| Posted: Mon Jun 02, 2008 4:54 pm |
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Hi Mark,
Thanks for your explanation. I found a few references where the
distinction between near and far fields is discussed. Can you point
me to any others?
By Googling (electromagnetic photon "near field") I found some
interesting looking papers. There is a bunch of stuff by Ole Keller
http://personprofil.aau.dk/Profil/107076 http://vbn.aau.dk/research/(9694)|publications?pageSize=500
regarding the near-field and the "birth of photons" - apparently at
the edge of this near-field, rather than at the radiating atom or
molecule itself. I haven't yet got copies of these papers, but I look
forward to reading them - including:
Near-field optics and quantum optics : an assignation arranged by four
kinds of photons
Optical near-field interaction on the basis of photon wave mechanics
(Abstract at end of this message - mentions "photon embryo state".)
On the emergence of a polychromatic photon from a single atom
Near-Field Optics : The Nightmare of the Photon
I don't have a satisfying understanding of what the term "photon"
means regarding emr being inherently quantized, though its absorption
and creation by matter often seems to be.
Googling "near-field optics" turns up mainly pages on microscopy, such
as: http://monet.unibas.ch/snom/
These paper might be relevant:
Near-field mapping of the electromagnetic field in confined photon
geometries
http://physics.technion.ac.il/~dg/PAPERS/PRB/prb02_3.pdf
The History of Near-field Optics, Lukas Novotny
http://www.optics.rochester.edu/workgroups/novotny/papers/history4.pdf
You wrote:
Quote: If the physical dimensions and separation of the
charged objects fit certain parameters, the time-varying
near field can shed loops which propagate away as
"far field radiation" aka photons comprising transverse
electric and magnetic field components.
Do you mean that the near-field is not comprised of photons?
Quote: (In free space those fields are solely transverse; in
dielectrics there can also be longitudinal field components).
I don't understand why there should be such a difference between free
space and a dielectric.
I read that in the near-field, the electromagnetic energy is not
necessarily radiating freely. So in this framework, photons are
quantized packets of electromagnetic energy which radiate freely,
independently of the transmit antenna, atom, molecule etc.
This seems to be a rather different view of "photons" from them being
particle like things which are absorbed or created by single atoms,
molecules etc. I wonder how the momentum of the emr (distinct packets
of momentum for each "photon") is coupled to the matter if the photons
themselves arise at the edge of the near-field. What then - if not
"photons" - is coupling energy and momentum inside the near-field?
Boo: I don't understand "white hole".
- Robin Whittle
http://astroneu.com (Trying to explain the heating of the solar
corona and acceleration of the solar wind.)
Optical Near-Field Interaction on the Basis of Photon Wave Mechanics
Ole Keller Journal of Nonlinear Optical Physics and Materials, Volume
12, Issue 04, pp. 393-417 (2003)
Near-field optical aspects of classical electrodynamics are brought
into focus by dividing the electromagnetic field into its transverse
and longitudinal vector-field parts. A transverse electromagnetic
propagator formalism thereafter is used to study the field-matter
interaction in the transverse current density domain, the birth domain
of the photon. Subsequently, a brief summary of photon wave mechanics,
the first-quantized theory of the photon, is given, paying particular
attention to the dynamics in the near-field zone of matter (atom,
molecule, mesoscopic particle). In the wake of a discussion of the
relativistic transformation properties of the covariant photon field
matrix the photon energy wave function is introduced. In a central
section, photon wave mechanics and near-field optics are brought in
contact, and the photon embryo state, the polychromatic photon
concept, and the quantum mechanical theory for the transverse one-
photon current density discussed. |
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