Re: Electromagnetic wave and photon spin

On Tue, 18 Sep 2006, rds wrote:

First off I want to say that I am not physics major.

Can someone explain to me the details of how electromagnetic radiation?
Actually occurs. I know the pat answers that the acceleration of
charged particles or change in direction, but that doesn't really
explain the process.

Why do ripples spread out when you drop a rock in a pond? You disturb the
surface of the water, and the disturbance spreads outward. Change the
electromagnetic field in a region, and a similar thing happens. If changes
in the EM field can't travel instantly, then you expect waves, as the
changes travel outwards at the speed of the waves.

[cut]

Lastly the description of EM wave as abstract photons (all energy no
mass) makes sense to me in that the EM wave propagates as an E field
(sin) + an M field (cos). Is there an actual difference in the nature
of these two fields?

Yes. The E field exerts a force on any charge, and the magnetic field
exerts a force on a moving charge; F=q(E+vxB).

Or are they indistinguishable from one another
--more like two E-Fields with a 90 degree phase shift

Not at all. Apart from the difference in effect, they don't have a 90
degree phase shift - in a plane electromagnetic wave, the electric and
magnetic fields are in phase with each other. Their directions are 90
degrees apart (eg E might be straight up, and B horizontal to the right).

[cut]

The spinning at the rate of the transmission frequency.
This totally describes the EM field without even referring to a
magnetic
field. As the plate rotates its positive to the 3 and 9 o'clock
positions it
represents the M field peaks, and at the 12 and 6 o'clock position it
represents the E field peaks.

What you describe is approximately what happens to the two components of
the electric field in a _circularly-polarized_ wave. But these are both
components of the _electric_ field. (The magnetic field of the wave
behaves in the same way, too.) For a plane polarised (or linearly
polarised) plane wave, the average spin of the photons is zero.

I would normally call this point a photon
and the rotation I would call spin. But I read that photon spin is
actually in the direction of travel, and so the spin must be something
different. Any comments clarrifications is welcome.

What is the direction of a rotation? If you have rotation about an axis,
what way does the axis point? There are two directions you could use; the
usual convention is: curl the fingers of your right hand, and stick your
thumb out; if the rotation is in the direction your fingers curl in, the
rotation/spin/angular momentum vector is in the same direction your thumb
is pointing in.

--
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
.

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