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Re: POLARIZATION




At 12:13 6/13/98 -0700, [James McLean] wrote:
....
If linear polarization can be described as a superposition of circular
polarization, and vice versa, how can one be more 'basic' than the other?

--James McLean

[bw]
" Here's a weak, quantum argument:

" A linear wave has twice the amplitude of its two component helical waves.
" A helical wave has the same amplitude as its component linear waves.
" In the limit, smaller helical waves are permitted than linear waves. QED


[bw]
This is a 'reductio' argument for the least possible magnitude of two
components of an electromagnetic wave which can yet synthesize a wave
of a combined type.

[Leigh]
Surely you realize that the "magnitude" of an electromagnetic wave
is quantity with which has not been generally recognized in the
physics community. Perhaps you should rethink what you say above.


I think that one or more of the words in Leigh's sentence may have been
lost in the transmission. As far as I can tell, Leigh points to the
'unknown magnitude' of the photon here, I think.

This is the clearly testable prediction of my position:
1) One can use two photons of 'unknown amplitude' but known frequency
to make a linearly polarized em wave of twice that 'unit' amplitude

2) One will find it IMPOSSIBLE to use one photon of that same frequency
to make a linearly polarized wave.

3) One will find it possible to use one photon of that same frequency
to make a helically polarized wave.

There; what could be a more clear-cut, understandable and falsifiable
prediction?

In the development you suggest above there is no limit to the
degree to which one may make "smaller" waves. It is not a quantum
argument at all.

This is not the case: I hope my prediction (above) will clear up this
error of understanding.


This analysis is seriously flawed, principally because of a common
conceptual error. If one wishes to think about photons (and I will
usually be the last to adopt a photon model to explain physical
phenomena) then one must recognize that the components of a photon
are not themselves photons.

Flawed it may be, or not? But here I state what I feel is obvious:
the irreduceable has NO components. Instead, it may be said to have
describable features.
It was Leigh who insisted that the photon was adequate to describe
polarization phenomena, after all!

"Components" refers to the terms in a
linear combination of wave functions. These are mathematical
entities; they are not physical ingredients which one may combine
to form composite systems. The photon itself is an indivisible
quantum, yet its wave function can readily be expressed as a
linear combination of wave functions.

This seems to be the root of Leigh's discomfort: I assert that either
polarization is an emergent statistical quality arising from large
populations of photons, or that it is experimentally tractable at the
single photon level. If the latter is the case, then my position is
that helical polarization can be demonstrated with less photons of a
given frequency (perhaps just one) than can linear polarization.


[bw]
I say here that if two helical waves can have smaller amplitude than
the least amplitude of a linearly polarized wave, then ipso facto the
helical waves may be said to be more basic or fundamental than linearly
polarized waves.
One concludes that some equivalent of circular polarization exists at the
quantum level, and one supposes this to be a 'spin' property.

I see that not only is this 'weak' but difficult to follow as well?

[Leigh]
I can't follow it; perhaps someone else can.


This is a topic that cannot be found in your texts. It does take an
effort to comprehend. Would anyone like to step up to the lab bench
to grab for the brass-ring?
It is I suppose just conceivable that a significant discovery
could be made of this. Nobel, anyone?
********************************************************************

[Moving on to the putative loss of momentum of a single photon...]

[Leigh]
OK. We adopt a photon model. (More on this later.)

Isystem = circularly polarized photon + ideal linear polarizer.
Polarizer is initially at rest; photon is propagating in direction
normal to polarizer. Photon has been prepared by dilution from a
beam of circularly polarized light and possesses nonzero linear
and angular momentum.

Interaction occurs.

Final states of system possible, both equally probable:

Fsystem1 = polarizer moving with initial linear and angular
momentum of photon. The photon has been absorbed by the polarizer.

Fsystem2 = polarizer moving with initial angular momentum of
photon, linearly polarized photon moving with undiminished linear
momentum.

The usual idealizations apply to these interactions.

Now I ask the question: what is the value of adopting a photon
model to explain this very simple wave phenomenon? It is not
difficult to do so, but where is the conceptual payoff? In what
way is this picture superior to the conventional picture? Is
there some inherently quantum mechanical aspect to the
phenomenon? In my opinion the gratuitous introduction of photons
encourages conceptual errors. Photons are a fine way to treat
the photoelectric effect; they don't help anyone to understand
polarizetion.

The value of the photon concept here is that if a mind experiment
leads to a result which demands a photon denuded of some essential
feature (such as this one), we can immediately say: that
mental-experiment is in error!


I apologize for not answering your previous posting of this
question, Brian. I was trying to remain consistent in my policy
of ignoring threads that were initially cross-posted to both
groups for no good reason (the POLARIZATION thread started that
way). I had changed the subject line to be helpful, but for some
reason you changed it back.

It is now my turn to give the written equivalent of a blank look.
I subscribe to just one physics list. I have consistantly responded
to a subject line of "re: POLARIZATION".
I have not changed this subject line in any way.

It is true that if a post of mine has not echoed back within a day
or two, I try again using the 'mailer.uwf' domain, rather than
'atlantis.uwf'. (Following a helpful suggestion from Ludwik.)
Does this in fact mean that I have skipped between two lists of the
same name, "phys-l"?

If so I apologize for my ignorance. It will be necessary for me to
know what two lists Leigh has in mind, for me to be able to avoid
inadvertantly careening between them....


Others may not object to gratuitous cross-posting; I do. I don't
require that others share my feelings, but I reserve the right
to ignore such crosspostings.

Leigh

I will post this to mailer.uwf, but I would appreciate a public or
private description of the several? manifestations of this list.....

Brian Whatcott Altus OK