Re: single photon through glass
- From: ajiko <ajiko2004@xxxxxxxxx>
- Date: Wed, 2 Jul 2008 02:01:24 +0000 (UTC)
"BW" <bjorn.wesen@xxxxxxxxx> wrote in message
news:cc79c846-5dc7-4041-a8b6-6f36334f52f0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jun 30, 7:53 am, "ajiko" <ajiko2...@xxxxxxxxx> wrote:
That's a core point for looking at this. I'm still not sure. Feynman'
setup uses a monochromatic source. His other examples were almost
always single photons. Not the case for the glass
reflection/transmission example.
There was a reflection analysis (grating) that required the originating
time at the source for the photon to vary. This implies that multiple
source photons went into the calculation for a single destination
photon.
Actually, I posted a related question about this here a couple of
weeks ago
but there were no takers :)
In case people don't know the passage I'm mentioning here, the story
is
that the photon propagator according to the first 100 pages of the
Feynmans
short QED book is described as having a phase component that varies
with
the pathlength. All kinds of experiments involving interference and
sum-over-
histories are described and explained from this principle. "The little
arrow
turns as the particle moves".
Then at around p 100, he in passing mentions that, oh by the way, the
propagator
does not modify the phase. It is really the light-source (emission)
which has a
*time-dependent* phase-rotation, thus combined with non-phase-
modifying
propagation it gives the *effect* of a path-length-dependent photon
propagation
phase. Is this correct btw ? I have never calculated with the real QED
propagators,
is the photon propagator phase-conserving ?
He used a coherent source to help with the reflection variation.
The reflection off deeper layers has longer path lengths so I think that
multiple source photons might be part of the reflection variation.
Don't really know.
The transmission variation looks at all paths straight through. These
all have the same length, but there is one absoption and re-emission
on each path (except one). (should be complex cascade of variations).
I'm not sure if this is supposed to be described as a time delay
or a phase shift. A time delay allows for multiple cycles. Is there a core
field theory concept here for only considering the phase?
A time delay immediately gives us slower travel. But the phase shift
seems to be the usual technique. As a time delay, the 90 degree time
equivalent would vary 2X from red to blue.
--Ned
Without knowing what went on, I assume he did it this way to avoid
having to
describe a space-time diagram directly at page 1 in the book (which
would be
necessary to describe interference-effects arising not only from the
spatial
configuration but also from the time-dependence of the emissions
etc).
So yes, according to that, multiple virtual emissions at different
times have to be
combined and interfered for the correct classical result to emerge..
/Bjorn
.
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