From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
Subject: Re: My son would like to know about photons...
Keywords: photon,size
Organization: Computer Science Department,  Stanford University.
Distribution:  sci.physics
Date: Mon, 7 Sep 1992 06:00:08 GMT

In article <17u6gpINNs9s@smaug.West.Sun.COM> Richard.Mathews@West.Sun.COM (Richard M. Mathews) writes:
>bhoughto@sedona.intel.com (Blair P. Houghton) writes:
>>But Daddy, why are some photons bigger than others?
>
>For every photon there exists a reference frame in which the photon is
>any given energy, so all photons are the same "size".  It is the matter
>they are passing which is different.

Down under, photon size is a spatial attribute.  At Narrabri in New
South Wales the Hanbury-Brown stellar interferometer uses two antennae
to measure the width of photons from a star.  (I can vouch personally
for this nifty gadget, having visited it in the 1960's when Don
Herbison-Evans was operating it---Don eventually saw the light so to
speak and became a computer scientist.)  When the antennae are closer
together than the width of a photon, interference fringing occurs due
to both antennae seeing the same photon.  The antennae are moved apart
until the fringing disappears, i.e. drops below a given threshold,
giving the nominal width of the photons.  Typical photon widths ranged
from a few feet to hundreds of feet.

Photon "width" is really the uncertainty in lateral position of the
photon, which by Heisenberg increases with increasing certainty of the
lateral momentum.  This latter is in turn proportional to the degree of
collimation of the light from that star, which by elementary geometry
is in inverse proportion to the angular diameter of the star as
perceived from Narrabri.  This was the primary function of the stellar
interferometer, namely to measure the angular diameter of stars.  But
it also served the pedagogical purpose of making QM seem more real.

So photons from stars are very wide pancakes, and the further they
travel the wider they get.

Now, I have a couple of questions for the QM experts out there (which
I'm not, I saw the light even before Don).  First, in what sense if any
do photons travel?  The idea of a photon as a *transmitted* or
*received* quantum of light is ok, but what about a photon in flight?
If a photon leaves a point on the surface of a star and is received at
the Narrabri interferometer, how can its width depend on the diameter
of the star unless it somehow merges with other photons from the rest
of the star to form an unquantized beam of light which is then
quantized at the receiving end?

It seems to me that photons can exist only at the transmission and
reception of light, and that once on the move they cease to be discrete
photons and blend into the overall EM radiation.  If so, is this
blending modeled by linear superposition of the respective wave
functions of the transmitted photons?  And is there any useful sense in
which one can say that photons have an independent existence while in
flight?

Question 2.  How thick is a photon?  (Measured along the direction of
propagation.)  Can the Narrabri interferometer measure photon thickness
by bringing the antennae together (to within a few feet) and then
moving one of them towards and away from the star to see over what
range the fringing persists?  Is that range a valid measure of photon
thickness?  And what does QM predict that range to be?


======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, zero was
======================================================| there when He arrived.
-- 
======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, but zero was
======================================================| there when He arrived.