Re: Coherence length and bandwidth of photons
- From: "Robin Whittle" <rw@xxxxxxxxxxxxxx>
- Date: Wed, 31 Jan 2007 21:42:19 +0000 (UTC)
Thanks Arnold and Igor for your helpful responses. I will
purchase a copy of "Optical Coherence and Quantum Optics".
I have printed and will read Paul Kinsler's message, his
diagrams and some other material:
http://www.lns.cornell.edu/spr/2000-02/msg0022377.html
http://www.lsr.ph.ic.ac.uk/~kinsle/QO/quncert1994/
http://www.lsr.ph.ic.ac.uk/~kinsle/QO/thirdom1995/
kinsler-1995nqec.pdf
Arnold wrote of photodetectors:
Thus the clicks are an artifact of photodetection caused
by the quantum nature of matter, rather than a proof of
photons arriving!!!
I agree. The atoms or molecules of the photodetector are
resonant systems with discrete energy levels. While I don't
think anyone understands exactly why there is a relationship
between energy and frequency, I understand how this leads
to discrete energy levels where electrons are captive in a
system with discrete resonances.
I don't think empty space with static or varying magnetic
fields is a resonant system and I do not currently believe
that electromagnetic radiation itself is quantized. I know
there are experiments which are widely believed to prove
this, but I don't yet fully understand these experiments,
their conventional interpretation or the various critiques
of these interpretations.
Its a mystery to me why a seemingly diffuse field from one
or more emitters can illuminate a surface, including very
dimly, and we perceive discrete deposition of energy at
localised points. But that doesn't make me think that emr
itself is quantized, because it seems the emr from multiple
synchronous sources can interfere perfectly well to produce
an emr field at a particular location which is
indistinguishable from that which results from a single
source. (There is a "photon" view of this - that to the
degree the sources are in phase, these emr waves can be
considered to be "the same photon" and so can interfere
according to QED - but I don't find this satisfying.)
Thus the talk about photons is usually done
inconsistently; almost everything said about photons
should be taken with a grain of salt.
The same could be said about "god", "the unconscious" or
"spiritual" which are well known words, without precise,
agreed definitions - concerning things which are not
amenable to scientific experiment. These words are
nonetheless widely used, with a subtly or wildly different
meaning for every occasion, because there is is a question
about something we don't understand or agree upon, but for
which we want an easy handle on, at least for the purposes
of discussion. The danger is in thinking that such things
really exist or that the terms have a precise meaning.
I am one of a long line of people trying to find a
satisfying theory of emr and of its interaction with matter.
Ignoring for the moment Willis Lamb and other malcontents, I
am trying to find if there is a single, agreed, notion of
photons within mainstream QM. What I find is a series of
statements in QM mathematical language which at best I only
partially understand (and intend to understand better). I
can't yet tell how consistent these descriptions are, or
how consistently they are applied by different people to
physical questions such as those I raised in my initial
message.
It seems the QM position is like Repo Man saying "You don't
want to look in the trunk, Officer." - that beyond a
certain point all that we know, and all we can ever know, is
most succinctly represented by specialised mathematics. So
attempts at physical questions such as mine are met with
responses along the lines that the questions are not right,
and the closest answers result from theoretical constructs
such as "photon number" and thinking of empty space with
electromagnetic fields as if it was really made up of
oscillators.
Since current physics cannot explain the heating and
acceleration of the solar corona, I suspect that there is
something about current electrodynamic theory which prevents
people from seeing how sunlight interacting with sparse
plasma is really the source of energy and momentum in the
corona and wind. Because I think there is something
inadequate about current electrodynamics, I still want to
look in the trunk.
I read some of Willis Lamb's "The Interpretation of Quantum
Mechanics", including especially "Anti-Photon". I need to
reread it once I understand QM more, but I couldn't derive
any physical satisfaction from what he wrote because I
perceive that he was still trying to describe emr in terms
of superpositions, wave functions and photon states.
Maybe I am mathematically challenged, but I expect
mathematics to be a precise adjunct to a fulsome, textual
and diagrammatic explanation of a theory about physical
reality - not the whole explanation. For instance, I don't
need a precise understanding of Maxwell's equations to have
a good, physically satisfying, notion of static and
cyclically changing electromagnetic fields - and therefore
electromagnetic radiation - in a classical sense.
Willis Lamb's statements about photons not existing are at:
http://web.archive.org/web/20040203032630/www.aro.army.mil/phys/
proceed.htm
I tried to ask questions about the physical state of emr
which has left a black body source, and its state after it
has passed through a narrow absorber. I tried to ask how
this would be understood in terms of photons, and what
experiments could be done to measure the two different
sets of emr in terms of photons, which I understand must
physically exist as discrete packets of energy, if the the
quantum theory of light is correct.
Arnold wrote, in part:
I think the most apt interpretation of an 'observed'
photon (in contrast to the photon formally defined as
above) is as a low intensity coherent state. Such a state
consists mostly of the vacuum (which is unobservable hence
can be neglected), and the contributions of the
multiphoton states are negligible compared to the single
photon contribution. With such a notion of photon, most
of the actual experiments done make sense.
When the above collapses into my brain, it does not deposit
sufficiently physical impressions for my satisfaction.
A black body emits emr. The emr propagates through vacuum.
This is a real thing, completely independent from whatever
gave rise to it and whatever may interact with it in the
future. It is a bunch of fluctuations of the
electromagnetic field, which we could make with antenna and
electronics if transistors were about a million times faster.
Describing emr as being in a single or multi "photon state"
does not help me understand what is meant by a photon,
assuming photons physically exist and that theories
concerning photons are absolutely essential to
scientifically explaining this aspect of Nature. If a
photon is just a handy way of thinking about certain
phenomena, than that's fine with me.
Igor indicated that it was not valid to ask about the
spectral bandwidth or coherence length of photons. He
pointed to his 2005 message:
http://groups.google.com/group/sci.physics.research/msg/
f45b1a725d517f62
In this, he explains photons theoretically by applying a
Fourier transform to the emr signal, but I find this
overly mathematical and reductionist. Fourier transforms
are only practical over a finite time, and break the signal
into evenly spaced buckets of frequency, each with an
amplitude and a phase. I am not convinced Nature can
adequately be conceived of in such terms. It seems that
the math-based theory can't model what happens, except with
certain assumptions and approximations - and with an
infinite time for emitting a photon.
Igor wrote:
. . . one can talk of absorption and emission of photons
only in idealized situations where we have an indefinitely
long time at our disposal to perform observations. In all
other contexts, photons are merely an approximation, . . .
and:
It is also important to realise that the photon
description of the electromagnetic field is most
appropriate (a) when the intensity of the light is very
very low, (b) when light interacts with a system with
quantized energy levels, such as an atom. In all other
cases, the classical description of the electromagnetic
field is most appropriate, where translation into the
photon description would be very cumbersome and highly
unhelpful.
My impression from the above is that a photon is an
idealised theoretical construct which is handy for dealing
with certain types of situations and not others. I am
happy with this. My initial questions were an attempt to
understand the thinking of people who see photons as
physical entities, which actually exist. If this is the
case, then I think a good theory of photons should have a
gutsy basis which works from DC to daylight and beyond to
gamma rays. If this is the case, I think there should be
robust experimental evidence for emr getting around the
place in these little packets, which either can or can't
(depending on the theory) interfere with each other.
It appears to me that most of your questions are best
addressed in the classical description, where the word
"photon" does not even come up, except maybe colloquially.
I agree. I was attempting to understand people's
understanding of photons as real things. To someone who
considers them to be objectively real, indivisible, aspects
of Nature, I wonder what the answer to my questions would be,
or why those questions would be seen as invalid.
(The fine print is that the classical description of the
EM field also breaks down at *extremely* high intensities,
where non-linear vacuum polarization effects become
important.)
Can you provide a reference for this? I understand there
is non-linearity in the movement of electrons, for instance,
in molecular or crystal structures, but I hadn't heard of
distortion in the absence of matter. I guess the energy
density could get so high that the space containing the emr
contains appreciable mass and that this causes some
interactions between the various types of emr, or even with
a pure sine wave signal itself - but this is just a guess.
"Photon" is merely a word that physicists use on paper
and in conversation.
OK!
What you should believe is that there are some experiments
that are hard to explain with classical electrodynamics:
black body spectrum, photoelectric effect, Compton
scattering, the laser, gyromagnetic ratio of the electron,
and many others.
I understand that either semiclassical or quantum
electrodynamics is required to explain the quantisation of
energy in resonant systems. My questions were about the
purported physical reality of particle-like "photons", as
separate, quantized items of electromagnetic energy, in
transit, quite separate from the quantized generation and
absorption of emr in atoms etc.
I am also interested in emr's interaction with systems of
matter which I consider to be non-resonant, such as
bremsstrahlung interactions between two charged particles,
and short coherence length emr interacting with sparse plasma.
I have been unable to find any conventional theory which
considers how such emr speeds up to light speed in the vacuum
between the particles (electrons, protons, ions etc.) and
then slows down for each particle. This is an inhomogeneous
medium, I am sure, since each particle is like a little cloud
of gas with > 1.0 refractive index. This means each little
ripple of emr couples momentum to the particle temporarily
as the particle temporarily slows it down. See my site
http://astroneu.com for where this might lead.
I need to understand the one or more approaches to
semiclassical electrodynamics, which I guess are best
explained in "Optical Coherence and Quantum Optics".
Another approach I am pursuing is looking at the experiments
which are widely believed to prove the physical existence of
"photons" == "quantized emr in transit" and the various
critiques of the experiments and their interpretation. These
have been discussed at length in the past on
sci.physics.research and I am still trying to read and
understand this stuff.
A few references follow.
- Robin
PDFs of original papers on EPR experiments and theory,
including the Einstein, Podolsky and Rosen 1935:
http://www.drchinese.com/David/EPR_Bell_Aspect.htm
http://en.wikipedia.org/wiki/Bell's_theorem
Thorn et al. 2004 with a modern version of the Grangier,
Roger and Aspect experiment of 1986:
http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
Hidden-Variable Example Based upon Data Rejection,
Philip M. Pearle 1970:
http://adsabs.harvard.edu/abs/1970PhRvD...2.1418P
Nightlight's critiques in spr 2005 thread "The time it takes
to emit one photon":
http://groups.google.com/group/sci.physics.research/browse_thread/
thread/1e3ae3b3697948db/7764006d43d65061
David Elm, Ratko V. Tomic and Ray Tomes in 1996:
http://www.cyclesresearchinstitute.org/physics/elm.html
.
- References:
- Coherence length and bandwidth of photons
- From: Robin Whittle
- Coherence length and bandwidth of photons
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