photon decay
sca...@utdallas.edu
changes occur to a photon comparing its' state upon emission by a
quasar and its' state upon entering the eye of an Earth observer? Do you
think it is beneficial to view the electromagnetic spectrum in terms of
other particles via creation and annihilation?
James H. Panetta
Ray Tomes <rto...@kcbbs.gen.nz> wrote:
>particles. They are not particles, they are not made from particles
>and they never manifest as particles. I defy anyone to give any
>evidence to the contrary.
Pair creation. A photon with energy > 2*0.511 MeV can create an
electron positron pair. This is not explainable using purely wave
arguments.
>What happens to electromagnetic waves between a quasar and someone's
>eye is that the vast majority of the energy goes somewhere else.
>This can be calculated by the inverse square law and the size of your
>eyehole compared to the quasar distance. The only exception is if
>intervening bodies absorb part of the wave or refract it.
What about the Doppler effect? Quasars tend to have a large redshift.
--Jim
--
--
My opinions are mine...not SLAC's...not Caltech's...not DOE's...mine.
(except by random, unforseeable coincidences)
pan...@cithex.caltech.edu pan...@slacvm.slac.stanford.edu
Kevin Sterner
> It is wrong to call electromagnetic waves (of the travelling kind)
> particles. They are not particles, they are not made from particles
> and they never manifest as particles. I defy anyone to give any
> evidence to the contrary.
Mike Kelsey just wrote a remarkably lucid piece on how a photomultiplier
tube works, you should check it out.
What do you make of the photoelectric effect? If what you say is true,
then you would expect that the likelihood of knocking one electron out
of a bit of metal would depend on the intensity of a light source and
the amount of time it has been shining on the metal. Instead, it
depends strongly upon the frequency of the light and not at all upon
the time. The interpretation is that light comes in discrete particles
(photons), and that if the photons from a source have a high enough
frequency (energy), they will knock out electrons immediately, whereas
if they don't have enough energy, they will never knock any electrons
out of the metal, regardless of the intensity. Do you disagree with
this interpretation?
> Matter has both a wave and a particle nature and em waves are always
> emitted and absorbed in discrete amounts by matter. These amounts
> are according to Planck's law E = h nu. This law is really the law
> for the energy of a particle.
I thought you said light was never manifest as particles. Why does
this equation work for light, then?
> Electromagnetic waves however spread out as they travel long
> distances, and can have any energy concentration whatsoever.
That is true. Similarly, carbon dioxide have any concentration whatsoever
in a body of water. Do you maintain that CO2 does not manifest itself
as discrete particles?
> What happens to electromagnetic waves between a quasar and someone's
> eye is that the vast majority of the energy goes somewhere else.
> This can be calculated by the inverse square law and the size of your
> eyehole compared to the quasar distance. The only exception is if
> intervening bodies absorb part of the wave or refract it.
Yes, but that doesn't speak to the question "what form does that light
take when it excites your retina?" The answer is "as discrete particles."
-- K.
-------------------------------------------------------------------------------
Kevin L. Sterner | U. Penn. High Energy Physics | Smash the welfare state!
-------------------------------------------------------------------------------
Ray Tomes
> The entire electromagnetic spectrum is made up of one particle? What
> changes occur to a photon comparing its' state upon emission by a
> quasar and its' state upon entering the eye of an Earth observer? Do you
> think it is beneficial to view the electromagnetic spectrum in terms of
> other particles via creation and annihilation?
It is wrong to call electromagnetic waves (of the travelling kind)
particles. They are not particles, they are not made from particles
and they never manifest as particles. I defy anyone to give any
evidence to the contrary.
Matter has both a wave and a particle nature and em waves are always
emitted and absorbed in discrete amounts by matter. These amounts
are according to Planck's law E = h nu. This law is really the law
for the energy of a particle.
Electromagnetic waves however spread out as they travel long
distances, and can have any energy concentration whatsoever.
Jonathan Scott
on 12 May 1995 15:20:25 GMT,
Kevin Sterner <ste...@sel.hep.upenn.edu> writes:
>In article <16395131.1...@kcbbs.gen.nz>,
> rto...@kcbbs.gen.nz (Ray Tomes) writes:
>
>
>Mike Kelsey just wrote a remarkably lucid piece on how a photomultiplier
>tube works, you should check it out.
I don't think this argument connected properly.
Everyone agrees that interactions between the electromagnetic field
and the systems which emit or absorb radiation are quantized.
This doesn't however necessarily directly mean that the field itself
is quantized, although it is very common to talk about photons as if
they were travelling particles.
As a trivial illustration, consider a tap dripping into a bowl which
is already full, causing drips to fall over the edge of the bowl.
I'm aware that there are some rather obscure interference phenomena
which are difficult to explain without the concept of a photon
travelling, but at the same time there are others which require half a
photon travelling along each of two paths by the same analogy.
If there are any results which support or contradict the idea of waves
travelling as discrete photons, I'd be interested to hear about them.
I seem to remember having seen a thread on this a long time ago but
I can't remember the conclusion.
Jonathan Scott
jonatha...@vnet.ibm.com or jsc...@winvmc.vnet.ibm.com
Michael Weiss
classical theory of E&M radiation will have with the photo-electric
effect. Just as important is the Franck-Hertz experiment and the
Compton effect. In fact the Compton effect had considerable
historical impact, demolishing the Bohr-Kramers-Slater proposal that
energy conservation held only statistically, not in individual
electron-photon interactions.
And there are always the billions of events recorded in the decades
that particle accelerators have been operating.
Kevin Sterner
> Everyone agrees that interactions between the electromagnetic field
> and the systems which emit or absorb radiation are quantized.
>
> This doesn't however necessarily directly mean that the field itself
> is quantized, although it is very common to talk about photons as if
> they were travelling particles.
The question of whether the EM field is quantized is a completely
separate question from whether an EM wave is quantized. The first is
quantized by virtual photons, the second by real photons.
> As a trivial illustration, consider a tap dripping into a bowl which
> is already full, causing drips to fall over the edge of the bowl.
Er...that's a fine analogy, but I don't see how it illustrates what
you are trying to say.
> I'm aware that there are some rather obscure interference phenomena
> which are difficult to explain without the concept of a photon
> travelling, but at the same time there are others which require half a
> photon travelling along each of two paths by the same analogy.
There is no such concept as "half a photon". The more traditional way
of saying it is that the photon took both paths, superposed.
Photons are fundamentally indivisible.
You can diffract electrons through two slits, too. Does that mean that
the electron "splits in two", with each half going through one slit? No.
> If there are any results which support or contradict the idea of waves
> travelling as discrete photons, I'd be interested to hear about them.
> I seem to remember having seen a thread on this a long time ago but
> I can't remember the conclusion.
Whether you see light as behaving as waves or particles depends entirely
upon the design of your experiment. The same can be said of electrons,
or any other subatomic particle. They behave like particles, and they
behave like waves. That's strange, but that's quantum mechanics.
Oz
>I'm not yet convinced that there is any obvious case in which it is
>necessary to use a particle model to describe electromagnetic radiation
>in transit.
1) Photons must surely always be 'in transit', by definition. So to
some extent at least must then always be waves.
2) Except, of course, when they deliver their energy up to another
'particle'. At this point the energy becomes 'localised'. I suppose you
might comment that the 'area' of localisation is of the order of a
wavelength anyway.
3) I was always intrigued that since photons travel at light speed *in
their frame of reference* the size of the universe is zero, and so they
should *in their frame of reference* get to all of it simultaneously.
Or put in another way, time runs at zero speed for them. This being the
case I have no difficulty in seeing how a photon can 'go' through both
slits simultaneously, in fact it would be obligatory. Also any
experiment that determined that they hadn't gone through one slit would
mean that they hadn't, destroying the interference pattern. Since *in
their frame of reference* the size of the universe is zero (or time
does not flow), there is no problem with any transluminal communication
problem either (I guess).
Oh dear, in this case they could be particles also. If the probablity
wave of a massive particle (like an electron) also propogates at light
speed then .....
--
-------------------------------
'Bye, Oz
"I am always doing things I can't do, that's how I get to do them": Picasso
Mike Kelsey
(Jonathan Scott) writes:
|>
|> I'm not yet convinced that there is any obvious case in which it is
|> necessary to use a particle model to describe electromagnetic radiation
|> in transit.
I'm no field theorist, just a knuckle-dragging experimentalist, but it
seems to me there is one "obvious case" -- photon self-interactions.
Classically, the linear Maxwell equations require that electromagnetic
waves pass through one another without effect. However, in QED, loop
diagrams such as
g * * g
*_____*
| e | e
e |_____|
* e *
g * * g
give photon-photon couplings, and hence two electromagnetic waves in
vacuum which intersect _can_ affect one another in ways contrary to
classical electrodynamics. Since this effect only appears in a
quantized theory (which I thought had been demonstrated with
ultra-high power lasers, though I can't find a reference), it seems to
me that it requires the in-transit EM field to be quantized, or
"particulate", if you want to use that term.
-- Mike Kelsey
--
[ My opinions are not endorsed by SLAC, Caltech, or the US government ]
"I've seen things you people wouldn't believe. Attack ships on fire
off the shoulder of Orion. I've watched C-beams glitter in the dark
near the Tannhauser Gate. All these moments will be lost in time,
like tears in rain." -- Roy Baty
Jonathan Scott
on 12 May 1995 20:04:23 GMT,
Kevin Sterner <ste...@sel.hep.upenn.edu> writes:
>In article <19950512....@vnet.ibm.com>, jonatha...@vnet.ibm.com (Jonathan Scott) writes:
>> This doesn't however necessarily directly mean that the field itself
>
>The question of whether the EM field is quantized is a completely
>separate question from whether an EM wave is quantized. The first is
>quantized by virtual photons, the second by real photons.
I guess you have a specific convention that "field" refers to something
approximately static and "wave" for changes propagating at c. I had
intended to include both in my reference to "field".
>There is no such concept as "half a photon". The more traditional way
>of saying it is that the photon took both paths, superposed.
>Photons are fundamentally indivisible.
I used this "half a photon" example to illustrate that a conventional
particle view is not adequate.
I'm not yet convinced that there is any obvious case in which it is
necessary to use a particle model to describe electromagnetic radiation
in transit. Of course, since emission of photons is a quantized
process, disturbances to the field will normally themselves be at least
approximately quantized, and for waves (at least below the energy needed
to create electron/positron pairs) it is only through interference
effects that the transmitted and received disturbances can avoid being
one to one equivalents. I don't count the Compton effect as proof
because it involves a quantized interaction, but I think that some
experiments relating to "empty waves" might convince me.
I'm basically just taking a "devil's advocate" position against the
particle model of waves, because I get the impression that it has not
really been shown to be necessary so far, and that using terminology
which assumes a particle model may possibly be placing unnecessary
constraints on our mental tools for understanding quantum theory.
Ray Tomes
> As a trivial illustration, consider a tap dripping into a bowl which
> is already full, causing drips to fall over the edge of the bowl.
detect the difference between the electromagnetic field being a pool
of water vs a bunch of raindrops (to continue the analogy).
The only example that I could think of was this:
Put a tiny mirror in free fall and bounce some short wavelength em
waves off it. Measure the change in momentum and repeat many times
to see if it is quantised.
Not only is it required to be a very sensitive measurement, but I
suspect that even if light is a pool, matter may still "like" to have
quantised momentum. If so, then it may be impossible to tell by
direct measurement.
Ray Tomes rto...@kcbbs.gen.nz
Bruce Sterling Woodcock
>Not only is it required to be a very sensitive measurement, but I
>suspect that even if light is a pool, matter may still "like" to have
>quantised momentum. If so, then it may be impossible to tell by
>direct measurement.
If I understand the arguments correctly, it seems like some people here are
casting some doubt on the wave-particle duality of light, at least to the
extent at which we start describing photons as actual particles. In this
approach, light is not actually quantized as photons, but when the waves
interact with matter, the latter "requires" a quantized value for things
like momentum.
I suppose the test of the proof, then, is to concot an experiment where
photons can act in a quantized manner with other, zero-mass particles.
I don't know if this is possible with two photons hitting each-other...
how about a photon/neutrino interaction?
It also begs the question that if an EM wave can have any arbitrary non-
quantized value, and then interacts with matter in a quantized fashion,
what happens to the "remainder" of the less than quantum energy? Does
a very weak "e-m" wave continue to spread out? If so, then it won't
interact with matter again (and thus be measurable) until we "reinforce"
it with another light wave, perhaps via interference, until it is back
up to "quantum" levels again. Perhaps an experiment can be constructed
along this design.
I realize, this all sounds kinda hokey, but I am trying to come up with
an experiment to test the hypothesis of others... being helpful, at least,
even if I don't think they can possibly be correct. :)
Bruce
--
Bruce Sterling Woodcock --- Systems Analyst / Admin --- ster...@netcom.com
The views and opinions expressed in this post | Power is being able
are not necessarily those of my former employer, | to sneak into your
NETCOM On-Line Communication Services. | house over a wire.
Oz
apologies.
The whole subject you are discussing has been tested ad nauseum. You
will find all you need in a decent text-book. It is inappropriate to
try to describe a photon using metaphors of the macroscopic world. It
has elements of its behaviour that are best described as waves, and
elements best described as a particle. In reality of course it is
neither or both, it is itself, a photon. The fact that it does not
behave like things we are used to in our macroscopic world is
unfortunate (particularly for physicists) but that's just the way it is.
Simply put, the 'frequency' of the wave description of the photon (how
it diffracts, reflects etc) is related to its energy (a rather more
particle-like description). High energy = high frequency. Basically
there's not much more to it than that! If you 'take' energy from a
photon it will drop in frequency (eg climbing up a gravity gradient),
and vice-versa. To increase the power in a red light beam you can do
two things (or a combination), increasing the number of photons gives
you a brighter red, increasing the energy of each photon gives you a
bluer light beam.
Mike Kelsey
|> Whoops, I really should have read the whole thread before butting in,
|> apologies.
|>
|> The whole subject you are discussing has been tested ad nauseum.
|>
|> Simply put, the 'frequency' of the wave description of the photon (how
|> it diffracts, reflects etc) is related to its energy (a rather more
|> particle-like description). High energy = high frequency. Basically
|> there's not much more to it than that!
You've missed the point of what some of us (actual practicing physicists)
were discussing.This issue has little more to do with "wave-particle
duality" than any other aspect of quantum mechanics, and the conventional
wisdom you quote, that different experiments show different results, is
immaterial. Similarly, the de Broglie relation for photons (E = hf)
_assumes_ that the propagating photon must be quantized, which is
precisely the question that is being asked. You therefore cannot use that
assertion to answer the question.
Jonathan Scott, in <19950513....@vnet.ibm.com> raised an
interesting and most certainly not "tested ad nauseum" point,
specifically, whether quantization of the propagating electromagnetic
field is necessary or not on experimental grounds. Experimentally, we
know that interactions with matter are quantized, but the logical,
theoretical question is whether that is due to quantization of the matter
fields, the EM fields, or both.
The conventional view, expressed in QED, is that all fields are
quantized, including an EM field "in transit" (as Dr. Scott put it). It
is nonetheless useful to ask whether that has been experimentally
tested, and what the results are. One of the few predictions of QED which
doesn't involve interactions with matter (which beg the question) is
the second-order photon-photon coupling through an electron loop. This
coupling should produce a measurable changes in otherwise classical EM
fields, compared to the predictions of classical electrodynamics. I would
be very interested in knowing whether an _experimental_ measurement of
this effect (for example, two intersecting laser beams) has been made.
Oz
Perhaps due to rude people (like me), butting in.
Perhaps you could explain in a little more detail the proposed
experiment with two high power lasers via electron loops. Although
electrons are particles and might interfere with the purity of the
experiment. What problem do you have with a quantised Em 'in transit'?
I am still not quite sure what you are really discussing, is it that EM
in transit is NOT quantised, but only becomes so when interecting with
matter? If so what is wrong with a small modification of a very old
experiment. The original self-interference of photons experiment had a
light beam where the experimenters could be confident that only one
photon was in the works at one time. Add a high speed shutter. This
shutter speed only opens for long enough to allow ONE photon wavepacket
to enter. Stick a photomultiplier at the end and see if you get
photons. Now if you do it's quantised, if not it isn't. You can easily
check the frequency by sending it through a diffraction grating first.
Essentially you are attempting to isolate a photon, 'in flight'.
Just in passing there is a (vaguely) amusing story told in Cambridge
about the original experiment. Apparently the Post-Grad doing it was
very lazy and had a large private income. He chose to do the original
experiment because he could live on the Riviera, and just come back to
Cambridge every three months for a couple of days to change and develop
the plates. Oh, times have changed!
Jonathan Scott
on 15 May 1995 16:47:45 +0100,
Oz <Oz> writes:
> The original self-interference of photons experiment had a
>light beam where the experimenters could be confident that only one
If energy is entering the field in quanta, then it isn't surprising that
it travels through it approximately in quanta, regardless of whether it
is necessarily strictly quantized in the field.
Even a photon-photon interaction via an electron-positron vertex could
be considered as an example of a quantized interaction with matter.
Perhaps the particle creation process might be quantized even if the
wave were not?
I have a feeling that I have previously heard some argument relating to
the Aspect experiment and Bell inequalities which shows that each photon
has a unique identity and hence that it is isn't just a quantized
interaction occurring locally. If I'd been convinced, I think I'd have
changed my mental model, so I guess I wasn't.
I'm still open to further suggestions of evidence that waves travel
in quantized form.
Stephen Burke
> Even a photon-photon interaction via an electron-positron vertex could
> be considered as an example of a quantized interaction with matter.
> Perhaps the particle creation process might be quantized even if the
> wave were not?
This argument is in danger of becoming metaphysical; if you disallow *any*
interactions, how is it meaningful to ask the question? (What does it do
when you're not looking?) However, there is a W+W-gamma vertex, which is at
least an interaction of gauge fields. I don't think it's been conclusively
measured yet, but it shouldn't be long.
BTW, someone in this thread made a distinction between real and virtual
photons. According to QED, *any* photon which is created and then absorbed
is virtual, it's just that ones which go macroscopic distances have very
small masses. Only a photon which travelled an infinite distance would
be exactly on-shell, and then you'd never detect it ...
--
e----><----p | Stephen Burke | Internet: bu...@desy.de
H H 1 | Gruppe FH1T (Lancaster) | DECnet: vxdesy::burke (13313::burke)
H H 11 | DESY, Notkestrasse 85 | JANET: bu...@uk.ac.rl.v2
HHHHH 1 | 22603 Hamburg, Germany | Phone: +49 40 8998 4564
H H 1 | "It is also a good rule not to put too much confidence in
H H 11111 | experimental results until they have been confirmed by theory"
Andy Boden
>
> The conventional view, expressed in QED, is that all fields are
> quantized, including an EM field "in transit" (as Dr. Scott put it). It
> is nonetheless useful to ask whether that has been experimentally
> tested, and what the results are. One of the few predictions of QED which
> doesn't involve interactions with matter (which beg the question) is
> the second-order photon-photon coupling through an electron loop. This
> coupling should produce a measurable changes in otherwise classical EM
> fields, compared to the predictions of classical electrodynamics. I would
> be very interested in knowing whether an _experimental_ measurement of
> this effect (for example, two intersecting laser beams) has been made.
>
> -- Mike Kelsey
Interesting question. From one "knuckle-dragger to another, here's
what I know:
0) For Mike and others, you can read about photon-photon processes
starting in Itzykson and Zuber, p 355. You should know that the
cross-section is tiny: for Nd:YAG lasers (omega=1.2 eV) you get
something like 10^-41 barns! The cross-section is an alpha^4 process
(of course), and goes like (omega / m_e)^6 (I&Z Eq. 7-101) so for
stuff in the near visible, this 6th-power hombre kills you. To make
this process go you need at least one of the photons to have gamma-ray
like energies, hence the designs I've heard for the photon-photon
collider up at SLAC.
1) In 1971 Adler worked out the implications of photon-photon in an
astrophysical context -- say in the neighborhood of a neutron star,
with one or of the photons being virtual (magnetic). These are called
"photon-splitting" processes, because you can end up with more photons
than you started with (there are issues of momentum conservation to be
dealt with, but apparently it all hangs together).
2) Recently a Rochester and FNAL optical test for axions (who couple
with virtual photons as well) failed to detect either axions or the
photon-photon coupling. The test was done by shining a laser (the
real photons) in a magnetic field (the virtual photons), and the
observable would have been a change in the polarization state of the
laser light. See Cameron et. al. Phys Rev D, V47 N9, p3707. The
failure was to be expected, the experiment did not have the required
sensitivity to see the QED effect.
3) An Italian experiment PVLAS (INFN) following the Rochester
experiment has been approved and is funded. They *should* have the
requisite sensitivity to see QED, and they should be turning on soon.
4) There are people talking about doing a Rochester-like experiment
with old SSC prototype magnets at FNAL. We'll see...
Hope this helps.
- A.B.
--
"...it doesn't matter how beautiful your theory is, it doesn't matter
how smart you are -- if it doesn't agree with experiment, it's wrong."
- R.P. Feynman
Kevin Sterner
> Just in passing I made a note (down the thread a bit) about photons and
> the size of the universe in their frame of reference. Surprisingly
> (given the high mindpower that should be on this thread), I got no
> comment at all. Somewhat (not much) off thread, but its surely worth
> something at least?
OK, here is what you said:
>3) I was always intrigued that since photons travel at light speed *in
>their frame of reference* the size of the universe is zero, and so they
>should *in their frame of reference* get to all of it simultaneously.
>Or put in another way, time runs at zero speed for them. This being the
>case I have no difficulty in seeing how a photon can 'go' through both
>slits simultaneously, in fact it would be obligatory. Also any
>experiment that determined that they hadn't gone through one slit would
>mean that they hadn't, destroying the interference pattern. Since *in
>their frame of reference* the size of the universe is zero (or time
>does not flow), there is no problem with any transluminal communication
>problem either (I guess).
First of all, the photon does not exist in an inertial frame of reference,
so any talk of the photon's frame of reference is technically off-base.
One *can*, however, consider what the universe might look like to an
observer as he approaches the speed of light (relative to a frame with
zero CMBR dipole anisotropy, for all you sticklers out there). This is
what you meant, I am sure.
So for the rest of this post, that is what I mean when I use the shorthand
"the photon's frame of reference".
The universe does shrink to zero length in the photon's frame of reference,
but only in the direction of the photon's travel. In the transverse
directions, the universe is the same size as it is in the "stationary
frame".
This poses a problem for your analysis, as the two slits are understood to
have a nonzero separation in a direction transverse to the photon's
direction of motion. You still have the problem of explaining how the
photon could have been in two *transverse* locations at once.
The second objection I raise to your interpretation is that nonrelativistic
electrons and neutrons will also exhibit two-slit interference phenomena.
The only way I know to describe the propagation of quantum particles
(e.g. photons, electrons, neutrons) through two slits using a purely
localized conception of these particles is the Feynman path integral
approach. In this formulation, the particle takes an infinitude of paths,
which are weighted and summed coherently to compute the wavefunction.
Mike Kelsey
|>
|> The only way I know to describe the propagation of quantum particles
|> (e.g. photons, electrons, neutrons) through two slits using a purely
|> localized conception of these particles is the Feynman path integral
|> approach. In this formulation, the particle takes an infinitude of paths,
|> which are weighted and summed coherently to compute the wavefunction.
As an interesting aside, you can actually apply the path integral approach
classically. Consider the old problem of refraction at a boundary between
two media (n1 != n2). If you consider every possible path from A to B,
where the light travels in a straight line from A to the boundary, then in
a second line from the boundary to B, and add the waves coherently, you
find that everything destructively interferes except for the the path we
all know and love. (See Feynman, _QED_, ISBN 0-691-08388-6, for example.)
Feynman's insight was to apply this same method, which is essentially just
Huygen's principle, to wavefunctions in quantum mechanics by adding a
weighting factor (for claissical light, the weighting factor is unity for
every path).
sca...@utdallas.edu
> >Jonathan Scott wrote:
> >I'm not yet convinced that there is any obvious case in which it is
> >necessary to use a particle model to describe electromagnetic radiation
> >in transit.
> 1) Photons must surely always be 'in transit', by definition. So to
> some extent at least must then always be waves.
> 2) Except, of course, when they deliver their energy up to another
> 'particle'. At this point the energy becomes 'localised'. I suppose you
> might comment that the 'area' of localisation is of the order of a
> wavelength anyway.
> 3) I was always intrigued that since photons travel at light speed *in
> their frame of reference* the size of the universe is zero, and so they
> should *in their frame of reference* get to all of it simultaneously.
> Or put in another way, time runs at zero speed for them. This being the
> case I have no difficulty in seeing how a photon can 'go' through both
> slits simultaneously, in fact it would be obligatory. Also any
> experiment that determined that they hadn't gone through one slit would
> mean that they hadn't, destroying the interference pattern. Since *in
> their frame of reference* the size of the universe is zero (or time
> does not flow), there is no problem with any transluminal communication
> problem either (I guess).
> Oh dear, in this case they could be particles also. If the probablity
> wave of a massive particle (like an electron) also propogates at light
> speed then .....
> --
sca...@utdallas.edu
Oz
the size of the universe in their frame of reference. Surprisingly
(given the high mindpower that should be on this thread), I got no
comment at all. Somewhat (not much) off thread, but its surely worth
something at least?
Just in passing, following the general refutation of the poor old photon 'in flight', I guess any interaction could be considered as operating at the photon (ie quantised) level. Seems to me that any EM interaction (even with another EM) will be regard
d as a quantised interaction, which of course it is. Should perhaps be in philosophy 'how to see the unobservable'.
Oz
>3) I was always intrigued that since photons travel at light speed *in
>their frame of reference* the size of the universe is zero, and so they
>should *in their frame of reference* get to all of it simultaneously.
>Or put in another way, time runs at zero speed for them. This being the
>case I have no difficulty in seeing how a photon can 'go' through both
>slits simultaneously, in fact it would be obligatory. Also any
>experiment that determined that they hadn't gone through one slit would
>mean that they hadn't, destroying the interference pattern. Since *in
>their frame of reference* the size of the universe is zero (or time
>does not flow), there is no problem with any transluminal communication
>problem either (I guess).
and Kevin Sterner replied (see recent back thread)
Whilst it is indisputably true that the photon does not exist in an
inertial universe (or is it the other way round?) I don't really think
that there is any evidence against considering it could see the
universe this way, simply taking relativistic views to the limit. One
can after all propose this for inertial particles to any arbitary
degree of accuracy as they approach C. Argueable anyway. What I am NOT
saying is that a photon 'is' a particle, it could equally be (probably
is) a wave propogating at C.
Now a light 'wave' (ex Mr Maxwell, but probably dated) propogates at C
IN ALL DIRECTIONS, the 'direction' of the photon or light beam being
the sum of all paths, in other words it's observable universe is only a
subset of the inertial universe due to mirrors/lenses/planets etc etc.
Within its universe, that universe has zero size. It is therefore
everywhere simulaneously in its universe, hence no problem with it
going through both slits simultaneously (cos its going in all
directions simultaneously). Please note that in this world view the
frequency of the wave is 'merely' the representation of its energy in
the inertial frame of reference.
There may be good arguments against this! (Particularly as it hasn't
really been adequately thought out).
Now we need to take on the wave aspect of inertial particles. I really
don't have enough knowledge any more to extend this adequately. My
knowledge of Feynman's technique is regettably 'popular' (which shows
my age). However could I ask if the summing over all possible paths is
regarded as the ACTUAL representation of the particle, or just a
convenient mathematical technique. I ask this because for quite a time
that was the view taken of relatavistic analyses, and also quantum
mechanical ones, whilst now these are regarded as real.
I hope you can find the energy to continue the discussion.
sca...@utdallas.edu
> Oz (O...@upthorpe.demon.co.uk) wrote:
> > >Jonathan Scott wrote:
> > >I'm not yet convinced that there is any obvious case in which it is
> > >necessary to use a particle model to describe electromagnetic radiation
> > >in transit.
In order to account for both the wave and particle nature of light a model
must be constructed under the assumption of simplicity i.e. that it
doesn't flippantly change it's behavior in mid-stream from its' wave
nature to its' particle nature (see #2 below).
> > 1) Photons must surely always be 'in transit', by definition. So to
> > some extent at least must then always be waves.
> > 2) Except, of course, when they deliver their energy up to another
> > 'particle'. At this point the energy becomes 'localised'. I suppose you
> > might comment that the 'area' of localisation is of the order of a
> > wavelength anyway.
The picture I see, if I may, is one in which e-m waves are drawn as
containing a localized wave inside a bulb traveling in a known direction at c.
These bulbs constitute "packets" of energy. If you have a coherent source
then the waves inside the so-called bulbs all have the same frequency and of
course for continuity would be "connected" amongst themselves by the
phase requirement.
> > 3) I was always intrigued that since photons travel at light speed *in
> > their frame of reference* the size of the universe is zero, and so they
> > should *in their frame of reference* get to all of it simultaneously.
> > Or put in another way, time runs at zero speed for them. This being the
> > case I have no difficulty in seeing how a photon can 'go' through both
> > slits simultaneously, in fact it would be obligatory. Also any
> > experiment that determined that they hadn't gone through one slit would
> > mean that they hadn't, destroying the interference pattern. Since *in
> > their frame of reference* the size of the universe is zero (or time
> > does not flow), there is no problem with any transluminal communication
> > problem either (I guess).
YES, YES, YES the stopwatch reads zero for photons. As for inertial
frames (I.F.), I think you might have created a bomb! - for the gamma
factor explodes at v=c.
> > Oh dear, in this case they could be particles also. If the probablity
> > wave of a massive particle (like an electron) also propogates at light
> > speed then .....
Sorry after writing the below I realized I was thinking of another thread...
Consider a Gaussian packet approaching the speed of light then as the
universe shrinks in the direction of its' motion things like tacheons and
the packet's phase velocity which are already traveling at superluminal
speeds increase their speed according to v=x/t (with the appropriate
relativistic correction, if they apply). But even in that I.F. along the
direction of travel the universe must be expanding because somewhere there
exists a quasar that has been radiating energy packets in all directions since
the big bang. How can energy be conserved in this case? I mean if
sometime prior to the big bang we consider the energy by enclosing the
big bang in a Gaussian sphere then at t=0 the big bang occurs. At t=0
the quasar is adjacent to the Earth observer. The Gaussian sphere
expands at t=small with photons at speed c. That means for the majority
of photons that are not observed there is a imprint of the quasar's
history for all time. Clearly collectively all this distributed energy
can not equal what was present in the Gaussian sphere. In fact some of
the energy has disipated but to where and to what end?
sorry i believe i lost my train of thought...thank you...must disconne
Ben Bullock
> 0) For Mike and others, you can read about photon-photon processes
> starting in Itzykson and Zuber, p 355. You should know that the
> cross-section is tiny: for Nd:YAG lasers (omega=1.2 eV) you get
> something like 10^-41 barns! The cross-section is an alpha^4 process
> (of course),
Not "of course", as I point out below....
> and goes like (omega / m_e)^6 (I&Z Eq. 7-101) so for
> stuff in the near visible, this 6th-power hombre kills you. To make
> this process go you need at least one of the photons to have gamma-ray
> like energies, hence the designs I've heard for the photon-photon
> collider up at SLAC.
The principle of a photon-photon collider isn't to bounce two photons
off each other to get more photons, it is to create new particles in
the collision. Therefore it is not an alpha^4 process. For example I
found the following papers in the last year of the Los Alamos bulletin
board:
hep-ph/9406395 [abs, src, ps] :
FOUR WEAK BOSON PRODUCTION IN TEV PHOTON--PHOTON COLLISIONS AND
HEAVY HIGGS SIGNAL (Talk given at the Workshop on gamma--gamma
colliders, March 28-31, 1994, Lawrence Berkeley Laboratory), by
George Jikia. LaTeX, 9 pages, 6 figures (compressed + uuencoded)
hep-ph/9406428 [abs, src, ps] :
QCD CORRECTIONS TO $b\bar b/c\bar c$ PAIR PRODUCTION IN POLARIZED
$\gamma\gamma$ COLLISIONS AND THE INTERMEDIATE MASS HIGGS SIGNAL
(Talk given at the Workshop on gamma--gamma colliders, March 28-31,
1994, Lawrence Berkeley Laboratory), by George Jikia and Avto
Tkabladze. LaTeX, 5 pages, 3 figures (compressed + uuencoded PS
file)
hep-ph/9411369 [abs, src, ps] :
Title: Higgs search in e+e- and gamma-gamma colliders
Author: Jan Kalinowski
Comments: LaTeX file, 25 pages, 13 figures not included, hard copy
can be obtained from Author,
Report-no: IFT 94/21
hep-ph/9503432 [abs, src, ps] :
Title: Quartic Anomalous Couplings in $\gamma\gamma$ Colliders.
Authors: O.J.P. Eboli, M.B. Magro, P.G. Mercadante and S.F. Novaes.
Comments: 15 pages, Latex file using ReVteX, 4 uufiled figures
included.
Report-no: IFT-P.017/95 and IFUSP-P 1131.
hep-ph/9504226 [abs, src, ps] :
Title: "Intermediate Mass Higgs Study at $\gamma \gamma$ Colliders"
Authors: Isamu Watanabe
Comments: compressed and uuencoded PostScript file, 15 pages
Report-no: OCHA-PP-58
hep-ph/9504309 [abs, src, ps] :
Title: Polarization Effects in Chargino Production at High Energy
$\gamma\gamma$ Colliders
Authors: Masayuki Koike ,Toshihiko Nonaka and Tadashi Kon
Comments: 7 pages, latex , 3 figures are available upon request
Report-no: RUP-5-95 ,ITP-SU-95/01
hep-ph/9504346 [abs, src, ps] :
Title: One-loop correction to the $\gamma W W$ vertex in the $e^-
\gamma$ collider
Authors: Jiro KODAIRA, Hiroshi TOCHIMURA, Yoshiaki YASUI, Isamu
WATANABE
Comments: 7 figures are included, 6 pages, Talk presented by Y.Yasui
at INS Workshop "Physics of $e^+ e^-$,$e^- \gamma$ and $\gamma
\gamma$ collisions at linear accelerators"INS,Tokyo,Japan
Report-no: HUPD-9509, OCHA-PP-57
hep-ph/9504348 [abs, src, ps] :
Title: ON MEASURING THE $\gamma \gamma$ WIDTH OF THE INTERMEDIATE-
MASS HIGGS AT A PHOTON LINEAR COLLIDER
Author: VALERY A. KHOZE
Comments: 7 pages, LATEX file, hard copies of 2 figs. availabl upon
request
Report-no: Talk at the Photon'95, Sheffield, April 8-15, 1995
The calculation you are refering to in I&Z's book is the "scattering
of light by light" where two photons collide and form a new pair of
photons through an electron "box diagram".
FWIW there's a clear "health warning" on I&Z's result (7.101) that it
only applies for omega << m: as should be fairly clear from unitarity
for omega >> m the formula (7.95) with the inverse square of omega
applies. Your comment about using gamma-ray like energies to "make
the process go" is totally wrong.
--
Ben Bullock @ KEK (National Laboratory for High Energy Physics) /
address: 1-1 Oho, Tsukuba, Ibaraki 305, JAPAN / TEL: 0298-64-5403 /
FAX: 0298-64-7831 / e-mail: b...@theory.kek.jp / DECNET: KEKVAX::BEN
[in Japanese]: ベン・ブロック＠高エネルギー物理学研究所 （つくば）
聞くは一時の恥、聞かぬは末代の恥。
Andy Boden
> Andy Boden (an...@elroy.jpl.nasa.gov) wrote:
>
> > 0) For Mike and others, you can read about photon-photon processes
> > starting in Itzykson and Zuber, p 355. You should know that the
> > cross-section is tiny: for Nd:YAG lasers (omega=1.2 eV) you get
> > something like 10^-41 barns! The cross-section is an alpha^4 process
> > (of course),
>
> Not "of course", as I point out below....
>
> > and goes like (omega / m_e)^6 (I&Z Eq. 7-101) so for
> > stuff in the near visible, this 6th-power hombre kills you. To make
> > this process go you need at least one of the photons to have gamma-ray
> > like energies, hence the designs I've heard for the photon-photon
> > collider up at SLAC.
>
> The principle of a photon-photon collider isn't to bounce two photons
> off each other to get more photons, it is to create new particles in
> the collision. Therefore it is not an alpha^4 process. For example I
> found the following papers in the last year of the Los Alamos bulletin
> board:
[very interesting list of pre-prints deleted]
>
> The calculation you are refering to in I&Z's book is the "scattering
> of light by light" where two photons collide and form a new pair of
> photons through an electron "box diagram".
which is *exactly* the radiative correction that Mike was refering to
in (at least my reading of) his question about colliding laser beams.
To wit:
Mike Kelsey writes:
>> The conventional view, expressed in QED, is that all fields are
>> quantized, including an EM field "in transit" (as Dr. Scott put it). It
>> is nonetheless useful to ask whether that has been experimentally
>> tested, and what the results are. One of the few predictions of QED which
>> doesn't involve interactions with matter (which beg the question) is
>> the second-order photon-photon coupling through an electron loop. This
>> coupling should produce a measurable changes in otherwise classical EM
>> fields, compared to the predictions of classical electrodynamics. I would
>> be very interested in knowing whether an _experimental_ measurement of
>> this effect (for example, two intersecting laser beams) has been made.
Lasers, at least the kind we think about when the term "laser" is
used, are in the IR-Visible-UV, i.e. omega of a few to a few tens.
The available, *readily detectable* final states for such an
interaction are two photons (perhaps you would care to suggest an
alternate final state, such as maybe neutrinos or axions). This is a
alpha^4 process, thus for *this* radiative correction my comments
stand. If you're going to flame, it would be best if you would
provide sufficient context to fairly judge your comments.
>
> FWIW there's a clear "health warning" on I&Z's result (7.101) that it
> only applies for omega << m: as should be fairly clear from unitarity
> for omega >> m the formula (7.95) with the inverse square of omega
> applies. Your comment about using gamma-ray like energies to "make
> the process go" is totally wrong.
>
Really. Of course, your statements about I&Z's admonition and
unitarity are obviously correct. But if higher energy photon's aren't
required, then what is the point of the up-conversion of laser photons
off electrons in the photon collider concept? By all means, feel free
to quote the high-energy production cross-section for your favorite
process. If the final state isn't two photons, then *of course* it
isn't an alpha^4 process (sigh).
I also note with some interest that the final states discussed in the
extracted titles you offered all are multi-GeV in energy. So by
energy conservation alone (and not the detailed form of the
cross-section, but perhaps this was the point you were trying to make)
photon up-conversion is necessary.
I can see how you might draw the inference that my statements about
the photon collider might imply my comments were based solely on I&Z's
formula. However, these are inferences, and given the fact you work
at KEK, you should have had the expertise to recognize the poetic
license I took in verbally extending the low energy formula above m_e.
It really annoys me when people disingenuously color the comments of
others to try to make themselves look smart (my *inference* on your
comments based on its tennor). It is, however, a pervasive
phenomenon...
- A.B.
> --
> Ben Bullock @ KEK (National Laboratory for High Energy Physics) /
> address: 1-1 Oho, Tsukuba, Ibaraki 305, JAPAN / TEL: 0298-64-5403 /
> FAX: 0298-64-7831 / e-mail: b...@theory.kek.jp / DECNET: KEKVAX::BEN
> [in Japanese]: ベン・ブロック＠高エネルギー物理学研究所 （つくば）
>
> 聞くは一時の恥、聞かぬは末代の恥。
--
Mike Kelsey
|>
[ Citations of "two-photon" physics in the technical sense used in the HEP
community, e+ e- -> e+ e- gamma* gamma*, where the two t-channel gammas
interact to produce some other final state, deleted. ]
|>
|> Now can someone lucidly explain to me what photon-photon elastic scattering
|> has to do with quantization of the electromagnetic field "in flight"?
I guess that you have jumped into this thread very late in the game.
Jonathan Scott, many, many cycles ago, raised the semi-experimental question
that although the interactions of photons with matter are typically
quantized, was there any _requirement_ that a propagating electromagnetic
field be quantized as well. In other words, despite the fact that we do
use a quantum description for the propagation, is that absolutely necessary?
I responded that there is one very good possible experimental justification
for quantizing the electromagnetic field, specifically elastic scattering
of photons. Classically, Maxwell's equations are linear and hence two
EM waves which pass through one another can have no lasting observable
effects on the outgoing waves: you can observe the interference fringes if
you put a detector at the point of intersection, but if you just look at
the waves after they go past one another, you can't see an effect.
QED, which quantizes the whole field, makes a definite prediction about the
existence of the order alpha^4 elastic scattering process, a prediction
which is in direct conflict with classical electromagnetism. Therefore,
an experimental measurement of such scattering, perhaps by way of a phase
shift in one of the beams when the second beam is on vs. off, would be a
direct, experimental validation of the QED hypothesis of quantizing the
"in flight" electromagnetic field.
My own literature searches have so far turned up no papers reporting such
a measurement. However, I *still* seem to recall a brief report in Physics
Today or the CERN Courier or something, back in 1991 or 1992. I would be
very interested in anyone who can provide a citation.
Ken McLean
(sorry for posting this to the wrong thread previously)
I'd just like to point out that, rather than having to consider
hypothetical processes at hypothetical ultrahigh energy photon photon
colliders, photon-photon collisions have been around for 25 years
as subprocesses at e+e- colliders. Witness the following article:
FORMATION OF THE PSEUDOSCALARS PI0, ETA AND ETA-PRIME IN THE
REACTION GAMMA GAMMA ---> GAMMA GAMMA.
By Crystal Ball Collaboration (D. Williams et al.). SLAC-PUB-4573, Mar 1988. 42pp.
Published in Phys.Rev.D38:1365,1988.
This describes a reaction where the colliding e+e- each radiate
a virtual photon in the t-channel, these virtual photons
collide to form resonant states (pi0,eta,eta') which subsequently decay
into two real photons. So the reaction is
e+e- > e+e-photon*photon* > e+e-pseudoscalar > e+e-photon photon
The final state e+e- are usually scattered at small angles and miss the
detector (since the initial state photons have a spectrum 1/Q^2 they are
"nearly real" for the most common two photon reactions, though highly
virtual photons can be tagged by looking for events with an electron or
positron in the detector that has been scattered thru a large angle).
The above reaction is actually alpha^6, though the two-photon subprocess
is only alpha^4 (the kinematics and singular nature of the bremstrahlung
spectrum of the photons compensates for the small coupling). In any
event, there have been many other papers on similar processes with
more complex hadronic final states (i.e. photon photon inelastic
scattering!) e.g. measurements of meson and/or baryon production and the
photon structure function.
Though the most interesting physics at e+e- colliders is usually the
annihilation channel where all the available energy is used. Photon photon
reactions represent a major part of the visible cross-section at these
machines, though they are easily separated from the annihilation channel by
cuts on the visible energy, except for final states where there are lots of
neutrinos (e.g. tau+tau- final states).
For a review of this type of physics try:
TWO PHOTON PHYSICS AT E+ E- STORAGE RINGS.
By Hermann Kolanoski (Bonn U.). BONN-HE-84-06, Feb 1984. 214pp.
Published in Springer Tracts in Mod.Phys.105:187,1984.
Though there are several newer reviews these usually are part of
conference proceedings.
Graviton
>I'm no field theorist, just a knuckle-dragging experimentalist, but it
>seems to me there is one "obvious case" -- photon self-interactions.
Photons have no *self interactions*. While whatever you have said is
perfectly okay the use of the word self interaction is not. Self
interactions are when photon couples with another photon. The
Langrangian of QED doesnot contain photon self interaction
terms. Other gauge bosons like Gluons have self interactions. This is
because QED is an Abelian Gauge Theory while QCD (whose Gauge bosons
are Gluons) is non-Abelian.
>Classically, the linear Maxwell equations require that electromagnetic
>waves pass through one another without effect. However, in QED, loop
>diagrams such as
> g * * g
> *_____*
> | e | e
> e |_____|
> * e *
> g * * g
>give photon-photon couplings, and hence two electromagnetic waves in
>vacuum which intersect _can_ affect one another in ways contrary to
>classical electrodynamics.
As I said before there is no "photon-photon" coupling in the above
diagram. Can you show me where a photon couples to another photon in
the above diagram ?? The diagram you drew is a vaccum polarization
diagram. In field theory you will never find any diagram of the form :
********************
* *
* *
*******************
for photons but you will find a similar diagram for gluons. The reason
is mentioned above. This sort of diagrams are called self
interactions.
But you are right. Photon-photon scattering (the one which you drew)
is predicted only by QFT. Its a fourth order QFT effect. There is no
classical prediction of Photon-Photon Scattering to my knowledge
(although I would welcome if anyone provides evidence to the
contrary). But anyway such photon-photon scattering diagrams (the
first diagram which you drew) is called VACCUM POLARISATION. Its not
SELF INTERACTION. VACCUM POLARISATION and SELF INTERACTIONS are
completely different.
> Since this effect only appears in a
>quantized theory (which I thought had been demonstrated with
>ultra-high power lasers, though I can't find a reference),
Yes actually you can Compton backscatter two laser beams and get
the effects of photon-photon collision. This is a quite well
established practice and it is being done for a number of years
now. If you want I can send you detailed references of the
experiments. Also for a theoretical overview of Photon *VACCUM
POLARISATION* you can read any book on QFT. I think Ryders book does
the complete QFT calculations of the photon self energy
diagrams. There are actually six such "self energy" diagrams but
because of gauge invariance and so forth you only need to explicitly
calculate three such diagrams.
Anyway as one final note : I donot have any objections to the views
you expressed. In fact I agree with them whole heartedly. I just
wanted to point out that self interaction is different from self
energy. Photons donot self interact. Photons being gauge bosons in QED
and QED being an Abelian theory there is no self interactions of
Photons. Gluons on the other hand being gauge bosons of QCD (which is a
non abelian theory) have self interactions. The self interactions of
Gluons are in a way responsible for the aymptotic freedom in
QCD. Without asymptotic freedom QCD would be not very practical as no
perturbative calculations could be done.
> it seems to
>me that it requires the in-transit EM field to be quantized, or
>"particulate", if you want to use that term.
> -- Mike Kelsey
Andy Boden
>
> QED, which quantizes the whole field, makes a definite prediction about the
> existence of the order alpha^4 elastic scattering process, a prediction
> which is in direct conflict with classical electromagnetism. Therefore,
> an experimental measurement of such scattering, perhaps by way of a phase
> shift in one of the beams when the second beam is on vs. off, would be a
> direct, experimental validation of the QED hypothesis of quantizing the
> "in flight" electromagnetic field.
>
> My own literature searches have so far turned up no papers reporting such
> a measurement. However, I *still* seem to recall a brief report in Physics
> Today or the CERN Courier or something, back in 1991 or 1992. I would be
> very interested in anyone who can provide a citation.
>
> -- Mike Kelsey
I don't believe there is such a measurement (if there is, several
groups around the world are wasting their time).
* Early work first by Halpern and then by Euler and Heisenberg led to
an effective QED Lagrangian:
O. Halpern, Phys. Rev. 44, 855 (1934)
W. Heisenberg and E. Euler, Z. Phys. 98, 714 (1936)
* The canonical paper about QED corrections to electromagnetic
propagation (which uses the Euler-Heisenberg Lagrangian) is by Adler
(work done in an astrophysical context):
S.L. Adler, Ann. Phys. (N.Y.) 67, 599 (1971)
* (As far as I know) the first experimental proposal to use optical
photons and the virtual photons of a magnetic field to detect both the
QED correction and the Pecci-Quinn axion is by Iacopini and Zavattini:
E. Iacopini and E. Zavattini, Phys. Lett. B85, 151 (1979)
and
L. Maiani, R. Petronzio, and E. Zavattini, Phys. Lett. B175, 359
(1986)
* Partially because this last reference by L. Maiani et. al. contained
numerical errors, a thorough exposition of the theory (that
concentrates on axions and gravitons, but also includes QED) is given
by Raffelt and Stodolsky:
G. Raffelt and L. Stodolsky, Phys. Rev. D37, 1237 (1988)
* The first codified experiment I know of was led by A. Melissinos
(U. Rochester), and included Zavattini. (The principle target was the
axion, but the QED effect would have been a byproduct.) The proposal
dates back to 1987, but the principle results are quoted in:
R. Cameron et. al., Phys. Rev. D47, 3707, (1993)
Neither the axion nor the QED effect were observed, but the QED is a
null result, since this experiment did not acheive the required
sensitivity to observe the predicted effects.
* As for current efforts, the one (in my opinion) most likely to
observe the QED correction is the PVLAS collaboration. You can get a
flavor for this experiment from (note that Iacopini and Zavattini are
members of this collaboration):
D. Bakalov et. al., Nuc. Phys. B (Proc. Suppl.) 35, 180 (1994)
However, there are active efforts in Taiwan:
W.T. Ni et. al., Mod. Phys. Lett. A 6, 3671 (1991)
and recently groups from Colorado State and SMU/JPL/Caltech/Taiwan (of
which I am a member) submitted proposals for conducting a
Cameron-style experiment using SSC string test magnets (the proposals
are unpublished). These proposals were not successful, but out of
their ashes there may be an effort mounted at FNAL (PVLAS' success is
not guaranteed).
- A.B.
Mike Kelsey
|> kel...@avocet.SLAC.Stanford.EDU (Mike Kelsey) writes:
|>
|> >I'm no field theorist, just a knuckle-dragging experimentalist, but it
|> >seems to me there is one "obvious case" -- photon self-interactions.
|>
|> Photons have no *self interactions*. While whatever you have said is
|> perfectly okay the use of the word self interaction is not.
No dispute about that; I was loose with my terminology. Loop diagrams of
the form I considered are vacuum polarizations, in exactly the same way as
modifications to the single-photon propagator by e+e- virtual pairs. I'm
a bit ticked off by your vituperativeness; the other theorists involved
in this discussion recognized and understood what I meant, and corrected
me less violently.
|> Yes actually you can Compton backscatter two laser beams and get
|> the effects of photon-photon collision. This is a quite well
|> established practice and it is being done for a number of years
|> now.
I'm confused by this. I thought that Compton backscattering was when you
collided a laser (for example) with an electron beam, not two lasers.
For example, that's how the polarization of the beams at the SLC is
measured: on the far side of the IP, a circularly polarized laser (using
a series of Pockells cells) interacts with the outgoing e- beam, and the
intensity of the scattered beam is measured, with both the electron and
laser polarizations being varied randomly and independently.
The expreiment you describe would be exactly the sort of thing I was
concerned with -- photon-photon interactions through higher-order
diagrams. Since it's an O(a^4) process, and all anyone else in this
thread has mentioned have been null upper-limit results -- I wouldn't
expect it to be a "well-established practice." Am I missing something?
Mike Kelsey
|> In article <3pt3ip$9...@azure.acsu.buffalo.edu>, ban...@acsu.buffalo.edu (Graviton) writes:
|> |> kel...@avocet.SLAC.Stanford.EDU (Mike Kelsey) writes:
|> |>
|> |> >I'm no field theorist, just a knuckle-dragging experimentalist, but it
|> |> >seems to me there is one "obvious case" -- photon self-interactions.
|> |>
|> |> Photons have no *self interactions*. While whatever you have said is
|> |> perfectly okay the use of the word self interaction is not.
|>
|> No dispute about that; I was loose with my terminology. Loop diagrams of
|> the form I considered are vacuum polarizations, in exactly the same way as
|> modifications to the single-photon propagator by e+e- virtual pairs.
I've thought about this a bit more, and you _can_ sensibly use the term
"self-interaction," from the viewpoint of an effective theory, rather than
the underlying Lagrangian. Let me argue by analogy to something I am a
bit more familiar with.
In the standard model, there are no direct flavor-changing neutral currents.
That is, both the photon and the Z^0 couple to particle-anti-particle pairs
only (Z0 -> e+ mu- or c u are absolutely forbidden). However, a Penguin
diagram, such as b -> W+* (u,c,t)* -> s, is a higher-order process which
gives the "external appearance" of a flavor-changing neutral current, and
in fact you can build an effective field theory where the W-t loop is
collapsed into a vertex with an effective coupling constant for the flavor
change.
Similarly, it seems to me that you can take the box diagram(s) for the
gamma gamma -> (stuff) -> gamma gamma interaction, and subsume them into
an effective four-photon vertex with some phenomenological coupling
constant. In such an effective field theory, this would indeed be a
photon "self-interaction," and only when you looked "inside the vertex"
would you see the underlying photon-fermion interactions that generate it.
Graviton
>No dispute about that; I was loose with my terminology. Loop diagrams of
>the form I considered are vacuum polarizations, in exactly the same way as
>modifications to the single-photon propagator by e+e- virtual pairs. I'm
>a bit ticked off by your vituperativeness; the other theorists involved
>in this discussion recognized and understood what I meant, and corrected
>me less violently.
Sorry if I sounded too harsh. It wasn't intended to be that way. I
just wanted to state that there are no photon self interactions. I am
indeed sorry it came out harsh. My apologies.
>I'm confused by this. I thought that Compton backscattering was when you
>collided a laser (for example) with an electron beam, not two lasers.
>For example, that's how the polarization of the beams at the SLC is
>measured: on the far side of the IP, a circularly polarized laser (using
>a series of Pockells cells) interacts with the outgoing e- beam, and the
>intensity of the scattered beam is measured, with both the electron and
>laser polarizations being varied randomly and independently.
>The expreiment you describe would be exactly the sort of thing I was
>concerned with -- photon-photon interactions through higher-order
>diagrams. Since it's an O(a^4) process, and all anyone else in this
>thread has mentioned have been null upper-limit results -- I wouldn't
>expect it to be a "well-established practice." Am I missing something?
> -- Mike Kelsey
I guess I covered this aspect in our private correspondence and once
again I agree wholeheartedly on what you had to say regarding photon
colliders. My apologies again. In my defence however I am just a
"knuckle dragging theorist" and not an experimentalist. :).
I would however like to thank the other person who provided some
references on photon-phton collisions.
Ben Bullock
> It really annoys me when people disingenuously color the comments of
> others to try to make themselves look smart (my *inference* on your
> comments based on its tennor). It is, however, a pervasive
> phenomenon...
Yes, it's just awful the way I tried to make myself look smart, that
pervasive phenomenon. Why don't you deny me the opportunity, by not
making incorrect statements?
Bob Jacobsen
discussion of how we know that photons themselves are quantized, as
opposed to just their interactions with matter.
So how about the Mossbauer measurements of gravitational redshift of
photons? Doesn't the energy loss tell you the "weight" of the photon?
Or is there some catch I haven't thought of?
--
Bob Jacobsen, (Bob_Ja...@lbl.gov, 510-486-7355, fax 510-486-5101)
Jonathan Scott
on Mon, 29 May 1995 19:38:57 -0800,
Bob Jacobsen <Bob_Ja...@lbl.gov> writes:
>...
>So how about the Mossbauer measurements of gravitational redshift of
>photons? Doesn't the energy loss tell you the "weight" of the photon?
>
>Or is there some catch I haven't thought of?
The apparent "energy loss" for photons going upwards is due to the fact
that the observer's time rate (clocks and everything else) is faster at
the top of a tower but the frequency of an electromagnetic wave being
transmitted from the top to the bottom does not vary, so the wave has
a lower frequency measured relative to clocks at the top of the tower.
The same frequency shift would be observed for any other periodic
effect, for example watching the same movie screen from the top or
bottom of the tower (although the shift would be undetectably small in
this case).
This doesn't seem to have anything to do with weight or photons to me.