Why don't photons interfere with each other in this setup?
Ioannis
Two light sources L1/L2 and two observers O1/O2 are placed as follows.
Observer O1 looks at source L1 directly and observer O2 looks at source L2
directly. The two light paths cross perpendicularly at the central junction
point "+".
If you like, consider the sources L1/L2 to be coherent light sources such as
laser beams, with the respective beams crossing each other perpendicularly
at "+".
(View with fixed width font please)
O2
|
L1---+---O1
|
L2
Or a similar setup with O1=O2 through a mirror M as follows:
O
|\
| \
L1---+--/M
|
L2
Or if you like, O1=projection on wall 1 and O2= projection on wall 2, both
viewed by a single observer.
My question is, why isn't there interference between the two beams at +?
That is, observer O1 for example, will see light source L1, as if beam L2/O2
doesn't even exist.
More naively put: How do the photons of beam L1-O1 (resp L2-O2) "know" how
to cross the junction + without interfering with the photons of beam L2-O2
(resp L1-O1)?
Thanks much in advance,
--
Ioannis
Jan Panteltje
<morp...@olympus.mons> wrote in <1154882814.533115@athnrd02>:
Because 'photons' do not exist as particle.
The _waves_ will only interfere (multiply) in a non linear medium.
There are experiments planned on photon - photon scattering,
http://2physics.blogspot.com/2006/03/photon-photon-scattering.html
but so far it has not been observed (as is logical regarding what I mentioned
above).
_However_ at high enough energy _something_ may happen, as space is not
totally empty, and it _could_ for example be EM waves are just a motion of Le
Sage particles.
An _non_ empty space likely is not 100% linear (nothing is), so....
This is all my personal view, many OneStonians here (if not all) will now
start to scream or cry of ejaculate insults, or whatever....
But it feels good to have my own theory that so far scores 100%.
You are free to listen to whom you believe most.
It is interesting that _humans_ learn largely by aping (imitating),
while _apes_ learn by analysis (this has been shown).
The advantage of 'aping' by humans is that you can learn to do a lot without
understanding (math is one way), this is what the school system uses too.
You _learn_ stuff, not from experience, but from parroting the teachers (or
teachings), be it formulas or theories.
Generation after generation, those 'apers' multiply, and man we no longer
can reach the moon, no fusion power....
Apes on the other hand (start joke) have been used to predict stock market,
and at some point scored better then the profies (end joke).
Anyways, genetically somebody will have the ability to think for themselves
some time during evolution... something will happen, something will be
invented, some new theory will be found, that then the next generation[s]
will parrot.
;-)
Sorcerer
"Ioannis" <morp...@olympus.mons> wrote in message
news:1154882814.533115@athnrd02...
| This may be a naive question, so please excuse it in advance.
|
| Two light sources L1/L2 and two observers O1/O2 are placed as follows.
| Observer O1 looks at source L1 directly and observer O2 looks at source L2
| directly. The two light paths cross perpendicularly at the central
junction
| point "+".
|
| If you like, consider the sources L1/L2 to be coherent light sources such
as
| laser beams, with the respective beams crossing each other perpendicularly
| at "+".
|
| (View with fixed width font please)
| O2
| |
| L1---+---O1
| |
| L2
|
| Or a similar setup with O1=O2 through a mirror M as follows:
|
| O
| |\
| | \
| L1---+--/M
| |
| L2
|
| Or if you like, O1=projection on wall 1 and O2= projection on wall 2, both
| viewed by a single observer.
|
| My question is, why isn't there interference
There is. My question is, why do you assume there isn't?
Androcles
Timo A. Nieminen
> Two light sources L1/L2 and two observers O1/O2 are placed as follows.
> Observer O1 looks at source L1 directly and observer O2 looks at source L2
> directly. The two light paths cross perpendicularly at the central junction
> point "+".
[cut]
> O2
> |
>L1---+---O1
> |
> L2
[cut]
> My question is, why isn't there interference between the two beams at +?
There is. Keep in mind by what "interference" is. When the total amplitude
of the wave (here, the electric field amplitude E) at some point, such as
your +, is given by E_total = E1 + E2, this is interference. This is also
linear, and waves 1 and 2 will continue on undisturbed after point +.
There is "interference" at +, in the technical sense of the word, but the
waves don't "interfere" with each other, in the common English usage of
the word.
> That is, observer O1 for example, will see light source L1, as if beam L2/O2
> doesn't even exist.
>
> More naively put: How do the photons of beam L1-O1 (resp L2-O2) "know" how
> to cross the junction + without interfering with the photons of beam L2-O2
> (resp L1-O1)?
Because photons are not like little billiard balls. Photons are not
_classical_ particles. Their motion is described the Maxwell equations, so
the fact that two waves crossing each other in a linear medium will
interfere where they cross, and then continue on undisturbed is enough to
explain it.
If you _really_ want a billiard-ball picture, the billiard balls are zero
size, and have no charge. How can they interact with each other when they
cross? Too small to hit each other, and no charge means no interaction at
a distance.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
Helpful person
They do not interfere with each other because they do not have the same
phase. Interference will only occur (under some conditions) if the two
light sources are originally from the same source.
Please visit my web sight at www.richardfisher.com
Timo Nieminen
> They do not interfere with each other because they do not have the same
> phase. Interference will only occur (under some conditions) if the two
> light sources are originally from the same source.
No, light from two sources can and does interfere. See, e.g.,
H. Paul, Rev. Mod. Phys. 58, 209-231 (1986).
To obtain a stationary interference pattern, then the sources need to be
monochromatic and maintain a constant phase relationship. That two lasers
with close but different frequencies generate a moving interference
pattern has been shown experimentally (a quick search of my bib db fails
to find it; perhaps Paul cites it, or is cited in it).
Helpful person
Timo Nieminen wrote:
> > light sources are originally from the same source.
>
> No, light from two sources can and does interfere. See, e.g.,
> H. Paul, Rev. Mod. Phys. 58, 209-231 (1986).
>
> To obtain a stationary interference pattern, then the sources need to be
> monochromatic and maintain a constant phase relationship. That two lasers
> with close but different frequencies generate a moving interference
> pattern has been shown experimentally (a quick search of my bib db fails
> to find it; perhaps Paul cites it, or is cited in it).
>
I believe the poster was interested in the creation of interference
fringes in a simple interferometer. This cannot be achieved with two
separate sources unless they are phase locked.
Interference between two sources with different frequencies can only be
achieved if they originate from a single source, allowing them to
remain phase locked.
Please visit my web site at www.richardfisher.com
Phil Hobbs
> On Mon, 6 Aug 2006, Helpful person wrote:
>
>
>>They do not interfere with each other because they do not have the same
>>phase. Interference will only occur (under some conditions) if the two
>>light sources are originally from the same source.
>
>
> No, light from two sources can and does interfere. See, e.g.,
> H. Paul, Rev. Mod. Phys. 58, 209-231 (1986).
>
> To obtain a stationary interference pattern, then the sources need to be
> monochromatic and maintain a constant phase relationship. That two lasers
> with close but different frequencies generate a moving interference
> pattern has been shown experimentally (a quick search of my bib db fails
> to find it; perhaps Paul cites it, or is cited in it).
Thirty years of laser phase-locking shows it, if nothing else. Locking
one laser to another requires that they interfere, and they do. There's
nothing spooky about the sources being different--it's all about
frequency differences, phase front matching, and measurement bandwidth,
just like radio.
Maxwell's equations are the complete, quantum-mechanically and
relativistically correct description of electromagnetic fields in vacuo,
up to intensities that can probably never be reached in the lab. You'll
never measure photons scattering off photons, period. Light waves pass
through each other without any interaction, and you calculate the fields
in the crossing regions by adding up the fields in the individual beams.
No worries.
Photons are a red herring, and reasoning about propagating light using
photon ideas will lead you wrong every single time. Count on it.
Nobody knows what a photon *is*, for one thing. There was a
supplementary issue of Optics & Photonics News a couple of years ago
where they asked a number of leading optical physicists to answer the
question, "What is a photon?" Everyone had a definite answer, but no
two of them agreed! "Often wrong, but never in doubt."
The one place where the concept of the photon is actually useful is in
the interaction of light with matter, i.e. absorption, scattering,
emission, and stimulated emission. In optical measurements, the main
consequence of the quantization of light is photodetction statistics.
People have written down quantum mechanical expressions for what happens
in photodetection, but that has nothing to do with what light does in
vacuo, which is to obey Maxwell's equations.
Cheers,
Phil Hobbs
Timo A. Nieminen
> Timo Nieminen wrote:
>>> light sources are originally from the same source.
>>
>> No, light from two sources can and does interfere. See, e.g.,
>> H. Paul, Rev. Mod. Phys. 58, 209-231 (1986).
>>
>> To obtain a stationary interference pattern, then the sources need to be
>> monochromatic and maintain a constant phase relationship. That two lasers
>> with close but different frequencies generate a moving interference
>> pattern has been shown experimentally (a quick search of my bib db fails
>> to find it; perhaps Paul cites it, or is cited in it).
>
> I believe the poster was interested in the creation of interference
> fringes in a simple interferometer.
From the original post, I got the impression the the OP was interested in
how/why the two beams could be undisturbed after crossing. Note that only
light from a single source fell onto each detector.
> This cannot be achieved with two
> separate sources unless they are phase locked.
>
> Interference between two sources with different frequencies can only be
> achieved if they originate from a single source, allowing them to
> remain phase locked.
It's been done with 2 sources, optically. Not easy to get stable fringes,
but it's been done. Easy to do at RF.
Yes, I agree that the phase difference must be kept (close to) constant to
get (almost) stationary fringes. But "interference" is far more than
"getting visible fringes". For example, interference is why
anti-reflection coatings work, with no fringes in sight.
Just the usual usenet nit-picking.
Timo A. Nieminen
> Nobody knows what a photon *is*, for one thing. There was a supplementary
> issue of Optics & Photonics News a couple of years ago where they asked a
> number of leading optical physicists to answer the question, "What is a
> photon?" Everyone had a definite answer, but no two of them agreed! "Often
> wrong, but never in doubt."
2003, vol 14, October, iirc.
And for more: Lamb, "Anti-photon", App Phys B, 60? ??-?? (1995?)
> The one place where the concept of the photon is actually useful is in the
> interaction of light with matter, i.e. absorption, scattering, emission, and
> stimulated emission. In optical measurements, the main consequence of the
> quantization of light is photodetction statistics. People have written down
> quantum mechanical expressions for what happens in photodetection, but that
> has nothing to do with what light does in vacuo, which is to obey Maxwell's
> equations.
To pick nits: expect in the case of energy and momentum and angular
momentum transport for non-large photon-numbers (although Maxwell gives
the correct probabilities). Assuming of course that transport of energy,
momentum, and AM is something "light" does.
Phil Hobbs
> On Mon, 7 Aug 2006, Phil Hobbs wrote:
>
>> Nobody knows what a photon *is*, for one thing. There was a
>> supplementary issue of Optics & Photonics News a couple of years ago
>> where they asked a number of leading optical physicists to answer the
>> question, "What is a photon?" Everyone had a definite answer, but no
>> two of them agreed! "Often wrong, but never in doubt."
>
>
> 2003, vol 14, October, iirc.
>
That sounds about right.
> And for more: Lamb, "Anti-photon", App Phys B, 60? ??-?? (1995?)
That's a brilliant article. Not too many people could have written
that, and even fewer could have gotten away with it.
>
>> The one place where the concept of the photon is actually useful is in
>> the interaction of light with matter, i.e. absorption, scattering,
>> emission, and stimulated emission. In optical measurements, the main
>> consequence of the quantization of light is photodetction statistics.
>> People have written down quantum mechanical expressions for what
>> happens in photodetection, but that has nothing to do with what light
>> does in vacuo, which is to obey Maxwell's equations.
>
>
> To pick nits: expect in the case of energy and momentum and angular
> momentum transport for non-large photon-numbers (although Maxwell gives
> the correct probabilities). Assuming of course that transport of energy,
> momentum, and AM is something "light" does.
>
Is there a way of measuring that that doesn't involve the interaction of
light with matter? I'd have thought that those were in more or less the
same class as photodetection statistics.
Cheers,
Phil Hobbs
Timo Nieminen
> Timo A. Nieminen wrote:
> > On Mon, 7 Aug 2006, Phil Hobbs wrote:
> >
> >> Nobody knows what a photon *is*, for one thing. There was a
> >> supplementary issue of Optics & Photonics News a couple of years ago
> >> where they asked a number of leading optical physicists to answer the
> >> question, "What is a photon?" Everyone had a definite answer, but no
> >> two of them agreed! "Often wrong, but never in doubt."
> >
> > 2003, vol 14, October, iirc.
> >
> That sounds about right.
>
> > And for more: Lamb, "Anti-photon", App Phys B, 60? ??-?? (1995?)
>
> That's a brilliant article. Not too many people could have written
> that, and even fewer could have gotten away with it.
I've been giving Lamb to my graduate coursework students for a couple of
years now for some light reading, I think I'll add the OPN supp as well.
Hanbury Brown had a nice section in "Boffin" on the reaction to the
intensity interferometer from the billiard-ball photon mafia. A nice
cautionary tale.
> >> The one place where the concept of the photon is actually useful is in
> >> the interaction of light with matter, i.e. absorption, scattering,
> >> emission, and stimulated emission. In optical measurements, the main
> >> consequence of the quantization of light is photodetction statistics.
> >> People have written down quantum mechanical expressions for what
> >> happens in photodetection, but that has nothing to do with what light
> >> does in vacuo, which is to obey Maxwell's equations.
> >
> > To pick nits: expect in the case of energy and momentum and angular
> > momentum transport for non-large photon-numbers (although Maxwell gives
> > the correct probabilities). Assuming of course that transport of energy,
> > momentum, and AM is something "light" does.
>
> Is there a way of measuring that that doesn't involve the interaction of
> light with matter? I'd have thought that those were in more or less the
> same class as photodetection statistics.
Detection is where you see it. One can even say that this is what
detection is all about. But the E/M/AM still needs to get from
point A to point B, at least with some interpretations of QM. This is a
great mystery of nature. Atom A emits radiation, atom B absorbs all of the
energy emitted by A, even if classically, the directivity of emission and
reception would not allow it. OTOH, while this is easily explained by a
simple corpuscular model, this then leaves even more mysterious the
question of why the Maxwell equations work for everything except detection
statistics. Moral: if you insist that Nature be simple and easy to
understand, disappointment lies in wait.
AES
Phil Hobbs <pc...@SpamMeSenseless.pergamos.net> wrote:
> Photons are a red herring, and reasoning about propagating light using
> photon ideas will lead you wrong every single time. Count on it.
Absolutely!
> Nobody knows what a photon *is*, for one thing. There was a
> supplementary issue of Optics & Photonics News a couple of years ago
> where they asked a number of leading optical physicists to answer the
> question, "What is a photon?" Everyone had a definite answer, but no
> two of them agreed! "Often wrong, but never in doubt."
Or there's Nobel Laureate Willis E. Lamb, Jr., "Anti-photon," Appl.
Phys. B, vol. 60, pp. 77--84, (February/March 1995).
Abstract: "It should be apparent from the title of this article that the
author does not like the use of the word "photon", which dates from
1926. In his view, there is no such thing as a photon. Only a comedy of
errors and historical accidents led to its popularity among physicists
and optical scientists. I admit that the word is short and convenient.
Its use is also habit forming. Similarly, one might find it convenient
to speak of the "aether" or "vacuum" to stand for empty space, even if
no such thing existed. There are very good substitute words for
"photon", (e.g., "radiation" or "light"), and for "photonics" (e.g.,
"optics" or "quantum optics"). Similar objections are possible to use of
the word "phonon", which dates from 1932. Objects like electrons,
neutrinos of finite rest mass, or helium atoms can, under suitable
conditions, be considered to be particles, since their theories then
have viable non-relativistic and non-quantum limits. This paper outlines
the main features of the quantum theory of radiation and indicates how
they can be used to treat problems in quantum optics."
Sample quote: "It is high time to give up the use of the word `photon',
and of a bad concept which will shortly be a century old. Radiation does
not consist of particles."
> The one place where the concept of the photon is actually useful is in
> the interaction of light with matter, i.e. absorption, scattering,
> emission, and stimulated emission.
But even there, only as a convenient unit of energy, right?
> In optical measurements, the main
> consequence of the quantization of light is photodetction statistics.
> People have written down quantum mechanical expressions for what happens
> in photodetection, but that has nothing to do with what light does in
> vacuo, which is to obey Maxwell's equations.
And even here, you're flirting with trouble. A photodetector
click does _not_ mean "a photon arrived", right?
Salmon Egg
siegman-485A97...@news.stanford.edu, "AES"
<sie...@stanford.edu> wrote:
> In article <44D761AA...@SpamMeSenseless.pergamos.net>,
> Phil Hobbs <pc...@SpamMeSenseless.pergamos.net> wrote:
>
>> Photons are a red herring, and reasoning about propagating light using
>> photon ideas will lead you wrong every single time. Count on it.
>
> Absolutely!
>
>
>> Nobody knows what a photon *is*, for one thing. There was a
>> supplementary issue of Optics & Photonics News a couple of years ago
>> where they asked a number of leading optical physicists to answer the
>> question, "What is a photon?" Everyone had a definite answer, but no
>> two of them agreed! "Often wrong, but never in doubt."
Just to argue with you, Tony, Richard Feynman liked photons. He went into
detail on how to calculate with photons. Of course, he delved into how waves
entered the picture, but sort of avoided them by talking probability
amplitudes. A link to his lectures on video is
http://www.vega.org.uk/video/subseries/8.
These are well worth watching although they are a bit on the long side.
To me, Feynman was a demigod when it came to physics and other science.
Bill
-- Ferme le Bush
Perre Mancenillier
"Phil Hobbs" <pc...@SpamMeSenseless.pergamos.net> wrote
> Maxwell's equations are the complete, quantum-mechanically and
> relativistically correct description of electromagnetic fields in vacuo,
> up to intensities that can probably never be reached in the lab. You'll
> never measure photons scattering off photons, period. Light waves pass
> through each other without any interaction, and you calculate the fields
> in the crossing regions by adding up the fields in the individual beams.
> No worries.
>
> Photons are a red herring, and reasoning about propagating light using
> photon ideas will lead you wrong every single time. Count on it.
>
> Nobody knows what a photon *is*, for one thing. There was a supplementary
> issue of Optics & Photonics News a couple of years ago where they asked a
> number of leading optical physicists to answer the question, "What is a
> photon?" Everyone had a definite answer, but no two of them agreed!
> "Often wrong, but never in doubt."
Wonderful, what I relief, I already sleep better at night!
Does that mean that we poor optical designers can go on with
our rays, our wavefronts and all our ad-hoc assumptions and
non-physical entities, our vectors and the like? ;-)
Which somehow have proved adequate for about 350 years?
Phil Hobbs
Except the optical transfer function. That one, you guys need to repent
for, or it's the slammer, no question about it.
The confusing names of aberrations in the wave and ray pictures (I
mentioned defocus in a previous post) is a venial sin, but you do need
to 'fess up in public and take your lumps.
Radiometry/photometry is a lost cause--if you want your insomnia back,
imaging being held responsible for metrelamberts and apostilbs...nits
and lumens...and 'intensity' meaning 'watts per steradian'.
But I digress. The photon is useful for book-keeping: predicting the
counting statistics of photoelectrons, sorting out the frequency shifts
in acousto-optic cells, that kind of thing. Just don't describe
propagating fields as ensembles of photons, and you'll be okay.
Cheers,
Phil "Photon Cop" Hobbs
Phil Hobbs
> Detection is where you see it. One can even say that this is what
> detection is all about. But the E/M/AM still needs to get from
> point A to point B, at least with some interpretations of QM. This is a
> great mystery of nature. Atom A emits radiation, atom B absorbs all of the
> energy emitted by A, even if classically, the directivity of emission and
> reception would not allow it. OTOH, while this is easily explained by a
> simple corpuscular model, this then leaves even more mysterious the
> question of why the Maxwell equations work for everything except detection
> statistics. Moral: if you insist that Nature be simple and easy to
> understand, disappointment lies in wait.
>
It's a deep philosophical and theological point as well, interestingly.
Since there is no way to know in advance which atom is going to do the
absorbing, information is appearing in the universe all the time. Where
does it come from, if anywhere? This new information plus the butterfly
effect is enough to allow divine providence to be totally consistent
with all known physical laws--you just have to posit somebody or
something working all those little control levers.
Physics Today published an old speech of Einstein's in 2005, in which he
claimed among other things that irreversible thermodynamics was enough
to restore determinism to physics, but he obviously didn't know about
sensitive dependence on initial conditions. More and more chaotic
amplifiers show up in different system all the time--it's not just the
weather, even billiard balls exhibit it.
Cheers,
Phil Hobbs
AES
> absorbing, information is appearing in the universe all the time. Where
> does it come from, if anywhere? This new information plus the >>> butterfly
> effect <<< is enough to allow divine providence to be totally consistent
> with all known physical laws--you just have to posit somebody or
> something working all those little control levers.
> sensitive dependence on initial conditions. More and more chaotic
> amplifiers show up in different system all the time--it's not just the
> >>> weather <<<, even billiard balls exhibit it.
You may have seen the recent op-ed piece I read somewhere pointing out
that if we want to prevent future Katrina-style disasters, we have to
track down that elusive butterfly out there somewhere that will trigger
the next such hurricane, and kill it. Except maybe terrorists are,
right now, training flights of butterflies that will go out and trigger
even bigger events . . . and so on.
Phil Hobbs
"That butterfly must be caught and stopped." Beautiful.
Cheers,
Phil Hobbs
p.ki...@ic.ac.uk
> But I digress. The photon is useful for book-keeping: predicting the
> counting statistics of photoelectrons, sorting out the frequency shifts
> in acousto-optic cells, that kind of thing. Just don't describe
> propagating fields as ensembles of photons, and you'll be okay.
OK, enough. Many workers in quantum optics describe
propagating fields as ensembles of photons, and have no
problems. Quantum optics photons are single excitations of
EM field modes, and are perfectly well defined objects that can
be thought about consistently, and used reliably in calculations.
I've no idea what your sort of "photon" is, but just because that
doesn't work very well, doesn't mean that all definitions have
that problem.
--
---------------------------------+---------------------------------
Dr. Paul Kinsler
Blackett Laboratory (QOLS) (ph) +44-20-759-47520 (fax) 47714
Imperial College London, Dr.Paul...@physics.org
SW7 2BW, United Kingdom. http://www.qols.ph.ic.ac.uk/~kinsle/
Jan Panteltje
p.ki...@ic.ac.uk wrote in <milsq3-...@delillo.lsr.ph.ic.ac.uk>:
>I've no idea what your sort of "photon" is, but just because that
>doesn't work very well, doesn't mean that all definitions have
>that problem.
My definition of elves has no problem either.
'Photon' is just a mathematical concept, and as that you can do math with it.
It is _not_ a real particle however.
Sue...
Do you shut off your cars headlamps when approaching an intersection
so you don't knock the beams from approaching traffic askew? :o)
<<Now, does not the prize to Einstein imply
that the Academy recognised the particle
nature of light? The Nobel Committee says
that Einstein had found that the energy exchange
between matter and ether occurs by atoms emitting
or absorbing a quantum of energy,hv .
As a consequence of the new concept of light quanta
(in modern terminology photons) Einstein proposed the
law that an electron emitted from a substance by
monochromatic light with the frequency has to have
a maximum energy of E=hv-p, where p is the energy needed to
remove the electron from the substance. Robert Andrews
Millikan carried out a series of measurements over a
period of 10 years, finally confirming the validity of this
law in 1916 with great accuracy. Millikan had, however,
found the idea of light quanta to be unfamiliar and strange.
The Nobel Committee avoids committing itself to the
particle concept. Light-quanta or with modern terminology,
photons, were explicitly mentioned in the reports on
which the prize decision rested only in connection with
emission and absorption processes. The Committee says
that the most important application of Einstein's photoelectric
law and also its most convincing confirmation has come from
the use Bohr made of it in his theory of atoms, which explains
a vast amount of spectroscopic data. >>
http://nobelprize.org/physics/articles/ekspong/index.html
Sue...
Sue...
p.ki...@ic.ac.uk wrote:
> Phil Hobbs <pc...@spammesenseless.pergamos.net> wrote:
> > But I digress. The photon is useful for book-keeping: predicting the
> > counting statistics of photoelectrons, sorting out the frequency shifts
> > in acousto-optic cells, that kind of thing. Just don't describe
> > propagating fields as ensembles of photons, and you'll be okay.
>
> OK, enough. Many workers in quantum optics describe
> propagating fields as ensembles of photons, and have no
> problems.
If they have no problems, then why must they fit their photons
with wrist watches and have them carry magnetic monopoles
and explore all paths?
http://www.physics.yorku.ca/undergrad_programme/highsch/Feynm4.html
Sue...
Edward Green
> p.ki...@ic.ac.uk wrote:
> > Many workers in quantum optics describe
> > propagating fields as ensembles of photons, and have no
> > problems.
>
> If they have no problems, then why must they fit their photons
> with wrist watches and have them carry magnetic monopoles
> and explore all paths?
To the best of my knowledge, the "explore all paths" business is just
an expression of Huygen's principle, and can describe any wave
disturbance.
Feynman was a showman, and liked the slightly outre description.
Phil Hobbs
> Phil Hobbs <pc...@spammesenseless.pergamos.net> wrote:
>
>>But I digress. The photon is useful for book-keeping: predicting the
>>counting statistics of photoelectrons, sorting out the frequency shifts
>>in acousto-optic cells, that kind of thing. Just don't describe
>>propagating fields as ensembles of photons, and you'll be okay.
>
>
> OK, enough. Many workers in quantum optics describe
> propagating fields as ensembles of photons, and have no
> problems. Quantum optics photons are single excitations of
> EM field modes, and are perfectly well defined objects that can
> be thought about consistently, and used reliably in calculations.
>
> I've no idea what your sort of "photon" is, but just because that
> doesn't work very well, doesn't mean that all definitions have
> that problem.
>
Yeah, I've done a bit of advanced quantum too, though never any publishable
stuff.
I'm really not intending to tell experts what to do or how to think. But
then you weren't the intended audience. The OP and almost all the photon
people we get on these NGs are very far from expert--and you can't get from
there to where you are without passing through Maxwell's equations and taking
them seriously.
One other point: When you talk about "ensembles of photons", you're talking
about second-quantized descriptions, no? That's a quite different idea from
the billiard-ball or high energy physics pictures of particles. The photon
is not an object that you can take out and examine, like an electron in a
Penning trap--it's a quantized excitation of a normal mode. You have to
stretch the idea of a 'thing' almost out of recognition in order for a photon
to qualify.
Hence the difficulty that Lamb and the OPN folk exhibit.
Cheers,
Phil Hobbs
AES
> propagating fields as ensembles of photons, and have no
> problems. Quantum optics photons are single excitations of
> EM field modes, and are perfectly well defined objects that can
> be thought about consistently, and used reliably in calculations.
>
> Dr. Paul Kinsler
> Blackett Laboratory (QOLS) (ph) +44-20-759-47520 (fax) 47714
I believe the problem lies in the 4th line above, which claims that
photons ". . are perfectly well defined objects . . ".
Quantum Trappists (the modern monks?) can put a single atom (which I
think we'd both agree is at least a "reasonably well defined object")
into one or another type of trap, at a time of their choosing, and
1) Confirm that it's there (without destroying it);
2) Make various, repeated, nondestructive measurements on it
(admittedly perturbing the results of other future measurements
on it in the process of doing so);
3) Confirm again, repeatedly, that it's still there; and
4) At the end take it out of the trap and maybe do something
else with it.
I suggest that these attributes are -- particularly the "repeatedly"
aspects -- are or ought to be, necessary attributes for something to be
thought of as an "object" (in common parlance at least).
But NONE of these things -- or at least, certainly not several of these
things -- can be done on a single photon, or a "single-photon pulse"
(not so far as I know, anyway).
Speaking of a "photon" as an "object" leads not only the readers of this
group, but professional audiences who listen to talks by people like
Marlan Scully, to think that photons are "objects", or at least have
attributes like the above that are commonly associated with objects --
and that's not true.
Arguments or examples to the contrary? Can you, even in principle, put
a single photon into an enormously high-Q cavity (or a fiber ring);
confirm repeatedly that it's there; repeatedly measure it or use it for
other measurements; and then take it out again?
P.S. -- With no resort to Mandel-type coupled signal + idler "photon"
concepts allowed. Those aren't separate signal and idler photons, as
separate objects; they're permanently and inextricably entangled photon
pairs.
[Historical note: When I and, independently, Louisell and Yariv first
proposed the existence of "parametric spontaneous emission" or
"parametric fluorescence" and calculated its properties in 1961, we
didn't anticipate all the wonders of Steve Harris' first observation of
it in 1967, much less all the more recent wonders of Mandel and
successors, but we understood the irremovable coupling involved in that
system.]
mme...@cars3.uchicago.edu
>Sue... wrote:
>> p.ki...@ic.ac.uk wrote:
>
>> > Many workers in quantum optics describe
>> > propagating fields as ensembles of photons, and have no
>> > problems.
>>
>> If they have no problems, then why must they fit their photons
>> with wrist watches and have them carry magnetic monopoles
>> and explore all paths?
>
>an expression of Huygen's principle, and can describe any wave
>disturbance.
>
Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"
p.ki...@ic.ac.uk
> I believe the problem lies in the 4th line above, which claims that
> photons ". . are perfectly well defined objects . . ".
> Quantum Trappists (the modern monks?) can put a single atom (which I
> think we'd both agree is at least a "reasonably well defined object")
> into one or another type of trap, at a time of their choosing, and
> 1) Confirm that it's there (without destroying it);
> 2) Make various, repeated, nondestructive measurements on it
> (admittedly perturbing the results of other future measurements
> on it in the process of doing so);
> 3) Confirm again, repeatedly, that it's still there; and
> 4) At the end take it out of the trap and maybe do something
> else with it.
> I suggest that these attributes are -- particularly the "repeatedly"
> aspects -- are or ought to be, necessary attributes for something to be
> thought of as an "object" (in common parlance at least).
I guess you demand a whole lot more of something described as an
object than I do ... perhaps I should have said " . are perfectly
well defined thingys . .". I think, though, that the average non
physicist will assume that even I replace "thingy" with "whatsit",
that the "object" has properties of a type inappropriate for the
definition of a photon used in quantum optics.
If I were to need a snappy but not-entirely-innacurate description of
photons, I'd call them "countable waves". I certainly wouldn't like
to call them particles.
> [...]
> Arguments or examples to the contrary? Can you, even in principle, put
> a single photon into an enormously high-Q cavity (or a fiber ring);
> confirm repeatedly that it's there; repeatedly measure it or use it for
> other measurements; and then take it out again?
If it were the right sort of measurement you could -- i.e. if it were
a (so called) non demolition measurement. Whether you could learn
anything useful is less certain: after all, what would thirty sucessive
quantum non demolition measurements of the same single photon state [1]
tell you? Even If I were to disregard the contraints on legal QND
measurements, I would still only imagine I'd get to see the same thing
thirty times, or some string of random results as you recollapsed it back
into a single photon number state. But I was never an expert on QND
measurements.
[1] Or something it had evolved into.
--
---------------------------------+---------------------------------
Dr. Paul Kinsler
Blackett Laboratory (QOLS) (ph) +44-20-759-47520 (fax) 47714
Skywise
<Snipola>
> If I were to need a snappy but not-entirely-innacurate description of
> photons, I'd call them "countable waves". I certainly wouldn't like
> to call them particles.
<Snipola>
Wavicles?
Brian
--
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p.ki...@ic.ac.uk
> p.ki...@ic.ac.uk wrote in news:6ei4r3-...@delillo.lsr.ph.ic.ac.uk:
> > photons, I'd call them "countable waves". I certainly wouldn't like
> > to call them particles.
> <Snipola>
> Wavicles?
Sounds better, I agree, but doesn't really impart any useful
intuition about how they (sort of) behave. What is it about
"icles" that suggests countability rather than spatial
localisation, for example?
Skywise
> Skywise <in...@oblivion.nothing.com> wrote:
>> p.ki...@ic.ac.uk wrote in news:6ei4r3-...@delillo.lsr.ph.ic.ac.uk:
>> > If I were to need a snappy but not-entirely-innacurate description of
>> > photons, I'd call them "countable waves". I certainly wouldn't like
>> > to call them particles.
>> <Snipola>
>
>> Wavicles?
>
> Sounds better, I agree, but doesn't really impart any useful
> intuition about how they (sort of) behave. What is it about
> "icles" that suggests countability rather than spatial
> localisation, for example?
Neither does "photon".
I don't think it's the word that matters, it's the definition.
Actually, that doesn't matter either. Whether one is more
classicaly inclined (for whatever reason) and think of
photons as little BB's bouncing around or the quantum
physicist who thinks of them as quantized packets of energy
with mostly (only) wave properties, the word 'photon' still
imparts the idea of energy transfer between two points.
The need dictates the definition used.
Heck, if we want to get really pedantic, there are no such
things as particles at all. Everything is a wave function
and has wave like properties. Is it still accurate to call
them particles?
Just some thoughts from an amateur. :)
Phil Hobbs
> Heck, if we want to get really pedantic, there are no such
> things as particles at all. Everything is a wave function
> and has wave like properties. Is it still accurate to call
> them particles?
>
> Just some thoughts from an amateur. :)
It's true that the behaviour of material particles is governed by their
wave functions. What isn't true is that photons possess the same sorts
of attributes as electrons or protons--so much so that they don't
qualify as 'things' in philosophical terms. However, we've pretty well
flogged the cover off this dead horse by now--let's go back to something
practical, like UFOs or perpetual motion machines.
Cheers,
Phil Hobbs
Skywise
@SpamMeSenseless.pergamos.net:
Agreed. At least those make sense. ;)