Photon Size: Direction & Probability

[Proof.Beh]

I wouldn't over-extend a statement to be a proof. I think you can say that 'you've introduced the sources your claim', but there is no (I believe) proof for the existence of size for photon.----and either marlon or ZapperZ probably have more knowledge in the area than (you + me) ^2.-- (and probably ^4)--(ok, even maybe even^6)

1- Perhaps they have more knowledge in the area than you (you can explain your personal opinion about your knowledge but it differs for me), but actually they did not prove your claim.

2- Now that I think more, see that their knowledge equal to (your [not my] knowledge)^n, n>>6. Note: everybody can explain his personal opinion about himself only.

3- Again, if you have any claim about this discussion, can introduce it. Of course your last post did not relate to "How big is a photon?" NONE.

Thanks.
Mr Beh

 

[rewebster]

I guess you miss a point. First of all, this session is not about “How big is a photon” but about how we handle a problem, since it was discussed in QP of PF several times and start to be boring even before the problem is formulated.

It is common point that one should use the references (the previous discussions in PF included). But kids are not used to do that. That is all point Zz made and suggested to use the “Standard References”. No doubt that he is right and everybody agree. Only we did not agree with the details of the technical realization.

Yeah,--- and I think 'how one thinks about a problem' is what sets people in different categories, too. People can "quote" this and "quote" that all day long--I think its the ability to correlate the ideas/information given/known and there's not a lot, it seems, on the 'size of photons'--(of which can be taken not 'too far'/'in new directions' here on the forum before it becomes 'speculative' and is not accepted on the forum--which is fine).

What information one believes one takes to be the basis of his belief system; and that's even here on the forum as there are different categories and sub-categories. Can you 'convert' a relativity person back into a Newtonian classic person? (it's hard to get them back on the farm, etc...). One person's 'standard' isn't another's. --and if you get deeper, which one is 'THE TRUTH' anyway?

Some sub-categories in physics, I don't think, will ever assign (the idea) of 'size' to a photon---so, which ones do? ---so, that logic of that area(s) will be more present in the (this) discussions.

However, I think that the problem is deeper. It should be clear to everybody that we are talking about field theory and fields (photon, electron, etc.) and the notions used by Greeks are not adequate. Even I. Newton already knew that.

From the pedagogical side, I do not agree with jtbell and agree with R.P.Feynman: it is meaningless to demonstrate more than once the same thing. If one did not understand it, the repetition will not help. In your post # 23 you randomly emitted the gold signal. Lack of understanding allowed you equally well to emit garbage (noise) in your post #35. jtbell explained it to you twice. I doubt that you understood. All that stuff in my POV one should do in his/her own. There is no other way to study physics. It is not picnic, it is spit blood.

Regards, Dany.

P.S. I guess that you do not read my posts and even if you do, the German word “eigenschaften” (alternative) tells you nothing. To those who are interesting, I used reference(that should be standard) to the “elementary particle” of information originally introduced by D.Hilbert and discussed by J. von Neumann in “Mathematische Grundlagen der Quantenmechanik”, Berlin, (1931), ch. III, par 5.

By the way, since after years of study I understand close to nothing, no doubt that you are right. But what wrong with jokes?

Is Newton wrong? Is Newton right?---

Part of the problem, I see, is that people look at peoples ideas sometimes that way--black/white, categorized, etc. ---but a lot do interpret, which I find too, to work best. ---but still, its interpreting with whatever knowledge base/category/'your theory of choice'.


so, maybe, the 'idea' that a 'photon has size' will ONLY be associated WITH certain theories, and that, those theories (your choice) should be referenced, if a 'reference' is made as to make the logic of the discussion more reasonable as some have done.

-------------------------------------------

mr. (dr.?) beh---I was wondering, do like the area of Logic more as in philosophy or as in math?

 

[jal]

Would someone care to refute the following
"As far as I can figure out, everyone assumes that an emitter of a photon cannot emit a photon bigger than what it is. Also, an absorber of a photon has got to be bigger than the photon."

 

[ZapperZ]

WHAT IS THE SIZE OF A PHOTON?
If you believe in extra dimensions then the size would be limited by the size of the extra dimension.
http://pdg.lbl.gov/2007/tables/sxxx.pdf
p.5
Constraints on the radius of extra dimensions for the case of two flat dimensions of equal radius
r < 90-660 nm (astrophysics; limits depend on technique and assumptions)
r < 0:22 mm, CL = 95% (direct tests of Newton's law; cited in Extra Dimensions
review)
-----------
Therefore, a maximum size of a photon would be either .22mm or 660nm. If you want to assume a way of making a bigger photon squeeze into a small hole (dimension) then you do not have any constraints on the size of a photon.
--------------
As far as I can figure out, everyone assumes that an emitter of a photon cannot emit a photon bigger than what it is. Also, an absorber of a photon has got to be bigger than the photon. So, from
http://pdg.lbl.gov/2007/tables/bxxx.pdf
PROTON
Charge radius = 0.875 ± 0.007 fm
NEUTRON
Mean-square charge radius R^2n_ = 0.1161 ± 0.0022 fm2 (S = 1.3)
-----------------
Therefore, we have another kind of constraint on the size of a photon. The size of a proton/neutron.

How small can a photon get? If quarks are proven to emit photons then this would be determined by the sizes of quarks.
----------------
http://arxiv.org/PS_cache/hep-ph/pdf/0611/0611005v1.pdf
QCD, New Physics and Experiment
Giuseppe Nardulli
01 Nov 2006
Abstract. I give a summary of Section E of the seventh edition of the Conference Quark confinement
and the hadron spectrum. Papers were presented on different subjects, from spectroscopy,
including pentaquarks and hadron structure, to the quest for physics beyond the standard model

For EM interactions one gets limit on the mass of a heavy electron: m∗ = 308±56 GeV and = 1253.2±226 GeV, correspondingfor the finite size of the electron a limit of to a
size r ≈ 16×10−18cm . For EW interaction the most stringent limits for the quarks are
rq < 2.2×10−18cm, for the leptons rl < 0.9×10−18 cm, and the form factor puts a limit
on the electron size of re < 28×10−18cm. Finally a scheme to describe all fundamental particles as extended objects of a finite geometrical size was presented by , J¨urgen Ulbricht.
--------------
QUARKS
http://pdg.lbl.gov/2007/reviews/quarks_q000.pdf

See my blog for more references.

There's a whole lot of errors and really far-our extrapolation in here. For example, you associated all of these based on the values of fundamental particles. Yet, in the PDG book that you cited, if what you said is true, why aren't the size of the photon defined this way? This is what I asked in the very beginning. If this is obvious, why aren't they in the standard references, especially the PDG handbook?

Secondly, you restricted the photon emission process only to the size of proton/neutron by assuming those are the photon emitter. That requires a whole lot of explanation because (i) atomic transition can emit a range of photons with different energies, so they are all of the same "size"? (ii) atomic transition involves electrons changing energy level, so why are proton/neutrons involved here? (iii) One can also get light when electrons combine with holes (that's how you get LED's) between the conduction and the valence bands in a semiconductor junction. So now what? The size of that photon is the size of an electron? (iv) I used to work at a synchrotron light source where I use photons coming from electrons passing through an insertion device such as an undulator. In fact, I can "dial in" the photon energy that I want simply by changing the periodicity of the undulator. So what's the size of the photon there?

Zz.

 

[ZapperZ]

It is common point that one should use the references (the previous discussions in PF included). But kids are not used to do that. That is all point Zz made and suggested to use the “Standard References”. No doubt that he is right and everybody agree. Only we did not agree with the details of the technical realization.

Several decades ago PDG did not included quarks for example. Obviously, it does not mean that the quarks did not exist. Moreover, during about 10 years physicists denied the experimental and theoretical evidence of them. Alternatively, in PDG was always written that the proton life time is infinite (stable). That did not prevent the very important experimental and theoretical investigations in HEP: QCD, experimental verification of the proton decay, GUT’s, etc. Since we did not agree with him, Zz even thought that we do not understand him: <ZapperZ bangs his head into the wall and cries>. But he is simply deadly wrong; he is not familiar with the “Bible” of ED, he does not know the modern electrodynamics.

But this is ridiculous. You made it sound as if the PDG isn't updated to reflect the BEST knowledge that we know of today. I did not make a reference to the PDG book from 10 years ago!

The PDG, CRC, and CODATA are update regularly to reflect the best known information. I can also say that many years ago, the value of "e" and "h" were determined via extremely primitive and experiments. It is only after we understand superconductivity and SQUIDs are those values now determined even more accurately.

I cited the latest edition of PDG handbook. This is THE best known and accepted values for all the elementary particles today. Will they change as our knowledge evolve. I hope so! But until then, these are the best we can determine. So from the best that we know of, the standard reference cannot produce neither a definition, nor a value, for the size of a photon.

Zz.

 

[rewebster]

"The Review includes a compilation and evaluation of measurements of the properties of the elementary particles. In the 2006 Review, the listings include 2,633 new measurements from 689 papers, as well as 21,926 measurements from 6,415 papers that appeared in earlier editions. Evaluations of these properties are abstracted in summary tables."

http://pdg.lbl.gov/2007/html/what_is_pdg.html


Pretty impressive, --really

----------------------------------------------------

I noticed what may be a mistake on their 'authors' page, though:


CERNY, V.


http://wwwppds.ihep.su:8001/doc_index.pp?a0:1

---is listed first, which is often caused when a 'space' is accidentally put in before the first letter, causing it to 'appear' first --which I think is the case here (just as a '!' or a '1'---numeric, or keyboard based layout does)

-----------------------------------------------------

http://wwwppds.ihep.su:8001/a.html

this seems the best page to start from, maybe

-----------------------------------------------

http://wwwppds.ihep.su:8001/doc_index.pp?p0:1


'dead particles'---funny

 

[Cthugha]

Par. 4.2 ?Some elementary concepts and definitions? p.148; specifically, p.149 and p. 151.

But only definitions are not enough. You should read the whole book.

Oh, thanks, but I am quite familiar with the book of Mandel and Wolf. It has been a faithful friend during my diploma thesis.

Anyway the section you mention is not about photon size, but spatial coherence (coherence length, coherence area, coherence volume,...). As these quantities are always characteristics of the emitting system, it is not sensible to extract a characteristic of a single photon from it.

 

[Proof.Beh]

"The Review includes a compilation and evaluation of measurements of the properties of the elementary particles. In the 2006 Review, the listings include 2,633 new measurements from 689 papers, as well as 21,926 measurements from 6,415 papers that appeared in earlier editions. Evaluations of these properties are abstracted in summary tables."
http://pdg.lbl.gov/2007/html/what_is_pdg.html
Pretty impressive, --really ...

Since Einstein introduced the relativity, if his name was in each author(s) part of a book, it was STANDARD book surely?

Since Bohr was one of the founders of atomic physics, if his name was in each author(s) part of a book, it was STANDARD book surely?

Since Witten introduced the ST, if his name was in each author(s) part of a book, it was STANDARD book surely?

No, there is no special reason for acceptation a book according to name of its aouthor(s). When we see a famous name on cover of a GR book such as P. A. Dirac, General theory of the Relativity, first point that reffer to our mind is that it will be a STANDARD book. But when read the whole of it, we understand that the books such as GENERAL RELATIVITY AND GRAVITATION VOLs (1vs2) by A. Held or GRAVITATION by J.A.Wheeler & C.H.Misner & K.S.Thorne must not be standard and valid because the aouthers of them are not as famous as Dirac!

You must assume that our references, writed by persons that they are your criterion of choosing of a STANDARD book. You must read whole of a book and when you confront a problem, answer it that is it correct or not. Work that we performed it, but did not see a safe solution.

Thanks.
Mr Beh

 

[marlon]

WHAT IS THE SIZE OF A PHOTON?
Constraints on the radius of extra dimensions for the case of two flat dimensions of equal radius
r < 90-660 nm (astrophysics; limits depend on technique and assumptions)
r < 0:22 mm, CL = 95% (direct tests of Newton's law; cited in Extra Dimensions
review)
-----------
Therefore, a maximum size of a photon would be either .22mm or 660nm. If you want to assume a way of making a bigger photon squeeze into a small hole (dimension) then you do not have any constraints on the size of a photon.
--------------

Hey, i would like to comment on your post.

How do those radii imply that a photon has to be "smaller" as their specific value? Also, a photon is considered to be a point particle in QM, so in QFT the photon STILL is a point particle, otherwise the Lorentz contraction gives strange particle behaviour. Just check the beginning chapters of Anthony Zee's "QFT in a Nutshel" for that matter ! Claiming that a photon is NOT a point particle violates the very foundations of QFT !

Besides, why is such a size value never mentioned in QM or the PDG data ? I have asked Proof Beh this but he refuses to answer ; How would we measure that size and what theoru describes the photon size (equation ?) ?

As far as I can figure out, everyone assumes that an emitter of a photon cannot emit a photon bigger than what it is. Also, an absorber of a photon has got to be bigger than the photon.

Err, i have NEVER heard of this and this is not mentioned in your references. What we DO know is that energy conservation needs to be respected. This implies constraints on the mass values of the emittor and absorber. I think you are referring to THAT.

How small can a photon get? If quarks are proven to emit photons then this would be determined by the sizes of quarks.

Why ?

For EM interactions one gets limit on the mass of a heavy electron: m∗ = 308±56 GeV and = 1253.2±226 GeV, correspondingfor the finite size of the electron a limit of to a
size r ≈ 16×10−18cm .

I agree with the stuff you say on mass but could you elaborate on how you acquire the radius ? Aren't these radii referring to the action radius of the interaction ? I mean, just like a form factor in effective field theories ? But such theories use non elementary particles as their degrees of freedom. This does not say ANYTHING about size but about strength of interaction (ie coupling constants expressed in terms of inter-particle distance).

In basic QCD, quarks are represented as point particles for the reason i stated above (basic QFT demand). The dual abelian higgs model gives a first guess for the proton and neutron structure by adding a linear potential (dominant at long range interaction or low energy scales) to the Coulombic exponential decaying potential (dominant at short range interactions). Never EVER is the notion of size used !

If you want a reference for this model, check the Elementary Particle's Presented Thread where i have posted a link to the PDF file.

greets
marlon

 

[Proof.Beh]

How do those radii imply that a photon has to be "smaller" as their specific value? Also, a photon is considered to be a point particle in QM, so in QFT the photon STILL is a point particle, otherwise the Lorentz contraction gives strange particle behaviour. Just check the beginning chapters of Anthony Zee's "QFT in a Nutshel" for that matter ! Claiming that a photon is NOT a point particle violates the very foundations of QFT !

Repetitive like always, But,

1- A photon can not be a point particle. Because when light of a lamp glints to a sheet of iron, we won't see any affect of photons and lighting under it. While we know that there are free spaces among moleculs of iron that can be sufficent for crossing the infinity number of these points (photons) that it is a paradox.

2- If read my fore posts, you see that I answered this question in other way. If we assume that a photon is a point particle then the propagator (for example a laser) must propagates infinity number of photons in an arbitrary time interval that it is an unreal imagination.

Thanks.
Mr Beh

 

[marlon]

Repetitive like always, But,

But, again you chose NOT to answer them. You are looking less credible by every post you make. Anyways, ...

1- A photon can not be a point particle.

This is a blunt violation against the basic principles of the standard model (read the webpage and check the references at the end of the article). You use laser physics later on to make your point but doesn't it strike you as being strange that you violate the very theoretical model that explains all this laser phenomenae correctly ?

Because when light of a lamp glints to a sheet of iron, we won't see any affect of photons and lighting under it. While we know that there are free spaces among moleculs of iron that can be sufficent for crossing the infinity number of these points (photons) that it is a paradox.

Newsflash, maybe that's because photons INTERACT with matter (atoms, phonons, etc) ?

If we assume that a photon is a point particle then the propagator (for example a laser) must propagates infinity number of photons in an arbitrary time interval that it is an unreal imagination.

Huh ? What do you mean by propagator ? Photons are the constituent particles of the laser. As a matter of fact "a laser" is just a bunch of photons with specific properties. I challenge you to show me ANY publication that explains the laser (from population inversion on...) using the photon size concept. In return, i will give you plenty of references where they DON'T :

Applied Physics B: Lasers and Optics (ISSN 0946-2171)
IEEE Journal of Lightwave Technology (ISSN 0733-8724)
IEEE Journal of Quantum Electronics (ISSN 0018-9197)
IEEE Journal of Selected Topics in Quantum Electronics (ISSN 1077-260X)
IEEE Photonics Technology Letters
Journal of the Optical Society of America B: Optical Physics (ISSN 0740-3224)
Laser Focus World (ISSN 0740-2511)
Optics Letters (ISSN 0146-9592)
Photonics Spectra (ISSN 0731-1230)

The Nobel Prize in Physics 1966 Presentation Speech by Professor Ivar Waller. Retrieved 1 January 2007.

Gould, R. Gordon (June 1959). "The LASER, Light Amplification by Stimulated Emission of Radiation". The Ann Arbor Conference on Optical Pumping.
Gould's conference presentation and the public introduction of the term laser are mentioned in:

Chu, Steven; and Townes, Charles (2003). "Arthur Schawlow", in ed. Edward P. Lazear,: Biographical Memoirs, vol. 83, National Academy of Sciences, p. 202. ISBN 0-309-08699-X.

G.P. Karman, G.S. McDonald, G.H.C. New, J.P. Woerdman, "Fractal modes in unstable resonators", Nature Vol. 402, 138 (11 November 1999).


marlon

ps : i am still waiting for that photon-size equation, though.

 

[Anonym]

I think its the ability to correlate the ideas/information given/known and there's not a lot, it seems, on the 'size of photons'--(of which can be taken not 'too far'/'in new directions' here on the forum before it becomes 'speculative' and is not accepted on the forum--which is fine).

The coherence length seems to me the adequate wave mechanical parameter or close to it. It is well defined mathematically measurable quantity that may be applied universally to CED, QED and at HEP (limit when Lt and Ls go to zero). Whether it is close to zero or exactly =0 for the structureless objects is now matter of the standard experimental and theoretical analysis.

Whether it is accepted on the forum or not interest me as the last year snow. I use the standard notion accepted in CED and QED during last 40 years.

Regards, Dany.

 

[Anonym]

But this is ridiculous. You made it sound as if the PDG isn't updated to reflect the BEST knowledge that we know of today. I did not make a reference to the PDG book from 10 years ago!

It sounds to you exactly opposite to what I wrote. What is “ridiculous”? My understanding of your intensions or the technical details? If I missed your point, I quit. If you do not agree with the specific technical realization, we may find eigenschaften.

Any way, my impression is that the quantity of home made philosophical considerations going up last time and the quality of the technical discussions going down. If I identify your intensions correctly, you want to stop that. I want let you know that I am ready to cooperate and help where I can. However, we are macroscopic bodies and therefore not identical (surely we have different longitudinal and transversal coherence lengths).

Regards, Dany.

 

[Anonym]

As these quantities are always characteristics of the emitting system, it is not sensible to extract a characteristic of a single photon from it.

How you define a single photon?

Regards, Dany.

 

[jal]

The coherence length seems to me the adequate wave mechanical parameter or close to it. It is well defined mathematically measurable quantity that may be applied universally to CED, QED and at HEP (limit when Lt and Ls go to zero). Whether it is close to zero or exactly =0 for the structureless objects is now matter of the standard experimental and theoretical analysis.

You might want to add

As far as I can figure out, everyone assumes that an emitter of a photon cannot emit a photon bigger than what it is. Also, an absorber of a photon has got to be bigger than the photon.

The size range of a photon seems to disturb this forum. Yet you all accept without argument the size range of an other dimensions.

Constraints on the radius of extra dimensions for the case of two flat dimensions of equal radius
r < 90-660 nm (astrophysics; limits depend on technique and assumptions)
r < 0:22 mm, CL = 95% (direct tests of Newton's law; cited in Extra Dimensions
review)

Maybe it's because you all want the size of a photon to be able to fill the size of our 3d universe.
I'm not doing the speculation... I'm just learning from the data presented by others

 

[rewebster]

Repetitive like always, But,

1- A photon can not be a point particle. Because when light of a lamp glints to a sheet of iron, we won't see any affect of photons and lighting under it. While we know that there are free spaces among moleculs of iron that can be sufficent for crossing the infinity number of these points (photons) that it is a paradox.

2- If read my fore posts, you see that I answered this question in other way. If we assume that a photon is a point particle then the propagator (for example a laser) must propagates infinity number of photons in an arbitrary time interval that it is an unreal imagination.

Thanks.
Mr Beh

so, which theory are you working from?


(and, please, mentors, don't lock this thread if he gives the answer that I think he may)

-----------------------------------------------------

The coherence length seems to me the adequate wave mechanical parameter or close to it. It is well defined mathematically measurable quantity that may be applied universally to CED, QED and at HEP (limit when Lt and Ls go to zero). Whether it is close to zero or exactly =0 for the structureless objects is now matter of the standard experimental and theoretical analysis.

Whether it is accepted on the forum or not interest me as the last year snow. I use the standard notion accepted in CED and QED during last 40 years.

Regards, Dany.

that's most of what is being discussed, I believe, whether it's zero or not.

(and, maybe, whose standard is correct/accurate)

 

[rewebster]

Included



As I mention in my post #136 above, people here use Greek’s notion of size. Apparently it is related to the question whether size is a matter. Indeed it is relevant parameter when we talking not QT but sex.

Regards, Dany.

Well, well, well---I have NEVER thought about PHOTONS being possibly male or female EVER before in my life--my, my, my...until now

maybe that's what FRA was talking about in another thread--evolutionary particles


so, Anonym, which of those theories carries the most weight in you mind for photons? (your post #153)

------------------------------------------------

Maybe it's because you all want the size of a photon to be able to fill the size of our 3d universe.

no comment

 

[Anonym]

which of those theories carries the most weight in you mind for photons? (your post #153)

I am not sure I understand the question. Each one is defined within its own range of validity and naturally connected with each other. The photon waveform is defined as usual by the boundary and the initial conditions.

Regards, Dany.

 

[Cthugha]

How you define a single photon?

What do you want to hear?

The basic quantity of the quantized em field?
Something represented by a n=1 fock-state?
A click on the detector?
A bright pixel on a CCD?
The "result" of a streak camera doing single photon counting?
The emission of a single photon source?
Light with $$g_2(\tau=0)=0$$?

Tell me, how you would like to combine photon size and coherence length and where the definition of a single photon comes into play. Just guessing what you are aiming at, makes it somehow hard to answer, especially as a single photon has different meanings concerning different experiments.
The meaning of "single photon" in single photon creation is different from the meaning of "single photon" in single photon detection for example.

 

[rewebster]

I am not sure I understand the question. Each one is defined within its own range of validity and naturally connected with each other. The photon waveform is defined as usual by the boundary and the initial conditions.

Regards, Dany.

it just seems that you like QED and I was thinking if you knew Feynman's stuff really well if he ever mentioned photon size (or, really, did he 'theorize' about anything close (real photons)--and not 'virtual photons' --I think I read something about smaller (?) than Compton's on the virtual(?)----QED -it is something else.
--------------------------------------------------

I like this one:

If a photon had no size in some way or another then it wouldn't exist. And as far as I'm concerned until someone proves that light has no mass or otherwise I'll reserve my judgement on the validity of any of the counter arguments

#90 post on about the same subject

https://www.physicsforums.com/showthread.php?t=104657&page=6

It's got to be of interest (to many) and frustrating (to many who have to try to re-explain it over and over again) as often as this subject comes up (everywhere) and if it was DEFINTE one way or the other--there wouldn't be any discussions like this SO often.

---------------------------------------
reilly has a good 'summation' post (#120) on same thread:

(just part below:)

" physics is all about challenging theories; that's what professional physicists do. But, do note that sometimes challenges are met with hostile reations, and the challenge become very contentious. So, at time, physics is a contact sport. And, surprise, many professional physicists are arrogant -- I know more than you do kind of stuff -- so they tend to ignore beginners, with whom they are not kindly disposed to discuss basis. "

 

[Anonym]

What do you want to hear?

The emission of a single photon source

The meaning of "single photon" in single photon creation is different from the meaning of "single photon" in single photon detection for example.

The philosophy leaves to you, please.

Anyway the section you mention is not about photon size, but spatial coherence (coherence length, coherence area, coherence volume,...). As these quantities are always characteristics of the emitting system, it is not sensible to extract a characteristic of a single photon from it.

Your statement is nonsense (philosophy) if you are not able to define a single photon.

Regards, Dany.

 

[Cthugha]

The emission of a single photon source

In the case of an ideal single photon source, the coherence volume is at least a volume, in which there is just one photon.
In that case it might make sense to at least take it as an upper bound for size, but the usual layman coming in here and asking for the size of a photon is certainly not interested in the very special case of subpoissonian statistics.

The philosophy leaves to you, please.

Your statement is nonsense (philosophy) if you are not able to define a single photon.

It is neither philosophy nor nonsense to mention, that there are different meanings of the term "single photon" in physics.
As the initial question of this topic was not specified, I was of course talking about what is experimentally measured by single photon counting, which is the more general case.

So in the case of poissonian or superpoissonian statistics the meaning of coherence volume reduces to the volume, in which individual photons are indistinguishable. And that is certainly not a quantity, which tells us something about the size of just one of them.

Anyway even in the case of single photon sources coherence times depend strongly on the system, but not much on the characteristics of the photon. In such devices antibunching is a signature of the system being unable to emit several photons at once (single molecules or single QDs for example). This is not comparable to antibunching of fermions, free electrons for example, where the effect of antibunching is a result of the characteristics of the particle itself.

 

[marlon]

How you define a single photon?

Regards, Dany.

As a chunck of energy

http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c3

marlon

 

[Schrodinger's Dog]

If a photon had no size in some way or another then it wouldn't exist. And as far as I'm concerned until someone proves that light has no mass or otherwise I'll reserve my judgement on the validity of any of the counter arguments

Well I'm not quite as controversial as I was a year and a half ago so shoot me Bit out of context anyway. I seem to vaguely remember I said that because I don't like people saying absolutely light has no mass beyond doubt, as much as I don't like people trying to prove it does, we're not in a position to claim absolute certainty of either, so it's a bit of a waste of time.

I'm not a big fan of absolute answers in science, I don't think scientists are, as they are not scientific. So you can probably find me on both sides of the argument in the forums history, depending on how it the answer is phrased, if it's phrased as an absolute condition that a photon has no mass then I say no that is not true explicitly.

If it says it is useless to discuss it, or we have an upper bound limit on the size of a photon of 6.6 nm or whatever etc, and we know from our equations that light cannot have mass, then I'm fine with that.

Leave the absolutes to religion is my philosophy.

That said I think it's best to say light has no mass, because it is useful, where as claiming it does have mass is impossible to prove and an exercise in philosophy, thus my current stance on the good side of the force .

 

[rewebster]

If it says it is useless to discuss it, or we have an upper bound limit on the size of a photon of 6.6 nm or whatever etc, and we know from our equations that light cannot have mass, then I'm fine with that.

Leave the absolutes to religion is my philosophy.

That said I think it's best to say light has no mass, because it is useful, where as claiming it does have mass is impossible to prove and an exercise in philosophy, thus my current stance on the good side of the force .

so, the well trodden path is the best way?

 

[DaTario]

I would like to share some reflections of mine.

First, coherence volume seems to be a good start point to this discussion (the size of a photon). Sources that shoot one photon at a time seems to be a well defined concept-apparatus. With this in hand, interferometric analysis seems to point to coherence volume as the FAPP volume of one photon.

Second. Objection to what is written above may comes from the following. Assuming photon as a wave train, it seems reasonable to consider it, most of time, a multi-frequency structure, presenting frequency variations along the line joining the source and the detector. I am considering here an atom in fluorescence process (point source) and the Weisskopf-Wigner theory for the spontaneous decay. If the wave train presents frequency variation in the logitudinal direction, of course the interference will diminish even if the photon is still going through the interferometer. Experiments with twin photons (photons generated by second order process k -> k' + k") allow us to detect one of these two photons and be aware that the other photon is entering (with certainty) the interferometer. I don't want to go far into this, but I would like to emphasize that non-monochromatic nature may lead to underestimation of photon's length via interferometric precudure.

Finally I would like to put in the same post my insatisfaction with these points:

1) If
E = h \nu
and
E = m c^2
how does one solve the question of not ascribing a mass to the photon.


2)
If one measures the photon length and area (photon volume). How does one correct it relativiscally to obtain proper length? Infinites will appear.

Thank all for the attention

Best wishes,

DaTario

 

[marlon]

I would like to share some reflections of mine.

First, coherence volume seems to be a good start point to this discussion (the size of a photon). Sources that shoot one photon at a time seems to be a well defined concept-apparatus. With this in hand, interferometric analysis seems to point to coherence volume as the FAPP volume of one photon.

Allow me to respond with my opinion on this. I have been reading quite some posts in which the idea of "coherence length is the photon length" is proposed. I think this is confusing because we are referring to a particle-like concept (length, volume) by using a wavelike concept (coherence length). This is another consequence of misinterpretation of the dual nature inherent to quantummechanics. I mean, why must we always mix the particle and wavelike notions ? To what purpose ? Both visions are dual and are not supposed to be used simultaneously. What purpose does that serve ? Looking at the QM formalism, one can only conclude that this is completely incorrect and therefore, this discussion is limited to a sequence of speculations for exactly THAT reason !

marlon

 

[rewebster]

Allow me to respond with my opinion on this. I have been reading quite some posts in which the idea of "coherence length is the photon length" is proposed. I think this is confusing because we are referring to a particle-like concept (length, volume) by using a wavelike concept (coherence length). This is another consequence of misinterpretation of the dual nature inherent to quantummechanics. I mean, why must we always mix the particle and wavelike notions ? To what purpose ? Both visions are dual and are not supposed to be used simultaneously. What purpose does that serve ? Looking at the QM formalism, one can only conclude that this is completely incorrect and therefore, this discussion is limited to a sequence of speculations for exactly THAT reason !

marlon

do you mean that you would like the thread moved to the Classical Physics area?

 

[DaTario]

Allow me to respond with my opinion on this. I have been reading quite some posts in which the idea of "coherence length is the photon length" is proposed. I think this is confusing because we are referring to a particle-like concept (length, volume) by using a wavelike concept (coherence length). This is another consequence of misinterpretation of the dual nature inherent to quantummechanics. I mean, why must we always mix the particle and wavelike notions ? To what purpose ? Both visions are dual and are not supposed to be used simultaneously. What purpose does that serve ? Looking at the QM formalism, one can only conclude that this is completely incorrect and therefore, this discussion is limited to a sequence of speculations for exactly THAT reason !

marlon

I think your point is quite good. Note however that we are searchig for spatial characterization of an phisically existent object. Or less, we want the photon to be an existing object, so we must offer such spatial characterization. At the background of this structure we call photon there is vacuum in the simplest case. Vaccum as we know, must be a configuration of EM with some noise, implying no interference and no coherence at all. For this reason, the experimental perception of coherence must have something to do with the spatial characterization of the photon.

But I still agree with you in that there seems to be a strange fusion of concepts related to the dual nature. Perhaps, understanding how one can mix this notions in a logical and sound manner, we can achieve a better analysis.

best wishes

DaTario

 

[pervect]

I would like to share some reflections of mine.

First, coherence volume seems to be a good start point to this discussion (the size of a photon). Sources that shoot one photon at a time seems to be a well defined concept-apparatus. With this in hand, interferometric analysis seems to point to coherence volume as the FAPP volume of one photon.

Second. ... I don't want to go far into this, but I would like to emphasize that non-monochromatic nature may lead to underestimation of photon's length via interferometric precudure.

Finally I would like to put in the same post my insatisfaction with these points:

1) If
E = h \nu
and
E = m c^2
how does one solve the question of not ascribing a mass to the photon.2)
If one measures the photon length and area (photon volume). How does one correct it relativiscally to obtain proper length? Infinites will appear.

Thank all for the attention

Best wishes,

DaTario

For starters, there might be some purposes for which "coherence volume" is not the practical thing one is interested in, but it's certainly a quantity that is of interest.

Note that the coherence length (and coherence volume) of a photon is not a property of the photon itself, it is a property of how the photon is prepared, i.e. what state the photon is.

For instance, using a stellar interferometer, the transverse coherence length of a photon from a distant star can be kilometers long - and its longer the further away the star is. A close star would have a much shorter coherence length than a further away star. So the coherence length of a photon depends on how it is prepared (and also on where it is being observed), it's not a property of just the photon. There are some related points I'll get into later.

The coherence length question is not at all related to the question of the mass of a photon. Even without defining a coherence volume, it is clear that photons can have a well-defined value of energy. The point is that E^2 - |p|^2 c^2 is an invariant for any isolated system, and that this invariant is the so-called invariant mass of the system. This mass is a property of the system itself, and for an isolated photon, this mass is zero. See any of the numerous threads on mass in relativity, or the sci.physics.faq http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

In contrast, the energy of a photon depends on the observer - an observer at rest will see the photon with one frequency, someone moving with respect to that observer will see the photon with a different frequency.

So the energy (aka relativistic mass) of a photon is NOT a property of the photon, but a property of the photon and a specific observer. The invariant mass of a photon is a property only of the photon, it does not depend on the observer (that's what makes it invariant under the Lorentz transform).

Also - a photon has no "proper volume", nor does it have a "proper frame". There isn't any way around the infinites that you point out. Nor should you expect there to be one.

To get back to something I promised to talk about earlier, in the case of a single photon, one can get away with thinking that the wavefunction of the photon occupies standard 3 dimensional space (ignoring the time dimension). If you start to study the quantum mechanics of more than one particle, you'll quickly come up against the fact that the wavefunction does NOT occupy physical space, but rather occupies a form of phase space.

For instance, the wavefunction of two photons requires a 6-d space, it cannot be described in a 3-d space.

 

[marlon]

do you mean that you would like the thread moved to the Classical Physics area?

Not at all, this thread is where it belongs.

I think your point is quite good. Note however that we are searchig for spatial characterization of an phisically existent object. Or less, we want the photon to be an existing object, so we must offer such spatial characterization.

Well, i understand your intentions but as i have mentioned before, this search starts off from "shaky foundations". I mean, the first question in this case is : "what is a photon" , Well, if we all would look at how a photon is defined in QM, we we would find out that a photon is defined as a piece of energy. QFT teaches us how photons arise if we quantise the EM field. In other words, if we adopt the QM definition of a photon, we are basically talking about a discrete amount of energy. That is all ! I don't understand where the "position" comes in. If we adopt the QFT photon definition, we know that a photon arises as a vibration of the EM field to which we can attribute particle like concepts like momentum. NOWHERE in QFT is there every anything mentioned on the spatial location of that EM-field vibration. Again i ask, why bringing in the notion of photon position ?

The closest thing you will get for a connection between photon and position will be this : the spatial coordinates in the EM waves equation. So, using the wavelike concepts of QM, one can indeed use cogerence length etc etc and connect it to spatial coordinates through its equation. But, once you start talking about photons, you are no longer in the wavelike world !

marlon

 

[rewebster]

-----------------------------------

"Well, i understand your intentions but as i have mentioned before, this search starts off from "shaky foundations". "

I've only been around (here) for about a year, and what I have seen is that there (almost definitely) will be a thread on:

how big is a photon?

What is a photon?

wave/particle whatever dilemma problem

are we having photons for dinner?


If you go by one 'definition', someone else brings in some 'old' definition, or some recently published blankety-blank; so, what's a father to do? beat the child, scold the child, teach the child, assume the child knows nothing, bless the child, or the one about you never know what will come out of the mouths of a child---and, then, on what level on what scale of from 1-100,---I don't know what is the age of the youngest PF member or the oldest (HA-ha)---


Personally---I like Photons---whatever they are and whatever we find them to be.

 

[marlon]

If you go by one 'definition', someone else brings in some 'old' definition

Well, that's a bit easy to justify all the speculations going on. What other photon definition is there except that quantummechanical one ? I mean, QM describes the photon and QED describes it's behaviour. Both definitions are more or less equivalent with respect to the theoretical background in which they are defined. So, that's that !

marlon

 

[DaTario]

First, Thank you Pervect, I agree to most of what you've said.

Not at all, this thread is where it belongs.


Well, i understand your intentions but as i have mentioned before, this search starts off from "shaky foundations". I mean, the first question in this case is : "what is a photon" , Well, if we all would look at how a photon is defined in QM, we we would find out that a photon is defined as a piece of energy. QFT teaches us how photons arise if we quantise the EM field. In other words, if we adopt the QM definition of a photon, we are basically talking about a discrete amount of energy. That is all ! I don't understand where the "position" comes in. If we adopt the QFT photon definition, we know that a photon arises as a vibration of the EM field to which we can attribute particle like concepts like momentum. NOWHERE in QFT is there every anything mentioned on the spatial location of that EM-field vibration. Again i ask, why bringing in the notion of photon position ?

The closest thing you will get for a connection between photon and position will be this : the spatial coordinates in the EM waves equation. So, using the wavelike concepts of QM, one can indeed use cogerence length etc etc and connect it to spatial coordinates through its equation. But, once you start talking about photons, you are no longer in the wavelike world !

marlon

Marlon,

Let me see if I understand well your picture with a somewhat simple example.

Consider two pendula coupled in a usual "graduation in physics" scheme. If just one of these pendula start moving, soon this one will be at rest and the other, at this time, will be moving with maximum amplitude. You will like to say that both pendula are exchanging quanta of energy (photons in a metaphorical sense) but you will be pleased to say that these quanta of energy has no well defined position.

Let me know if this example and the presented explanation are well suited to your understanding of this photon issue.

best wishes

DaTario

 

[marlon]

Consider two pendula coupled in a usual "graduation in physics" scheme. If just one of these pendula start moving, soon this one will be at rest and the other, at this time, will be moving with maximum amplitude. You will like to say that both pendula are exchanging quanta of energy (photons in a metaphorical sense) but you will be pleased to say that these quanta of energy has no well defined position.

That's exactly what i meant when i referred to the QFT picture, yes.

Additionally, one can certainly not make any claims on photon position based upon the photon definition which has NOTHING to do with spatial coordinates but with "bits of energy".

marlon