Photon Size: Direction & Probability

[rewebster]

Well, that's a bit easy to justify all the speculations going on. What other photon definition is there except that quantum mechanical 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

In your mind, then, all the questions are answered and there shouldn't even have be a discussion?---this is all a waste of time (in a way, except for, maybe, 'teaching' purposes)?


or do you have any questions?

 

[DaTario]

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

I myself like to go back to the particle to particle picture and see photon as just a quantum of exchange in interactions.

 

[marlon]

In your mind, then, all the questions are answered and there shouldn't even have be a discussion?---this is all a waste of time (in a way, except for, maybe, 'teaching' purposes)?


or do you have any questions?

I have many questions, some of them i would like to solve during my PhD. But one must not ask any question, one must ask the right question. In other words, one can ask questions but if the content of those questions violates some basic theoretical principles, then one should be honest enough to admit that the question itself does not solve any problem, it only confuses.

I myself like to go back to the particle to particle picture and see photon as just a quantum of exchange in interactions.

Me too, photon position or photon size has been proven NOT to exist in that theoretical framework.

marlon

 

[lightarrow]

1. I agree a photon is a quantum of electromagnetic field.
2. I agree it has not a definite position.
3. As Pervect notices: "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."
4. Many people seems to agree on the fact photon has no size.

Conclusion: a photon IS NOT AN OBJECT TRAVELLING FROM SOURCE TO DETECTOR.

If someone thinks this conclusion is wrong, please explain.

 

[LorentzR]

1.
Conclusion: a photon IS NOT AN OBJECT TRAVELLING FROM SOURCE TO DETECTOR.

If someone thinks this conclusion is wrong, please explain.

I'm convinced that lightarrows conclusion is correct.

I believe there is a growing body of evidence to suggest that light does not require a carrier particle.

The most notable being that if the constituents parts of matter cannot have exact locality relative to our inertial reference frames then the "photon's raison detre vanishes. The justification for such an object cannot be sustained.

Something can only have size if it physically exists in the first place.

It is inherent uncertainty of position and time of the charged "particles" of matter that allows them to interact directly?

 

[ZapperZ]

I'm convinced that lightarrows conclusion is correct.

I believe there is a growing body of evidence to suggest that light does not require a carrier particle.

Are you saying there are no photons? And what "growing body of evidence"? Can you give some references (and no Wikipedia, please)?

The most notable being that if the constituents parts of matter cannot have exact locality relative to our inertial reference frames then the "photon's raison detre vanishes. The justification for such an object cannot be sustained.

But one can say that to every single quantum particles. A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence. At the same token, photons and other quantum particles have definite position upon measurement, or else, what exactly are those photomultipliers and CCD detectors measuring? So what exactly do you mean by "cannot have exact locality" here?

Zz.

 

[rewebster]

But one must not ask any question, one must ask the right question. In other words, one can ask questions but if the content of those questions violates some basic theoretical principles, then one should be honest enough to admit that the question itself does not solve any problem, it only confuses.
marlon

When I read this part of your statement, I thought it sounded like part of some politician's speech.

(like 'what does "is" mean?')

I have many questions, some of them i would like to solve during my PhD.

so---do you have a theory?----or is it an 'interpretation'?

 

[rewebster]

At the same token, photons and other quantum particles have definite position upon measurement, or else, what exactly are those photomultipliers and CCD detectors measuring? So what exactly do you mean by "cannot have exact locality" here?

Zz.

Are you saying that photons have size?

(I haven't read all of your Posts: 8,981)

 

[ZapperZ]

Are you saying that photons have size?

(I haven't read all of your Posts: 8,981)

Nope.

However, there seems to be a misconception here in this thread. Saying that one can measure a position doesn't mean that one knows the size of the object. There are no "size" operator in QM, at least, not that I know of. There is, however, a position operator. When photons hit a photographic plate, you can tell where that is and its position. However, that has nothing to do with its size because of the nature of the interaction and the nature of the detection. The $\Delta(x)$ is the spread in a repeated measurement of the position of the object, not the width of the object.

Zz.

 

[rewebster]

Nope.

However, there seems to be a misconception here in this thread. Saying that one can measure a position doesn't mean that one knows the size of the object. There are no "size" operator in QM, at least, not that I know of. There is, however, a position operator. When photons hit a photographic plate, you can tell where that is and its position. However, that has nothing to do with its size because of the nature of the interaction and the nature of the detection. The $\Delta(x)$ is the spread in a repeated measurement of the position of the object, not the width of the object.

Zz.

doesn't that make it a good question then? could it be addressed in some way? ('some way' that hasn't been addressed so far) ---is HUP complete?--or is it just an 'unknown' factor/parameter/?? at the present time?

----------------------
(this was one of the areas I was beating around the bush in the HUP/accelerator thread)

 

[ZapperZ]

doesn't that make it a good question then? could it be addressed in some way? ('some way' that hasn't been addressed so far) ---is HUP complete?--or is it just an 'unknown' factor/parameter/?? at the present time?

What doesn't make a good question? What is the size of a photon? I think I've said this a few times already. That's like asking how sharp is green.

And I don't know what you mean when you ask if HUP is complete.

Zz.

 

[rewebster]

What doesn't make a good question? What is the size of a photon? I think I've said this a few times already. That's like asking how sharp is green.
And I don't know what you mean when you ask if HUP is complete.

Zz.

'How sharp is green?'---that's the direction I was heading--

-that statement 'How sharp is green?' is incomplete. Green is a descriptive term for an object/something. Just because you made an incomplete statement ''How sharp is green?'' doesn't mean that it means 'nothing'--it's just an incomplete statement/thought.

(I could have a pile of 20 knives for sale, of which 'one' is green--and someone looking at them could ask that question)



are you (z) completely satisfied/happy/content/'no problem-be happy' with the 'photon' as far as QM (theory)/classical (dealing with accelerators) as it stands now? What are you (z) looking for (as far as photons are concerned)?

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

'cooling lasers' are part of this too, I believe.

 

[Anonym]

doesn't that make it a good question then? could it be addressed in some way? ('some way' that hasn't been addressed so far) ---is HUP complete?--or is it just an 'unknown' factor/parameter/?? at the present time?(this was one of the areas I was beating around the bush in the HUP/accelerator thread)

Zz is right. Instead disputing you should reformulate your question using the standard terminology. For example, that is how your question looks like when you use the standard references:

How BUNCH length is related to the (longitudinal/transversal) coherence length?(my post #12)

How do you know the difference between the classical electrons bunches and the coherent wave packets?(my post #18)

Regards, Dany.

 

[rewebster]

Zz is right. Instead disputing you should reformulate your question using the standard terminology. For example, that is how your question looks like when you use the standard references:

How BUNCH length is related to the (longitudinal/transversal) coherence length?

How do you know the difference between the classical electrons bunches and the coherent wave packets?

Regards, Dany.

good questions and maybe he can answer those

but that's limiting; and limited as far as to answering specific situations(usually accelerators, I'm thinking--I can guess not always though);

and using terminology (bunches) that is specific to an area limits the overall idea, to me at least.

 

[ZapperZ]

'How sharp is green?'---that's the direction I was heading--

-that statement 'How sharp is green?' is incomplete. Green is a descriptive term for an object/something. Just because you made an incomplete statement ''How sharp is green?'' doesn't mean that it means 'nothing'--it's just an incomplete statement/thought.

(I could have a pile of 20 knives for sale, of which 'one' is green--and someone looking at them could ask that question)

But your "knives" CAN have a property of being green. A photon was never defined as a particle with a size in real space. It is, however, defined in energy (and momentum) space. Thus, when you ask for the size of a photon, you are asking for something it was never defined with. That is why that question is similar to asking "How sharp is green?". Those are two things never associated with each other.

are you (z) completely satisfied/happy/content/'no problem-be happy' with the 'photon' as far as QM (theory)/classical (dealing with accelerators) as it stands now? What are you (z) looking for (as far as photons are concerned)?

I am pretty happy with photons and what we can know using it. There are many things that can't be explained using anything else (I've listed several already a few times on PF).

Zz.

 

[lightarrow]

But one can say that to every single quantum particles. A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence.

So coherence length is related to a particle's position, so the same for a photon's position?

 

[rewebster]

But your "knives" CAN have a property of being green. A photon was never defined as a particle with a size in real space. It is, however, defined in energy (and momentum) space. Thus, when you ask for the size of a photon, you are asking for something it was never defined with. That is why that question is similar to asking "How sharp is green?". Those are two things never associated with each other.

Zz.

never defined?---maybe not, but it has been theorized a lot about (and for a long time):

http://en.wikipedia.org/wiki/Light

down to the 'Theories about light' section. (ol' wiki is good for some things)

--and "defined in energy (and momentum) space" may be only the first steps in the process.

Thus, when you ask for the size of a photon, you are asking for something it was never defined with. Zz.

I don't think I have asked 'what is the size?' yet here--but I do find the question interesting to think about as far as its possibility; and, as it (the question) has a long history, with no definitive answer---and that may be why the thread is a long one (even though variations of it have come up probably (x^n)-1 times on the PF.)

I am pretty happy with photons and what we can know using it. There are many things that can't be explained using anything else (I've listed several already a few times on PF).

Zz.

that's good---and that being in an area where the defined knowledge that there is on the photon has to be used to do what you do, that makes sense---but you also would use any 'new' knowledge that was found, too, wouldn't you?
----------------
isn't this one a 'fun' one

from wiki:

Ptolemy (c. 2nd century) wrote about the refraction of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes

 

[ZapperZ]

never defined?---maybe not, but it has been theorized a lot about (and for a long time):

http://en.wikipedia.org/wiki/Light

down to the 'Theories about light' section. (ol' wiki is good for some things)

--and "defined in energy (and momentum) space" may be only the first steps in the process.

Hey, you should know by now to never use a wikipedia reference with me.

If you have an exact citation where the size of a photon has been defined, I'd like to know about it.

I don't think I have asked 'what is the size?' yet here--but I do find the question interesting to think about as far as its possibility; and, as it (the question) has a long history, with no definitive answer---and that may be why the thread is a long one (even though variations of it have come up probably (x^n)-1 times on the PF.)

Is a question that has no clear-cut definition interesting? I suppose philosophically, it is. But in physics? Just think. In all those particle detectors that not only detect the various particles but also "EM showers" (i.e. photons), never once is there any kind of discussion whatsoever about the size of the photons they detected. Why do you think that is? I mean, these ARE the people who study elementary particles, after all. They either don't seem to care, or they think this is not a meaningful question. Again, when I asked this to an experimentalist who works at the ATLAS detector at CERN, his reply was "what in the world is a photon size and why do we care?" Yet, on here, we seem to be tripping over each other to discuss this. Do we know more than they do, or something that they don't? You answer that, because I can't.

that's good---and that being in an area where the defined knowledge that there is on the photon has to be used to do what you do, that makes sense---but you also would use any 'new' knowledge that was found, too, wouldn't you?

Sure, but is this really the issue here?

I don't work in studying fundamental particles. I do, however, USE the knowledge to accomplish something else. I used photons to study spectroscopy of strongly correlated system, and I use photons to generate electrons for a particle accelerators. As far as I can tell, the issues that I deal with are fully described by what we know now.

Zz.

 

[ZapperZ]

So coherence length is related to a particle's position, so the same for a photon's position?

Er.. why are you asking me this? I'm not the one who holds that opinion.

Zz.

 

[Anonym]

isn't this one a 'fun' one

from wiki:

Ptolemy (c. 2nd century) wrote about the refraction of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes

Compare (post #21):

you would indeed observe that there is a photon present through its interaction with the detector (emitted EM radiation for example: the light signal from the detector).

I guess that this is an important idea developed in his Ph.D. thesis.

Regards, Dany.

 

[Anonym]

but that's limiting; and limited as far as to answering specific situations(usually accelerators, I'm thinking--I can guess not always though).

I do not agree with you. I guess that Zz is doing the most general case (in ED). And in contrast with the possible fantasies he is doing real life.

Regards, Dany.

 

[lightarrow]

Er.. why are you asking me this? I'm not the one who holds that opinion.

I'm asking you because you were the one who wrote: "But one can say that to every single quantum particles. A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence".
From this I deduce you put in relation position and coherence lenght.

 

[ZapperZ]

I'm asking you because you were the one who wrote: "But one can say that to every single quantum particles. A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence".
From this I deduce you put in relation position and coherence lenght.

.. and I wrote that in reply to this:

The most notable being that if the constituents parts of matter cannot have exact locality relative to our inertial reference frames then the "photon's raison detre vanishes. The justification for such an object cannot be sustained.

.. which means that if we apply that logic, even Cooper Pair "cannot be sustained". This is obviously absurd. However, it is you who made the connection with "size", since that is what you asked me. I made no such connection, and thus, that question shouldn't be associated with me.

Zz.

 

[lightarrow]

.. which means that if we apply that logic, even Cooper Pair "cannot be sustained". This is obviously absurd. However, it is you who made the connection with "size", since that is what you asked me. I made no such connection, and thus, that question shouldn't be associated with me.
Zz.

I haven't talked about size, I'm simply trying to understand the meaning of your statement: "A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence".
If the meaning it's not what I have deduced from your statement, then explain which is it.

 

[ZapperZ]

I haven't talked about size, I'm simply trying to understand the meaning of your statement: "A Cooper Pair has no exact position in the superfluid - that's the whole reason why it has long-range coherence".
If the meaning it's not what I have deduced from your statement, then explain which is it.

A BE condensate, by definition, has long-range coherence. You can naively think of a cosine or sine function, or even something of the form exp(ikx) that extends for a very long distance when compared to the lattice spacing of the solid. This is how you describe such a system in the simplest form. The particle you are describing is spread out all over that range. If you don't believe me, try to find <x> for something having that kind of a wavefunction.

... and it is this description that is the cause of the existence of a supercurrent.

Zz.

 

[rewebster]

Hey, you should know by now to never use a wikipedia reference with me.
Zz.

whoops--let's see--now, ---do I remember that?



I can maybe see a line in your signature:

"Don't EVER use a Wiki reference with me--I have an accelerator--and I WILL use it.

If you have an exact citation where the size of a photon has been defined, I'd like to know about it.
Zz.

nope, I don't of one---its not in the standard references from what I read here.---you can try jal 's blog

Is a question that has no clear-cut definition interesting? I suppose philosophically, it is. But in physics? Just think. In all those particle detectors that not only detect the various particles but also "EM showers" (i.e. photons), never once is there any kind of discussion whatsoever about the size of the photons they detected. Why do you think that is? I mean, these ARE the people who study elementary particles, after all. They either don't seem to care, or they think this is not a meaningful question. Again, when I asked this to an experimentalist who works at the ATLAS detector at CERN, his reply was "what in the world is a photon size and why do we care?" Yet, on here, we seem to be tripping over each other to discuss this. Do we know more than they do, or something that they don't? You answer that, because I can't.

Zz.

They may have thought about it when they were younger (and ignorant).

I think one of the reasons is that you DO know more (have specific knowledge to do what you do). I don't know how to turn an accelerator on--good chance I never will --(in that area--I'm ignorant). --but, that's not saying that someone else asking the question, or reading it here may not develop an interest, even indirectly, enough to take over your job when you retire. Do you think most of us will ever run an accelerator (or even have the interest, or the years of schooling and training to do so?)

Z, are you telling me that you've never (ever) wondered what light is?--what magnetism is? and so on.

I remember some physicist at some school saying when I asked him about 'what causes magnetism' -- He said (paraphrased for the forum here), "don't think about that stuff, it will drive you crazy". I think he was paraphrasing someone else even. Most of the "professionals" have been around long enough, and have thought about it long enough, and knew that if, even Einstein couldn't figure it out--they better quit while they're ahead and not jeopardize their job by going off the deep end.

I don't have to worry about that (yeah, I got it--you think I'm there already and along with most of these 'other' lunies who 'argue' with you). It's not because we're ignorant---it's because we're (me, anyway) enthusiastic STILL. ---we (including me) aren't jaded yet, and talk with the possibility that the answer is out there still---(and, here's the part that you probably find sad for us (me) being us (me and some others, maybe)--'if we only thought about it in a different way'.---and, hopefully, the forum will be a forum and not just a 'cut and paste' from the 'standard references'.

Sure, but is this really the issue here?

I don't work in studying fundamental particles. I do, however, USE the knowledge to accomplish something else. I used photons to study spectroscopy of strongly correlated system, and I use photons to generate electrons for a particle accelerators. As far as I can tell, the issues that I deal with are fully described by what we know now.

Zz.

different people have different philosophies about physics for what they want to do

 

[ZapperZ]

I wonder about A LOT OF STUFF! That is irrelevant to what we are discussing here. Just because I tow the "party line" and refuses to speculate about things on here should not be associated with the notion that I do not wonder about such things. I too follow the PF guidelines, and it is inappropriate if I start spewing off all the stuff that I "think" is neat and wonderful, yet I have nothing to back them with.

Zz.

 

[rewebster]

I do not agree with you. I guess that Zz is doing the most general case (in ED). And in contrast with the possible fantasies he is doing real life.
Regards, Dany.

yeah--I know--that is the difference---but at the same time, not all who aren't in the profession are nut cases either.

(so, does that mean that some who are in the profession could be nut cases?)

 

[rewebster]

z--I appreciate the guidelines (especially after being on here about a year)---I've gone to some (most) other sights for a visit to 'see' and have never went back.

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

I agree some may step out of line (spew)--but most of us (who are not in the profession, yet, ever,etc.)--appreciate what is being done here.

--------------------------------
most of post 202 was in response to your question:

"Do we know more than they do, or something that they don't? You answer that, because I can't."

 

[DaTario]

1. I agree a photon is a quantum of electromagnetic field.
2. I agree it has not a definite position.
3. As Pervect notices: "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."
4. Many people seems to agree on the fact photon has no size.

Conclusion: a photon IS NOT AN OBJECT TRAVELLING FROM SOURCE TO DETECTOR.

If someone thinks this conclusion is wrong, please explain.

It seems quite to provoke us that light quanta has no position but travels with c and moreover obeys kinematics rules dictated by correlation experiments with twin photon's (spontaneous down conversion for instance).

Of course we may learn this lesson and start building the ontology of photon with such subtleties. Photon is not a typical kind of being and etc...

Weisskopf Wigner theory for spontaneous emission produces the notion of a particle like object, which is corroborated by ecrans. If we are to abandomn this particle view, we should provide explanation for ecrans and the punctual appearance of light on it.

best wishes

DaTario

 

[Anonym]

yeah--I know--that is the difference---but at the same time, not all who aren't in the profession are nut cases either.

That is not what I have in mind. Zz is the experimentalist.

the forum will be a forum and not just a 'cut and paste' from the 'standard references'.

You need the standard references and standard terminology if you want that the other people will understand what you want to say.

I don't know how to turn an accelerator on--good chance I never will.

What is the problem? Just apply to Zz at ANL to be his student/assistant. They need youngers with one leg in the experiment and the other in the theory.

Regards, Dany.

 

[lightarrow]

A BE condensate, by definition, has long-range coherence. You can naively think of a cosine or sine function, or even something of the form exp(ikx) that extends for a very long distance when compared to the lattice spacing of the solid. This is how you describe such a system in the simplest form. The particle you are describing is spread out all over that range. If you don't believe me, try to find <x> for something having that kind of a wavefunction.

... and it is this description that is the cause of the existence of a supercurrent.

Zz.

What does "The particle you are describing is spread out all over that range" mean? Does it mean that a particle with very short-range coherence has a quite well defined position in space? And the same for a photon? It's just a question.

 

[ZapperZ]

What does "The particle you are describing is spread out all over that range" mean? Does it mean that a particle with very short-range coherence has a quite well defined position in space? And the same for a photon? It's just a question.

Er.. I did ask you to find <x>, didn't I? Try it. When you find <x>, that is exactly what I meant when I said it is spread out.

We are trying to put "words" to something that is essentially and foremost, has a mathematical description. So unless you understand what it looks like mathematically, word description will never be able to convey the exact scenario.

Zz.

 

[lightarrow]

Er.. I did ask you to find <x>, didn't I? Try it. When you find <x>, that is exactly what I meant when I said it is spread out.

We are trying to put "words" to something that is essentially and foremost, has a mathematical description. So unless you understand what it looks like mathematically, word description will never be able to convey the exact scenario.

Zz.

Since it seems difficult for you to give direct answers, I will give them for you (so, you can criticize them, it's your preferite task).

1. A photon's wavefunction with coherence length L represents an object traveling in space with a position indefiniteness = L, that is, the photon can be detected in a any instant of time from when the initial end of the train wave reach the detector to when the last end reach it.

1.B The photon is pointlike. But in this case, why differently prepared photons (of the same energy) can have different L?

2. A photon's wavefunction with coherence length L represents an object traveling in space which is spread out in the length L, that is, the entire photon is actually located in all that lenght.

3. ...


Which do you prefer?

 

[ZapperZ]

It is difficult because the term "coherence length" has different physical meaning in different scenario, very much like "phase space". In superconductivity, it is of the scale of the extent of the Cooper Pairs wavefunction.

Since I wasn't the one who introduced the coherence length concept for a photon, I shouldn't be the one who has to answer this for you as far as photons are concerned. All I have given you is some generic concept on how something with long-range coherence can be considered to be spread out all over that range. That's all. If you wish to look beyond this but not care about the mathematical formalism, then I am the wrong person to ask.

Zz.