How big is a photon and what does it look and behave like?

[marlon]

All I did was state the correspondence principle in my own words. I don't understand why you object to it.

Tom,

After some consideration of my own words...Well...Just forget about my remark. Let's be clear, you are right in saying that large quantumnumbers should reduce QED to classical EM. Which ofcourse, it does. I do realize that that was your point.

The only thing i wanted to make sure is that others don't interprete your words as "many photons make up an EM field". This is a popular misconception that has occurred many times in the nuclei and particles subforum.

Just to be clear, it was not my intention to oppose to the content of your post.

regards
marlon

 

[Physics101]

This [not requiring the uncertainty principle] is impossible. Keep in mind what the Q in QED stands for.

While I understand that the Uncertainty Principle is a fundamental fact of nature in QM, I was merely referring to what Richard Feynman noted in one of his footnotes which I'm quoting below. Please read the following and see where I might have misunderstood.

Page 55-56 of QED: footnote 3

This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

 

[quantumdude]

Marlon,

I was sloppy with my terminology. I thought that it would be clear from the context, but evidently it was not. Let me explain.

Hoovooloo said that photons could be thought of as a pair of sine waves. Since plane wave solutions to the classical EM wave equation are sine waves, the claim on the table strongly suggested to me that photons could be pictured classically. Ben seemed to buy into this claim, and that is what I was opposing. When I said, "EM field" I meant "classical EM field". I cut the term short because I thought that Hoovooloo's comment put us in that mode of thinking.

So my comments should be read as follows: The photon cannot be viewed as a pair of sine waves (that is, as a classical EM wave) because classical electrodynamics only applies in the limit of large n, as per the correspondence principle.

I was sloppy by leaving off the adjective "classical". You are correct to point out that just because the EM field intensity reduces to a single photon, it doesn't mean that the field ceases to be an EM field. I wasn't trying to contradict that fact, which I see you now realize.

 

[quantumdude]

If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

I think I understand why he said that. There was a time when all the ideas of quantum theory were still forming and the uncertainty principle seemed to stand alone as a physical postulate. As quantum theory developed it was found that by disposing of all the old ideas of how particles and light behave and introducing new, revolutionary postulates, it was no longer necessary to postulate an uncertainty principle.

But the uncertainty principle is still deducible from the newer postulates. So while it is not a necessary first principle, it remains a necessary consequence of first principles, and hence is still with us.

 

[ZapperZ]

While I understand that the Uncertainty Principle is a fundamental fact of nature in QM, I was merely referring to what Richard Feynman noted in one of his footnotes which I'm quoting below. Please read the following and see where I might have misunderstood.
Page 55-56 of QED: footnote 3
This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

To add to what Tom said, the HUP is really not a "principle" in the same vein as the principle of conservation of momentum, etc. It isn't the starting point. Most peole who have not studied QM does not know that, i.e. they don't that the HUP is a consequence, not the source. What is more important here is the commutation relation of 2 observables, i.e. [A,B], which in some cases is called the First Quantization.

It is based on this mathematical formulation that you get the HUP. The HUP falls right out of the mathematics. It is not inserted by hand and not some ad hoc introduction into the QM description. Most of us don't even deal with it when we use QM under normal working conditions, because we know it will be taken care of by the formulation.

Zz.

 

[marlon]

This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

Well Tom, explained Feynman's point in post 54 so i don't have anything to add. But do keep in mind that Feynman is NOT saying that even when starting from the "new revolutionary QM ideas" one cannot retrieve the HUP from these new ideas. Basically he is saying that when trying to describe QM phenomena in terms of classical physics (as far as that were possible ofcourse), you will be making mistakes and errors. In other words, classical physics does not work. That's were the HUP comes in. The HUP just expresses that classical physics should not be used to describe QM stuff.

marlon

marlon

 

[Rade]

2) the most complete picture : QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field.

I have a question about your comments above, something that is unclear to me. When you say that a photon "arises" from the EM-field, are you then saying that the EM-field is a thing that exists independent of the observation of the photon arising ? I know, perhaps more philosophic than physics, but I really do have a hard time grasping the nature of this EM-field, e.g., its essence. A photon I can "sort of" get a handle on (e.g. a quantized packet of pure energy), but it is the EM-field that is the "cause" of the photon "effect" that I just cannot grasp mentally. Or, is it a case as discussed by Zapper that one really cannot grasp EM-field as a "mental concept", that it can only be grasped using "mental mathematics" ? Thanks for any comments, and a very happy new year 2006 to all.

 

[Sherlock]

I have a question about your comments above, something that is unclear to me. When you say that a photon "arises" from the EM-field, are you then saying that the EM-field is a thing that exists independent of the observation of the photon arising ? I know, perhaps more philosophic than physics, but I really do have a hard time grasping the nature of this EM-field, e.g., its essence. A photon I can "sort of" get a handle on (e.g. a quantized packet of pure energy), but it is the EM-field that is the "cause" of the photon "effect" that I just cannot grasp mentally. Or, is it a case as discussed by Zapper that one really cannot grasp EM-field as a "mental concept", that it can only be grasped using "mental mathematics" ? Thanks for any comments, and a very happy new year 2006 to all.

In lieu of Marlon replying to your query, you can do a Google book search ... typing in, "rodney loudon the quantum theory of light". Loudon discusses 'the photon' right at the beginning, and somewhere later on (I don't remember exactly where) he talks about the essence of the quantized em field, I think. You can get some good references there for further study also.

Eventually, I suspect you will come to the conclusion on your own that, as some mentors have noted, ZapperZ especially, trying to envision the physical reality of what a photon corresponds to independent of its operational definition (ie., independent of its existence as a measurement result and mathematical construction pertaining to photon measurement) in terms of some set of classical analogies will be inadequate (ie., there isn't any classically analogous picture that can be generally, unambiguously applied).

Trying to envision what an em field corresponds to in 3D physical space might be even more problematic. I don't know enough to comment definitively, but so far I don't have any sort of working physical picture of either photons or fields. I think that part of what the mentors' want to communicate about this is that as we students learn more (and become more fluent in the mathematical methods), then the reasons for the absence of a visualizable understanding (in terms of analogies with our ordinary experience) will become clearer and the need for such visualizations will diminish.

Nevertheless, the attempt to develop a picture of the deep nature of light doesn't seem to me to be an entirely metaphysical one ... and it seems likely that there will continue to be efforts to build such a picture from the data.

Do Bell's Theorem and the results of Bell-type tests tell us that such efforts are doomed from the start? Well, given the extant data and experimental capabilities, yes ... but, who knows what the future holds.

By the way, also coming to mind are discussions from the archives of sci.physics.research (and sci.physics ??). One had to do with the 'length of a wavetrain of a photon', and the other was called 'photon schmoton' or something like that.

 

[member 11137]

... as some mentors have noted, ZapperZ especially, trying to envision the physical reality of what a photon corresponds to independent of its operational definition (ie., independent of its existence as a measurement result and mathematical construction pertaining to photon measurement) in terms of some set of classical analogies will be inadequate (ie., there isn't any classically analogous picture that can be generally, unambiguously applied).
Trying to envision what an em field corresponds to in 3D physical space might be even more problematic. I don't know enough to comment definitively, but so far I don't have any sort of working physical picture of either photons or fields. I think that part of what the mentors' want to communicate about this is that as we students learn more (and become more fluent in the mathematical methods), then the reasons for the absence of a visualizable understanding (in terms of analogies with our ordinary experience) will become clearer and the need for such visualizations will diminish.
Nevertheless, the attempt to develop a picture of the deep nature of light doesn't seem to me to be an entirely metaphysical one ... and it seems likely that there will continue to be efforts to build such a picture from the data...

What a beautiful and exact commentary... Even if I didn't took part to your present conversation, I continue to read the discussions on these forums and I feel concerned by this discussion about the HUP and the Feynman's approach. Not only because of my own approach on the independant research forum where I personnaly try to explore the existence of other mathematical procedures able to give back this HUP. I recently discovered some stuff to understand the Feynman's paths integrals and the photonic crystals that are indeed concrete applications of these metaphysical "Überlegungen" "pensées" "thoughts". Happy new year 2006 to the team and to every body here.

 

[jtbell]

By the way, also coming to mind are discussions from the archives of sci.physics.research (and sci.physics ??). One had to do with the 'length of a wavetrain of a photon', and the other was called 'photon schmoton' or something like that.

I turned up those threads myself a couple of weeks ago when I was searching for material on the "nature" of photons. They have lots of interesting material, and I highly recommend them to anyone with plenty of time on their hands. Be prepared to do a lot of digging, because Google Groups doesn't collect all the postings from one thread together. I think it's because some people changed the subject line along the way, which confuses Google. As is usually the case with Usenet, some posters are more "trustworthy" than others, so you need to evaluate people's comments and apparent expertise.

 

[Nereid]

A most interesting thread!

There was a comment in marlon's first post (in this thread) that I feel merits further discussion:

[snip]

If you have done this, you answer me this : "do photons mutually interact ?"
Hell, i will even give you the answer
Answer : NO in first order but they do interact indirectly in higher order.

What does this mean? That photons can 'collide'? That a photon can split into two?

More generally, we've discussed the 'how big?' and 'what does it look like?' parts of the OP; I feel we've somewhat neglected the 'how does it behave?' part.

The many different two-slit experiments are good examples of the 'behaviour' of photons (and electrons, and atoms, and ...) - the 'photon' behaves just like a (QED) photon in these experiments (it does NOT behave 'like' a classical wave, or a classical particle, when you attempt to account for the experimental results in their totality).

What are some of the other notable (quantum) behaviours of photons? In particular, what are some 'extreme' behaviours, some of the most counter-intuitive behaviours, predicted by theory, later observed?

 

[ZapperZ]

I do not know how accurate it is but here is what I was taught about what a Photon may look like and why it acts like both a particle and a wave function.

But this is shown in CLASSICAL E&M! You get something like this by solving the Maxwell Equation. Such a picture that you were "taught" didn't come out of nowhere. It came from a classical wave picture. So where's the "particle" behavior? And not only that, this is valid only for a plane-polarized light.

And please don't pursue the line of discussion that you started that was locked earlier.

Zz.

 

[quantumdude]

There was a comment in marlon's first post (in this thread) that I feel merits further discussion:What does this mean? That photons can 'collide'? That a photon can split into two?

No, photons can only couple to matter currents. What happens is that two photons can momentarily be converted to e^+e^- pairs, and those can collide. By default this is a higher-order process since it requires multiple vertices in the Feynman diagram. If you have Jackson's Classical Electrodynamics, check out the introduction. You'll find a Feynman diagram for this process with the caption, "Scattering of Light by Light" or some such.

 

[Intuitive]

But this is shown in CLASSICAL E&M! You get something like this by solving the Maxwell Equation. Such a picture that you were "taught" didn't come out of nowhere. It came from a classical wave picture. So where's the "particle" behavior? And not only that, this is valid only for a plane-polarized light.
And please don't pursue the line of discussion that you started that was locked earlier.
Zz.

Kindly disregard the posting that I deleted about Classical EM.

P.S. It is much more difficult to draw a Photon(QED), Classical was easier.
I have been studying Java applets that represent Photon(QED) with various slits and Wave Guides, It's very fasinating indeed, I love the
thought of Photon Duality and Multiple Universes, It's keen.

 

[Nereid]

No, photons can only couple to matter currents. What happens is that two photons can momentarily be converted to e^+e^- pairs, and those can collide. By default this is a higher-order process since it requires multiple vertices in the Feynman diagram. If you have Jackson's Classical Electrodynamics, check out the introduction. You'll find a Feynman diagram for this process with the caption, "Scattering of Light by Light" or some such.

Thanks Tom.

It sounds like a thoroughly non-classical thing - has it been confirmed by (direct) observation?

I don't have the book to hand, does anyone know of a brief overview that's on the net?

 

[masudr]

The classic experimental observation that one talks about when discussing QED is the magnetic moment of an electron. The theoretical prediction and observed experimental result agree to unprecedented levels (to one part in 10^{11}, I think).

EDIT: The prediction for this value involves higher-order corrections, which is when a photon becomes a e^+e^- pair etc.

 

[Boffin]

I don't see a reason for the amount of opposition QM receives from some people. At times this forum seems to be a place for desperate attempts to disprove QM rather than a way to learn about it. https://www.physicsforums.com/showpost.php?p=501143&postcount=12

Hmm, OK I read that?

Take comfort in Feynman's quote, which sums up all of the problems with QM: Nobody understands quantum mechanics. https://www.physicsforums.com/showpost.php?p=501089&postcount=8

But somehow I think we can do a little tiny bit better than this. I haven't been adding to this thread because I've been reading several books and articles on this issue. I did some major reading of the Physics Forum past threads also. Here are some of the highlights of related threads. It'll give some of you something to do.

Electromagnetic wave in quantum physics? https://www.physicsforums.com/showthread.php?t=91205
Light, Wave or Particle? https://www.physicsforums.com/showthread.php?t=101692
Light and photon, confused https://www.physicsforums.com/showthread.php?t=63802
Picturing wave/particle duality https://www.physicsforums.com/showthread.php?t=57911
Wave/particle duality https://www.physicsforums.com/showthread.php?t=56561
What is light? https://www.physicsforums.com/showthread.php?t=83859
Particles or Waves? https://www.physicsforums.com/showthread.php?t=77457
Light = Particle or Wave? https://www.physicsforums.com/showthread.php?t=35854
What's Wrong with QM? https://www.physicsforums.com/showthread.php?t=68265

But after reading all these messages I still didn't even come close to answering one basic question. There seems to be a disagreement on the following issue on this forum even among the following mentors as to whether a wave in Quantum Mechanics can be a single photon or does it need to be composed of multiple photons?

The photon frequency \nu is the frequency of oscillation of the EM field that would result if there were a large number of those photons. https://www.physicsforums.com/showpost.php?p=868561&postcount=36

QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field. https://www.physicsforums.com/showpost.php?p=868582&postcount=38

No, the single photon can manifest as an EM plane wave, for example. Wave particle duality is serious, not just a statistical phenomenon of little bullets "doing the wave". https://www.physicsforums.com/showpost.php?p=766621&postcount=2

Here is what is puzzling to me. A single photon in Quantum Mechanics is able to interference with itself. Isn't then a single photon wavelike and have a frequency? Normal light is incoherent light. There is no regular spacing between photons to arrive at any frequency? Does not a prism separate light photons by their individual wavelengths or frequency? Am I being confused with definitions or imperfect models? Please enlighten me!

 

[quantumdude]

But after reading all these messages I still didn't even come close to answering one basic question. There seems to be a disagreement on the following issue on this forum even among the following mentors as to whether a wave in Quantum Mechanics can be a single photon or does it need to be composed of multiple photons?

There's no disagreement at all. We all agree that classical EM fields (that includes EM plane waves) aren't manifested by a single photon. This was clarified by the exchange between marlon and myself.

Here is what is puzzling to me. A single photon in Quantum Mechanics is able to interference with itself. Isn't then a single photon wavelike and have a frequency?

A single photon does have a frequency, but it is not a classical EM wave. See the following website on single photon interference.

http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/SinglePhotonInterference/SinglePhotonInterference.html

The familiar interference pattern emerges only after many photons have hit the screen. But each individual photon hits the screen in only one location.

 

[vanesch]

There's no disagreement at all. We all agree that classical EM fields (that includes EM plane waves) aren't manifested by a single photon. This was clarified by the exchange between marlon and myself.
A single photon does have a frequency, but it is not a classical EM wave.

The relationship between the classical EM description and the quantum mechanical description is rather subtle, and depending on what aspect you want to look at, things can be formulated differently and sometimes, at first sight, even contradictory. But when you understand the relationship well enough, you can often see the veracity of the different apparently contradictory statements.

Here are a few.
The classical EM description is of course a 4-vector field over spacetime:
A(x,y,z,t), from which the more well-known description in terms of E and B fields can be derived: E(x,y,z,t) and B(x,y,z,t).

The quantum description of the free EM field consists of Fock space, which has the basis:
|0> the vacuum
|k1,e1>, |k2,e2>, ... the 1-photon states
|k1,e1,k2,e2>,|k3,e3,k4,e4> ... the 2-photon states
...

Each (k,e) pair corresponds to a momentum vector (3-dim) and a polarization vector e which can take on 2 different values.

The above basis is the basis of a hilbert space called "Fock space" and all possible linear combinations of the above states are the possible quantum states of the free EM field.
But only very special combinations correspond to classical EM waves ; these are called "coherent states". Coherent states are special superpositions of the vacuum, 1-photon, 2 - photon ... 28621-photon... states which are eigenvectors of the so-called destruction operator. There is a coherent state that corresponds to each possible state of the classical EM field, but in Fock space there are many, many more possible states.

However, the 1-photon state |k,e> behaves *in certain respects* as the classical EM wave that corresponds to a plane wave with wave vector k and polarization e. It is not the equivalent (that would have been the coherent state, which CONTAINS |k,e> as a term, but has many others in them too) of the classical EM wave, but for certain aspects, one can *pretend* it to be like the classical EM wave. For instance, the probability of detection on a screen of the one-photon state is proportional to the intensity of the classical EM wave with wavevector k and polarization e. So IF YOU KNOW WHAT YOU'RE DOING, you can jump back and forth sometimes between a 1-photon state and a classical EM wave, because things come out the same for certain quantities.
And then people get sometimes a bit sloppy, and say that "the 1-photon state interferes with itself like the classical EM wave" and things like that. In the proper context, this is an operationally correct statement.
And then others say that 1-photon states are totally different from a classical EM wave which needs many photons to be correctly described. This is ALSO correct (the coherent state contains n-photon states).

cheers,
Patrick.

 

[jtbell]

the probability of detection on a screen of the one-photon state is proportional to the intensity of the classical EM wave with wavevector k and polarization e.

The electric portion of a classical EM plane wave is

\vec E = \vec E_0 \cos (\vec k \cdot \vec r - \omega t + \phi_0).

By "intensity" do you mean {|\vec E|^2} or {|\vec E_0|}^2? I suspect the second one, because in the absence of interference I don't expect the probability of detecting a photon to vary with the wave's oscillation.

 

[Boffin]

There's no disagreement at all. We all agree that classical EM fields (that includes EM plane waves) aren't manifested by a single photon. This was clarified by the exchange between marlon and myself. https://www.physicsforums.com/showpost.php?p=879499&postcount=68

I asked the question again, because in my opinion it wasn't clearly defined in the previous statements of you and Marlon shown below. These statements left a lot of concepts undefined in my mind.

Tom, the most complete picture : QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field. So, if you want to be talking about the relation between "fields" and photons, you should have said this. https://www.physicsforums.com/showpost.php?p=868582&postcount=38

Tom, The only thing i wanted to make sure is that others don't interpret your words as "many photons make up an EM field". This is a popular misconception that has occurred many times in the nuclei and particles sub forum.https://www.physicsforums.com/showpost.php?p=869050&postcount=51

Marlon, The photon cannot be viewed as a pair of sine waves (that is, as a classical EM wave) because classical electrodynamics only applies in the limit of large n, as per the correspondence principle. You are correct to point out that just because the EM field intensity reduces to a single photon, it doesn't mean that the field ceases to be an EM field.https://www.physicsforums.com/showpost.php?p=869781&postcount=53

A single photon does have a frequency, but it is not a classical EM wave. https://www.physicsforums.com/showpost.php?p=879499&postcount=68

Here is what I understand, based on the above. It seems that viewing a single photon as a pair of sine waves is the old classical way of viewing the photon. I don't know how classical this is, as before about 1900 scientists were still thinking of light as sound type waves in an aether, in which case individual photons didn't exist. So I'm assuming that perhaps the single photon viewed as a pair of sine waves came after Einstein's quantum theory but before Quantum Mechanics was accepted? I understand that EM fields which was mentioned in the posts repeatedly is a classical term and concept that is not used in Quantum Mechanics. Is this all correct so far?

So let's fast forward to modern day interpretations, which I take it are based on the QM or QED interpretation of how a photon behaves, at least in this Physics Forum. Tom said that a single photon has a frequency. So here are some more questions.

1. A single photon has a frequency so does a QM single photon have a corresponding wavelength?

2. We can polarize light with filters so I'm assuming light can be polarized even in the QM view. What does QM say that polarization is?

3. I take it that the QM view of a photon is a probability wave. Brian Greene in his book The Fabric of the Cosmos says p91.1 that "there is still no universally agreed-upon way to envision what quantum mechanical probability waves actually are." and p98.3 "As an example, take a look at a particularly simple probability wave for a particle, the analog of a gently rolling ocean wave". Can a QM single photon be described or thought of as a type of sine wave after all, just not exactly like the classical form?

4. Is the QED version of the probability wave just like the QM probability wave, except that each photon is actually billions or more of duplicate photon copies going all over space, and when the first photon copy is absorbed by any electron, the other billions of copies dissapear? Is the energy in each photon copy the amount finally absorbed? Or is the total energy of a photon really distributed around the billions of copies around the universe and when the first photon is absorbed the energy travels in some cases thousands of light years away in an instant to the electron that absorbs the first photon?

 

[reilly]

I asked the question again, because in my opinion it wasn't clearly defined in the previous statements of you and Marlon shown below. These statements left a lot of concepts undefined in my mind.
Here is what I understand, based on the above. It seems that viewing a single photon as a pair of sine waves is the old classical way of viewing the photon. I don't know how classical this is, as before about 1900 scientists were still thinking of light as sound type waves in an aether, in which case individual photons didn't exist. So I'm assuming that perhaps the single photon viewed as a pair of sine waves came after Einstein's quantum theory but before Quantum Mechanics was accepted? I understand that EM fields which was mentioned in the posts repeatedly is a classical term and concept that is not used in Quantum Mechanics. Is this all correct so far?

(RA)Read a history of 20th century physics, or a beginning QM, or Atomic Physics or nuclear physics text, then answer your own question.

..........
So let's fast forward to modern day interpretations, which I take it are based on the QM or QED interpretation of how a photon behaves, at least in this Physics Forum. Tom said that a single photon has a frequency. So here are some more questions.

(RA) QED is the application of quantum theory to electromagnetic phenomena. Planck's frequency relation falls out nicely from the QED approach -- describes free photons. It's a big subject; consider Wolff and Mandel's bible, Optical Coherence and Quantum Optics -- some 1100 pages -- and Weinberg's Quantum Theory of Fields -- say, 2000 pages, -- and, for historical interest, Schweber's QED and the Men Who Made It -- 700 pages. To make any informed judgement on QED, you really should have a sense of the contents of these almost 4000 pages. And, many professionals in the field have such knowledge -- much of which comes from the hard work of studying,.. So you remember, in XYZ's QM text there's a great explanation of LS coupling in atomic physics, or there's solid discussions of path integrals in books A, B, ...

.........

1. A single photon has a frequency so does a QM single photon have a corresponding wavelength?
2. We can polarize light with filters so I'm assuming light can be polarized even in the QM view. What does QM say that polarization is?

(RA) See any freshman physics book for the answers to 1 & 2.
............
3. I take it that the QM view of a photon is a probability wave. Brian Greene in his book The Fabric of the Cosmos says p91.1 that "there is still no universally agreed-upon way to envision what quantum mechanical probability waves actually are." and p98.3 "As an example, take a look at a particularly simple probability wave for a particle, the analog of a gently rolling ocean wave". Can a QM single photon be described or thought of as a type of sine wave after all, just not exactly like the classical form?

Again, read about waves and wavefunctions in most any freshman physics book. If you do so with some energy you'll be able to answer your own question.

...............

4. Is the QED version of the probability wave just like the QM probability wave, except that each photon is actually billions or more of duplicate photon copies going all over space, and when the first photon copy is absorbed by any electron, the other billions of copies dissapear? Is the energy in each photon copy the amount finally absorbed? Or is the total energy of a photon really distributed around the billions of copies around the universe and when the first photon is absorbed the energy travels in some cases thousands of light years away in an instant to the electron that absorbs the first photon?
.........

Poetic to be sure, but totally void of any connection to reality.

It seems to me a bit odd that you have such a strong stance on QED, when, by your questions and comments, you know virtually nothing about the subject. Don't forget that all of atomic and molecular physics falls under QED, as does the physics of supercondutivity, semiconductors, and regular metallic conductivity, crystal structure, ... Toss these in, and we're up to 6000 to 7000 pages of basic stuff, and all empirical results contained therein must hold true for any alternative theory.


There's the truth, ugly for some, those 7000 pages are pretty compelling. Particulalry when, QM, QED or whatever, has never, repeat never, been found wanting empirically. So to me, as a student, and now, I want to see how this stuff works, why is it so successful, what are the problems, does it make sense -- we're talking physics, in which you can do a lot worse than correctly predict experimental results.

Regards,
Reilly Atkinson

 

[Boffin]

See any freshman physics book for the answers to 1 & 2.

If all the answers are plain as day in the textbooks, and the textbooks are totally right, and we should not question the theories, what is the point of having this website or even professors? There is absolutely nothing to discuss.

Particularly when, QM, QED or whatever, has never, repeat never, been found wanting empirically.

Empirically: relying on experience or observation alone often without due regard for system and theory.

Obviously we are interested in different issues, but why do you want to prevent someone from wanting to know something you don't happen to be interested in? I what to know how the model works, and you just want to know that it works. These are different issues.

Even famous scientists have these questions. I quote from Brain Greene's book The Fabric of the Cosmos p182.5 "You might wonder how literally you should take the sum over histories description. Does an electron that strikes the detector screen really get there by traveling along all possible routes, or is Feyman's prescription merely a clever mathematical contrivance that gets the right answer? This is among the key questions for assessing the true nature of quantum reality, so I wish I could give you a definite answer. But I can't. Physicists often find it extremely useful to envision a vast assemblage of combining histories; I use this picture in my own research so frequently that it certainly feels real. The point is that quantum calculations unambiguously tell us the probability that an electron will land at one or another point on the screen, and these predictions agree with the data, spot on".

As Greene points out just because the model works, that doesn't mean the theory behind it is correct.

But I know you don't like questioning here. I didn't make any negative comments about QM or QED in my last posts, I was only asking questions.

I just want to know how some of you interpret the basic model of QM And QED. Surely there are slightly different interpretations? For example are you in the camp that thinks QM and QED is a perfect model, or are you in the camp that thinks that many of the ideas are just models. Surely that should be considered to be an acceptable enough type question here. Isn't Greene a peer reviewed published scientist? He is asking questions, so why can't I repeat the same question here? I don't see how I can get that out of a textbook, as I'll likely just get the authors viewpoint.

 

[Schrodinger's Dog]

Asking questions is key to understanding no matter how silly any of the "knowledgeable" think they are ask anyone here and I'm sure they'll tell you this. Keep asking questions silly or otherwise. I do and I find the answers more revealing than if I'd asserted something everyone already knows. The scientific community dmeands rigoroussness and proof. Sometimes I get the impression they tend to shy away from new or radical thinking because of the mind set that is science, then I meet a scientist with a versatile and free thinking mind who isn't afraid to ask stupid questions and postualte ridiculous theories on his way top finding the truth. When I meet someone like that I say, you're a research scientist. then there are those who like experimentation they are experimental phsysists, then there are those who do both, they're the real scientists IMHO.
Wake up with a theory disprove it over breakfast is a good saying. Anyone who says I have all the answers or this is absolutely correct is missing the whole point of science. Not that I'm accusing anyone of that here. But there will always be those who dismiss something out of hand, because it doesn't agree with the establisment. Question these people they're not really scientists there just wolves in sheeps clothing
No one has all the answers, particularly not me. Light is not a wave and a particle it's just something very much like it depending on how you want to look at it. It may have infinitesamal mass and it may have none, who really knows?
I'm still waiting for someone to tell my why light is bent by a difraction grating. Nothing is more likely to anoy someone than showing them just how ignorant they really are which is why I irritate myself so much

 

[reilly]

Ben --Questions are cool; ask away by all means. But, do not expect the answers to be forthcoming under all circumstances. It makes little sense to answer directly questions to which the answers are well known. Rather, like the parable of the fishes, it is far better to suggest a path to get the answers. How can you expect to discuss QM intelligently if you are not certain of the range of validity of Planck's frequency relation?, or are uncertain about wavefunctions.

Note that I said nothing about your questions. Rather my comments had to do with answers. I have virtually infinite belief in "Do your homework." In this forum, those that do their homework get more help than those that don't.

If you are sceptical of empiricism, then forget about physics. Physics is fundamentally an empirical science, rightly or wrongly. Philosophy is not, nor does it require much serious math. In philosophy, it's the argument that counts; in physics it's the experment that counts. And all that is unlikely to change.Indeed, ask questions. But, you will learn much by figuring out how to get the answers, rather getting others to do your work.Schrodinger's Dog -- Why is light "bent" by a diffraction grating? The whole story is laid out in great detail in Vol II of Halliday and Resnick. The basics of classical diffraction have been well understood for over a century. What's the issue?

Light == wave &/or particle? Do your homework -- why do lasers work? what goes on with the photoeletric effect?, what's a coherent state? These and other questions have a great deal to do with waves and particles. The controversy about waves and particles is not much in evidence in most of the physics community, and, in part, because few see any controversy.

Regards,
Reilly Atkinson

 

[leandros_p]

First of all, this is my first message in this forum, so hello everyone. It is very nice to be here.

Dear Ben Wiens,

You asked a legitimate question and you got some unpleasant replies that turned your question into a personal conversation about your competence or your incompetence to understand the issue of quantum physics.

Now, according to my understanding which is conditioned by my temperament, the reply “GO STUDY QED” is a product of irrational discussion. It is a tautology, meaning “you will find the answer in the answer”, which makes no sense!
The purpose of the science of Physics is to provide answers to any question. It was also indicated to you that ‘asking “how big is a photon" is similar to asking "how painful is purple?” ’. The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists. The scientific quest in finding the answer for the “size of a photon” eventually provided by the mathematical knowledge that: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* Although this answer seems that it does not construct a specific “morphology” for a specific photon (I use the term “specific” with the conventional meaning, although it is not a proper scientific term for photons), it is the linguistic “expression” of a (mathematical) structure provided by science and constructed by scientists, which indeed provides the “morphology of the photon", which is just the answer that was asked by the specific question.

So, Physics do provide the answer; just ask and science will provide the answer !

Let me remind you that a Greek philosopher by the name of Gorgias formulated a law about “knowledge” that in later centuries the science of Physics accepted in a scientific form by the formulation of the Principle of Heisenbergh. Gorgias formulated a law by three axioms:

(a) Nothing exists
(b) If anything existed, it could not be known
(c) If anything did exist, and could be known, it could not be communicated

(site about philosopher Gorgias: http://www.iep.utm.edu/g/gorgias.htm )

Regarding your initial question let me use an analogy that may help you seek further your quest (please read the following as an analogy only, you can not apply it to photons): The question “What is the physical space of a specific customer in a specific shop at a specific time?” seems to deserve a specific answer. The term “specific customer” defines a specific pinpointed person that performs the act of consuming/buying products, as a “customer”, at a specific shop at a specific time. According to conventional trade the question is valid and it may get a specific answer. But, according to contemporary trade a person can perform as a customer through internet, so his physical presence in the shop is equivalent to his physical absence and we can say about the customer from the internet that: “a specific customer is actually anywhere at the same time, but we can not define its position nor can we predict his physical space” – this sounds like the answer that we got for the size of photons. Of course, we can trace the network path of his connection through which he made the transaction and by that we can find his address, but then we can not define nor can we predict his actual physical “space” of his body, like we can do in a traditional shop. I repeat again that, this example is not about photons, which are certainly not customers at a shop, but it can provide an analogy to free our minds about the way that we mentally “hammer” physical reality.

(EDIT) PS: It is true that an "answer" is filtered by the listener, so he/she must apply the proper filter in order to realize the meaning of the answer. Just the same, a question is also filtered by the listener and when his/her filter is "polarized" the answer is polarized too.

Leandros

 

[ZapperZ]

The purpose of the science of Physics is to provide answers to any question. It was also indicated to you that ‘asking “how big is a photon" is similar to asking "how painful is purple?” ’. The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists.

Such as who? What scientific results that have been worked on methodically that have answered this question?

And do you think "How painful is purple" is also a legitimate question even when it makes no sense whatsoever?

Your post would have been perfect and dandy weren't it for one important fact: this is the physics section of the forum and NOT the philosophy section. You never even once addressed the ANSWER to the question, but rather went on an "in depth" analysis of the question and the responses. So let's cut to the chase, shall we? How BIG is a photon? You claim that this has been answered, so what is it?

If you wish, I can split this off to the philosophy section and you can over-analyze this all you want there. But if you want to continue this here, then there has to be concrete physics content from established sources, rather than hand-waving argument with no citations to back up your points.

Zz.

 

[Boffin]

You (leandros_p) never even once addressed the ANSWER to the question, but rather went on an "in depth" analysis of the question and the responses.

Half his whole post was in answer to the question. What's your problem Zapper? Just because someone doesn't describe something the way you would, that doesn't make it wrong or non existent.

 

[Schrodinger's Dog]

Thanks Ben I'll take a look at the link when I have more time
I think ZZ is right to question the use of philosophy in a physics thread(philosophy has it's place in science, in fact it is what science was: it merged alchemy,astronomy etc into one mass with philosophy taking a passing interest) But let's not bring it to a Physics thread eh?

 

[ZapperZ]

Half his whole post was in answer to the question. What's your problem Zapper? Just because someone doesn't describe something the way you would, that doesn't make it wrong or non existent.

Ah, now I get it!

If THAT is the TYPE of answer that you are looking for, then it isn't a physics answer that you want. I didn't realize that you WOULD buy hand-waving answers with no concrete foundations. (For example, it obvously doesn't bother you that someone would claim that photon sizes have been the subject of "methodical" work without offering to point out what they are).

Please restart this thread in the Philosophy section. You will get just the very type of answers and responses to satisfy your requirement.

Zz.

 

[Boffin]

Ah, now I get it! If THAT is the TYPE of answer that you are looking for, then it isn't a physics answer that you want. I didn't realize that you WOULD buy hand-waving answers with no concrete foundations. (For example, it obviously doesn't bother you that someone would claim that photon sizes have been the subject of "methodical" work without offering to point out what they are). Please restart this thread in the Philosophy section. You will get just the very type of answers and responses to satisfy your requirement.

Philosophy: investigation of the nature, causes, or principles of reality, knowledge, or values, based on logical reasoning rather than empirical methods-Answers.com

Physics: science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe-Britannica Encyclopedia

I would not consider explanations of the results of empirical science to be philosophy. That is what I'd like to see in this thread. That is what I took Leandros' message as also. But when human beings want to classify information, there is of course some overlap. This causes problems when the rules are too defined. What happens when part of the explanation is empirical physics and partly philosophical. It doesn't make sense to separate a single message into two forums, no one would make any sense of it. Personally I think it makes sense to put messages into the physics section when the message is mostly physics. I think that was done in this case. At least that's the way I see it.

 

[ZapperZ]

Philosophy: investigation of the nature, causes, or principles of reality, knowledge, or values, based on logical reasoning rather than empirical methods-Answers.com
Physics: science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe-Britannica Encyclopedia
I would not consider explanation of the results of empirical science to be philosophy. That is what I'd like to see in this thread. That is what I took Leandros' message as also. But when human beings want to classify information, there is of course some overlap. This causes problems when the rules are too defined. What happens when part of the explanation is empirical physics and partly philosophical. It doesn't make sense to separate a single message into two forums, no one would make any sense of it. Personally I think it makes sense to put messages into the physics section when the message is mostly physics. I think that was done in this case. At least that's the way I see it.

Exactly WHAT message is "physics" or "emprical", especially in Leandros's mesage? This is what I object to. People are TOO QUICK to make statements like that without establishing the validity of such statements. This occurs a lot in this thread.

Hand-waving arguments are not empirical. Try looking in Answer.com if you don't believe me. All I have seen are SPECULATIONS and guesswork. When I asked for specific citations and exact results, I get nothing. At what point do we stop making things up as we go along and actually LOOK in either physics textbooks or peer-reviewed papers?

Justify what you are claiming. Back it up with either specific theory, experiments, citations, phenomenon, etc. If not, you are making unjustified or unverified statements. If that is what you want, I can make things up as well as the next guy, and not only that, I can disguise it with impressive mumbo-jumbo almost as well as Alan Sokal to fool enough people. Is this what you wish to have?

Zz.

 

[leandros_p]

Such as who? What scientific results that have been worked on methodically that have answered this question?
And do you think "How painful is purple" is also a legitimate question even when it makes no sense whatsoever?
Your post would have been perfect and dandy weren't it for one important fact: this is the physics section of the forum and NOT the philosophy section. You never even once addressed the ANSWER to the question, but rather went on an "in depth" analysis of the question and the responses. So let's cut to the chase, shall we? How BIG is a photon? You claim that this has been answered, so what is it?
If you wish, I can split this off to the philosophy section and you can over-analyze this all you want there. But if you want to continue this here, then there has to be concrete physics content from established sources, rather than hand-waving argument with no citations to back up your points.
Zz.

Dear ZapperZ,

I appreciate your point of you. Let me express my fear that you missunderstand my post. Therefore I am responding in order to clarify it.

First, let me provide some answers to your requests. I am referring to the following scientists that worked methodically in order to provide the answer to the question of “the size of the photon”: "http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

I have already provided the ANSWER to this question, that is: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* You can find the same ANSWER also in the following lecture: http://galileo.phys.virginia.edu/classes/252/uncertainty_principle.html , meaning that if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum.

You may argue, that some “words” that were used in the previous paragraph, coming from reality as we know it in real life dimensions through human experience, like “size”, “space”, “individual” and “isolation” are not the proper words to express the quantum physic’s term of photon. I agree with you on this. But this language can be used by the inquiring mind in making it to realize that under the contradicted enunciation of the answer, there is a more substantial layer of knowledge.

Let me disagree with you in the absolute way that you reject the question because it is turned upside down by the answer. When the scientists, mentioned in the above link, were looking for answers like “what is the time?”, “what is the length”, “what is the size of photon”, they were looking for an answer to a specific question and after methodological work they found that the provided answer was creating a new question and then the new answer created another new question and so forth.

I think that the misunderstanding you are making in reading my post is that you presume that when I said that “the question about the size of the photon is a legitimate question which got its answer by the methodical work of many scientists” I refer to a definite calculated answer in analogy to the question. Well, I never said, nor I ever imply, that the provided answer was a definition of position and volume or any other type of time-space related formulation. Neither have I said that the answer is following the analogies of the question.

The question “how big is a photon?” does has an answer, although the answer is not in harmony with the framework that the question is initially placed into. This answer provides an exodus from the framework. I think that you already tried to say that in your posts and you tried to warn about the disaster that awaits those who insist to defend the framework of their question, but you proposed the exodus from the framework of the question, while rejecting the question itself. I think that your proposition is problematic; the inquirer will never abandon his question just being advised to do so. There must be a framework in order to achieve an exodus from it, or else “exodus” is a meaningless move. There must be a certain physical meaning for “big” and for “size” in order to transcend these meanings in photon, or else photon itself is meaningless.

Finally let me also say that, according to my understanding, your posts are more philosophical compared to my first post – which had none philosophical enouncement. The laws of Gorgias are actually enouncing “The Uncertainty Principle” of Heisenberg, by using the scientific language of ancient age. Greek philosophers were all “scientists”, by the contemporary definition of word. Aristotle is called ‘philosopher’ but he is the founder and the father of Science, as the world have known Science since the scientific work that he composed. The proper place to discuss ancient Greek science is actually a physic’s forum. Unfortunately modern scientists think that science started 200 years ago, or that all “contemporary ideas” are new. Some scientists are surprised to know that “contemporary ideas and theories” were initially expressed by ancient scientists through the peculiarity of the ancient scientific language- quantum physics being no exception to that.

Of course, it depends on how you define philosophy. For me, it seems that a methodology that orders the knowledge to precede questioning belongs to the field of philosophical discussion indeed. And I understand that you proposed such a methodology for quantum physics. Scientific knowledge always follows questioning and it never precedes it - according to my understanding. I live in Greece and I have to follow the ancient Greek tradition on this issue.

For science there be none wrong question; there are only wrong answers.

 

[Rade]

...For science there be no wrong question; there are only wrong answers.

Although your post was directed to ZapperZ, I would like to input here that this statement is just plain incorrect. There are many "wrong" questions for science such as "how exactly did God form Adam from dust of Earth and energy from God breath--the dynamics?"; "what is truth?"; "why is the number line infinite and not finite?"...the list goes on and on and on.

 

[ZapperZ]

Dear ZapperZ,
I appreciate your point of you. Let me express my fear that you missunderstand my post. Therefore I am responding in order to clarify it.
First, let me provide some answers to your requests. I am referring to the following scientists that worked methodically in order to provide the answer to the question of “the size of the photon”: "http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html
I have already provided the ANSWER to this question, that is: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* You can find the same ANSWER also in the following lecture: http://galileo.phys.virginia.edu/classes/252/uncertainty_principle.html , meaning that if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum.

Sorry, but what you just said is a bastardization of the uncertainty principle. The HUP NEVER claim such a thing. If I make a measurement of the position of a photon, the MOMENTUM isn't infinite. It is just my ability to predict the value of the momentum I will measure AFTER the measurement of position will have a large POSSIBLE range of value. This is EXACTLY the effect that one sees in a single-slit diffraction experiment. The more precise that I can determine where the particle is, the more uncertain I am what its momentum will be. But this doesn't mean it doesn't have a DEFINITE value of a momentum when I measure it. I strongly suggest you read my entry in my journal on the Misunderstanding the HUP.

So again, where are these people who have made a methodical measurement of the length of a photon?

You may argue, that some “words” that were used in the previous paragraph, coming from reality as we know it in real life dimensions through human experience, like “size”, “space”, “individual” and “isolation” are not the proper words to express the quantum physic’s term of photon. I agree with you on this. But this language can be used by the inquiring mind in making it to realize that under the contradicted enunciation of the answer, there is a more substantial layer of knowledge.

Or in this case, a complete misunderstanding of a physics principle.

Let me disagree with you in the absolute way that you reject the question because it is turned upside down by the answer. When the scientists, mentioned in the above link, were looking for answers like “what is the time?”, “what is the length”, “what is the size of photon”, they were looking for an answer to a specific question and after methodological work they found that the provided answer was creating a new question and then the new answer created another new question and so forth.
I think that the misunderstanding you are making in reading my post is that you presume that when I said that “the question about the size of the photon is a legitimate question which got its answer by the methodical work of many scientists” I refer to a definite calculated answer in analogy to the question. Well, I never said, nor I ever imply, that the provided answer was a definition of position and volume or any other type of time-space related formulation. Neither have I said that the answer is following the analogies of the question.
The question “how big is a photon?” does has an answer, although the answer is not in harmony with the framework that the question is initially placed into. This answer provides an exodus from the framework. I think that you already tried to say that in your posts and you tried to warn about the disaster that awaits those who insist to defend the framework of their question, but you proposed the exodus from the framework of the question, while rejecting the question itself. I think that your proposition is problematic; the inquirer will never abandon his question just being advised to do so. There must be a framework in order to achieve an exodus from it, or else “exodus” is a meaningless move. There must be a certain physical meaning for “big” and for “size” in order to transcend these meanings in photon, or else photon itself is meaningless.

But "big" and "size" are for particles that have a defined boundary in real space. A photon is not a "particle", nor was it ever defined to have a boundary in real space, the same way purple was never defined to have an amount of pain. You can argue all you want about not abandoning such a line of question, but don't expect a rational answer from an irrational question. Again, rather than going through your semantic gymnastics, why don't you show me WHERE exactly is a photon defined with a spatial dimension. Please look at Einstein's photoelectric effect paper and show me where he defined the size of his corpuscular entities.

Finally let me also say that, according to my understanding, your posts are more philosophical compared to my first post – which had none philosophical enouncement. The laws of Gorgias are actually enouncing “The Uncertainty Principle” of Heisenberg, by using the scientific language of ancient age. Greek philosophers were all “scientists”, by the contemporary definition of word. Aristotle is called ‘philosopher’ but he is the founder and the father of Science, as the world have known Science since the scientific work that he composed. The proper place to discuss ancient Greek science is actually a physic’s forum. Unfortunately modern scientists think that science started 200 years ago, or that all “contemporary ideas” are new. Some scientists are surprised to know that “contemporary ideas and theories” were initially expressed by ancient scientists through the peculiarity of the ancient scientific language- quantum physics being no exception to that.
Of course, it depends on how you define philosophy. For me, it seems that a methodology that orders the knowledge to precede questioning belongs to the field of philosophical discussion indeed. And I understand that you proposed such a methodology for quantum physics. Scientific knowledge always follows questioning and it never precedes it - according to my understanding. I live in Greece and I have to follow the ancient Greek tradition on this issue.
For science there be none wrong question; there are only wrong answers.

Then you pay way too much attention to the philosophy aspect than trying to get the physics right. Which is why your answer belongs more in philosophy than in physics. The one citation you gave to back your answer, you had a faulty understanding of. I am not interested, nor do I care, about philosophical ideas that match so-and-so in this thread. If you cannot back what you claim with the physics, and so far you haven't, then this isn't physics. If you think I can write a physics paper and publish it in a physics journal using the way you have written your post, then you haven't seen or read that many physics papers.

Zz.

 

[leandros_p]

Dear ZapperZ,

It seems, according to my peculiar understanding, that you are following a polarized path seeing my mesages in a predetermined way. My phrase "...if you come to know the precise momentum of a photon, then its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum" clearly refers to "a photon" as an individual member of a class and NOT to a specific individual instance of “a photon”. In this context, I do not refer to a single instance of an individual photon, but to a member of a structured class of physical agents that we name "photons", as an individual member of the class.

In your article, which is very interesting and perfectly lucid, you write that “If I shoot the particle one at a time, I still see a distinct, accurate "dot" on the screen to tell me that this is where the particle hits the detector. However, unlike the classical case, my ability to predict where the NEXT one is going to hit becomes worse as I make the slit smaller.” This is exactly the same thing that I have tried to express in my post! (obviously I have failed, since you read somethine else in my post) The uncertainty refers to the physical characteristics of the members of a class, and not to an isolated instance. I understand that an isolated event has no physical meaning. Only members of a physical class can be described by physical terms.

I also agree with your declaration that “physics involves the ability to make a dynamical model that allows us to predict when and where things are going to occur in the future”. In this context, let me also add that physics is not just future telling, but it is a dynamic intellectual structure that classifies physical “agents”, which is confirmed by the applied observation.

Leandros

 

[ZapperZ]

Dear ZapperZ,
It seems, according to my peculiar understanding, that you are following a polarized path seeing my mesages in a predetermined way. My phrase "...if you come to know the precise momentum of a photon, then its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum" clearly refers to "a photon" as an individual member of a class and NOT to a specific individual instance of “a photon”. In this context, I do not refer to a single instance of an individual photon, but to a member of a structured class of physical agents that we name "photons", as an individual member of the class.

If this isn't a semantics gymnastics often practiced in philosophy posts, I don't know what is. Maybe next time, before you post, that you figure out what exactly are the physics defintion of the things you so-freely use before you use them out of context. I mean "structured class of physical agents that we name "photons""?

And we wonder why the Alan Sokal hoax worked so well!

I am still waiting for you to find for me the definition of a photon that involves a physical size AND this methodical work by some physicists that have actually measured the length of a photon.

Oh wait, maybe you don't mean ONE single photon, but rather a whole structured class of photons. This then is irrelevant directly to the original question which ASKED for A PHOTON.

And people wonder why I dislike philosphical discussion of physics by non-physicists!

In your article, which is very interesting and perfectly lucid, you write that “If I shoot the particle one at a time, I still see a distinct, accurate "dot" on the screen to tell me that this is where the particle hits the detector. However, unlike the classical case, my ability to predict where the NEXT one is going to hit becomes worse as I make the slit smaller.” This is exactly the same thing that I have tried to express in my post! (obviously I have failed, since you read somethine else in my post) The uncertainty refers to the physical characteristics of the members of a class, and not to an isolated instance. I understand that an isolated event has no physical meaning. Only members of a physical class can be described by physical terms.

No, you had it wrong. If you agreed with my description of the HUP, then there's no such thing as "length" of a photon, nor a size of any kind. You wouldn't have made that kind of a statement in the first place. Please re-read your previous posts and see for yourself the kinds of claims that you have make and the erroneous application of the HUP that you have used, dispite of what you claimed here.

Zz.

 

[reilly]

Quite a few years ago, Newton and Wigner proved that photons do not have position operators. So the issue of photon size is, at best, a bit tricky. One approach is simply to define a number operator for a finite volume. Not so easy to do, but it is all explained by Mandel and Wolf (Optical Coherence and Quantum Optics) in Chapter 12. One of the more interesting results, at least to me, is that the spatial distribution of a photon's energy goes as r**-7, and can extend into regions in which the "wave function' of the photon is zero. Wild stuff, indeed.

Regards,
Reilly Atkinson

 

[leandros_p]

If this isn't a semantics gymnastics often practiced in philosophy posts, I don't know what is. Maybe next time, before you post, that you figure out what exactly are the physics defintion of the things you so-freely use before you use them out of context. I mean "structured class of physical agents that we name "photons""?
And we wonder why the Alan Sokal hoax worked so well!
I am still waiting for you to find for me the definition of a photon that involves a physical size AND this methodical work by some physicists that have actually measured the length of a photon.
Oh wait, maybe you don't mean ONE single photon, but rather a whole structured class of photons. This then is irrelevant directly to the original question which ASKED for A PHOTON.Zz.

Dear ZapperZ,

There is a difference between the phrases: “a class” and “a member of a class”. I do not use terms out of context. You fail to read the word “member” in my text, which makes all the difference.

As for your remark: “…waiting for you to find for me the definition of a photon that involves a physical size…”, as I have tried to said in every one of my posts, the question is valid, while the provided answer makes the question obsolete. So, you keep asking the same question. I never said that there is a physical “size of a photon”, although I defend the legitimacy of the searching of a “size of photon” from the "ignorant".

I will provide later some questions about your article.

 

[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

 

[ZapperZ]

Dear ZapperZ,
There is a difference between the phrases: “a class” and “a member of a class”. I do not use terms out of context. You fail to read the word “member” in my text, which makes all the difference.
As for your remark: “…waiting for you to find for me the definition of a photon that involves a physical size…”, as I have tried to said in every one of my posts, the question is valid, while the provided answer makes the question obsolete. So, you keep asking the same question. I never said that there is a physical “size of a photon”, although I defend the legitimacy of the searching of a “size of photon” from the "ignorant".
I will provide later some questions about your article.

Let's review, shall we?

The purpose of the science of Physics is to provide answers to any question. It was also indicated to you that ‘asking “how big is a photon" is similar to asking "how painful is purple?” ’. The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists. The scientific quest in finding the answer for the “size of a photon” eventually provided by the mathematical knowledge that: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* Although this answer seems that it does not construct a specific “morphology” for a specific photon (I use the term “specific” with the conventional meaning, although it is not a proper scientific term for photons), it is the linguistic “expression” of a (mathematical) structure provided by science and constructed by scientists, which indeed provides the “morphology of the photon", which is just the answer that was asked by the specific question.

And the specific question here is how BIG is a photon, i.e. the size.

First, let me provide some answers to your requests. I am referring to the following scientists that worked methodically in order to provide the answer to the question of “the size of the photon”: "http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

I have already provided the ANSWER to this question, that is: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* You can find the same ANSWER also in the following lecture: http://galileo.phys.virginia.edu/cla...principle.html , meaning that if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum.

You just said that ITS SIZE WOULD BE INFINITE. Of course, you are referring a "class of photon", whatever that is physically.

And this is where I pointed out that you made a complete mess of the understanding of the HUP. You somehow think that POSITION is equivalent fo SIZE. The size of a single slit defines the uncertain in POSITION of the photon, NOT the size of the photon. But somehow, you read this differently and thought that it was relevant to the question of the SIZE, or else why did you enter this thread armed with that wrong "evidence"?

I'm not going to answer your questions till you answer mine.

Do you STILL claim that "...The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists"? If you do, please provide peer-reviewed citations.

If you can't provide me a definitive answer on that, then do not expect a free physics lessons on here from me.

Zz.

 

[Schrodinger's Dog]

Clarifictaion of your understanding itself and then clarification of what and why it is your asking such a question. Define your reasoning citing why you believ what you believe and considering the points made in counter and then ask a valid question.

Can I have a free physics lesson ZZ?

And that volume you refer to Ben is there a website or link I could go to to find out more, I'd hate to have to pay out a lot of money to find out something that can be summed up in 5 pages of text.

 

[leandros_p]

Let's review, shall we?

And the specific question here is how BIG is a photon, i.e. the size.

You just said that ITS SIZE WOULD BE INFINITE. Of course, you are referring a "class of photon", whatever that is physically.
Zz.

Dear ZapperZ,

I can not understand why you constantly censor my texts, in your answers. By censoring my original phrase you take out of context the following certain part: “its size would be infinite”, and by that you are falsify my message. If you preserve the integrity of my phrase which is: "if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space", then it is obvious that I am talking about the position of the photon, since the phrase "it could be located" means that the uncertainty in momentum means uncertainty in location/position.

Also you use the phrase "…class of photon…", although I used the term "…member of class…" and if you do not understand the difference, at least do not make omissions in transcription”.

One more time, for the last time, I say that I support the legitimacy of asking about the “size of a photon” coming from an inquirer/student of physics, while I support that the answer to this question turns the question upside down making the question “size of photon” to become groundless.

If I am allowed to use your pattern of attitude, I am getting tired of giving free English speaking lessons, considering that English is not my native language.

You were also constantly asking about the scientists that were searching for the “size of photon”. Please see the following article from the website of American Institute of Physics: ( http://www.aip.org/history/heisenberg/p07c.htm ) that cites a dialogue between Heisenberg and Einstein:

-Heisenberg: "We cannot observe electron orbits inside the atom...Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."

--Einstein protested: "But you don't seriously believe, that none but observable magnitudes must go into a physical theory?"

--Heisenberg asked in some surprise…: "Isn't that precisely what you have done with relativity?"

-Einstein : "Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same...In reality the very opposite happens. It is the theory which decides what we can observe."

Was Einstein’s question a meaningless question, just like the question about the “size of photon”? Einstein and Heisenberg were not talking about the size of photon, but Einstein’s question and reasoning is just the same meaningless in quantum physic's world as the question about "the size of photon". Was Einstein a fool, who was just babbling ? Surely the answers, provided by quantum physics, to Einstein’s question turn his question and his reasoning upside down, but he had the right to ask this question!

 

[Schrodinger's Dog]

You know you're right, you do make a different point from that portrayed. There are no stupid questions in science only stupid answers

 

[ZapperZ]

Dear ZapperZ,
I can not understand why you constantly censor my texts, in your answers. By censoring my original phrase you take out of context only the following certain part: “its size would be infinite”, and by that you are falsify my message. If you preserve the integrity of my phrase which is: "if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space", then it is obvious that I am talking about the position of the photon, since the phrase "it could be located" means that the uncertainty in momentum means uncertainty in location/position.

And you ignored the point that I made that a LOCATION of an object does NOT signify the SIZE of the object. If you have understood this, why in the world are you entering a discussion on the SIZE of a photon, when the position of the photon give ZERO CLUE on its size?

You have entered this thread under the WRONG premise by confusing the HUP as an indication of the SIZE of a photon. And I QUOTED your entire paragraph before I highlighted that one phrase, so I did NOT take it out of context. To indicate that the size of a photon is INFINITE, whether one has made a measurement of its momentum or not is INCORRECT no matter the circumstances of the measurement! The size of a photon is NEVER INFINITE, especially when there is no such thing as a "size" of a photon. Your application of the HUP to this situation is what I called the bastardization of the HUP.

Also you use the phrase "…class of photon…", although I used the term "…member of class…" and if you do not understand the difference, at least make do not make omission in transcription”

And if you do not know enough physics, do not make a faulty application of it.

One more time, the last one, I say that I support the legitimacy of asking about the “size of a photon” coming from an inquirer/student of physics, while I support that the answer to this question turns the question upside down making the question “size of photon” to become groundless.

And how is this different than my original reply to indicate that asking for the size of a photon is the same as asking for how painful is purple? No where in here did I said that such a question SHOULDN'T be asked! I was illustrating WHY such a question is meaningless. So what is the problem with that? And for your benefit, I wasn't the one who made the suggestion to go look up QED, even though it is a valid suggestion.

If I am allowed to use your pattern of attitude, I am getting tired of giving free English speaking lessons, considering that English is not my native language.

That makes two of us. I am getting tired of having to dig up BASIC physics and pointing out how people simply use physics principles in ways it shouldn't be used and then have the gall to use it against me.

You also constantly were asking about the scientists that were searching for the “size of photon”. Please see the following article from the website of American Institute of Physics: http://www.aip.org/history/heisenberg/p07c.htm that cites a dialogue between Heisenberg and Einstein.
-Heisenberg: "We cannot observe electron orbits inside the atom...Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."
-"But you don't seriously believe," Einstein protested, "that none but observable magnitudes must go into a physical theory?"
-"Isn't that precisely what you have done with relativity?" I asked in some surprise…
-"Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same...In reality the very opposite happens. It is the theory which decides what we can observe."
Was Einstein’s question a meaningless question, just like the question about the “size of photon”? Einstein and Heisenberg were not talking about the size of photon, but Einstein’s question and reasoning is just the same meaningless in quantum physic's world as the question about "the size of photon". Was Einstein a fool, who was just babbling ? Surely the answers that quantum physics provides to Einstein’s question turn his question and his reasoning upside down, but he had the right to ask this question!

1. Point out to me where I prohibited the asking of such a question. It appears that it is YOU who misread what I wrote.

2. Are you implying that you cannot tell the difference in the situation here, and the situation you are quoting? HONESTLY?

3. An electron has the PRECEDENT to be considered as a classical particle. Classical E&M treated it that way. It was initially DEFINED to be that way. It is a legitimate question to ask "well, why I can't I treat this the same way in an atom?" And such questions HAVE been asked already MANY TIMES in PF. Go to the Atomic physics forum if you don't believe me. Did you see me responding with "how painful is purple?"? On the other hand, a photon WASN'T defined EVER as a classical particle. It isn't even a "particle" (Einstein called it corpuscular!) other than the fact that experimental observations were indicating it to have similar qualities - that is when the description of "particle" stuck! But nowhere in its definition is there a SIZE! THIS was my original response that you seem to have a problem with, and you took that and jump to the conclusion that this isn't a question that should be asked!

If that is how you understood things, no wonder you could make such horrible misinterpretation of the HUP.

Zz.

P.S. Where, again, is this methodical study of the size of the photon?

 

[Dense]

What a fun thread! Who knew that the humble photon could cause such vitriolic arguments, such ire, such conflict!

I wonder if other threads on this site are as entertaining.




PS -- this is my first post, so hello all! This place is so much better than other sites I've been to.

 

[Boffin]

And that volume you refer to Ben is there a website or link I could go to to find out more, I'd hate to have to pay out a lot of money to find out something that can be summed up in 5 pages of text.

Are you referring to my quotes from Brian Greene's book The Fabric of the Cosmos? Actually the quotes I used are really about all there is in the book on Quantum Mechanics that's relevant to this discussion. You can't read books like this on the Internet, but Google allows you to read pages in a book search. But while Brian Greene's book The Elegant Universe appears in a Google book search, The Fabric of the Cosmos does not, at least not yet.

 

[leandros_p]

I'm interested in various views on this issue. How far away from a hypothetical center point does the energy of a photon extent? Photons do travel at the speed of light in a vacuum. At an instant of time how far foreward and backwards does a photon interact in some way with the environment around it? Over a brief period of time how far to each side? Is it vibrating or is it a fixed point particle? If it is vibrating, are the vibrations spread out in space or zero width? One or more vibrations? If it is vibrating is it vibrating in reference to a background entity?

Dear Ben Wiens,

You can find an interesting analysis about the “size of photon and what does it look” in the pages of the book:

Physics, the Human Adventure: From Copernicus to Einstein and Beyond,
by http://www.fas.harvard.edu/~hsdept/faculty/holton/index.html"

Interestingly, page 401 of the book is web-published by Google under license at http://books.google.com/books?ie=UT...wave+dilemma&sig=7AAK90sPQDyTlbbBak84oHuwTkg" (just register to google and you will be given access to the page)

In this page, the authors of the book ask questions similar to your questions, and they present in the same page the framework that might help you understand the answers.

First, the questions: “…Indeed, the inconvenience of having to deal with photons is very real. They represent bundles of energy without having ordinary rest mass; in this they differ from the Newtonian corpuscles of light, leaving only a faint analogy between them, even through it is customary to refer to Einstein’s photon theory as the corpuscular or particulate theory of light. But our minds tend to insist on a good picture, and it requires great self control to visualize a quantum of energy without bringing in some matter to which it can be attached. It was a little easier to think of Maxwell’s light energy spread evenly through the “field” along a wave front.

Then, too, there are other questions: How large in volume and cross sectional area is the “spot” on the wave front where the photon is located? What could be the meaning of “wave-length” and “frequency” of light, which determine the energy content of the photon by E=hv, if the photon is, so to speak, only a dot on the wave front and not part of a whole wave train?

By what mechanism does the wave determine the path of the photon in such a wave phenomena as interference and polarization? How does an electron absorb a photon?...”

Then, the analysis: “In the past, one partial answer to such questions has been to hold concurrently two separate views on light – the wave and the photon model- and to apply one or the other as required by the problem. The more recent answers, which allows, in principle, a solution of every problem with physical meaning in this field, involves combining both views and assuming that the photons are distributed over the wave front in a statistical way, that is, non individually localized at a particular point. But at our level the more practical solution is, first of all, to realize that some of these questions, while possibly very disturbing, are asked on the basis of a mechanical view of atomic phenomena that may stem from an erroneous transfer of experience with large bodies obeying simple Newtonian laws. Thus the “size” of the photon is not a concept that we should expect to have the same meaning as the size of marbles and projectiles. Furthermore, photons (and atomic particles as well) differ from water waves, pebbles, and other large-scale entities in that one cannot make various experiments on the same subatomic entity. One can localize and measure and weight a stone, find its velocity, etc. and all the while it is the same unchanged stone. But a photon, after it has been sent into a Geiger counter or a photographic emulsion, is no more; two photons on which we impress different experimental conditions – for example, in searching for wave and for corpuscular properties – are not, strickly speaking, the same entities….”

Max Born’s remark at the end of the page continues to the next page, and it should be: “The ultimate origin lies in the fact (or philosophical principle) that we are compelled to use the words of common language when we wish to describe a phenomenon, not by logical or mathematical analysis, but by a picture appealing to our imagination. Common language has grown by everyday experience and can never surpass these limits. Classical physics has restricted itself to the use of concepts of this kind; by analyzing visible motions it has developed two ways of representing them by elementary processes: moving particle and waves. There is no other way of giving a pictorial description of motions – we have to apply it even in the region of atomic processes, where classical physics breaks down.

Every process can be interpreted either in terms of corpuscles or in terms of waves, but, on the other hand, it is beyond our power to produce proof that it is actually corpuscles or waves with which we are dealing, for we cannot simultaneously determine all the other properties which are distinctive of a corpuscle or of a wave, as the case may be. We can therefore say that the wave and the corpuscular description are only to be regarded as complementary ways of viewing one and the same objective process, a process which only in definite limiting cases admits of complete pictorial interpretation”

Max Born, 1946

Leandros

 

[Boffin]

Interestingly, page 401 of the book is web-published by Google under license.

Thanks, sometimes a person gets lucky.

 

[Boffin]

What theory does this thinking belong to?

There have been lot's of theories about the photon over the ages, Descartes ether theory, Newtons particle theory, Young's wave theory, Planck's quantum theory, Quantum Mechanics, Quantum Electrodynamics, Quantum Field Theory, String Theory, The Standard Model and other's inbetween. With all the theories that have been proposed over the ages, it's hard to get a handle on exactly what ideas belong to what theory, and how much of that theory is still retained in new theories. How much of this thinking below is still retained by Quantum Mechanics or Quantum Electrodynamics? How much of this is still valid?

Excerpt from http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c5
According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite "bundles" of energy which we call photons. The quantum of energy for a photon is not Planck's constant h itself, but the product of h and the frequency. The quantization implies that a photon of blue light of given frequency or wavelength will always have the same size quantum of energy. For example, a photon of blue light of wavelength 450 nm will always have 2.76 eV of energy. It occurs in quantized chunks of 2.76 eV, and you can't have half a photon of blue light - it always occurs in precisely the same sized energy chunks. But the frequency available is continuous and has no upper or lower bound, so there is no finite lower limit or upper limit on the possible energy of a photon. On the upper side, there are practical limits because you have limited mechanisms for creating really high energy photons. Low energy photons abound, but when you get below radio frequencies, the photon energies are so tiny compared to room temperature thermal energy that you really never see them as distinct quantized entities - they are swamped in the background. Another way to say it is that in the low frequency limits, things just blend in with the classical treatment of things and a quantum treatment is not necessary.