Lightarrow – In what follows, please do not confuse “frustration” with “irritation.” While I am frustrated by what appears to be being ignored in the examples I’ve posted or that you’ve only posted assertions without proofs, I’m not irritated in the sense that I’m angry at you. I mention this because it seems that some people in physics forums read certain emotions or intentions into posts that aren’t really there. They seem to be based on what the reader assumes about the person who’s doing the posting. Okay?

It's very simple, and I let you do it: define "photon".

I gotta tell ya, lightarrow, I'm quite baffled as to why you’ve been ignoring the examples I've posted for you. I'm not posting them for my health ya know.

They are meant as counter arguments to the assertions that you've been sticking to. If you ignore them then I see no reason to discuss the physics anymore.

You also seem to be ignoring the fact when I explained that when photons are used in classical derivations its always an approximation to real life, i.e. quantum effects or precision is either ignored or not important.. Such approximation are not unique to photons or the examples I've posted but apply to ever classical treatment in physics that there exists since all real life particles have real life uncertainties in their locations. Why you've singled out photons among other particles such as electrons and protons and why you've been ignoring the usage of classical trajectories such as those in particle physics like tacks in bubble chambers of elementary particles is something I don’t understand.

Anyway … asking for the definition of the term "photon' is not a proof that they can’t be used in the manner which you say they can’t. When photons were first discovered by Einstein and Planck there was no uncertainty principle in existence and there was no need for such even though the particle nature of photons has been used long before the term "photon" was invented. Einstein and Planck came up with the concept of "bundle of energy" long after the first thoughts of "particles of light" were first being thought of and before they knew of the exact nature of them.

In any case the definition of Photon is - a discrete bundle (aka "quantum") of electromagnetic radiation where its kinetic energy E is related to its momentum p by E = pc. Einstein called these discrete bundles "light quanta". When the term “photon” was actually first defined by Gilbert Lewis they didn't have the same properties that we now assign to them.

Here the term "bundle" does not mean that they have a finite extension in space that one might visualize when one hears the term "bundle." The term “bundle” means "finite amount," nothing more and nothing less.

Also I’ve used the term “classical photon” or referred to the use of photons in a classical setting as approximations to the more exact expressions given in quantum mechanics. In your responses its like you’re not reading the parts of my posts which clarifies this point. When the wave aspect of the photon is under consideration or more precise measurements are being considered then the classical approximation of the photon is no longer being used.

I've already given you many examples of how and where photons are used in derivations using the conservation of center-of-mass. In particular, they are used in the article

**Inertia of energy and the liberated photon ** by Adel F. Antippa, Am. J. Phys. 44(9), September 1976

A similar derivation is given in

**Special Relativity** by A.P. French on pages 17 and 27.

In each version of the derivation the correct results are obtained.

I'd like to remind you that it was you who first claimed that the center of energy of a system of photons couldn’t be defined because you claimed that photons couldn’t be localized. In that response you had your mind fixed on quantum mechanics and were ignoring the sense in which that person was using the concept of photons. To be 100% accurate, nothing in the universe can be localized to a mathematical point. What you failed to mention was the fact that any particle can be localized to a finite region of space. If that weren’t true then nobody would have ever referred to anything in physics, quantum or classical, as a “particle.” In fact even in quantum mechanics photons can be localized to an arbitrarily small region of space. One just has to give up some uncertainty in the knowledge of the photons momentum. If we were to take your response literally then you would have said that the center of mass of electrons, protons, neutrons etc. couldn’t be defined because no elementary particle can be localized in space. I’m rather baffled at where you got the notion that photons can’t be localized. No treatment of quantum mechanics would ever make such an assertion. Perhaps you made the mistake of thinking that “localized” meant “located at a mathematical point.” If so then no elementary particle can be localized. But that’s not what is meant by “localized” in quantum mechanics.

I think this point needs to be clarified more (Let’s use the double slit experiment and for purposes of illustration we’ll only look at the x-component). When one is considering experiments like Young’s double slit experiment then what is observed under low enough intensity of the light source are photons hitting the screen at finite locations. That means that if the screen were made of a huge amount of very small pixels (let’s say 0.001 mm) then only one pixel at a time is illuminated at one time. The location of that pixel is then recorded. After a large number of photons hit the array of pixel what you’ll have a bunch of numbers that denote locations where the photons were localized to a region 0.001 mm long. If you were to plot the number of photons at a particular x-location verses the x-coordinate what you’ll have is a graph which has the general shape of a bell curve. Let the width of that curve be dx. This is the dx that the uncertainty principle uses, not the size of a pixel. That is precisely what it means to localize a photon and precisely why photons are said to behave like particles in this experiment.

In case you ignore my previous posts and explanations (since you failed to explain why you’re claiming that all these physicists who use these explanations got them all wrong) I’ll have to repeat myself. I showed you how physicists are and have used photons in a classical context. You didn’t acknowledge the derivations that I’ve shown you. They are at -

http://home.comcast.net/~peter.m.brown/sr/einsteins_box.htmGo down to below Eq. (

and you'll see how they are used in the classical sense, i.e. by using them in the center of mass theorem. This is how Antippa and French used them.

Other uses are when physicists draw spacetime diagrams and show the worldliness of photons. Such worldliness are classical approximations to what photons actually do on a small scale. When such diagrams are drawn the worldliness used as example are often of the order of a meters. The uncertainty of the photon’s location along such paths is negligible when drawing such diagrams. Thus the width of the worldline is approximated to be zero rather than the real life finite width one would get in experimental data. When one draws a classical trajectory like this for a particle then, in all cases, one is using an approximation. Just like when the path of an electron is drawn as a circle in a cyclotron. That path is also an approximation to real life.

From now on I’ll have to wait until you give a proof and a valid reason why the above approximations cannot be used. Until then I won’t respond to any other post of yours in this thread since all you’ve been doing is making a claim that they can’t be used with no acknowledgement that (1) I’ve been referring to approximations and (2) that they are indeed used by physicist and appropriated so and without error in their derivations.

One last comment before I end this post: To drive this point home let me remind you that what I’ve been, i.e. what I’ve labeled a “classical photon” which you seem to keep mistaking for a “quantum photon” which has a wavelength L related to its momentum p by p = h/L and whose position and momentum satisfy the uncertainty relationship. Now recall what a “classical electron” is. It’s a particle with charge e = 1.60x10^(-19) C, a proper mass of 9.11x10^(-31)kg which moves on a well-defined trajectory. This is different than a “quantum electron” which has a wavelength L related to its momentum p by p = h/L and whose position and momentum satisfy the uncertainty relationship.

With this in mind recall the definition of "classical photon" a quantum of radiation whose kinetic energy is related to its momentum by E = pc which moves on a well-defined trajectory. From this definition it follows that the proper mass of a photon is zero.

Do you understand what I meant by "classical photon" now?