Talk:Photon/Archive 5

This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

The article is missing a section on photon origins, i.e., how they are created.--LeyteWolfer (talk) 11:17, 13 March 2019 (UTC)

You are not allowed to ask that question. Photons are quantum particles, obeying the rules of quantum mechanics, and QM restricts which questions you can ask. But OK, photons are created (and destroyed) by accelerating charges, except when they aren't. You might look at interpretations of quantum mechanics to start understanding which questions you are allowed to ask. Gah4 (talk) 15:02, 13 March 2019 (UTC)

Actually, I think that how photons are created is a great question, and he's allowed to ask. Within the QM paradigm, there's not really an answer, is your point, yes? Still, a useful question, and answerable in some paradigms. Dicklyon (talk) 05:20, 5 May 2019 (UTC)

The article on photon ignores several issues such as:
The fact that photons as a straight line is bent by the gravity of the Sun as Mercury shines past it, sending the ray of photons.
How is it that the photon disappears and the electron appears instead in the case of photovoltaics? The case isn't so clearcut as the article seems to say.
How is it that photons are emitted as the stars burn up and there is a conversion of matter into photons by E=mc(2) and the photons are supposed to be massless still? Surely a paradox with matter disappearing in "thin air"!
Cheers! 109.189.66.223 (talk) 18:40, 10 August 2017 (UTC)

These "issues" are covered in General Relativity, photoelectric effect and invariant mass. — dukwon (talk) (contribs) 11:47, 11 August 2017 (UTC)

The point is that they have zero rest mass. They still have an energy mass m = E/c^2. Dicklyon (talk) 22:21, 16 April 2018 (UTC)

@Dicklyon: as DVdm would say, photons don't have a rest frame, so no rest mass. A hypothetical thing having zero mass in its rest frame has zero energy as well. Does it like a photon? Mobilize your brains to get why doesn’t “rest mass” cover everything having a well-defined “invariant mass”. Incnis Mrsi (talk) 20:41, 3 May 2019 (UTC)

Sure, brain mobilized. Photon has 0 rest mass and 0 energy, in the limit, in system moving fast in the direction of the photon's travel. In any other inertial frame, it has positive energy, and a corresponding energy mass. What's the confusion?

— Preceding unsigned comment added by Dicklyon (talk • contribs) 05:15, 5 May 2019 (UTC)

An interesting argument, but it should be formulated “a photon’s energy mass vanishes in the limit of reference system moving in the direction of the photon’s travel”. Whereas in any(!) inertial frame it has positive energy. You can obtain such limit for the photon’s energy, but such sequence(family) of Lorentz transformations does not converge to any non-degenerate linear operator, hence no frame in the limit. A decent mathematical imagination in required to (re)formulate your argument avoiding wrong assumptions or corollaries. Incnis Mrsi (talk) 12:22, 5 May 2019 (UTC)

This can be defined: https://arxiv.org/pdf/quant-ph/0508202.pdf Biggerj1 (talk) 15:17, 4 June 2019 (UTC)

If this page is reached as a result of a redirect (and not a 'common spelling correction', etc.) then shouldn't a brief explanation involving that word appear upon the page ?

I am not a Nuclear Physicist so I can not comment upon the correctness of the Term but would it make sense for there to be a short statement indicating that the usage of the word "Antiphoton" is incorrect and that it is the SAME as a "Photon" (since that is what the redirect suggests, but whether this is a Fact is somewhat less clear).

Can we have some Expert write about the Antiphoton and to explain that it is the same as a Photon OR to explain how it is different (in the usual Wikipedia most informative manner). ;)

96.48.222.120 (talk) 16:35, 21 March 2016 (UTC)

I second that --93.231.233.116 (talk) 17:39, 29 July 2017 (UTC)

It's a misconception. To have anti-particles of the force carriers posits anti-forces, which is (currently) incoherent (as opposed to say charge or other polarized concept). Also antiparticle states that the photon is its own antiparticle, a way of saying the same thing (no anti-photon as such or maybe an ordinary one moving back in time). 98.4.124.117 (talk) 07:05, 29 July 2018 (UTC)

On the OP's point, the section Photon § Physical properties does contain the statement "Seen another way, the photon can be considered as its own antiparticle." It might be helpful if this was made more apparent to someone following that redirect (it might be a bit much to expect the reader to search for "antiparticle"). On the other hand, the term "own antiparticle" is what would generally be used. —Quondum 11:23, 29 July 2018 (UTC)

Acknowledged, didn see that § when comment made. 98.4.124.117 (talk) 15:22, 29 July 2018 (UTC)

Today I was bitten by this as well. I've just changed:

Seen another way, the photon can be considered as its own antiparticle.

to:

{{anchor |antiphoton}}Seen another way, the photon can be considered as its own antiparticle (thus an "antiphoton" is simply a normal photon).

and changed antiphoton to redirect to the new anchor rather than doing an unhelpful bare redirect to photon. --Dan Harkless (talk) 14:22, 26 June 2019 (UTC)

Hmm. To be honest, it's a little confusing to click on a link and be dumped three-quarters of the way through a long section, then have to dig around for why. From the point of view of the least surprise principle, I'm not quite sure whether it's better or worse than arriving unexplained at the top of the article, but certainly neither one is great.

Maybe there's enough to say to make a little subsection on antiphotons and why they're the same as photons. If there isn't, then maybe it's best to delete the redirect. --Trovatore (talk) 00:12, 27 June 2019 (UTC)

Yeah, I wasn't super-happy with putting the anchor in the middle of a paragraph, but it didn't seem to me that the "Seen another way..." sentence through the end of the paragraph made for a natural paragraphal unit. But preventing confusion by making the redirect land on the beginning of a paragraph is probably worth that non-ideal paragraphization, so I've just added a paragraph break at that point. I'm not the right person to expand on antiphotons, so hopefully this measure will be acceptable as the lesser evil (particularly in comparison to removing the redirect altogether) for the time being. --Dan Harkless (talk) 12:50, 27 June 2019 (UTC)

Today in the W and Z bosons article, I came across the phrase "is its own antiparticle" wikilinked to truly neutral particle. I've followed suit here, changing "the photon can be considered as its own antiparticle" to "...as its own antiparticle". That article is only a stub right now, but there's enough detail that people wanting to know more about why there isn't a separate "antiphoton" can link over there for more info on the general concept, which may eliminate the need for additional discussion of antiphotons in this article to further justify the antiphoton redirect. --Dan Harkless (talk) 17:22, 3 July 2019 (UTC)

Looks like it should actually be ≥hbar/2 — Preceding unsigned comment added by 128.223.131.219 (talk) 05:59, 20 April 2013 (UTC)

In most cases, if you have to ask that question, you are doing something wrong. The uncertainty principle isn't certain. With a Gaussian wave packet, and its Gaussian Fourier transform, you can appropriately define the widths, and then from that the uncertainty. But then it depends on what you use for a width. FWHM, HWHM, etc. Gah4 (talk) 21:05, 12 July 2019 (UTC)

@Dan Harkless: If we add that then we can equally add every other application of light. A light bulb! It uses photons to illuminate a room. This can't be the right approach. All the other applications are cases where individual photons matter - things that would not work if light couldn't be quantized. These fit in here. Solar sails do not, and an obscure concept that has never been used in practice is WP:UNDUE in addition. --mfb (talk) 21:10, 16 July 2019 (UTC)

Adding to that point, Dan's edit summary pointed out that "the propulsion is from the momentum of the photons". But if I'm not mistaken, classical (that is, non-quantum; might be relativistic if necessary) electrodynamics also predicts that EM waves carry momentum, with no need to invoke photons. Which I think is mfb's point, not stated explicitly in the above. --Trovatore (talk) 21:34, 16 July 2019 (UTC)

Correct. It was known that electromagnetic waves have momentum even before the concept of photons was around. --mfb (talk) 23:01, 16 July 2019 (UTC)

Okay, I can see your points on that, though for a physics non-SME, at least, it's quite a striking visualization, to see that the mere "pressure of photons beating against a surface" can move an object that quickly. It's also conceptually nice how the reflecting and reusing of the beam, not done with solar sails, intuitively translates to more photons hitting the object, and thus faster movement. Presumably the momentum transfer could in theory be quantized down to the individual photon level.

I think the graphic is the sort of thing that can get general readers excited about a science topic, while still being relevant enough to the topic to be considered encyclopedic. But if everyone else feels strongly that it should be removed, I won't revert the change again. --Dan Harkless (talk) 00:27, 17 July 2019 (UTC)

I'm not necessarily utterly opposed to some mention. It's more intuitive to think of this force as mediated by photons bouncing off of something than as the action of continuous waves, notwithstanding that the two descriptions are both valid, and as you say there's potential for some eye candy. But it should probably be clarified that it can also be thought of as wave action, and the text definitely needs copyediting. --Trovatore (talk) 04:08, 17 July 2019 (UTC)

It might look nice, but that doesn't make it on-topic. I'm serious with the light bulb. I feel strongly that solar sails and an obscure concept without an application don't belong here because they don't rely on the existence of photons. But if people think they belong here then all other applications of light have an equal right to be in the article - or even more, because they are more widely used. And that won't fit. --mfb (talk) 22:04, 17 July 2019 (UTC)

The article says all bosons obey Bose–Einstein statistics (whereas all fermions obey Fermi–Dirac statistics. This seems backwards. Particles that obey Fermi-Dirac statistics are called Fermions, and ones that obey Bose-Einstein statistics are called Bosons, because they obey those statistics. Gah4 (talk) 21:04, 29 August 2019 (UTC)

The Photons in Matter section seems a little confusing. The first sentence mentions transparent materials, while the second uses the sun as an example. It seems to me that the sun is not a good example of a transparent material. Also, collision is confusing in describing photons. They get absorbed and emitted in a variety of interactions, though that can include emitting a photon of the same frequency in a different direction, which might be called elastic scattering. Otherwise, photons traveling through transparent, or close to transparent, materials cause photons to be absorbed and emitted such that the result looks like a lower speed. The article tries to say this, but not so well. Gah4 (talk) 21:12, 29 August 2019 (UTC)

"Otherwise, photons traveling through transparent, or close to transparent, materials cause photons to be absorbed and emitted such that the result looks like a lower speed." - no it doesn't. That is a very misleading approach to make an analogy. I moved the sentence about the Sun to a separate paragraph. --mfb (talk) 03:21, 31 August 2019 (UTC)

Agree with the two editors above. Early authors brought confusion between transparency (where photons are essentially quasiparticles) and translucency. The problem of conceptual description of transparency also has a remove relation to talk:Elementary_particle #Scarce_on_conceptual_properties. Incnis Mrsi (talk) 04:52, 31 August 2019 (UTC)

I might not say much more about it, but my favorite description is the one from Feynman. Gah4 (talk) 06:03, 31 August 2019 (UTC)

There seems to be much editing regarding Planck. As well as I know, and currently reading Quantum,^[1] Planck did not believe in quantization of the EM field, or the (not named at the time) photon. He believe that it was a mathematical trick that gave the right result, maybe that emission was quantized, but not that the EM field was quantized. For a long time, it was mostly Einstein arguing for actual quantization and photons. Gah4 (talk) 00:54, 21 October 2019 (UTC)

• I agree with you but my edit was reverted (with a rather aggressive edit summary). Graham Beards (talk) 08:44, 21 October 2019 (UTC)

@Graham Beards: - I sincerely apologize. I have no problem with the current text and shouldn't have reacted like that. Interferometrist (talk) 17:41, 21 October 2019 (UTC)

(@Interferometrist: ... and I should have explained my actions more clearly. Best wishes, Graham Beards (talk) 18:53, 22 October 2019 (UTC))

Much of science as taught in school isn't historically accurate. It is taught in the way that makes it easier to learn, which most of the time is the reason for learning it in schools. I think I have always known that Planck didn't initially believe in quantized light, but hadn't thought that he might not yet believe in it 23 years later. The book also has details of the disagreements between Heisenberg and Schrodinger on matrix and wave mechanics that aren't usually covered in textbooks. Continuing, it was Einstein's belief in quantization that led to his paper on the photoelectric effect, not the other way around (as we might believe from many books). Gah4 (talk) 21:11, 21 October 2019 (UTC)

OK, I now undid all three in reverse order. I now have the above linked book, but I suspect many others explain this. Looking back now, it is hard to explain how people understood it at the time. From the above book: Planck's oscillators forced him to slice and dice radiation energy so as to feed them the correct "bite-sized" chuncks of hv. He did not believe that the energy of radiation was really chopped up into quanta. It was just the way his oscillators could receive and emit energy. Gah4 (talk) 11:37, 21 October 2019 (UTC)

As the book explains, and I believe is well explained in many physics textbooks, it was Newton who first advocated for particles of light. Even though it was hard to go against the master, Young advocated for waves, and wrote a book about it. He sold only one copy, to Fresnel. Maybe I don't need to explain Fresnel here. Gah4 (talk) 11:53, 21 October 2019 (UTC)

Planck came up with his blackbody curve, and his constant, in 1900. Even by 1909, it was only Einstein and Stark who believe in actual quantized EM radiation. Many (and maybe not Planck) believed emission/absorption was quantized, but not EM field itself. Even by 1922, when Einstein received his 1921 Nobel prize, it was for the law of the photoelectric effect, the formula, but still not that light was quantized. Again, only emission and absorption. So, 22 years after Planck's constant, and still no quantized photons. Gah4 (talk) 12:14, 21 October 2019 (UTC)

Photons have a non-zero mass. Divide their momenta by their respective speeds. You obtain the mass. Photons have relativistic mass. So, please correct the article. Somebody400 (talk) 19:34, 18 January 2020 (UTC)

Yes photons have relativistic mass, but they don't have (rest) mass which is what always goes in the "mass" section of infoboxes. That is, for photons ${\displaystyle E^{2}-p^{2}c^{2}}$ is zero. Yes it is strange that they just call it mass instead of rest mass, but that is tradition by now. Gah4 (talk) 22:03, 18 January 2020 (UTC)

The concept of relativistic mass is of course as good as obsolete. The multiple accounts abusing Somebody probably has the equation ${\displaystyle p=mv}$ in mind, but the relativistic momentum is given by

${\displaystyle p={mv \over {\sqrt {1-{\frac {v^{2}}{c^{2}}}}}},}$

which is actually valid for sub-light-speed particles only. Indeed, when attempting to apply it to anything with speed v=c, the denominator is zero, and the only way to end up with a finite momentum, is to have a zero numerator too, in other words m=0, aka zero mass. Pretty elementary and compelling plausibility argument. So for light we just have ${\displaystyle p=h\nu /c.}$ It's all in the article, and wp:TPG I don't think it should be discussed here, so this discussion should probably be closed. - DVdm (talk) 23:10, 18 January 2020 (UTC)

This article obtained FA over thirteen years ago. Since then the standards for citations in Featured Articles have improved. The article is now found wanting in this regard. There are substantial sections that are not supported by citations and dated citations for others. Can this be addressed to avoid nominating the article at WP:FAR?Graham Beards (talk) 09:51, 21 February 2020 (UTC)

I have nominated Photon for a featured article review here. Please join the discussion on whether this article meets featured article criteria. Articles are typically reviewed for two weeks. If substantial concerns are not addressed during the review period, the article will be moved to the Featured Article Removal Candidates list for a further period, where editors may declare "Keep" or "Delist" the article's featured status. The instructions for the review process are here. Graham Beards (talk) 09:43, 12 March 2020 (UTC)

You linked to the older review. here is the current one. --mfb (talk) 03:49, 13 March 2020 (UTC)

The article says: it can behave as a particle with definite and finite measurable position or momentum. This seems strange to me for a (rest)massless particle. A photon has a definite position when it is absorbed by something, but then it doesn't exist anymore, so it has no position and no momentum. A single photon, as well as I know it, is a Gaussian wave packet with appropriate, Heisenberg satisfying, spatial and momentum distributions. In the usual cases that I think of, it is multiple photons in the same state that have more definite position or momentum. It needs to be explained satisfying quantum mechanics more than I think I can do now, though. Gah4 (talk) 23:48, 23 March 2020 (UTC)

The introduction says: "it always moves at the speed of light in a vacuum."

I wasn't sure if that meant

• it moves at c (the speed of light in vacuum) while it is in a vacuum
• it always moves at c, even in matter, where it's slower, but phycicists found a way how it can always move at c even while being slower than c.
• It always moves at c, but, oops, we forgot/neglected it's slower in matter

I almost tried to clarify that by changing it to "it always moves at 299,792,458 m/s, the speed of light in a vacuum." but that did't fully resolve the issue(s).

How about "it always moves at 299,792,458 m/s, the speed of light in vacuum, even while moving through matter." ? That would be news for many people, requiring explanation, possibly later in the article or in another article, perhaps in speed of light.

Also, "photons, being light, move at the speed of light" ... reminds me of "Chlorine-releasing compounds [...] are a family of chemicals that release chlorine." Darsie42 (talk) 11:31, 23 March 2020 (UTC)

There are two ways to explain this, which are usually called the microscopic and macroscopic Maxwell's equations. In the macroscopic version, light (photons) moves slower in materials (usually dielectrics). In the microscopic version, light moving through materials (again, usually dielectrics) causes their electrons to vibrate, emitting new photons. The result, then, looks like light moving at a slower velocity. There is an explanation of this in Feynman (I believe) volume II. Most books try to avoid the discussion, as the math is complicated, and still comes up with the easy answer. It gets especially interesting when you have materials that, at a specific frequency (wavelength) have an index of refraction less than one, or sometimes even less than zero. You have to explain very carefully in those cases. Gah4 (talk) 17:28, 23 March 2020 (UTC)

Ok. Any idea how to improve the sentence in the introduction? Darsie42 (talk) 00:49, 25 March 2020 (UTC)

To do it right takes more quantum electrodynamics than I can explain, and pretty likely too much for the introduction of the article. Electrons are easy to think of as particles, which move around from forces, but, for reasonable energies, are not created or destroyed. But photons are easily created and destroyed. From QED, two electrons sitting near each other experience a force due to virtual photons going back and forth. Given all that, I believe that as written it is about as good as can be. Gah4 (talk) 05:21, 25 March 2020 (UTC)

OK, in a vacuum it moves at the speed of light in a vacuum, like the chlorine releasing compounds above. Should we link to special relativity or mention that it inertial reference frame invariant? Gah4 (talk) 15:45, 25 March 2020 (UTC)

The current version ("in vacuum it always moves at 299,792,458 m/s, the speed of light in vacuum") looks okay, just a bit redundant. Suggestion: "The speed of light in vacuum is always 299,792,458 m/s". --mfb (talk) 06:40, 26 March 2020 (UTC)

Well it is, but the number is, by definition, the speed of light in vacuum. In most cases, it is the speed of a collection of many photons, not just a single photon, the subject of the article. Less obvious, as we usually mean it, it is the group velocity of the wave packet that is a photon. Also left out is that this applies to all photons, that is, electromagnetic waves, of any frequency. But also, people are so used to knowing the difference between particle and wave, that it is not so easy to explain when it is both. Gah4 (talk) 09:36, 26 March 2020 (UTC)

"The speed of photons in vacuum is always the speed of light in vacuum, 299,792,458 m/s"? --mfb (talk) 11:46, 26 March 2020 (UTC)

I would like to give the speed of one photon, not of a group of photons. The speed of one photon is always the speed of light. For a not very good analogy, consider that the speed of air molecules is about 1000 mi/h. (or 1600km/h, depending on what country you are in.) Even so, there might be a slow wind, or no wind. In an almost vacuum, air molecules will travel a long distance at that speed, but at atmospheric pressure, on average, they don't get very far before colliding. The analogy isn't so good, as photons are easily created and destroyed, where air molecules aren't, but the cases of slower light are ones with multiple photons, carefully arranging things to look like they are going slow. Visible light photons are just the right energy (wavelength) to make this interesting. Gamma rays have frequency high enough that electrons pretty much can't follow it, so are mostly not bothered by them. Radio waves, say AM radio at 1MHz, photons have such low energy that only large collections of them are measurable. Gah4 (talk) 17:04, 26 March 2020 (UTC)

You might be interested in this one where Feynman talks about light. Gah4 (talk) 20:39, 26 March 2020 (UTC)

Well, the whole video is worth watching, but the question here comes up at 1:20:00 to 1:22:00 when he brings up the topic for the next lecture. But he does say in words that light (photons) always move in a vacuum, and are absorbed and emitted by matter... Gah4 (talk)

And the next lecture is here! Gah4 (talk) 22:22, 26 March 2020 (UTC)

A photon doesn't even have a position operator. For everything that has a measurable speed it's c in vacuum and something else (and more complicated) in a medium. --mfb (talk) 07:57, 27 March 2020 (UTC)

DVdm, 'they also have zero mass in every reference frame' is only a valid statement if you have a priori taken "mass" as a synonym for "rest mass", which makes the argument circular. I would like to see a reference for the claim '"rest mass" is becoming more and more obsolete' outside of the disciplines of particle physics and quantum mechanics; as a counterexample, the concept of "rest mass" (or rest mass density) makes no sense in a classical analysis of EM, whereas local density of mass does with respect to any frame of coordinates. Not to mention that your claim does not fit with the definition of mass in E = mc², which would imply awful lot if rewriting. In short, to use the term "mass" without a qualifier or explanation of its meaning for your average reader (who has more familiarity of special and general relativity than of the aforementioned disciplines) is maybe inappropriate on WP. —Quondum 12:36, 2 May 2020 (UTC)

Hi Quondum, indeed the part of my edit summary remark about obsoleteness of the term rest mass was not really on the mark. The term relativistic mass is becoming obsolete, and mass and rest mass are becoming synonyms — see the subsections of Mass in special relativity#History of the relativistic mass concept.

Now, rest mass is usually defined as something's "mass in its own rest frame" — see for instance, Mass in special relativity#Rest mass: "The term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object". No observer can do that with a photon. As indeed "a photon has no rest frame" (Google scholar and Google books), for photons the concept of rest mass cannot be meaningfully defined, and is i.m.o. therefore meaningless. - DVdm (talk) 14:16, 2 May 2020 (UTC)

I find Mass in special relativity § History of the relativistic mass concept rather unconvincing (it uses arguments from the early days of grappling with the problem), so I do not buy this as an argument for current terminological trends. Mass, however it is defined, is a slippery concept, including invariant mass. One point I think is fairly clear: linking to the article Mass is singularly unhelpful in this context. Far more helpful articles to link to in the context are Invariant mass or Massless particle, since these explains all the concepts and issues. What about "Photons are massless, ..."? —Quondum 15:16, 2 May 2020 (UTC)

For obvious reasons, no problem whatsoever . Perhaps we should wait for others' comments, but if nobody objects, go for it. - DVdm (talk) 15:30, 2 May 2020 (UTC)

Whatever - as long as it agrees with the way "mass" is used in physics, i.e. what was called "rest mass" at a time when the relativistic mass was still used sometimes. --mfb (talk) 16:20, 2 May 2020 (UTC)

There does need to be a clear statement that the "relativistic mass" of the photon is not zero, and this should be done without disparaging the concept of relativistic mass, which is a perfectly well-defined and useful notion, just sort of redundant with energy. It's fine to explain as well that relativistic mass is not typically what "mass" means in current scholarly usage.

As for "massless", I agree that this word is used as shorthand for "having mass zero", but it's a little unfortunate because the particle does have a mass; it just happens to be zero. --Trovatore (talk) 20:06, 2 May 2020 (UTC)

This statement can be somewhere in the article but it shouldn't be in the introduction, or at least not at the place where the photon mass is first discussed. Relativistic mass is an archaic concept that is properly treated in its historical context in other articles. --mfb (talk) 20:49, 2 May 2020 (UTC)

I think this is a good use case for an explanatory footnote. It should be treated the first time photon mass is mentioned. Otherwise a lot of readers will get confused. But it doesn't have to be inline; an explanatory footnote (clearly distinguished from a reference footnote with an "nb" or something) should be adequate. --Trovatore (talk) 20:55, 2 May 2020 (UTC)

@Trovatore: as a photon really has no rest frame, I still feel a bit uneasy with the mention of "rest mass" in your footnote, so I slightly amended it: [1]. We can also just leave the "(also called rest mass)" out altogether, but the current version is alright with me. - DVdm (talk) 16:32, 3 May 2020 (UTC)

I think that's a bit of a quibble, but fine. If it did have a rest frame, its mass in that frame would be zero, so I think "rest mass" is fine, but I won't stick on the point. --Trovatore (talk) 16:46, 3 May 2020 (UTC)

I'm not arguing any changes here, but I'd like to note that terms have definitions that often do not exactly match with how we would parse a phrase. When terms have obvious generalizations that are useful, we tend to use the term for the generalized meaning. There are a lot of other points one can quibble about: e.g., classically speaking, the CoM of an EM wave never travels at the speed of light because it is never a true plane wave. Quantum mechanically, I'm not sure that the statement "a photon always moves at the speed of light" even makes sense. An article such as this is inherently riddled with this kind of shorthand. What we should be focusing on not confusing and on conveying to the reader a useful meaning, all while remaining readable. Quantum mechanically, by "particle mass" I expect we also usually mean a parameter to a field equation, not the CoM mass, so we have yet another definition in the mix. —Quondum 17:19, 3 May 2020 (UTC)

Interesting points. I think the notion of a photon always moving at c makes sense quantum mechanically — if you're using the Feynman-diagram formulation. But if you're not, then I'm not so sure; kind of outside my wheelhouse. I'd be interested to hear more in some other forum; here of course we need to keep some connection to improving the article. --Trovatore (talk) 17:56, 3 May 2020 (UTC)

This gets back to why it took so long for most to believe in quantization of the EM field, and photons. Maxwell's equations worked so well, with a continuous EM field. But otherwise it gets back to microscopic vs. macroscopic. In the macroscopic form, ignoring that matter is made of individual atoms, you can consider light moving slower, and index of refraction. But at the microscopic scale, it is individual atoms, and electrons on those atoms that move. Watch the Feynman videos like this one , and it will be more obvious where the problems come from. But mostly, the macroscopic form doesn't work well when considering individual photons, but only large collections of them. Gah4 (talk) 19:04, 3 May 2020 (UTC)

OK, there is a whole article Electromagnetic_radiation that explains both classical E&M, and also much of QM and photons. Probably this article should link there close to the top, for those who need that. But being titled Photon, it should mostly cover the QED version of quantized photons. Gah4 (talk) 19:31, 3 May 2020 (UTC)

Mention above of Chlorine-releasing compounds occurred to me when I saw: all bosons obey Bose–Einstein statistics (whereas all fermions obey Fermi–Dirac statistics). That is, the definition of bosons and fermions is based on which statistics they follow. Gah4 (talk) 19:21, 2 August 2020 (UTC)

In the current (17.10.2020) Wikipedia article, a photon is defined as a particle. The implication of this definition is that it is characterised by all the aspects ascribed to particles. In their massive tome "Optical Coherence and Quantum Optics" by Professors Emeritus of Rochester University Emil Wolf (one-time collaborator of Max Born in their 'Principals of Optics') and the late Leonard Mandel (regarded by many as 'the-father-of-quantum-optics') the autors contradict the Wikipedia definition of a photon. In section 10.4.2 (page 480 et sec. of the 2008 corrected reprint edition, Cambridge UP, ISBN 978-0521-41711-2 hardback) these distinguished authors define a photon as 'quantum excitations of the normal modes of the electromagnetic field', associated with which are plane waves of definite wave vector and definite polarisation. The authors then go on to add that plane waves 'have no localisation in either space or time' and therefore deduce that the excitation of the one-photon state 'must be regarded as (itself) be distributed over all space-time. The authors go on to agree that it is sometimes useful to regard a photon as a pseudo-particle, but with the proviso (amongst others) that a photon cannot have an exactly precise location, whatever state it is in. In proof of this lattermost point, citation is made to Newton and Wigner (1949 Rev. Mod Phys.21 400) and to Amrein W O (Helv. Phys. Acta.42 149). A suggested practical view of a photon is a 'wave-packet' about whose centre at a given time there is a Gaussian distribution, so that the vector has no definite value. Further discussion of 'localisation' by Mandel and Wolf is to be found in Section 12.11 of the book, 'The problem of localising photons', page 628 of the hardback edition. This Section is based on the treatment presented by a previous paper by Leonard Mandel (1966, Phys. Rev.144 1071). I must close here, so that the reader must refer to the above sources for that further discussion. Hoping that this correction of the current article about photons is seen as important, I am yours sincerely etc. Geoffrey Harding BSc., Welwyn Garden City

I am not sure about the all the aspects ascribed to particles. They have the quantum mechanical aspects of particle, such that they are emitted and absorbed in whole. There is no good way to reconcile the quantum meaning of words with the everyday meaning. Gah4 (talk) 14:28, 17 October 2020 (UTC)

There is no contradiction. The only problem is coming from your assumption that particles need well-defined positions. They do not. --mfb (talk) 16:33, 17 October 2020 (UTC)

I actually agree wholly with the objection raised to calling the photon a "particle" which also extends to other so-called particles. The objection is that the English language meaning of "particle" is a point-like object with a well-defined position, whereas the "size" of a photon depends on its spatial and temporal coherence (or I could have just said the extent of the wave that it is a quantization of). Unfortunately I do not think that Wikipedia is the proper place to correct the vocabulary that has come into place regarding the quantization of fields using the word "particle." As I said, this also applies to other "particles," in particular bosons which can have a quantum number >>1, thus a large energy in a mode, implying identical "particles", all of which obviously do not exist at a single point in space. To me, the only "particle" aspect of photons (or other "particles") is that the energy and momentum of one particle (hν of the field's energy) when it is transferred to another particle (field) occurs at one point in space and time; otherwise violation of energy conservation would be observed in at least some intertial frames. However when not interacting with matter, there is nothing particle-like about the photon (or phonon, plasmon, etc.). For instance, in order for interference to take place "a photon" from a distant star is captured simultaneously by two telescopes of an Astronomical interferometer 100m apart, which would be impossible if the "size" of the photon were much smaller than 100m. The "length" of "a photon" from my single-mode laser is likewise many meters in order to explain interference over such a path delay. It is only when a detection of ONE photon of the laser light (consisting of gazillions of photons in ONE MODE) takes place that a precise position and time-stamp can be assigned to that "photon" as if it had always been a property of THAT photon.

I think it would be great if you or someone wanted to -- carefully! -- add the insights of Wolf and Mandel to the photon article, understanding that what is really being discussed is the, to put it mildly, difficult issue of wave-particle duality, and the linguistic error of using "particle" to describe the quantization of the EM field even when there is no localization. I would also say that there is a great deal of language in this and other Wikipedia pages which I'd object to on these grounds, but which almost any attempt to edit would also be controversial, so I wouldn't venture to. But Wolf and Mandel are among the most authoritative theoretical physicists in this area, so a fair airing of their wisdom in a Wikipedia page would be quite justified. Also, have you looked at the paper "Anti-photon" by Willis Lamb? Interferometrist (talk) 21:31, 18 October 2020 (UTC)

Please read WP:TALK. Talk pages are not for personal opinions on the topic. The Wikipedia article follows the literature, and photons are unambiguously particles in that literature. If you want to change that, first convince most physicists to change that. You might first work on the arguments. I'm one of these physicists and you didn't convince me at all (and no, this is not the right place to try further). --mfb (talk) 23:55, 18 October 2020 (UTC)

Listen, I don't have any "personal opinions" about scientific issues. My opinion, if you will, is about the English language meaning of "particle" and I believe most linguists would agree with me, that calling something a particle implies that it is point-like with a definite position which you admit yourself no particle can have in view of the uncertainty principle. As you have been told, there are other physicists of great repute who also see that linguistic problem. Since science students also learned language before studying physics, that means that using a misleading word will convey the wrong impression, and I hear every day (including from my own mouth) references to photons as if they had definite positions (not just when they interact with other fields) or independent properties (other than belonging to a mode that the speaker has defined), such as one talks about the "frequency of a photon" or that a photon enters a piece of glass and the same photon comes out the other end. I realize that questions of this sort are often relegated to the field of "philosophy," but as you are aware, there are physicists who we regard as RS's who do think these are within the realm of discussion among physicists, and thus within the realm of the Wikipedia page's content. Interferometrist (talk) 16:28, 19 October 2020 (UTC)

I think the article should mention the size of a photon. this is very important. anyone agree? As for what is the size of a photon, we can use a section to describe it. And in the section, we say it is not measurable, but there are some effective sizes, such as Compton wave length, or de brogie wave length. And we can give some examples. User:DVdm will you delete this again? Jackzhp (talk) 13:41, 10 December 2020 (UTC)

@Jackzhp: Yes, the article can mention the size of a photon, if and only if there are some solid sources to support it. Yes, per the talk page guidelines, if again you add some chat to talk pages as you did here, I will revert and give some higher level warnings on your user talk page — see User talk:Jackzhp#Talk pages and User talk:Jackzhp#Talk pages revisited. - DVdm (talk) 15:41, 10 December 2020 (UTC)

I don't know about the previous post, but I do try to give some leeway that a post is meant for improvement of the article.

As for size, it is pretty much the wavelength. deBroglie is mostly for things we otherwise consider as particles, but otherwise is the same. In the case of the electron, the size is, as well as is known, zero, such that the deBroglie wavelength is what is useful. You can take an electron (more usually a bunch of electrons) accelerate them to high energy, and so high momentum and short wavelength. You can do this pretty much as far as you can afford. CERN took it about as far as can be with LEP. But you don't take photons and accelerate them, they always go at the same speed, so there is no fair comparison. The article should mention wavelength, but more details probably gets into too much quantum mechanics for this article. Gah4 (talk) 20:31, 10 December 2020 (UTC)

This isn't the place to discuss it, but no, the wavelength has nothing to do with the size. The "size" of a photon, to the extent it makes sense, is zero as far as anyone can tell (unlike, for example, the proton, which does have a finite size). This is relevant to the path-integral formulation of quantum mechanics. --Trovatore (talk) 20:44, 10 December 2020 (UTC)

I think it is very important to mention size in the article for completeness. If I change the article directly, you guys will just delete it rather than "improve it". To Trovatore: I want to mention size in the article, where to discuss this? Jackzhp (talk) 08:27, 26 December 2020 (UTC)

If you can present a solid reliable source that says something about the size of a photon, and we all agree on its relevance, then we might mention something in the article. Otrherwise you are disrupting the talk page. Be careful. - DVdm (talk) 11:38, 26 December 2020 (UTC)

There is no clear "size of a photon". --mfb (talk) 05:06, 27 December 2020 (UTC)

Perhaps I missed it, but I don't see this addressed in prior discussions: The infobox shows two values for mass: 0 and < 1×10⁻¹⁸ eV/c² . The Gluon article infobox appends the characterizations (theoretical value) and (experimental limit) to these, respectively. Any objections to doing likewise here? Secondarily, the experimental value and cite for the latter value is not addressed in the Experimental checks on photon mass §. Suggestions for reconciling that? Humanengr (talk) 08:00, 29 July 2021 (UTC)

As well as I know it, gluon physics is much less certain. Without actually doing the math, I presume the 1e-18 comes from quantum uncertainty and the life of the universe. In any case, any mass measurement has a quantum uncertainty, but that isn't included in the stated mass. Gluons are much harder to do measurements on, and so more deserving of an uncertainty. Gah4 (talk) 11:50, 29 July 2021 (UTC)

As it stands, there are two numbers indicated in the infobox with no indication of what distinguishes them. That is perplexing to the reader. The Experimental checks § begins: Current commonly accepted physical theories imply or assume the photon to be strictly massless. In line with that, the infobox, as a summary, should indicate the 0 is a theoretical value. The experimental limit ascription would be consistent with both your surmise re the 1e-18 value (I see the Amsler cite doesn't indicate its rationale) and the text in the Experimental checks § re the values given there. This is not a matter of how much uncertainty there is relative to a gluon; only an indication that there is a non-zero experimental limit to the mass of a photon. Humanengr (talk) 15:05, 29 July 2021 (UTC)

This is the second of two discussions I am submitting regarding the mass of a photon. These two discussions demonstrate flaws in the assertions as to a hypothetical massless state of photons.

https://www.sciencedirect.com/science/article/pii/S2211379719330943 2600:1700:8A00:64E0:CD76:5C45:1F15:CCB3 (talk) 21:21, 10 December 2021 (UTC)

It is desirable that the article "Photon" be supplemented as follows: A system consisting of two gravitationally interacting photons has inert and heavy properties and describes the phenomenon of inertia (resistance to acceleration) and gravity (fall) of this system by the difference in momentum of photons moving towards each other , when accelerating the system or when it is in a gravitational field. And one more thing: a system consisting of two gravitationally interacting photons with the Planck energy ${\displaystyle E_{P}=10^{19}}$ GeV collapses and turns into a Planck black hole with a gravitational radius ${\displaystyle r_{s}=10^{-33}}$ see Rationale: Article [2], §§ 2-3. — Preceding unsigned comment added by 178.120.69.72 (talk) 12:24, 10 February 2022 (UTC)

Not a wp:reliable source. - DVdm (talk) 12:49, 10 February 2022 (UTC)

This article de-emphasizes the wave-like nature of photons to a fault.

Extensive discussion of the mathematics surrounding the invariant mass of a photon overlook the wave/particle duality of a photon. It needs to be clarified that a photon is a wave carrying mass in addition to a particle at rest carrying zero mass.

For every photon, *both* of the following are *always* true:

E ^ 2 = ( mrst ^ 2 ) ( c^ 4 ) + ( p ^ 2 ) ( c^ 2 )

and *also*

E = ( mrel ) ( c ^ 2 )

Where mrst is invariant or resting mass and mrel is the mass of the photon while moving at the speed of light relative to the observer.

The wave/particle duality must be introduced earlier and more clearly in the piece, because as written now it obfuscates the reality that photons carry mass as they traverse the Higgs field, thus is entirely deceptive.

I say this, even though many physicists would prefer this truth to be hidden. 99.191.33.30 (talk) 21:04, 10 December 2021 (UTC)

(I had removed this message, and explained the reason at User talk:99.191.33.30. The message was restored here.)

Article talk pages serve as a place to discuss the quality of the article, based on reliable sources—see wp:Talk page guidelines. This is not the place where we discuss the subject itself, or where we explain parts of the subject. See also some more links about this in the message I have put on your IP-user talk page. - DVdm (talk) 00:11, 11 December 2021 (UTC)

Sometimes discussing the subject, helps to understand the article and possible improvements. Sometimes. Gah4 (talk) 21

53, 23 February 2022 (UTC)

Thanks to Constant314 for keeping an eye on changes in Wikipedia!
But I think, my edit from 17:30, 22 February 2022 regarding the electromagnetic interaction should be at least considered and discussed.
This section hopefully helps to clarify my thought processs.

In particle physics, electromagnetic interactions are described by the exchange of photons - the carrier of the electromagnetic force.^[1]
Furthermore, electromagnetic interactions happen only betweeen electrically charge particles.
Since photons do not hold any charge and photon-photon interactions in the sense of contact (Feynman) diagrams do not exist ^[2], I would argue, that photons do not interact electromagnetically.

Yes, they interact with charged particles - "the sources of the fields" - but not in a electromagnetical sense.

Any thoughts on this topic are more than welcome!
--DakiwipieRuse (talk) 08:58, 23 February 2022 (UTC)

Photons do interact, but mostly at high energy. See Two-photon physics. Even if that were not so, photons mediate the electromagnetic force, hence electromagnetic should go in the info-box. Constant314 (talk) 16:46, 23 February 2022 (UTC)

Thanks for your response!

Hmm, I am still not conviced, that photons interact directly with each other. To my knowledge, they can only interact via the exchange of charged particles like charged leptons or W bosons.

Hence, a definition via the existence of contact diagrams would not work for the electromagnetic (em) interaction.

We had a very interesting discussion [3] about that same topic over in the german wiki.

Bottom line was that photons should still be described as interacting electromagntically since they participate in a 3 particle vertex which is defined as a vertex of em interaction.

But this means, that they neither interact weak (only 'weak' interaction is with W bosons, simply because they hold an em charge i.e. this is still em interaction) nor via gravitation.

Both should be excluded from the list of possible interactions.

--DakiwipieRuse (talk) 11:35, 25 February 2022 (UTC)

This article was the subject of a Wiki Education Foundation-supported course assignment, between 29 August 2022 and 14 December 2022. Further details are available on the course page. Student editor(s): Amarz1234, Lexinawara (article contribs).

— Assignment last updated by Yonderling (talk) 00:44, 16 December 2022 (UTC)

This topic seems to have been discussed several times in the past, but there is still something off in the current article. In the section Photon#Orbital_angular_momentum it is claimed

[...] spin angular momentum of light of a particular photon is always either +ħ, 0, or −ħ.

My insight is based on [4]https://physics.stackexchange.com/a/265646/348563 which is not considered a valid reference. The linked answer explains the difference of massive and massless particles within relativistic QM. While for massive particles the spin can be associated to SU(2) representations which essentially carries over from classical QM, this is no longer the case for massless particles. Massless particles like photons have to be treated with representations of SO(1,3), this is termed helicity. Now helicity has an observable represented by a matrix that is similar to a spin observable. So by abuse of notation one speaks of the spin of a photon when one really means helicity.

The quote above should be interpreted as "helicity" in place of "spin angular momentum". The meaning of "spin angular momentum is some numbers" is rather vague. I interpret it as "the eigenvalues of the helicity observable are always from these numbers". In this sense the claim is false. Helicity only has eigenvalues +ħ and −ħ, but not 0.

Note e reffered to in the same section tries to clarify this point, but still doesn't get it right. Again it is claimed that

The possible spin states of a spin J = 1 boson are +1, 0, and -1.

Obviously the units changed to natural units - as an aside it would be nice if the units would be uniform across all of the article. It is correct that the spin has these eigenstates for massive (classical or relativistic) spin=1 bosons. As explained above this is not true for photons which are massless relativistic spin=1 bosons and the helicity observable only has eigenstates +1 and -1.

The comment goes on to introduce linear polarized light as superposition (of equal parts) of +1 and -1 eigenstates.

Spins +1 and −1 are the distinct circularly polarized states. Also, it can be introduced 'zero' spin: the spin state = 0 case can be interpreted as a linearly polarized wave, with no circular polarization, or equivalently as a superposition of two +1 and −1 circular states.

It is true that repeated helicity measurements on several linear polarized photons would have averaged helicity 0, but this does not imply that 0 is an eigenvalue with an associated eigenstate state = 0. To give another example, one could look at "slightly left polarized photons", which are proper superpositions with more +1 contribution than -1 contribution. The averaged helicity might come out, e.g., 0.3, but this does also not imply that 0.3 is now an eigenvalue of the helicity observable with an associated eigenstate state = 0.3.

I tried to improve the article from my insight gained from the PSE (physics stack exchange) answer. However I do not know of a good proper reference and so my revision got reverted, which is OK. I call for someone with a better knowledge of the literature who corrects the above citing proper references. Tpreu (talk) 21:46, 6 August 2023 (UTC)

If the article claims are referenced, then the content can be checked against the references. If the article claims are not referenced, they can be deleted. The rational is the same as applied to your reverted-edit: we want reliable content as evidenced by sources.

I encourage you to delete unreferenced content you think is questionable and challenge those who support the content to produce references.

Johnjbarton (talk) 21:53, 6 August 2023 (UTC)

I have a copy of Hecht "Optics" 3ed so I can fix these areas. Johnjbarton (talk) 22:10, 6 August 2023 (UTC)

Fixed please review. Johnjbarton (talk) 22:38, 6 August 2023 (UTC)

A paragraph in the momentum sections starts To illustrate the significance of these formulae... But no such formula precede the paragraph. I guess this got mangled. But I don't understand what these paragraphs are getting at anyway. Johnjbarton (talk) 23:04, 6 August 2023 (UTC)

A correct article would cite heavily Nobel Laureate Professor Dr. Richard Feynman "The Strange Theory Of Light And Matter" where he explains probability amplitudes and the interaction of electrons and photons. QED explains electric fields and magnetism in terms of photons, everything except gravity and radioactivity.

Photons are particles that are best explained by quantum electrodynamics as taught to 3rd-yr graduate students. There is no wave-particle duality. Now that we have devices sensitive enough to detect a single photon, it is obvious that photons are particles. A photomultiplier clicks when struck by a photon. In bright light, it clicks faster than in dim light. We now have laser devices that can emit a single photon. One photon, two adjacent photomultipliers: if photons were wave-like, both would click, instead of one or the other. The cited books may have been written before such technology was invented?

The dual-slit experiment is a popular hand-waving way to prove wave-like photons. This is incorrect. When you add a second slit, you change the experiment. Probability amplitudes calculated for a double slit agree with experimental results, as do amplitudes calculated for a single slit. No waves, no interference, different experiment with different probability amplitudes. All references to waves in the article are explained by particles. Newton was correct. Hpfeil (talk) 18:31, 28 May 2023 (UTC)

WP:BEBOLD Constant314 (talk) 22:33, 28 May 2023 (UTC)

Sorry, I believe you misunderstand wave-particle duality. The equations of motion for QM are wave equations. Low intensity photon diffraction demonstrates wave-particle duality. Single slit experiments also show interference. And so on.

I would challenge any edits along these lines.

Resolved

Johnjbarton (talk) 22:43, 6 August 2023 (UTC)

Please read "QED The Strange Theory Of Light And Matter" where you will learn that your notions of waves is incorrect. Photons are particles. Newton had it right, corpuscles. When correct calculations are carried out, single slit experiments agree with particles, not waves. Theoretical calculations agree with experiment to the degree of accuracy on the order of the width of a human hair compared to the distance between Los Angeles and New York. You can cling to your false notions which were disproved by the invention of single-photon laser gates and photomultipliers sensitive enough to trigger by a single photon. You misunderstand the fact that photons are particles. There is no interference if you correctly identify the terms of the experiment. Every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles. Go talk to any 3rd-year graduate student in physics if you don't believe Nobel Laureate Doctor Richard Feynman. There is no splitting of particles that go off in different directions. "If you put a whole lot of photomultipliers around and let some dim light shine in various directions, the light goes into one multiplier or another and makes a click of full intensity. If one photomultiplier goes off at a given moment, none of the others goes off at the same moment." If you design your slit experiments correctly and perform the correct calculations, there is no wave behavior. "It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves. I'm telling you the way it does behave -- like particles." (Quotes are from pages 14-15 of the cited lecture transcription.)

Hpfeil (talk) 18:44, 10 September 2023 (UTC)

I apologize for the the double-paste. When I returned from the login-page, the reply window was blank, so I pasted my reply again. I am unable to delete the second pasted content. Hpfeil (talk) 18:49, 10 September 2023 (UTC)

Do you have any specific recommendations? Constant314 (talk) 19:22, 10 September 2023 (UTC)

I took the liberty of removing the extra copy. Johnjbarton (talk) 21:22, 10 September 2023 (UTC)

Feynman's QED lectures are very interesting and adding material based on that source would be awesome.

He does say that light behaves like particles, but he does not say many of the things you assert.

Notice that in your quote from Feynman he specifically says "light behaves like particles". He does not say "the fact that photons are particles". The wave-particle duality is exactly that we observe fully particle behavior in some experiments and wave behavior in others.

"There is no interference if you correctly identify the terms of the experiment" is simply incorrect. Your excitement about single-photon experiments is justified, but not your conclusions. The probabilistic quantum transitions are particle behavior, but the pattern the probability amplitude makes is wave behavior.

You might reflect on the title of Feynman's lectures: "QED The Strange Theory Of Light And Matter". If light was just particles then the theory of light would not be strange would it?

You might be interested to read the article in Further reading by Willis Lamb, also a Nobel prize winner, entitled "Antiphoton" in which he uses field theory to show that particles are no needed to explain light behavior, exactly the opposite conclusion. He does talk about quantum transitions. It's fairly readable.

You can also look up work by a more recent Nobel winner, Anton Zeilinger. Most of his work is around entanglement, a property extremely hard to explain with a simple particle model.

Again I want to emphasize that additions of content from the QED lectures would be welcome. But a wholesale re-write to your particle concept won't be verifiable. Johnjbarton (talk) 21:42, 10 September 2023 (UTC)