Talk:Photon

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Contents

[edit] Welcome and README

Dear newcomers,

Hi, and welcome to Photon! There's still lots that can be improved here and we encourage you all to be bold. However, some topics have been debated at length already, and we encourage you to consider the consensus of previous generations of editors. It is not always easy to see why things have been written as they are, or the repercussions of editorial choices

  • The term "light" refers to all forms of electromagnetic radiation, for both beauty and brevity.
This is a standard usage among physicists and in most technical contexts (see Wikipedia's Light article). Here at Photon, the general concept of "electromagnetic radiation" is used throughout the article, and it is easier and more illuminating for readers to see a familiar 1-syllable word such as "light" rather than a 10-syllable phrase such as "electromagnetic radiation". So we have chosen to clarify in the header that light is defined as EM radiation to help those readers for whom this definition is not obvious or familiar. Although it might be tempting to replace "light" with "electromagnetic radiation" in the header and throughout the article, it would unfortunately make the article longer and more tiresome to read, less clear without being more precise.
If this is standard usage among physicists then where is the good reference to this? Last time I read this article there was a poor reference to a website where I couldn't find anything about it (though I have to admit I didn't look for long!). This concept seems adsurd to me. You might as well say that the term "radio waves" refers to all forms of electromagnetic radiation. While it might be standard usage in the academic community, it certainly isn't in the world at large. Remember - this article is not for physicists, it's for the man in the street. Arcturus 19:24, 17 October 2006 (UTC)
The unqualified term "massless" refers to invariant mass. No one disputes that photons can contribute to the invariant mass of a system, or to the stress-energy tensor (and, thus, photons gravitate). It's just that we have agreed to reserve "massless" for the intrinsic property of invariant mass, and to use more nuanced wording for those other conceptions of mass. The Talk archives above contain the discussion of this point at length.

For further information, please consult the Talk archives above, the scientific and non-scientific peer reviews and the Featured article discussion. Thanks and good editing! :) Willow 09:48, 3 October 2006 (UTC)

[edit] What's still needed? Please help!

  • A reference for how the photon came to be symbolized by γ, or a link to a scientific nomenclature committee that has established it so.
I don't have a ref handy, but I'm pretty sure it derives from the early days of radioactivity, when the three differently-behaving rays were named alpha, beta, and gamma. Later, when the gamma rays were discovered to be the same as X-rays and light, that is, electromagnetic, the name propagated to all EM "rays". Dicklyon 03:00, 4 October 2006 (UTC)
  • Do we need more on the interactions of photons with matter? It's dangerous because it's a huge field and the article is already long. Moreover, it doesn't pertain intrinsically to photons, although this is debatable. Your thoughts are welcome!
I think not much more, if any. Depends, though; someone might write something I like. Dicklyon 03:00, 4 October 2006 (UTC)
  • A detailed account of photon spin is missing. It should include the 1935 Beth experiment. It should mention the problems with a gauge-invariant expression of photon spin and with its conservation law.Aoosten 22:46, 12 December 2006 (UTC)

[edit] Some problems in the introduction?

I understand that this article has been trhough a couple of peer-review rounds, and I'm probably a bit late in pointing this out, but I have a problem with a couple of the statements made in the introduction of this article:

"It mediates electromagnetic interactions and is the fundamental constituent of all forms of electromagnetic radiation, that is, light."

Is the term "light" really used for electromagnetic radiation in general? I have never heard it used that way. To me, "light" refers to electromagnetic radiation that can be detected by the human eye, and perhaps some wavelengths bordering on this region of the spectrum (e.g. "UV-light", and "infrared light"). Using the term "light" e.g. for radio- or microwaves seems odd to me. I would like to delete the last part of the sentence.

this is a good point - the introduction should start off not with this technical talk, but something to help the ordinary reader understand the diffrence between common usage (photons are part of sunlight...) with scientific usage (em radiation). —The preceding unsigned comment was added by 24.60.137.141 (talk • contribs) 14:22, 15 October 2006 (UTC).

Furthermore:

"The photon has zero rest mass and, in empty space, travels at the speed of light;[...]"

To me, this is also quite odd. As I understand things, light/electromagnetic radiation always travels at "the speed of light", but "the speed of light" varies depending on the medium. In empty space this speed has the value which we usually identify as c (that is 299,792,458 metres per second). A more correct (or at least meaningful) statement would be "The photon has zero rest mass and, in empty space, travels at a speed of the speed of 299,792,458 metres per second;[...]" or something similar.

- O. Prytz 13:11, 1 October 2006 (UTC)

As I understand it, the photon always travels at c, even when in a medium. However, when in a medium it interacts with the other particles and hence spends some of the passage time in the interactions, and the overall bulk speed is less than c. The same happens to a certain extent in a vacuum, thanks to quantum mechanics (a perfect vacuum can never exist), so it possibly travels faster than the standard speed of light. I could be wrong, though, and this may fall into OR. Mike Peel 15:52, 1 October 2006 (UTC)
Perhaps, but wouldn't the passages I cited still be misleading? O. Prytz 21:51, 2 October 2006 (UTC)
Hi, O. Prytz, sorry for not replying immediately, I was away when you first wrote. For the reasons outlined above, we do need that little phrase "that is, light" — the definition makes the article a lot better, and the wording was worked out over several iterations to avoid making the header "clunky", i.e., to avoid breaking the flow of the writing too much.
Your point about "the speed of light" is well-taken; that could indeed be confusing to some readers, which we editors had overlooked. Thanks for improving the article! I hope you don't mind, I tweaked your wording a little, to stress its constancy of c and to replace the number with the variable; the number seemed like it could be daunting to some readers and a little distracting due to its length and units. Thanks again and welcome to Photon! :) Willow 10:08, 3 October 2006 (UTC)

Make clear that "electromagnetic" refers to two very different aspects: non charged, non magnetic photons that travel at the speed of light, and charged, magnetic particles that usually (always) have non zero rest mass. This confused me for a long time - how could the electrically neutral photon have anything to do with electricity. —The preceding unsigned comment was added by 24.60.137.141 (talk • contribs) 15:47, 14 October 2006 (UTC).

[edit] Wording of "light = electromagnetic radiation"

In the intro, perhaps we should change the fundamental constituent of all forms of electromagnetic radiation, that is, light to read including light rather than that is, light. Radio antennae don't emit light, and light doesn't travel across electric circuits. --Wjbeaty 04:18, 10 October 2006 (UTC)

I second that. I despise the use of "that is". -Ravedave (help name my baby) 04:58, 10 October 2006 (UTC)
Hi, I'm sorry that you all don't like the use of "light" to stand for all forms of electromagnetic radiation (including radio waves!) and our little verbal finesse with "that is" instead of "i.e." (Laura deprecated my Latin inflorescences. ;) These issues have been debated at length here and in related articles, producing the article that you see here. We need to define "light=EM radiation" (which is routine anyway in scientific circles), since there seems to be no practical way of writing an FA article (which must have sparkling, easily intelligible prose) if you replace a 1-syllable, familiar word such as "light" throughout with the 10-syllable phrase "electromagnetic radiation". Aside from being more clunky and less intuitive, "EM radiation" may even be seen as less accurate by some readers; since we argue that the electromagnetic field itself is produced by photons, it might seem like circular reasoning if we seem to call photons "particles of electromagnetic radiation". It's probably better to make a cleaner break in terminology with the classical Maxwellian model, don't you agree? And "light" is simply a beautiful, direct word. Hoping that you see things in the same light ;) Willow 08:32, 10 October 2006 (UTC)
I am just against 'that is' I don't really care how it gets changed. I find that sentences using that connector are hard to read. -Ravedave (help name my baby) 13:29, 10 October 2006 (UTC)
You may be right. Laura and I had been trying to give the opening paragraph a light touch with lots of flow, so we replaced the original clunky spelling out of "light=EM radiation" definition with the "that is", or "i.e." version after some tinkering. Unfortunately, the light touch may be doomed; I foresee that many readers will be unfamiliar with scientific customs and, not understanding that we intentionally set "light=EM radiation", will feel the impulse to change "that is" to "including", or "such as", or some such. To save ourselves a lot of explanations with new users, how about we spell out the definition and its rationale in the article itself:
...all forms of electromagnetic radiation, which we call "light" for brevity.
Although it uses the first-person "we", this version seems lighter and more direct that the passive voice version
...all forms of electromagnetic radiation, which will be called "light" for brevity.
What does everyone else think? Willow 20:54, 10 October 2006 (UTC)
I like including or perhaps beeing or wich is ty
I do agree that the "this is" formulation might have been too short, so that readers might not realize that a definition was being made for the rest of the article. On the other hand, we definitely don't want to make the lead paragraph clunky and worded overly technically. So I'm trying out a solution where we raise the definition into the pre-article (italics) section; how do people like this approach? Willow 13:58, 13 October 2006 (UTC)
In my opinion the key is communicate to the layman is that visible light, radio waves, the microwaves that heat up frozen pizza, and the UV that gives you sunburn is all the same thing. I don't think its a matter of finding the right words, but a matter of saying that explicitly and prominently. If someone was only going to take one thing about light from the article, I'd vote that the most important. TRWBW 19:35, 17 October 2006 (UTC)

[edit] Can light slow down?

Can light slow down? Notice that if we define "medium" as meaning "a transparent uniform medium" in the classical physics sense, then light really does change speed, e.g. when it enters glass. In particular, EM waves slow down and change wavelength when propagating through a medium. The atomic spacing of glass is far smaller than the light wavelength, so to a first approximation the glass behaves as a uniform medium for light, and light does propagate more slowly in glass than in vacuum.

On the other hand, if "medium" refers to a more modern model where the vacuum contains a crystal array of atomic nuclei with electron clouds between, then light is *always* travelling in a vacuum, even when it travels through glass, and the very idea of "medium" barely applies. Analogy: humans cannot pass through solid wood, therefore they can never travel within a forest; they can *only* travel through space between trees! I'm not trying to be funny, in fact I've encountered many people who respond with rage when they hear the suggestion that light "slows down" within transparent materials. I suspect their trouble is with concrete thinking versus abstract thinking. The terms "EM waves" and "transparent media" are abstracts, since all that really exists are photons and subatomic particles. A purely concrete thinker might discard the ideas of light waves and glass, then insist that light (meaning photons) can never change speed. This has consequences: they're essentially denying that the "transparent material" concept and the "EM waves" concept has any utility. In their world, glass is a vacuum, "light waves" are a misleading illusion, and photons travel only in the vacuum between particles and can never be inside a "transparent medium." --Wjbeaty 04:13, 10 October 2006 (UTC)

See Wave-particle duality. Both pictures are useful for different things. Glass is never a vacuum, though - it still has nuclei in it, plus all sorts of fields, with which the photons interact. Hence why light slows down in it (or alternatively, the time between the waves/particles entering and leaving the material is increased). It's transparent as the interactions don't change the properties of the photons - i.e. it doesn't shift them in colour/wavelength/frequency. Mike Peel 05:22, 10 October 2006 (UTC)

[edit] Preparation for onslaught of vandalism

Today's Main Page article, Enzyme, became a featured article not long before Photon. Unfortunately, its prominence has attracted a horde of vandals, who are hitting it roughly every 15 minutes. Photon is due to appear on the Main Page in three days (Saturday, the 14th) and I, for one, am totally unprepared for the onslaught. I probably won't even have access to a computer then. What can we do to prepare for the attacks — maybe lock the article for a day or so? I had no idea that it would be so bad, and I hate to think of all the time wasted on repairing the damage. Hoping for some good ideas and quickly, Willow 14:51, 11 October 2006 (UTC)

Does not the attraction of a lot of attention include the attention of lots of people who are willing to notice and revert vandalism? Dicklyon 15:06, 11 October 2006 (UTC)
It does indeed. No worries Willow. Caffeine got hit pretty hard when it was on the front page. I asked to have it locked but apparently there is policy not to do that. Overall caffeine actually ended up gaining some good material. So sit back, install Wikipedia_talk:Tools/Navigation_popups and squash some vandals. -Ravedave (help name my baby) 15:30, 11 October 2006 (UTC)

That's true, Dick, but why should they have to? It's a horrible waste of time. If we know in advance that we're going to be vandalized, we should probably prepare for it, don't you agree?

There will be roughly 300 edits to Enzyme today, and although the new editors aren't all malicious, it's probably safe to assume that none of those edits will improve the article significantly. However, once Enzyme is off the Main Page, it seems likely that the vandals will turn their attention elsewhere, whereas those who care to improve the article will remain. So why not proactively "freeze" the article until it's faded from the memory of those who might want to damage it? We'll save ourselves and other well-meaning people a lot of pointless work that way. While the article is frozen, we could encourage people to leave suggestions here on the Talk page on how to improve the article. But other ideas are welcome as well! Willow 15:48, 11 October 2006 (UTC)

My guess as to why featured articles aren't locked is that when someone comes to Wikipedia for the first time, odds are the first article they'll look at will be the featured article. If they then go to edit that article (after all, anyone can edit any article at Wikipedia), and find that they can't, then they'll be a bit non-plussed. If it happens to be a journalist that finds that out... well, that'd be some bad publicity for Wikipedia.
In the long-term, it doesn't really matter either way - someone can always come along on Sunday or so and revert to a pre-vandal onslought version. So I wouldn't let it stress you too much - possibly consider taking the day off Wikipedia on Saturday, then repairing any damage on Sunday. Mike Peel 16:40, 11 October 2006 (UTC)
Agree with Mr.Peel. There are plenty of very very good vandal fighters. Take the day off, when you come back it will still be there in the same shape if not better. -Ravedave (help name my baby) 17:15, 11 October 2006 (UTC)
(copied from Willow's talk page)
Congratulations on getting Photon featured! It's not easy to do for any article, but for a highly technical subject, with some conflict in its history, is truly an accomplishment. You and your collaborators deserve many kudos for that! Reading over it now, I see a truly fine piece of work, and I'm glad to know my essay may have helped in some small way.
As far as protection goes, please see User:Raul654/protection, where Raul lays out his reasons why featured articles should not be protected. It is painful to watch an article you've worked hard on be vandalized, and it's frustrating if you can't watch over it constantly yourself, but rest assured that many people add featured articles to their watchlists for the day it's on the main page, even if they're not otherwise interested in the subject. Between those who already watch it and those who will add it, and the ever-vigilant bots who watch for blanking and bad words, any graffiti will be reverted in seconds. On the other hand, many featured articles gain many improvements during their day in the sun, and a great many new editors get their start by dipping their toe in experimenting with whatever's on the main page. I agree with Raul that the benefits outweigh the inconvenience, and that a dynamic article that is responsive to users (both good and bad) does a better job of illustrating "what Wikipedia is all about" than a pristine and untouchable piece of scholarship, no matter how good. And remember that everything can be undone -- even if nothing good came out of being unprotected and attacked all day, after twenty-four hours it would be exactly back where it started, with no greater harm done.
Hope that helps to calm your fears; I will add the article to my own watchlist today to help do my part. Good luck to you! — Catherine\talk 16:51, 11 October 2006 (UTC)

(deep breath) Whew.... Thank you all for helping me calm down; sorry for getting panicked by Enzyme! But it was also good for me; I appreciate now even better the nature of Wikipedia and the collective strength of Wikipedians. It's thought-provoking and makes me realize how much I still don't understand. Willow 17:47, 11 October 2006 (UTC)

Cool. I decided to check on Enzyme to see how bad the vandalism was and I did a diff from yesterday to today. There were actually more improvements than I thought. [1] -Ravedave (help name my baby) 19:06, 11 October 2006 (UTC)
Indeed, the passing editors on Enzyme caught three typos, one serious mistake and two unclear and misleading sentences. Two figures were upgraded to SVG versions and the lead brought to a better format. Overall, a very positive experience. TimVickers 15:02, 13 October 2006 (UTC)

Thanks, everyone! I really appreciate the supportive words and the reassurance that Photon will benefit from its day on the Main Page. Enzyme improved significantly, and Photon will probably make a "quantum leap". ;) I'm at least hoping that somebody will track down the missing article titles or a good historical reference for the first time that a photon was symbolized by γ. Less than two hours to go — good luck, everyone! Willow 22:28, 13 October 2006 (UTC)

[edit] Some questions about Photons I am trying to locate answers for

Would appreciate help I have had little success finding satisfactory explanation of these, please excuse me if I am being green:

1. If photons are destroyed when detected, is it safe to assume that light actually travels like a particle at all? Because we can only know of an event creating a photon at some point in space and then measure the photon arriving at some other point, for any individual photon we make the assumption that it travelled in a straight line, but this is impossible to verify by experiment. We can test many photons in a stream and move a target to demonstrate that a light beam appears to move in a straight line, or shine a beam through smoke. However this tells us nothing about the path taken by any individual photon from its creation to its destruction. This actually throws into question whether the photo travels at all. It might just appear where detected after a time delay dependent on distance between emission and detection without actually travelling at all. Additionally if it is true that it is impossible to measure the existence of a photon without destroying it, this raises some interesting questions about the assumptions made generally, and regarding chains of observation. (If we detect a photon by measuring the impact of it on a smoke particle, will this destroy the photon?)

You're quite right that it might not make sense to talk of the "path" of the photon between its creation and destruction. Indeed, if we accept the Feynman conception, then all possible paths of the photon are tried out by Nature.
There are some disturbing consqeuences of this of course... if we accept that nothing can travel faster than the speed of light then light can only arrive from A to B at the speed of light be going in a straight line. So I would guess that there is no actual path tried at all. It kind of leads me to think that a photon is actually just information and information takes time to propogate... or something. But it also gets very confusing when you set up an experiment where dependent on the 'path' taken the photon might be detected at one of two detectors at different distances... say one at a distance of 1 light second and the other at a distance of 1 light year. The interesting aspect of this is that the probability field resolves time as well as location. If the experiment somehow succeeds in limiting the options to one or the other, I guess you can know for certain it will hit the more distant target if it does not hit the nearer in one second. Does this mean that the probability field is resolved before the photon is detected? Have we not detected the photon without destroying it in this case? Further my guts tell me that the only way to resolve this kind of problem is if the photon is already destined to resolve in a certain way at the moment of its creation, which would possibly have severe philosphical consequences regarding free will.

2. Is there a coherent explanation of Young's Double Slit experient as yet? Particularly the question of how a single particle can apparently interfere with itself? I have failed to locate one as yet.

There is no need for an explanation in terms of a more basic mechanism. Physicists now accept quantum mechanics as the way the world works, as a fundamental mechanism with no need of further explanation. The classical conception was that Nature had a secret "notebook" in which she records the position and velocity of all particles, a notebook that is intrinsically impossible to read beyond a certain refinement, as shown by Heisenberg. Experiments have shown that such hidden-variable models of Nature are incorrect. The modern conception is that position and momentum (which seem so natural to us, thanks to the philosphies of Aristotle, Newton and others) are not actually part of reality until they are measured; there is no secret notebook beyond the reach of any experiment.
This appears to be a case of "We give up". I suspected that there was no full explanation of Youngs Slits. Has it not occured to anyone that it might be possible that a grand unifying theory can not be found where there is no explanation of such a simple experiment?

3. Where can I find an explanation of why the speed of light changes when passing through a medium?

Ummm, doesn't the article itself discuss that? Maybe we should make that more clear.
It mentions slowing down due to "interactions" but I found this a bit vague... what interactions exactly? The only thing I can think of is that as photons do not interact with anything except gravity, unless the effect of slowing down in a medium is due to some kind of absorbtion and re-emission on collisions with particles (which I believe can be ruled out on grounds of such collisions being too rare and also because there would be a randomisation of the travel time and actual scattering of light as well) then I can only conclude that the effect is due to minute bends in space caused by the matter of the medium and that the light is actually navigating rippled space that as a consequence is a longer distance to travel. Thus the light doesn't not slow down, it just has further to travel. Does that make any sense?
Oops scratch all that I missed the actual explanation in the article confused it with statements in the talk. The official explanation I see is quite complicated. Are there models that predict the refractive index of a material given it's compsition? Dndn1011 17:01, 14 October 2006 (UTC)

thanks - Dino Dndn1011 12:57, 13 October 2006 (UTC)

Hoping that this helps with your excellent questions, Willow 14:18, 13 October 2006 (UTC)
Thank you very much for your help Dndn1011 15:55, 13 October 2006 (UTC)

[edit] Decay

Do photons decay? Intutivly I would say no, they last forever, but that doesn't seem right. -Ravedave (help name my baby) 16:48, 13 October 2006 (UTC)

You know that old joke about time being nature's way of keeping everything from happening all at once? Well, for photons, everything really does happen all at once, including emission and absorption. Since technically no time passes for a photon while it's in flight, there's not way to know if it would be unstable if it traveled slower and had time to decay. In any event, no particle traveling at the speed of light can decay, because in order to decay, you have to have time in which to do it. SBHarris 17:03, 13 October 2006 (UTC)
Photons are indeed stable, as far as I know; they don't decay spontaneously, although they may be absorbed or produce antiparticle pairs upon interacting with matter. Since photons can cross countless light years from distant galaxies and have survived as microwave radiation from the Big Bang itself, it seems that photons are at least very long-lived.
I understand Dr. Harris' point of view, which may very well be valid. However, I don't share his opinion that the stability of photons is an apodictic truth; the history of physics teaches us to be cautious about being certain of anything. Willow 17:12, 13 October 2006 (UTC)
Well, I think it's fair to say that since the decay half-life of particles can only be computed in particle's proper time (ie, rest frame), and since photons, gluons and gravitons (if they exist) have no rest frame, the two concepts are incompatable. Asking what the lifetime of photons would be if they didn't travel at the speed of light is like asking how massive a spaceship would be if it traveled at the speed of light. It's one of those "divide by zero" questions. The proper answer is what your programming language tells you if you attempt it: some variation of ERROR. The fact that photons travel on for light-years in OUR reference frame is not relevent to their life, any more than the fact that cosmic ray muons go through tens of km of atmophere and rock, even though their half-distance to decay would be 0.66 km without relativistic effects. Of course, there are relativistic effects. SBHarris 17:59, 13 October 2006 (UTC)

My head asplode! So to fill in the infobox would decay time be stable, 0, undefined or infinity/0? Has anyone actually done research on that exact thing? -Ravedave (help name my baby) 17:15, 13 October 2006 (UTC)

As SBHarris points out, the notion of a lifetime for photon is essentially inapplicable. They are usually called "stable" even though they can not exist for any nonzero amount of proper time. Personally, if a lifetime was called for, I'd put zero (but when I did that here a while back, it didn't survive, because the offical line is "stable", which also makes no sense at all). This problem goes away in the transactional interpretation where photons aren't really particles, I think. Those "photons" that have survived since the big bang are really just transactions, at zero interval in spacetime, between our detectors and some emitters in the big bang; those photons are absorbed by our detectors "immediately" on being emitted in the big bang, from their own point of view. They are not "stable" in any normal sense, but completely ephemeral. This viewpoint was articulatd by Gilbert Newton Lewis, I believe in 1926 before he coined the term "photon". It is consistent with everyhing we know in modern physics, just a different way of looking at it. Dicklyon 17:27, 13 October 2006 (UTC)
Cerebral asplosion — funny! Modern physics is dangerous for your mental health ;)
The Eidelman et al. table (ref #46c in the main article) says that the mean lifetime of the photon is "stable", which is the mainstream nomenclature among particle physicists. "Zero" is less than ideal, since that could be interpreted as saying that the photon decays instantly in our frame, which is evidently not true. "\infty" is likewise problematic for the opposite reason; the photon itself experiences zero proper time, as Dr. Harris and Dick Lyon point out. So "stable" is the generally accepted compromise. Willow 17:47, 13 October 2006 (UTC)

From a theoretical POV you can't have decaying photons, but you can have mixing interactions where photons get transformed into other zero or low mass particles (such as axions or gravitons) in the presence of strong electromagnetic fields.see e.g. here pdf of preprint. Count Iblis 18:30, 13 October 2006 (UTC)

[edit] doesn't every particle exhibit duality?

The article implies to me that there is something special about the photon's wave/particle duality. Historically the photon is important in this regard, since its ubiquity and relatively large wavelength made its wave behavior obvious, and led to the realization that all particles show duality. But that's a different then what is implied here. TRWBW 00:32, 14 October 2006 (UTC)

Yes, that's right. Perhaps one should write "Like all particles, Photons exhibit both..." Count Iblis 01:36, 14 October 2006 (UTC)
Hi, I don't want to start an edit war when Photon is on the Main Page, but that's not strictly true. Yes, electrons and photons both exhibit wave/particle duality, but their wave-particle dualities are different in kind. Electrons and other material particles are described by the Schrödinger equation; the photon is not. The photon is described instead by quantum electrodynamics, the quantum field theory (QFT) first developed by Paul Dirac; an electromagnetic wave is different in kind from the Schrödinger wave. We do not use ΔxΔp in connection with photons but, as the article states, ΔnΔφ. Unfortunately, this is a common misunderstanding and difficult to clear up, since it involves the highly technical field of QFT. We could, if we like, plunge into QFT and talk about the Dirac equation and the electron field, but (I believe) that is not what most people will understand by "wave-particle duality". Willow 10:17, 14 October 2006 (UTC)
There is a ΔxΔp inequality for photons, where x and p are the transverse momentum and position, or ΔfΔt, where f and t are frequency and time. And, although the Schrodinger Equation does not apply to photons - there is a direct conversion between it and Maxwell's equations. (e.g. the core of an optical fiber can be thought of as a finite potential well for photons). In most circumstances, you can think of photons in the same way as electrons just with a different dispersion relation. I support Count Iblis' wording.--J S Lundeen 16:25, 14 October 2006 (UTC)

Since Willow reverted my changes, I don't want to revert them back, but I stand by my point. The current wording implies that wave-particle duality is specific to photons, which is simply not true. If you are saying a photon's duality is different from other particles, okay, craft a sentence that that says how it is different. Something like "Photons, like all bosons, ..." TRWBW 17:07, 14 October 2006 (UTC)

The Count's wording is fine with me — thanks again, Count! I'm sorry for reverting TRWBW's edit, but I couldn't agree that the wave-particle duality of electrons and photons was "identical", at least not as that identity would be usually understood. (Inspired by the analogous two-slit experiments, most lay-people assume that Maxwell's equations are just another version of Schrödinger's equation.) That particular edit also had a few minor flaws that might not be obvious: its wording was historically inacccurate (quantum electrodynamics was developed around 1930, whereas the photon correlation experiments were in 1970-1980), it could be mis-read that the photon was a material particle, and it had a weak connection to its paragraph. I tried to find a compromise wording, but that apparently made the problem worse rather than better. I'm sorry for not explaining myself more on Saturday (and any hard feelings that might have raised); it was difficult for me to get computer access, so I had to be as brief as possible. I'm sure that, together, we can find a compromise that is scientifically accurate and elegantly worded. One element of a solution might be to substitute "quanta" for "particles" in a few key places, so that we don't end up saying "particles have wave and particle properties", which might sound bogus to lay-people. Anyway, other ideas are welcome! Willow 11:04, 16 October 2006 (UTC)
It's been a while since I studied quantum, I could be totally wrong on this. But is there a way to explain it to folks in the middle who understand a curvature tensor but may not be up on the latest? Where do photon's currently fit in the theory? Is QFT enough? If you are distinguishing the wave-partical duality of photons from other particles, where is the line? Bosons? Gauge bosons? Or are photons in a class by themselves? TRWBW 02:59, 17 October 2006 (UTC)
Of course photons are in a class by themselves ;) You don't see editors lavishing so much love on those massive gauge bosons, do you? We're special. :D
Kidding aside, I should say upfront that I'm not an expert and would welcome other viewpoints and insights. I guess I was distinguishing between the wave/particle duality of first-quantized theories (exemplified by the Schrödinger wave equation for persistent, massive particles) — which is what most people understand by wave/particle duality — from the wave/particle duality of a second-quantized quantum field theory and, more specifically, from the QFT of gauge bosons. But I didn't want to go into too much detail in the article, because we'd already been criticized for being too technical. The best solution might be to clarify the issue in wave-particle duality. What do other people think? Willow 12:30, 17 October 2006 (UTC)

[edit] Intro

Hiya. I feel it should be mentioned (especially since this article is on the front page) that the speed of light is 3x10^8 m/s. Or however you want to measure the velocity. Wikipedia should appeak to a non-geeky audience, if you are going to say its speed is "c" you might as well say what c is. Kinda an important quantity, and should be in the bit thats featured on the main page. 24.222.116.79 03:54, 14 October 2006 (UTC)

Hiya, 24.222.116.79, thanks for the suggestion! We thought about that, but in the end we chose not to give the numerical speed precisely because we have a non-geeky audience. Instead, there's a link to the excellent article speed of light. The numerical value isn't essential for the Photon article, so we don't need to give it much "air time" in the lead. Willow 10:35, 14 October 2006 (UTC)
Hiy'all. True, but the current text does this in a bit underhanded way: "The photon ... travels at a constant speed c." Won't the readers think: "I know what constant speed means; that's one link I don't need to follow"? I suggest this:
The photon ... travels at a constant speed c, the speed of light.
 --LambiamTalk 09:31, 16 October 2006 (UTC)
Thanks, Lambiam, that's a good suggestion! I'll make the change right away. We had an objection earlier to wording similar to yours, roughly: "Why shouldn't a light particle move at the speed of light? Isn't that a tautology?" which led to the, umm, presently understated version. ;) BTW, thanks very much for fixing up those references; that was one of the happiest things on Saturday! :) Willow 11:12, 16 October 2006 (UTC)

[edit] Newton/Maxwell

I reverted this edit because it seems to say that corpscules are part of the "modern concept of the photon," which I don't think is accurate. The "modern concept" seems to refer to the photon as a quantum of light which exhibits wave-particle duality. Newton's idea are worth mentioning, to be sure, but I don't think that was the right place. -- SCZenz 17:24, 14 October 2006 (UTC)

Thanks for opening this on the talk page. I was prepared to back off for a few days, but the photon concept really does embody many of the main tenets of the corpuscular theory, notably the quantization of energy and momentum (which Newton predicted but nobody could measure until the turn of the 20th century). Einstein's work is notable in a historical context mainly because it unified the wave and particle theories of light, which had both existed since Huygens & Newton's time. I don't like the intro (and wish I'd noticed it before it was FA/FP) because, like so many physics discussions, it fosters ignorance of the long history of the concept being explained. For light, that is particularly tragic as the story is so interesting. I'd like to see at least a short reference in the intro. The wave/particle history is covered moderately well in the wave/particle duality article; rather than putting a full historical section in Photon it would be better to refer to an augmented history there. zowie 17:35, 14 October 2006 (UTC)

I think improving the history is a great idea, and noting that light had previously been seen as a particle, and then a wave, is also very good for the intro. My concern is with the idea that corpscules really share many properties with the modern photon. The intro says the modern concept was invented by Einstein, and that's right; obviously the "modern concept" was an outgrowth of older ideas, but that's not quite what the edit said. I think it was just the wording I objected to, and there's something similar that we can all agree on. Any ideas? -- SCZenz 04:33, 16 October 2006 (UTC)

I'm fond of history, too, and almost too eager to honor the dead by remembering their contributions. But I also feel that we should be historically accurate and specific to photons, not light in general. To my (admittedly limited) knowledge, the corpuscular theory of Newton does not include the energy quantization equation E = hν, which is for me the quantitative cornerstone of the modern theory. Also, Newton was not the first to pose the qualitative hypothesis that light is composed of particles; most theories of light up to that era did that, even ones in the ancient world. As I recall, Newton proposed that the light particles had some quasi-gravitational interactions with matter to explain Snell's law of refraction (then recently discovered); that law seemed at odds with the naive idea that light should travel more slowly in denser media, not faster. The Maupertuis article that was translated recently has a short discussion of the scientific debate surrounding light in that era. But is it Newton's contributions specifically that you'd like to add, or something more general about earlier particle theories of light? Interested and open to other ideas, Willow 11:53, 16 October 2006 (UTC)

[edit] Photons in matter - group velocity misleading?

This claim appears vaguely misleading:

In some cases, the dispersion can result in extremely slow speeds of light.

Has it been scientifically peer reviewed? My problem is that the section starts giving some views on the observed reduced speed of photons through matter (a wave classical and a particle view). The polariton is introduced and before the equation there is:

The polariton propagation speed v equals its group velocity, which is the derivative of the energy with respect to momentum.

Although a new concept to me, it makes sense so far, even for a single photon. However the link to slow light seems misleading because it describes a "group property" (if that is the right jargon), and not a property of separate photons. In other words the slow light article appears merely to describe the optical equivalent of the audio beat effect. I can visualise two beams of coherent photons, being phase shifted in such a way that they create such a beat effect, but would that affect a separate photon (or a cascade of non-coherent photons) passing through? -213.219.184.15 01:14, 15 October 2006 (UTC)

While certainly good science, non-technical descriptions can use "speed" and "velocity" in ways that are misleading. As in the famous thought experiment, the blades of a pair of scissors can close faster than the speed of light, but that doesn't mean anything is travelling faster than light. TRWBW 02:39, 15 October 2006 (UTC)

[edit] Proof

"further experiments proved Einstein's hypothesis that light itself is particulate." Is it right to say the experiment proved the hypotheisis? AS a science article should we not be more careful with such a word? Would it not be more accurate to say that "futher experiments provided evidence which supported Einstein's hypothesis that light itself is particulate over the others."? Or something like that? Phoenixis 15:24, 15 October 2006 (UTC)

I changed it to "light itself has particulate properties". --HappyCamper 15:26, 15 October 2006 (UTC)
There is a fine line between keeping it simple and dumbing it down. Either saying light has particulate properties or light has wave properties doesn't hit the key fact. Both the wave model and the corpuscle model are approximations of quantum theory. We should find some wording that captures this. Quantum theory is a contender for the greatest achievement of mankind, and we shouldn't downplay it. TRWBW 01:33, 16 October 2006 (UTC)
I have a worry that too many readers won't understand that "particulate" is just an adjective for the noun "particle" and not a particulate synonym of the word "particular". Also, the flow of thought and how what is connected to what is perhaps not as clear for neophytes as it could be. A suggested replacement text:
..., further experiments confirmed Einstein's hypothesis that it is light itself that is quantized. The quanta constituting light are what we now call "photons".
In particular the last sentence is a missing link. (I know it comes back later, under Nomenclature, but by the time the reader is there they may have lost the relevant connection with Einstein's hypothesis.)  --LambiamTalk 09:52, 16 October 2006 (UTC)
Again, excellent wording, Lambiam! :) I had been trying to contrast the semiclassical hypothesis (light itself is continuous, but systems that emit/absorb light are quantized) with the "light itself is quantized" hypothesis of Einstein. Since all semiclassical theories were disproven by those elegant photon-correlation experiments mentioned in the text, it seems fair to say simply that the "light itself is quantized" — "confirmed" is much better than "proved". Willow 15:12, 16 October 2006 (UTC)
Would it be fair to say that the resolution of the classic wave versus particle argument was the discovery that both are approximations of quantum theory, just like the ideal gas law is an approximation of statistical mechanics and newtonian gravity is an approximation of general relativity? TRWBW 06:02, 17 October 2006 (UTC)
That sounds fair to me at least, although one needs to specify what is meant by "quantum theory". Semiclassical models are quantum theories, but do not require that light itself is quantized. Such theories do a good job of accounting for most phenomena, so it wasn't trivial that they were ruled out in the 1970's.
As I understand it, the only first-principles quantum theory of light is quantum electrodynamics, begun by Paul Dirac and amplified by so many others. It's a quantum field theory, so it's an extension of the normal Heisenbergian quantization of a one-dimensional simple harmonic oscillator to the infinite-dimensional case. The photon is not a persistent particle with position&momentum operators in the same sense as the electron is in the Schrödinger equation; instead, it's a quantum of an electromagnetic mode, one of those infinitely many simple harmonic oscillators used to describe the electromagnetic four-potential field.
I usually think of valid 19th-century theories as limiting cases of some variable, such as c→infinity (Galilean relativity) or h→0 (correspondence principle). But the creation and annihilation of a photon seem like discontinuous processes that would not disappear until h=0, not in the continuous limit h→0. So I'm not sure if I understand the sense in which you're using the word "approximations"; maybe "aspects" is better? I guess I haven't really thought it through :( What do other people think? Willow 12:07, 17 October 2006 (UTC)
I meant approximation, not limit. The older theories get the right answer too, mostly, most of the time. But on second thought, my phrasing could be misleading, since it could imply that newer theories are the final truth. They are all approximations, the new ones are just more better approximations. Saying phrase wave/particle duality without explaination bugged me, since it could imply that particles behave one way sometimes and another way at different times. Aspects is pretty good. TRWBW 02:25, 19 October 2006 (UTC)

[edit] External links

At the moment, we have three external links. I'd like to see two of them removed. The links we currently have are:

The first one of these points to what looks like an interesting document, however it seems that you need to be a member of the Optical Society of America to read it. That probably isn't the case for most readers, so it's a bit pointless having it there. The second one of these looks useful, although the link needs cleaning up a bit. I'd suggest it changes to:

i.e. using a cite template. The final link points to part of a Physicsweb article, but is very short - the content from that could easily be integrated into the article, if it isn't there already. Does anyone have any objections to me removing the two links (the first and third)?

Also, are there any other relevant links that could be added? They don't have to be web links - if there's useful journal articles, or any other media, that would be useful to readers, then it can be added into this section (possibly changing the title to "further reading"). Are there any relevant chapters in Feynman's Lectures, for example? (sadly, I don't have a copy to hand, so I can't check. ) Mike Peel 19:46, 15 October 2006 (UTC)

Lots of journal articles on the internet have restricted access. They are generally not available to the public, but if you go directly to a university library to request them, they will give it to you... --HappyCamper 20:48, 15 October 2006 (UTC)
Can we put it (with a full bibliographic citation like "OPN Trends (special issue), Guest editors: Roy & Roy") under Additional references, adding something like "Online version (OSA membership required)"?  --LambiamTalk 10:01, 16 October 2006 (UTC)
I haven't seen the OPN article, but it doesn't seem true to the spirit of Wikipedia to list a subscriber-only source, don't you think? Perhaps we should see if it has good content before suggesting that others pursue it? The other two sources don't seem to add that much to the article, so I'm OK with their deletion if you'd like. Feynman mentions photons a few times, esp. in Chapter 2 in volume I and Chapter 4 in volume III, but they're unfortunately very brief. There must be a good elementary discussion of photons out there somewhere, though! I looked at the Britannica and a few other encyclopedias a few weeks ago, but their treatment of photons was surprisingly meagre, nothing worth referencing. Willow 16:05, 16 October 2006 (UTC)
I added that OPN article as a reference originally. Then later I added the direct link to a page which took $15 credit card orders for a paper copy of that backissue. Now it appears that the website is no longer selling it, but instead only offers a download to OSA members. Also, I don't recall moving the ref into External Links. It's not a link, it's a reference. If such a thing is too confusing, then we should turn it back into a reference only, with no URL. BTW, it's an excellent set of articles discussing cutting-edge photons concepts, written by physicists and aimed at the technical public. Some of the articles are the sort of thing we'd have seen in Scientific American back in the 1960s, others are a bit more advanced. A valuable reference for anyone who wants to know what physicists now think. --Wjbeaty 01:24, 17 October 2006 (UTC)
I followed HappyCamper's advice and got ahold of the journal at my local library. I haven't studied the articles in depth, but they do seem very good and aimed at the technical public, as Wjbeaty says. In particular, they have some thought-provoking ideas about the wave-particle duality of photons, complementary to the usual presentation of quantum electrodynamics. We should definitely digest these articles (which at first blush don't seem perfectly consistent with one another) to improve the article. They also point to some articles by Willis Lamb that seem like they should be considered as well. Overall, though, they do seem to agree with the article as it is now (whew! ;) Willow 16:45, 17 October 2006 (UTC)
I think the OPN issue is a worthwhile read. The reason the articles 'don't seem perfectly consistent with one another' is because they are all written from a different physics subdiscipline's perspective. Each sub-discipline views the photon differently: the particle physicists view is often quite different from the quantum optician's view. Part of what makes this wiki article difficult to write is incorporating all these ideas coherently. Overall, though, I would say you guys (especially Willow) have done an excellent job making the article clear and educational. Waxigloo 19:08, 17 October 2006 (UTC)
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