Talk:Photon

Random questions it might be worthwhile to answer in the article?
How does a photon have momentum (p=mv) if there is no mass? (the article has some equations, but not much explanation.)

How would a Kugelblitz (astrophysics) form if there is no photon mass? or does energy also bend space-time? (perhaps better addressed in the Kugelblitz article?

If a particle and anti-particle annihilate, how many photons does one get? Just one? or a bunch of them?

Could we say the number of photons in the universe vastly out-numbers the amount of matter particles? I think I read someplace 99% of matter in the early universe annihilated when particles met their anti-particles, presumably making a lot of photons.

A radio wave might be many meters long - how do they manage to interact with very tiny electrons? I suppose the wave is composed of innumerable photons which are of a "size" to interact? Feldercarb (talk) 23:05, 6 October 2023 (UTC)


 * The momentum should be explained I agree.
 * Kugelblitz already has a page and that's more than it deserves IMO.
 * The article answers annihilation: at least two photons, "never" one. But it's complicated. Seems like more detail would go on Annihilation.
 * The number of photons and their relationship to the cosmic background would be a good addition see [https://physics.stackexchange.com/questions/276890/how-to-calculate-how-many-photons-are-in-the-universe
 * 
 * A section pointing out that for many problems (eg radio) continuous field models are much better than photons would be welcomed. (the wavelength relates to the antenna size, photons are essentially irrelevant).
 * The key for each case is to find a suitable reference to back up additions to the article. All of these should be covered in textbooks I presume. Johnjbarton (talk) 00:16, 7 October 2023 (UTC)

Do photons interact with each other? If two photons "collide" for example? Since don't follow exclusion principle I would presume nothing happens. But then we have wave interference. Obviously I am muddled.

Can an energetic photon "decay" or "transform" into several photons of lesser energy, such that the total energy remains the same? (2xE --> E + E). Or two lesser energy combine into a single more energetic one?

A bit unclear where both electric and magnetic components of an EM wave arise. Single photon carries or mediates both? Or there are "E" photons and "M" photons. As pointed out above, I suppose sometimes the wave model work for some things, photon model for others? Feldercarb (talk) 23:05, 6 October 2023 (UTC)


 * Perhaps you should take your questions to Quora, they are prefect for that venue. Johnjbarton (talk) 02:55, 8 October 2023 (UTC)

Re: 'Random questions it might be worthwhile to answer'
Under the heading 'annihilation' it would be useful to explain when four or six (or 2n) photons would be produced. The received wisdom seems to invariably prefer 'two', but in 3-dimensional space, six - mutually at rightangles - seems an obvious choice. If there were four they'd need to be in the same plane, but we'd then need to address the question of what their polarization and the angle of that plane should be; (if we were really struggling we'd no doubt dismiss that as 'random'). Also, traditionally photons don't interact in the absence of an electric charge but I understand that exceptionally they can, usually if one is a very high energy ɤ and the other of much lower energy. Assuming that this is the case, it might be helpful to confirm that. It might also be useful to explain when axions can be produced (if such esoteric items exist!). Paul Renshaw (talk) 15:16, 2 November 2023 (UTC)
 * If you want to ask questions about a topic, not the article, then the reference desk is a better place. All numbers except 1 are possible in an annihilation reaction. The outcome is random, 2 photons is by far the most common case. Four photons don't have to be in the same plane. There is no reason why 6 should be common, or what would be special about right angles in this context. Photon-photon interactions get more likely with higher photon energies, that applies to both photons (technically: with a higher center of mass energy). --mfb (talk) 19:33, 2 November 2023 (UTC)

No mention of Orbital Angular Momentum (OAM)
An article describing photons should mention that in addition to Spin Angular Momentum, photons also have Orbital Angular Momentum 50.38.13.172 (talk) 03:35, 19 November 2023 (UTC)


 * To make the addition easier, could you provide a reference which discusses this (there might be subtleties)? Best kind of reference would be some established textbook. Jähmefyysikko (talk) 04:20, 19 November 2023 (UTC)
 * My copy of Hecht 3rd makes no mention of orbital angular momentum for photons. Johnjbarton (talk) 15:39, 19 November 2023 (UTC)
 * So-called orbital angular momentum is a property of extended wave fields or beams of light. Unlike helicity, there is no simple single-particle-like model to think about. Consequently the scientific issues fit poorly in an article about "photons". For example, it is incorrect to say "photons also have orbital angular momentum", its a property of a system.
 * Here is an excellent reference: Chen, Jian, Chenhao Wan, and Qiwen Zhan. "Engineering photonic angular momentum with structured light: a review." Advanced Photonics 3.6 (2021): 064001-064001. https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-3/issue-6/064001/Engineering-photonic-angular-momentum-with-structured-light-a-review/10.1117/1.AP.3.6.064001.pdf Johnjbarton (talk) 15:54, 19 November 2023 (UTC)