Talk:Photon

In my opinion including the full image of the article is inappropriate for an encyclopedia. Johnjbarton (talk) 23:09, 13 May 2025 (UTC)

Also, there are some discarded ideas about photons. Constant314 (talk) 23:33, 13 May 2025 (UTC)

I noticed in the subsection "Relativistic energy and momentum" that the equation deriving a photon's momentum explicitly defines the variables and describes most the constants, but the Planck constant h is not explicitly labeled. At first I considered that the editors may have believed the inclusion of the description for the reduced Planck constant ħ was good enough, but then I considered how π was also an unreferenced constant used in the equation. A belief that the value of π is common knowledge seems like the justification for not labeling it, and I personally agree. I then considered that the same line of reasoning may extend to the Planck constant. I'm sure π is much more generally familiar than the Planck constant, but where does one draw the line on explicitly listing values of constants in relation to their recognition among readers? The reduced Planck constant is defined which would contradict any guidance to only define variables (and never constants), so I'm left wondering if this is an editorial oversight, or if I'm generally expected to be less dumb. Nom de vileplume (talk) 17:52, 20 June 2025 (UTC)

I made a small change to address this issue. In general symbols should be explained but I suppose level of the treatment of the topic can result in different choices. Any reader of "energy and momentum" can be, I suppose, familiar with pi. But in the context of Photon, the Planck constant should be called out, rather more than we do here. The so-called derivation ("This derives from the following relativistic relation, with m = 0:") is bogus. The Planck constant is that fundamental constant of nature which derives from the photon. Johnjbarton (talk) 22:24, 20 June 2025 (UTC)

I made more changes to this section, see if it helps. Johnjbarton (talk) 23:09, 20 June 2025 (UTC)

Currently the intro says

• Photons are massless particles that can only move at one speed, the speed of light measured in a vacuum.

This claim subtly contradicts the article section Photon#In matter which says

• Light that travels through transparent matter does so at a lower speed than c, the speed of light in vacuum.

I thought a less ambiguous form would be clearer:

• Photons are massless particles; in vacuum they can only move at one speed, the speed of light.

This form is silent on the speed not-in-a-vacuum. My change was reverted by @Ponor with the edit summary This could imply that for massless photons there is v other than c. This implication is not how I read the two sentences but we could use a period if that is better:

• Photons are massless particles. In vacuum they can only move at one speed, the speed of light.

(The subtle issue is that "light" and "photons" are not equivalent.) We can discuss other forms but the current one is not reasonable. Johnjbarton (talk) 16:41, 21 February 2026 (UTC)

Your statement does not imply that for massless photons there is v other than c. Constant314 (talk) 16:53, 21 February 2026 (UTC)

I have been corrected on this before when I tried to change the article to say that photons move more slowly. Their speed is always c whether they are in a vacuum or not, so saying that in vacuum they can only move at one speed implies that in material photons move at a slower speed. You can leave the vacuum word out out entirely. In matter if photons are moving, rather than having been absorbed temporarily by an electron as they interact with the medium for example, they are moving at c. StarryGrandma (talk) 19:48, 21 February 2026 (UTC)

The story is, I believe, that a photon plus other matter is no longer that one photon (same with electrons, which become quasielectrons, even massless relativistic particles in some matter). If light moves through a crystal at v, that does not mean that the photon that entered it went through unchanged; you'd need to draw many weird Feynman diagrams between that photon coming in and the photon that (and if) comes out; there would be some light-matter modes traveling and what not. We know that *a photon* moves at speed c. That speed is called "the speed of light in a vacuum". If we say "In vacuum they can only move at one speed, the speed of light." you'll be asked "but what when they are not in vacuum", and then we'll be in trouble. (Matter is mostly vacuum anyway.) Is this convincing enough: https://van.physics.illinois.edu/ask/listing/1354? Ponor (talk) 20:13, 21 February 2026 (UTC)

I believe the "one-speed" bit is in contrast to massive particle with differing speeds limited by the maximum speed, both compared in vacuum.

I found one source that measured the speed of photon in vacuum. As far as I know we have no sources to summarize about their speed in "matter" whatever that might mean. Our current content is therefore incorrect: we only know about speed in a vacuum. In my opinion, what possible questions we might be asked is not relevant to the content. We don't speculate. By coupling massless with speed in a material we are creating "information" which we cannot source.

So to speculate ;-), the theory that relates photons and matter is QED. In Feynman's QED: The Strange Theory of Light and Matter page 89 and 90 he talks about speed of light. In QED over short distances there is quantum amplitude to have speed higher or lower than c; over longer distances only the single speed is significant.

Page 89- It may surprise you that there is an amplitude for a photon to go at speeds faster or slower than the conventional speed, c. The amplitudes for these possibilities are very small compared to the contribution from speed c; in fact, they cancel out when light travels over Hong distances. However, when the distances are short as in many of the diagrams I will be drawing...

He goes on to discuss interaction with matter which follows the "weird Feynman diagrams" bit above. However, photons are absorbed and photons are emitted in these diagrams. There is no "that photon".

To sum up, in my opinion "Photons are massless particles that can only move at one speed" is unsourced should not be included in the intro. Johnjbarton (talk) 04:19, 22 February 2026 (UTC)

I don't think that Feynman was saying that photons can go faster than c. I think that he was saying that you assume that they do that with a certain amplitude for the purpose of calculating the end point probability. Constant314 (talk) 05:41, 22 February 2026 (UTC)

Oh I agree that this is passage is "just" about the theory. However physical concept is valid as far as I know, it is analogous to the uncertainty principle arguments used for vacuum fluctuations. The whole summing up of virtual particles bit seems like a math exercise that got out of hand, and yet actual physics is made out of these ideas. I think everyone agrees do not have a perfect, lucid model for light+matter. What we have is QED and what we don't have is little bullets of light we can measure within a material. That is why I don't think we will find a source for the speed of a photon in matter. Johnjbarton (talk) 17:32, 22 February 2026 (UTC)

There is a good explanation here that also shows how we tailor our view of physical problems (classical to quantum) depending on the problem. See "View No. 2: Its all particles".

• Sutter, Paul. "How does light slow down?". Space.com. Retrieved 22 February 2026.

StarryGrandma (talk) 19:06, 22 February 2026 (UTC)

Hm... maybe because there's no such thing as "the speed of *a* photon in matter", the speed being pretty much a classical concept, while everything "in matter" is very much quantum (uncertain). If we send a (bunch of) photon(s) from here to Mars, at distance d, I'm pretty sure we can say they all arrived there in time d/c. But when a photon can interact with "matter", we cannot really consider the photon-in and photon-out as one. (apple in, pear out - what's the speed of plum?) When we can actually measure their speed, when they travel freely without being absorbed, their speed is c, even according to our featured article. Ponor (talk) 16:01, 23 February 2026 (UTC)

I lean that way. The speed of a photon in matter is probably an ill defined concept.

If we have a source that can reliable emit a single photon and a detector that can detect a single photon, then we can measure the time and distance between emission and detection. We can measure the speed of the effect and if we say that a photon is responsible for the effect, we have a meaningful speed of a photon. If we put a slab a glass between the source and the detector, we can measure the added delay in infer a speed of the effect in the glass. But there is no way to assure that the detected photon is the emitted photon. Constant314 (talk) 17:01, 23 February 2026 (UTC)

• "But there is no way to assure that the detected photon is the emitted photon."

In

• Chen, J., Jeong, H., Loy, M. M., & Du, S. (2013). Optical Precursor of a Single Photon. In Optical Precursors: From Classical Waves to Single Photons (pp. 65-74). Singapore: Springer Singapore.

they use heralded single photon source to get two related photons and coincidence between detection allows timing and presumably statistical evidence of being the same photon. They talk about single photons from fast switching pulses,

• The fast changing envelope excites a broad spectrum around the carrier frequency of the incident light, and the extremely high and low frequency components give rise to precursors since these parts of light barely interact with the medium.

This is the team that reported single photon speeds of c. But as far as I can tell without reading the whole book they are only interested in proving that no photon has a higher speed. Johnjbarton (talk) 23:33, 23 February 2026 (UTC)