Talk:Speed of light

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Light speed versus light wavefront speed[edit]

In the third paragraph, "For example, in videos of an intense lightning storm on the Earth's surface taken from the International Space Station, the expansion of light wavefronts from individual flashes of lightning is clearly visible, and allows estimates of the speed of light to be made from frame-to-frame analysis of the position of the light wavefront."

What does this even mean? How can you see light that is not the wave of light? How can the wave of light propagate without it being visible? In my reckoning, when a light wave front reaches your eye, you see it... what deviation is there between a light wavefront and the light itself? — Preceding unsigned comment added by (talkcontribs) 22:42, 9 January 2014 (UTC)

Imagine you fly with a spaceship and look downwards onto the clouds of a thunderstorm. A sudden flash of lightning occurs in the center of view. The light propagates away in all directions and causes stray light from the clouds as it moves along. It is that stray light which gives the appearance of light moving from the center outwards. (talk) 21:21, 13 February 2014 (UTC)

Please sign your talk page messages with four tildes (~~~~). Thanks.
Please ask at wp:reference desk/Science. Here we discuss the article, not the content—see wp:talk page guidelines. Cheers. - DVdm (talk) 07:35, 10 January 2014 (UTC)
This is relevant inasmuch as the phrasing should be changed. The word "wavefront" without qualification should not be used here. We are observing light scattered from the expanding wavefront, which creates the perception of viewing an expanding source of light. Imagine you observe the same phenomenon from an acute angle. What we observe is the further scattering zone apparently retreating at a little over half the speed of light through the medium, and the nearer portion of the scattering zone apparently moving at a superluminal speed. It might be helpful to speak of observation of when light scattered from the expanding wavefront reaches the camera. —Quondum 03:56, 14 February 2014 (UTC)
I came here for this exact reason. I think this description is not helpful for the layman. It sounds very interesting though and I think if someone could explain it better for the layman (in the article) that would be fabulous. (talk) 19:44, 29 April 2014 (UTC)
I am actually not sure how suitable this bit is for the lede of this article. Sure, the visuals provide a powerful illustration of the finite speed of light. However, without the actual visual images it just becomes a hard exercise in imagination. Not to mention that it is unsourced and not further discussed in the rest of the article. This example would be much better suited for the main body of the article. Especially, if we can public domain video images to go along side it.
Speaking of the stuff in the lede that is not discussed in the main body. Right after this the paragraph continues " This transit time is what causes the Schumann resonance". Besides being a cryptic useless statement for the lede, this is not discussed in the main body.TR 08:16, 30 April 2014 (UTC)

Clarification of invariability of massless particles' speed in vacuum[edit]

Was not until recently I gained the understanding that was never made clear that particles without mass _always_ travel at exactly c through vacuum. I've noticed this is not made clear in almost any description of the nature of such particles. Recommend this addition for insight & clarity, from 1st paragraph:

"light) travel in vacuum."


"light) always travel in vacuum." — Preceding unsigned comment added by (talk) 19:55, 27 April 2014 (UTC)

Travelling energy and fields?[edit]

Rewording in the lede has lede to the nonsensical statements that a) energy (a quantity not a thing) is travelling, and b) that fields are travelling. Why? TR 21:48, 27 April 2014 (UTC)

I agree with the sentiment that energy (and fields) cannot be considered as "travelling". How does one even define in the classical setting what this is, in terms of the energy–momentum tensor? This is enough to consider it suspect for use in the lede. —Quondum 22:21, 27 April 2014 (UTC)
(Not to mention that energy can "leak" out of a light cone (e.g. black hole evaporation, but also quantum tunneling effects). So, if you would find a suitable ways of saying that energy travels, it probably won't be limited by the speed of light).TR 08:07, 28 April 2014 (UTC)
That's quite a subtle point, but not necessarily conclusive: if energy density can be negative (as it is sometimes regarded to be briefly or over short ranges in quantum mechanics), this could be interpreted as negative energy "travelling" at less than the speed of light in the opposite direction in the case of black hole evaporation. In the case of tunnelling, the interpretation of faster than light tunnelling is almost certainly not valid. —Quondum 13:38, 28 April 2014 (UTC)
I know. That is a approximately what happens. There is certainly no field excitations travelling faster than the speed of light. My point was that even in the weakest sense that one could define "travelling of energy", i.e. by looking at the flow of energy, that flow won't necessarily respect light cones. This all illustrating the point that the notion of "energy travelling" is problematic and not very useful. (It is "things" with energy that travel, energy itself is not a "thing".)TR 13:52, 28 April 2014 (UTC)
So, how do we change the wording so that it is actually saying something concrete rather than talking about an ill-defined concept? The lede is trying to say that the speed of light in vacuum is a sort of cosmic speed limit for "everything", but is not succeeding. The best that I can come up with is that causality that is constrained in this way. Classically, objects travelling are considered constrained in this way, but objects are ill-defined in a field theory. —Quondum 16:38, 28 April 2014 (UTC)
1)Remove energy from the the list in the 4th sentence (i.e. make it a statement about matter and information).
2)Remove "travel" from the 5th sentence. (i.e. talk about the speed of massless particles and fields) If felt necessary "travel" can be replaced by "propagate".
Given most changes to the lede have been battled to death in this article, I did not want to stir the hornest nest by just making them. However, if there are no objections I'll make those changes later.TR 16:56, 28 April 2014 (UTC)
Heh – yes, I can imagine that changes have been and may be contested. It is reasonable to make the statements a little inexact in the lede if they communicate the gist accurately enough (as with "matter"), especially when being precise would make it less readable. I support both your suggested changes. I would further want to replace "fields" in the fifth sentence with something like "field perturbations". —Quondum 18:22, 28 April 2014 (UTC)
If you work with classical (nonquantum) theories, you can define the speed of light for waves by talking about the manner in which small (or large) local changes in the wave get propagated. That is, if two waves are equal at the start time t=t_0 except on a set U, then they remain equal at later times except on the future of U (the union of the future light cones of all points in U).
This works in flat (Minkowski) space, but applies equally well in general relativity. Indeed, it also applies to the evolving space-time itself viewed as a field, as it must since it is now coupled to the other fields. Inside the future of U, the two space times will differ because of interactions with the differing matter fields (thus you should call these two futures by two different names since they are in two different worlds with different shapes) -- but outside the future of U the spacetimes and their contents will be identical.
While I agree that it is difficult to localize energy in general relativity without choosing a frame of some kind (long discussion here !!!!), nonetheless the heuristic statement "no energy crosses the light cone toward the outside" is given a good meaning by the above discussion. After all, energy is mass and vice versa, so it is entirely contained in the description of the matter fields together with the spacetime. So if the matter fields and spacetime don't change, the amount of energy hasn't changed, even if getting a mathematical function called "energy density" is problematic.
Another way to give meaning to "no energy crosses the light cone toward the outside" is via a positive energy condition of some kind, but this is deep inside the theory and is more difficult to give an operational meaning to. (talk) 21:14, 24 May 2014 (UTC)


The intro says: "the expansion of light wavefronts from individual flashes of lightning is clearly visible". Could this be explained further? It's not clear (to this layman at least) what this means. - (talk) 19:41, 29 April 2014 (UTC)

I've reworded this. See whether it makes more sense now. —Quondum 23:37, 29 April 2014 (UTC)

Semi-protected edit request on 11 May 2014[edit]

Please change the text in the article from 1 to 2 in the article: 1: The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass

2: The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass. However, there are some mathematical challenges within this underlying logic (reference link) (reference link):

Tapan Paramhans Siwach (talk) 04:08, 11 May 2014 (UTC)

X mark.svg Not done per wp:reliable sources. Blogspots don't belong in that category. - DVdm (talk) 07:50, 11 May 2014 (UTC)

The time it takes light to travel one meter?[edit]

Stephen Hawking says that the time it takes for light to travel one meter is 0.000000003335640952 seconds, as measured by a cesium clock. But I don't know if this is accurate? Should I edit this into the page? — Preceding unsigned comment added by BangBangClubUK (talkcontribs) 03:17, 17 May 2014 (UTC)

I note that you've removed the signature added by the bot, so I've added it back. Please adhere to talk page guidelines, and sign your posts with four tildes (~~~~).
The infobox already gives this information, albeit to a lower precision, and I see no reason to make a change. The exact speed of light is given in the article, allowing anyone to easily calculate of the time takes to travel a metre to arbitrary precision. —Quondum 03:27, 17 May 2014 (UTC)

Semi-protected edit request on 9 June 2014[edit]

"the refractive index of air for visible light is 1.000293, so the speed of light in air is 299705 km/s or about 88 km/s slower than c." -but the speed of light in air also depends on pressure, temperature and humidity. Source: - Department of Physics - University of Illinois at Urbana-Champaign (talk) 15:02, 9 June 2014 (UTC)

X mark.svg Not done because not a specific request. - DVdm (talk) 15:46, 9 June 2014 (UTC)

Accurate Earth to Moon time[edit]

is there a particular reason you can't have the more accurate figure of 1.28 seconds
rather than the current "from Moon to Earth 1.3s" figure? (talk) 18:24, 12 June 2014 (UTC)

You are looking at a table of rounded, approximate times, simplified for convenient use in situations where additional precision is not needed. The Moon to Earth travel time given is also only an average; due to the eccentricity of the Moon's orbit the actual time varies by more than 10% between closest and furthest approach. Hertz1888 (talk) 21:08, 12 June 2014 (UTC)

Speed Of Light Possibly Wrong[edit]

In the past few weeks, a few scientists have come to the conclusion that the speed of light might actually be slower than we thought. I do not have the expertise to deal with this subject, but I posted a Google link to articles that I think might be helpful, along with three separate articles that I thought were useful, along with a Youtube video that I thought explained it pretty well. Please, feel free to make any comments in this, and if you think is necessary, please adjust the article. Also, the original article link is below. If you wish to see the original article, click on the PDF link embedded in the website link below. Original Article: Google Link: Article 1: Article 2: Article 3: Youtube Video: NaseemHDH (talk) 22:16, 14 July 2014 (UTC)

No. This idea is almost certainly wrong. There are a lot of fringe ideas like this. Wait for some acceptance of the idea. Roger (talk) 16:08, 15 July 2014 (UTC)


Rupert Sheldrake argues that the actual measured values of C has changed, based on published reports by recording officials. Is this view worth reporting? (talk) 14:36, 15 July 2014 (UTC)

Not here. WP is an encyclopaedia, not a news channel. —Quondum 15:30, 15 July 2014 (UTC)