Talk:Variable speed of light

Einstein's view
Now that the Einstein digital papers are online, it's possible to search various papers to determine Einstein's opinion about the speed of light. For example you can find this where in the second paragraph Einstein says this:

"Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields".

That wasn't written in 1911. It was written in 1920. I'm afraid it's a popscience myth, or a rewriting of history, to claim that Einstein abandoned the variable speed of light in 1911. You can find other examples yourself. Note that in some of the translations you see the word "velocity" instead of "speed". Some will tell you that this is the vector quantity velocity. It isn't, it's the common usage, as per high-velocity bullet. Have a read of this Baez article written by Don Koks for another example. See the general relativity section. JohnDuffield (talk) 19:14, 2 January 2015 (UTC)

- All you need to do is look at the math in Einstein’s book "The Meaning of Relativity" 5th edition from 1953. A calculation of alpha (α) follows equation 107 and makes an unambiguous use of variable scalar light velocity (L) both as the argument of a partial differential function (proving a variable) and as the denominator in a fraction (proving not a vector) both in the same integrated quantity. Division by a vector is not defined, so there is no other way to interpret the velocity of light in this usage except as a variable scalar speed.

(1/L)(∂L/∂x1)dx3

In other calculations Einstein explained that he used velocity as a vector when it was measured as components of a coordinate system.

Peter Bergmann did not agree with Einstein, but left the dispute out of his earlier book "Introduction to the Theory of Relativity" 1942 to get Einstein’s endorsement. After Einstein died Bergmann wrote a new book "The Riddle of Gravitation" 1968 claiming that vector light velocity could change direction but not speed. This has become a prevailing opinion in science, but not in agreement with Einstein’s unambiguous math.

Max Born agreed with Einstein and very clearly stated both speed and direction of light change in a gravity field. Einstein’s Theory of Relativity 1923 in English, 1920 in German. Dover editions 1965 page 357. Astrojed (talk) 01:43, 24 June 2015 (UTC)

I added references to 6 books that show Max Born and Richard Tolman agreed with Einstein while Peter Bergmann did not. The Bergmann view has prevailed in much of educationAstrojed (talk) 06:46, 30 December 2022 (UTC)

Style corrections were made in response to a request. Astrojed (talk) 06:59, 30 December 2022 (UTC)

Conflation of unrelated concepts
This article is aware in some places, and unaware at others, that "VSL" groups a number of completely unrelated proposal. This is of course a result of content-creep over time. Atm, I can see at least three unrelated topics here:
 * "Einstein and Dicke" ideas of frequency/wavelength dependency of c: all criticism related to "but c is dimensionful" is off topic to this, because it is about an upper limit on the ratio of c of light of two different frequencies. If c were frequency-dependent, the SI definition would become meaningless unless a specific frequency was specified.
 * massive photons: this is not a proposal that c as a fundamental parameter in relativity etc. is variable, but that actual light travels at variable speeds slightly below c. Again, this would be about the ratio of the speed of a given photon to c
 * cosmological things: Here I am not sure whether "VSL" is just a convenient way of phrasing this for the purposes of cosmologists who are perfectly aware that their "variable c" is equivalent to a variation of alpha and/or G. The problem is apparently that the editors responsible for this section were themselves not sure/aware of this.

tl;dr / bottom line: it is useful to point out the "dimensionful" problem to the reader of the article, but it is beside the point to portray this as a "criticism" because it simply doesn't apply to any actual proposals discussed in the article. It also isn't possible to present any one "criticism of the VSL concept" because there isn't any single "VSL concept". --dab (𒁳) 13:58, 12 May 2016 (UTC)


 * Thank you for response. I believe you are correct about there being a number of different opinions about VSL, some of which I added to the page.
 * Tolman used an integrated equation largely in agreement with Einstein's differential. Born went further while agreeing with Einstein about the affect of gravity. The Born view if accurate may assist in eventual travel between stars with local light speed faster than standard c of flat space, while the other representations do not.
 * Influence of Peter Bergmann is very great in education, and well documented to be in disagreement with Einstein, Born, and Tolman.
 * Please notice there is a well documented disagreement as shown in the six book references, old books by famous writers.
 * On the page I feel the major disagreement should be documented as I have done for the most famous original writers. I find many college graduates quoting the Bergmann view, and verry few quoting Einstein, Born, and Tolman.
 * As always you are welcome to edit what I wrote.
 * Thank you again for reply. Astrojed (talk) 21:20, 30 December 2022 (UTC)

Sign error in Dicke's refraction formula?
If c_0 is the speed of light absent of gravity, should not c --- the speed of light in presence of gravity --- be smaller? The formula then should rather have a minus sign as in 1-2GM/rc^2, converging nicely to 0 as r approaches the Schwarzschild radius r_s=2GM/c^2 from above? — Preceding unsigned comment added by 46.92.251.76 (talk) 08:20, 17 September 2017 (UTC)


 * In Alexander Unzicker and Jan Preuss, "A Machian Version of Einstein's Variable Speed of Light Theory", the same formula is given as c_0/c whereas this page gives it as c/c_0. I believe the former is correct as the speed of light is deemed to be lower in the vicinity of mass.TonyP (talk) 14:43, 18 March 2024 (UTC)


 * Einstein referenced it first in his notes from 1907 as the "spatial variability of c" when referencing the effects of mass on the speed of light, but he was also equally referencing variability over time. Dicke was expanding that idea to cosmological Redshift; I think c_0/c would imply exclusivity of mass in affect toward c. I'm not sure, though. -- Sleyece (talk) 01:23, 24 April 2024 (UTC)

Einstein's VSL - re:Unzicker's Real Physics video
As discussed in this YouTube video, it seems that mentions into Einstein's exploration of VSL have been unjustifiably removed. While I disagree with some of the language in the video, I think he demonstrated well that Einstein's VSL definitely belongs in this article. //Yuval Talya; My contributions; Let's talk// 10:50, 26 July 2022 (UTC)


 * Cranks are gonna crank. The article contains a perfectly cogent discussion of the speed of light in general relativity; the removed sections were entirely synthesis from primary sources.  Such a discussion (of early 20th century views on the speed of light) should instead be sourced to published literature on the history of science.  If Unzicker wants to write such an article and get it through peer review at a real journal, more power to him; ranting youtube videos are not a substitute. JBL (talk) 19:17, 26 July 2022 (UTC)
 * Everything removed from this article (repeatedly, and by multiple editors) was about something else, shoved in here in order to make borderline fringe ideas look more respectable. XOR&#39;easter (talk) 17:48, 27 July 2022 (UTC)
 * @XOR'easter except of course, the references that were removed were to primary sources. The article should be renamed to Dicke's variable speed of light theory if the actual basis and history of the concept is being deliberately excluded. 2.31.141.178 (talk) 11:10, 4 August 2022 (UTC)
 * The removed references were about ordinary general relativity, while the article is about alternatives to general relativity. And not just Dicke's proposed alternative, but a whole host of them (none of which have gained significant traction). There is a very clear dividing line. XOR&#39;easter (talk) 16:08, 4 August 2022 (UTC)
 * @XOR'easter nowhere does this article state that it is an alternative to general relativity, nor is it mentioned or linked in the article on Alternatives General relativity. perhaps you could rectify that. As a summary of variable speed of light theories, omitting Einstein's papers, and articles that discuss this, appears to be a deliberate act of reinforcing the idea that this was a misguided flight of fancy primarily instigated by Dicke.
 * I'll leave you to ponder why this is more about orthodoxy and suppression of information than scientific thought. 2.31.141.178 (talk) 09:04, 5 August 2022 (UTC)
 * nowhere does this article state that it is an alternative to general relativity How about the second sentence of the article? Also, the article does not omit a discussion of general relativity -- it's in the section called "Background".  What it omits is the prior questionable synthesis of primary sources. JBL (talk) 18:06, 5 August 2022 (UTC)