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Do mass and energy refer to the same physical quantity? If it does, then speed of light would be dimensionless according to the famous formula. Therefore, the formula itself implies ironically that mass and energy cannot be the same. So, I think the first sentence of the section "conservation of mass and energy" should be edited. Somebody400 (talk) 20:02, 15 January 2020 (UTC)
Unless I'm mistaken, c is not the speed of light. It is the universal speed limit, or universal constant. It happens to also be the speed at which light travels in a vacuum because light has no mass. But the speed of light varies with the medium, whereas c does not vary. --18.104.22.168 (talk) 22:37, 6 March 2020 (UTC)
Anyone who has seriously studied relativity knows this. But it becomes tiresome to constantly say "Energy equals mass times the square of the speed of light in a vacuum". In most transparent media, such as air, the speed of light is very close to c anyhow, so it does not matter much. JRSpriggs (talk) 01:52, 7 March 2020 (UTC)
c0, or c, is indeed the speed of light in a vacuum. The ratio of the speed of light in a vacuum to the speed in a nominated material is the refractive index of the nominated material. As JRSpriggs points out, the refractive index of gases exceeds 1.000 by only a very small amount. (eg. The RI of carbon dioxide is 1.00045.) Dolphin(t) 07:09, 7 March 2020 (UTC)
OK, if you guys are happy with that, then! Personally, I think it should be explained, even though these things are equivalent. --22.214.171.124 (talk) 22:53, 12 March 2020 (UTC)
Points: (i) "universal speed limit" is not commonly used in literature, and "universal constant" could refer to any number of constants; (ii) saying "in vacuum" gives the false impression that "the speed of light" refers to anything other than the speed in vacuum. Note the use of the word the in "the speed of light", rather than a which could mean the speed in various media. --Jules(Mrjulesd) 23:20, 12 March 2020 (UTC)