# Talk:Speed of light

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 Definition:  A physical constant c describing the speed of electromagnetic radiation in vacuum. In the International System of Units the metre is the distance light travels in classical vacuum in 1/c seconds, using the defined value c = c0 ≡ 299  792  458 m/s (exact). [d] [e]
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## The definition of the metre

The article makes the statement "noticing that the uncertainty in the determination of the meter was about as large as the uncertainty in the measurements of the speed of light, recommended the definition...". This statement is misleading. Two things are involved, not one.

The first is that the definition of length was changed to transit time of light. That is a logical change of viewpoint.

The second thing is that this change of definition was seen to be practical from a metrology standpoint.

The logical switch makes c a defined quantity, the accuracy issues make the change of stance to a defined value a practical advantage.

In principle a switch in definition to measure length as time of transit could have been implemented at any time, and even could be based upon (say) the speed of sound in air under specified conditions. However, that was not done for metrology reasons: it would be a hassle to prepare the standard conditions for the standard speed of sound. Because the speed of light is theoretically a universal constant (whatever its value may be) preparation of free space where this universal value is obtained is considered to be a simple task that any observer can manage without hassle. The improved accuracy in time measurement means that time of flight has an accuracy advantage (assuming the standard speed of light is readily reproduced) over counting fringes of a standard wavelength of light. John R. Brews 14:19, 29 November 2010 (UTC)

I'm not sure accuracy is a good word choice in this context, because the actual value of the speed of light is irrelevant here, and only length comparisons are involved. Maybe precision is more appropriate? John R. Brews 15:43, 29 November 2010 (UTC)

## Less than c

I have read somewhere that the speed of light in vacuum is less than c. This is not widely known since the difference is below the accuracy of feasible measurements. It is a theorwtical prediction, not an experimental fact. But still, of some interest. The point is that the sole quantum of electromagnetic field, being massless, has the speed c. However, quantum electrodynamics is the theory of two interacting fields, electromagnetic and Dirac. The bare photon is (assumed to be) massless (that is, with no rest mass), but the real ("dressed") photon is no more massless because of the cloud of virtual particles (electrons and positrons). Could some physicist find this in the literature? Boris Tsirelson 16:18, 29 November 2010 (UTC)

That is, in the theory, c is the constant appearing in Lorentz transformations, and not the speed of (real, dressed) photon. In practice, of course, the two are the same, and metrology need not bother. Boris Tsirelson 16:30, 29 November 2010 (UTC)

Hi Boris: This topic is closely related to the fact that the quantum vacuum has a susceptibility greater than 0 (relative permittivity > 1). See footnote here. Of course the speed of light in a medium is inevitably reduced by its susceptibility, including the quantum vacuum. This topic rapidly evolves into complexity, and I don't understand it. John R. Brews 16:45, 1 December 2010 (UTC)
Thank you for the interesting information. Boris Tsirelson 17:33, 1 December 2010 (UTC)