An EM particle-wave propagates in a vacuum at a constant speed $c$, independent of the source/receiver but dependent on most mediums it moves through. Since the vacuum is a medium, and if logically permissible, all mediums affect the speed of light, these affects of a medium can:

  1. alter the speed of light.

  2. possibly alter the distance light has to travel between source/receiver.

  3. subsequently alter the interval of time necessary to travel that distance.

Therefore the vacuum will affect the speed of light, and the speed light is measured to move at will depend on its medium, the vacuum. If each medium has a refractive index measuring this affect on light speed in a medium, then in a vacuum it would be at $c$. Is it possible that light has a true speed beyond what we observe in a vacuum? Also, since light travels the shortest distance/in the shortest possible interval of time through any medium, despite changes of lights speed in that medium,: does the speed of light actually slow down in a medium or just appear to slow down because of path deviations caused by a medium that alter distances thereby altering time intervals?


Is the speed of light affected by all mediums it travels in?

Yes. Including space, but don't get distracted by virtual particles. The speed of light varies with gravitational potential. Search the Einstein digital papers on "speed of light" or "velocity of light" for examples like this:

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Also see Shapiro's 4th test of General Relativity along with The Deflection and Delay of Light by Ned Wright and this PhysicsFAQ article by Don Koks:

"Einstein talked about the speed of light changing in his new theory. In the English translation of his 1920 book "Relativity: the special and general theory" he wrote: "according to the general theory of relativity, the law of the constancy of the velocity [Einstein clearly means speed here, since velocity (a vector) is not in keeping with the rest of his sentence] of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [speed] of propagation of light varies with position." This difference in speeds is precisely that referred to above by ceiling and floor observers."

There is however an issue with current teaching wherein the speed of light is usually taken to mean the locally measured speed of light. That's always the same because of a tautology wherein we use the local motion of light to define our second and our metre, which we then use to measure the local speed of light. See http://arxiv.org/abs/0705.4507. What Einstein/Shapiro/etc referred to as the speed of light nowadays tends to be called the coordinate speed of light, which is IMHO most unfortunate.

if light travels the shortest path/in the shortest possible interval of time, does it ever actually slow down?

Yes. Light goes slower near the floor than at the ceiling. If it didn't, optical clocks wouldn't go slower when they're lower, light wouldn't curve, and your pencil wouldn't fall down.


Wavelength, speed and frequency of light: V=c/lambda, v being the frequency. Light emitted or radiated from a MONOCHROMATIC source has a range of wavelengths and velocity, the frequency is invariant, The speed of light actually is slowed down. That's why we define refractive index as c/v. Example: Light will travel in air having a refractive index 1.00029 with v=c/1.00029, as compared to vacuum in which v=c/1.


protected by Qmechanic Sep 14 '16 at 18:45

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