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While the speed of light in vacuum is a universal constant ($c$), the speed at which light propagates in other materials/mediums may be less than $c$. This is obviously suggested by the fact that different materials (especially in the case of transparent ones) have a particular refractive index.

But surely, matter or even photons can be accelerated beyond this speed in a medium?

If so, what are the effects?

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This is the sort of question that needs to be closed and discouraged if we are going to raise the level of discussion of this site. –  Daniel Bingham Nov 25 '10 at 22:17
@Daniel: To quote from the FAQ "Physics - Stack Exchange is for active researchers, academics and students." It' is as much a student level question (and perceptive one at that) as a pop-sci one. –  dmckee Nov 25 '10 at 22:38
@dmckee I disagree, the highest level of student this is at would be high school. Even most high schoolers are aware that the speed of light in a vacuum cannot be exceeded conventionally. I do not believe the question was referring to medium where the speed of light is lower than vacuum, as it makes no mention of them. Rather is speaks about the potential of a cataclysm resulting from breaching the speed of light. That is absolutely pop-sci level. –  Daniel Bingham Nov 25 '10 at 22:42
@Daniel: first, this site isn't intended for research anymore (see discussions on meta) but rather for undergraduate/graduate level questions (with proviso that interesting high school and research level questions should also be allowed). This general point aside, most people probably heard about speed of light but basically no one believes it is fundamental. Therefore this question deserves to be answered. Also relation to Cherenkov radiation shows that there is some depth in the question, intended or not. –  Marek Nov 25 '10 at 23:04
@Marek If you edit the question to make it more about Cherenkov radiation, then I wouldn't have a problem with it. But its current phrasing puts it squarely in the popular science category and therefor off topic even for a site dedicated to undergraduate questions. –  Daniel Bingham Nov 25 '10 at 23:08

2 Answers 2

up vote 19 down vote accepted

Speed of light is indeed lower when light propagates through materials (as opposed to vacuum). This doesn't mean that individual photons go slower but rather that the apparent speed of light pulse is lower due to interactions with atoms of the material. So in this case it is possible for some objects to go "faster than light" and indeed very similar effect to sonic boom, called Cherenkov radiation, appears.

Note that for most materials the apparent speed of light is still huge (of the order of speed of light in vacuum) so you need very energetic particles to generate Cherenkov radiation. So this effect is mainly relevant for high-energy particle physics, astrophysics and nuclear physics.

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Indeed, Lene Hau at Harvard has frozen light--see Lene Hau's publications at her lab webpage:deas.harvard.edu/haulab/publications/HauPublications_All.htm and many links are available for reading, e.g.- –  Gordon Jan 25 '11 at 7:17
Lene Vestergaard Hau Frozen Light Scientific American 285, 52-59 (July 2001) and Special Scientific American Issue entitled "The Edge of Physics" (2003). –  Gordon Jan 25 '11 at 7:23

There is a phenomenon somewhat analogous to an "optical boom" know as Cherenkov radiation which occurs when charged particles pass through a medium at a speed greater than the speed of light for that particular medium.

This produces a beautiful blue glow often seen in pictures of the cores of nuclear reactors. See the wikipedia page for pictures.

Not entirely sure if this answers your question but it may be of interest to you.

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I was just coming to post this...in addition to the pretty color it is useful in terms of detector technology. It is the basic detection technology for the ultra-large scale neutrino detectors (SuperK, IceCube, ...) and is useful for particle ID in some compound detector systems at accelerators. –  dmckee Nov 25 '10 at 22:35
This, I did not know. Interesting stuff, I'd not heard of IceCube project - thanks for the information. –  xenon Nov 25 '10 at 22:42
It definitely interests me! I may be a computer science grad but physics was and always will be my first love. :) Thanks. –  Robin Maben Nov 26 '10 at 5:51

protected by Qmechanic Feb 10 '13 at 23:32


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