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?


2 Answers 2


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.

  • $\begingroup$ 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.- $\endgroup$
    – Gordon
    Jan 25, 2011 at 7:17
  • $\begingroup$ Lene Vestergaard Hau Frozen Light Scientific American 285, 52-59 (July 2001) and Special Scientific American Issue entitled "The Edge of Physics" (2003). $\endgroup$
    – Gordon
    Jan 25, 2011 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.

  • 2
    $\begingroup$ 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. $\endgroup$ Nov 25, 2010 at 22:35
  • $\begingroup$ This, I did not know. Interesting stuff, I'd not heard of IceCube project - thanks for the information. $\endgroup$
    – xenon
    Nov 25, 2010 at 22:42
  • 1
    $\begingroup$ It definitely interests me! I may be a computer science grad but physics was and always will be my first love. :) Thanks. $\endgroup$ Nov 26, 2010 at 5:51

Not the answer you're looking for? Browse other questions tagged or ask your own question.