[Stephen VandeCarr]

Stephen VandeCarr asked the Naked Scientists:

If light speed is slightly variable as in "near vacuums", as suggested by Richard Feynman, how does this relate to the Lorenz Factor which goes to infinity at "light speed." Does this result hold for any actual value of "light speed" ?

What do you think?

[Soul Surfer]

The only significant light speed is the real one ie light in a normal vacuum.  It is quite possible for particles to exceed the mean speed of light in solifs liquids and gases with a refractive index and therefore a lower velocity of light.  when particles enter such media at speeds greater than the local speed of light the emit Cerenkov radiation until they drop below the local light speed.

see  http://en.wikipedia.org/wiki/%C4%8Cerenkov_radiation