I read somewhere that Einstein's interest in the properties of light stemmed from a question he posed to himself "What would light look like if you matched it for speed?" This then led to his theories of relativity. The question though is still valid, because although he determined that the speed of light in a vacuum is a universal constant, light moves slower through other mediums depending on the refractive index of the medium. In fact it is even possible to travel faster than light for example electrons can through water which has a refractive index of 1.33. So is it possible to answer Einstein's original question and if not why not ie is there a law in physics preventing such knowledge, is it only an apparent reduction in the speed of light as a consequence of it taking a longer path due to ricochets off molecules or is it just a matter of limitations in our technology preventing us from conducting such an experiment? IMHO I imagine it is only an apparent reduction due to a longer path taken. Can anyone confirm this?