Could the speed of light be variable?

[Dremmer]

Could the speed of light be variable? (not always travel at the same speed)

 

[kaleidoscope]

the speed of light varies from substance to substance. For example, it travels slower in water than in air than in vaccum. It has to do with the refractive index of the substance http://en.wikipedia.org/wiki/Refractive_index#Speed_of_light

 

[mathman]

The speed of light (in current theory) in a vacuum is constant. However, some possibilities have been raised that it might have varied over time, but the evidence is at best scanty.

 

[mrspeedybob]

This is a related question...

Could the speed of light through water be exceeded by an object traveling through water. In other words does the C change for all of the relativistic equations if the experiment is conducted in a medium other then vacuum?

 

[russ_watters]

Yes: http://en.wikipedia.org/wiki/Cherenkov_radiation