[lmilcin]

The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.

[blueprint]

Where does there exist a vacuum?

How does a particle or its environment measure the particle's traversal of a truly empty space?