If matter is mostly empty space how does a particle know when its going faster than light in water and emit Cherenkov radiation? How if there are two immiscible fluids one on top of the other and particle is traveling top down will the particle emit cherenkov radiation twice once in the first fluid and again when it enters the second fluid if the speed of light in the second fluid is less than the first? How thick must the fluid be for the particle to be required to emit the light, or if the fluid is thin enough the particle just say 'eh screw it' and just passes through with out emitting the cherenkov radiation.