All matter moves through space-time at the universal constant— the speed of light— but this speed is shared between motion in space and the flow of time. Objects don’t speed up beyond light ...

In water, light travels at 75 % the speed it would in the vacuum of outer space, but the electrons created by the reaction inside of the core travel through the water faster than the light does.

Light travels slower in any medium more dense than a vacuum, which has no density at all. For example, light travelling through glass slows to two-thirds its speed in a vacuum.