The absolute speed limit of the universe is the speed at which light travels in the vacuum of space – 299,792 kilometres (186,282 miles) per second, or ‘c’. Nothing can go faster than this as doing so would enable it to violate the basic laws of cause and effect. Einstein’s special theory of relativity explains the effect this has on the laws of physics – eg if you had a spaceship that was powerful enough to travel close to c, you’d find accelerating those last few per cent got harder and harder, as the craft got heavier and heavier. No object with mass can ever reach c itself – light can only travel so fast as it is massless. That said, light travels more slowly when it passes through materials such as water and glass, hence why it bends, or refracts.
Answered by Giles Sparrow.