Einstein’s theory predicted that time and space would have to stretch or contract as someone traveled with increasing speed. And out of special relativity popped a cosmic speed limit: nothing could ever exceed the speed of light.
There’s actually a minor clarification necessary here: Einstein’s speed limit is the speed of light in a vacuum. Light slows down when it moves through a material like water or glass, and then it’s perfectly possible to exceed this reduced speed of light – up to its speed in a vacuum, of course. Anything moving faster than light in water or glass produces the luminous equivalent of a sonic boom, called Čerenkov radiation. It’s what gives underwater nuclear reactors their attractive blue glow.