Everything in the universe is moving at light speed. Normally we move primarily through a temporal dimension, rather than a physical dimension. When we exert energy to move in space we can make the vector veer off from the temporal dimension, but we can't make it go faster than light-speed no matter how hard we try (which is how time slows/space contracts). Most of the complexity in relativistic physics follows from there.

Not yet up to where the question of superstrings and their extra dimensions come in, just beginning to work on reading the section on gravity.