Space-time. Conceptual artwork of a clock face on a background of stars. This represents the concept of space-time, where time is not invariant and is dependent upon the structure of space. If space is warped by strong gravitational fields (as near a black hole) or by near-light speeds, then time can vary in duration. Space and time have to be recast as parts of a 4-dimensional structure that is called space-time. This unification of space and time was predicted by Einstein's theories of Special and General Relativity.