Why does time run slow near a massive body like a black hole?
Clocks which are far from massive bodies (or at higher gravitational potentials) run faster, and clocks close to massive bodies (or at lower gravitational potentials) run slower (slow is low). This is because gravitational time dilation is manifested in accelerated frames of reference or, by virtue of the equivalence principle, in the gravitational field of massive objects. It can also be manifested by any other kind of accelerated reference frame such as an accelerating dragster or space shuttle. Spinning objects such as merry-go-rounds and ferris wheels are subjected to gravitational time dilation as an effect of their angular momentum. This is supported by the general theory of relativity due to the equivalence principle that states that all accelerated reference frames are physically equivalent to a gravitational field of the same strength. For example, a person standing on the surface of the earth experiences exactly the same effect as a person standing in a space ship accelerating at 9.8 m/sec2 (that is, generating a force of 9.8 N/kg, equal to the gravitational field strength of Earth at its surface). According to general relativity, inertial mass and gravitational mass are the same. Not all gravitational fields are "curved" or "spherical"; some are flat as in the case of an accelerating dragster or spacecraft. Any kind of g-load contributes to gravitational time dilation.