A second is measured thus: "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom." What if the cesium atom itself is moving close to the speed of light? Would that not change the time measured? Also - at the centre of a black hole .... time stands still.. As one moves away from the black hole ... time starts moving again because less gravity .... if we keep moving away from heavy objects into an area of space where there is NO gravitational pull ... does time then run at *infinity*? Whats the upper limit of how 'fast' time flows? I am getting a little confused when I think about there being no absolute time. Because even the duration of x periods of an atom - surely that 'duration' will change depending on if a) the atom is moving close to the speed of light, and b) what if it is at the centre of a black hole?