Before posting reply, please think about time as a concept that is used to describe a change relative to some other change. For example, a traveler will arrive on a destination after earth completes x rotations. At the beginning, people used earth rotation as a reference change and called it a "day". Today we use change processes on a mechanical clock or on atomic clock as a reference change . The main point here is that it is all about change. Theory of relativity postulates that internal changes in an object are slowed down when that object moves very fast. For example, If one twin leaves the earth, travels very high speed then returns to the earth and finds that his twin aged 50 years more than he did then that means that all changes in his body (cells life cycles, electrons travelling through neurons, brain synapses building, etc) were slower compared to the changes in his twins body. One of the experiments that was used to prove it was comparison of changes on an atomic clock that orbits the earth with same kind of changes on an atomic clock that is on the earth. Since our ability to observe changes on the subatomic level is very limited, what are the chances that some other factors caused the difference in between observed changes on the atomic clock on the earth compared to observed changes on the atomic clock that was orbiting earth? Is this experiment sufficient to conclude that all subatomic changes , including the atom spin and subatomic particles movements , are slowed down compared to the changes in the atom that moves with slower speed. Are time dilation experiments so far sufficient to allow general application on a complex system such as a living organism?