Time has historically been closely related with <answer>space</answer>, the two together merging into <answer>space</answer>time in Einstein's special relativity and general relativity. According to these theories, the concept of time depends on the spatial reference frame of the observer, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion. For example, if a <answer>space</answer>ship carrying a clock flies through <answer>space</answer> at (very nearly) the speed of light, its crew does not notice a change in the speed of time on board their vessel because everything traveling at the same speed slows down at the same rate (including the clock, the crew's thought processes, and the functions of their bodies). However, to a stationary observer watching the <answer>space</answer>ship fly by, the <answer>space</answer>ship appears flattened in the direction it is traveling and the clock on board the <answer>space</answer>ship appears to move very slowly.
Historically, time has been closely related with what?
space