text: time and the 4th dimension? why is time considered the 4th dimension of space? why not something else? how is it related to length, breath and height? why also we can only move forward in time and not back if it is a dimension?  a 4 dimension object is called a hypercube is it not? how then is this hypercube related to time? first of all: time is directly related to space in a way.  according to special relativity, time and space both dilate with gravity and speed.  also, to give the specific location of an object, time should be included.  for example, if i say to meet me at the corner of the street, it would be unlikely that you would find me because i did not mention when.  time is not a spacial dimension.  travelling back in time isn't possible due to many things, such as thermodynamics.  basically, time can slow down, and even stop, but not reverse.  the why isn't clear to me, but i haven't seen a plausible way for time travel to occur.  this hypercube thing uses extraspatial dimensions which are theorized to exist.  the difference here is that it is a spatial dimension, not time.  there are supposedly up to 13 different dimensions in which this fourth dimension could be.