I read on a website (http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html): [Broken] "The world we live in consists of four dimensions, the three space dimensions and one that is not exactly time but is related to time (it is in fact time multiplied by the square root of -1)." In my layman's world, as I think about multidimensional spaces, I envision basis vectors that have the same unit quantities. They all describe direction and distance (in inches say). Is this too narrow? Do we really have three dimensions that are length and another dimension that is some other beast altogether? Or, is the nature of this fourth dimension indistinguishable from the other three? Is sqrt(-1)T a unit of distance?