Time is treated as part of a unified 4-dimensional "spacetime" in relativity (shouldn't this have been posted in the relativity forum?), but for technical reasons I don't think it's quite right to treat it as a fourth spatial dimension, see this thread for some discussion. On your other thread you asked for book recommendations and one I offered was General Relativity from A to B, this is a very good nontechnical introduction to the meaning of "spacetime" and why it makes sense to say that time is treated as a "dimension".-=Red=- said:So do we know for sure what the 4th spatial dimension is? I have read in many places that it is time. I would say this makes sense because we are perceiving time in 3 dimensional slices in our dimension. The same way a 2D being would perceive 2D slivers of a 3D object in its own dimension.