I know I am probably missing the point, but something is not clicking for me in understanding time as a dimension. Isn't time just an arbitrary measurement of distance? It's always just us 'counting hippopotamuses' while watching something moving from point A to point B (we don't have to use seconds or minutes, because we just made those up for our convenience, didn't we?) I understand that if we want to accurately describe an event, we need to give an x,y,z and a "time dimension" measurement. But how does measuring or describing something make it a dimension? I guess I don't really understand the concept of "past" either. All the matter in the universe exists in the state we see around us right now. The matter no longer exists in the state it was in yesterday. You could never go back in time because all of the matter has changed. The time we experience is just the accumulation of our current, "now" experience of the matter around us. I'm on the verge of a long ramble, maybe someone can set me straight before I confuse myself too much more?