Being about to get on a plane, I get around to thinking about time and space. Actually the same thing, or so the physicists tell us, although in day to day life they seem pretty distinct.
Until you fly. I've always been enchanted by air travel - how it brings home the utter irreversibility of time. You get up, it is morning, you are in London. You get on a plane. By evening you are in New York. This morning you were in London. Mental. Mind boggling. Just this morning, but it is already 3,000 miles away. Unreachable, untouchable, from your now present evening.
Of course the days that we fly just make very clear what is always true: By every evening that morning is as far away from us as New York is from London. You can't go back there any more than you can reach back across the Atlantic. We just don't notice most of the time.
So time and space are the same thing, after all. Except in one crucial aspect: Distance can be travelled both ways, but for the most part time moves in only one direction. I say for the most part as I gather (I hesitate to say understand) that if you are in a black hole time can move backwards.
You can come back to London from New York if you are minded to spend another day travelling to do so, but, as I'm not in a black hole and I'll take a guess that neither are you, we cannot come back to this morning from this evening. The present is a place to which we will never return.
