Time

Clocks are apparently simple, happy things with no worries. Like feathers in the wind, or perhaps going down a river.

Clocks give us the completely artificial impression that time is cyclic. That if we wait they will come back to the same position they were at before, and that they will continue to do so over and over and over again.

However, time, as far as we know it, is not cyclic (we don't really know what "time" actually "is", but let's forget about that "tiny" detail). Time is not like a watch, nor even a river. Water in a river can potentially come back to the same river over and over again, but time does not go back to before. It just runs in a single direction, apparently towards what we classify as infinity.

That's actually quite a big problem for what we would call "time-travelling". Even if we were to find a way to go "back in time", right here, we would quickly realise how that simply wasn't a very good idea. Even if we were to travel just a few days back in time, as we came out of our time-machine, we would simply die. We would be outside our planet, in a position that the Earth would occupy in a few days, but still really far away from it. But maybe we could go back inside, or have a really nice space-suit to protect us. And then we would probably think: well, ok, so let's just go back exactly one year back in time - surely the Earth goes back to the same position, so then we will be able to land on Earth.

We would soon find out that's not really the case. The Earth would be even further apart from where we would be. The problem is that not only does the Earth rotate around the Sun, the Sun also revolves around the centre of our Milky Way. And there's more: our Milky Way is moving towards Andromeda at even greater velocity; and both are moving towards a very strong potential well at even greater speeds.

So there you have it. Despite our silly/human views of how everything is cyclic and how time is just like that, reality is very, very different. We may not be able to see it at first, because our lifespan is so so small compared to the actual passage of time and its effects in the real Universe, but it is there.