New little series I'm going to call, Causality4. An examination of causality in terms for specific groups of people.
I won't even start by explaining causality. You are probably wonder why? Well because a generic definition isn't a good way for anyone to learn about it. I think everyone can better understand it by applying the concepts to things they already know. Since the new Harry Potter movie came out I'm going to start by examining causality in the movie (book).
Causality for the Harry Potter fan!
I think this is a great place to start, since so many people dig the book (I don't but I still watch the movies). In the latest movie we find that Hermione is given a little trinket that enables her to get to all of her classes by going back in time. She uses the trinket knowing full well that there are some rules surrounding the process, rules which in the movie at least, aren't discussed. The rules are actually the rules of causality and bring up some small paradoxes, like what happens if you kill your grandfather or your father before you were ever born. Won't go into that, so back to the movie.
Well, as you watch the movie (or read the book) you get a chance to see the story unfold from the perspective of Harry and Hermione as they are in the story-line. Some strange things happen during this time, like tossed rocks, and a mysterious person in the bushes. Later you get get to see them go back in time, just the two of them, and they get to see all of these prior events actually happening. Now there is a causality issue here. I mean what happens if they *break* the series of events that led them to go back in time in the first place? Those are the casuality rules that Hermione is of course following to make sure bad things don't happen. Well it turns out that all of the *strange things* they experienced the first time were actually the things they wind up doing in order to change things. There was ever only a single reality, and that reality, even before they go back in time, assumes that at some later point they did.
I actually enjoy the way they handle this whole process because I believe they accidentally define a universe in which you can't break the rules of causality. If you had killed your own grandfather then you couldn't have gone back in time in the first place. No matter how hard you try to go back and whack him, there doesn't exist a universe in which you are capable of doing that. In other words, there is no series of events that would allow you to go back in time and kill your grandfather because doing so would have prevented the event from happening in the first place.
That kind of ends causality in Harry Potter, but maybe you get an idea of what it is all about now. Everything that happens is a direct cause of everything that happened before. You can't short-circuit the process because there doesn't exist any set of events that would allow you to do so. Note this assumes you can only go one way in time, backwards. That is truly the only direction they went in Harry Potter, and at the point where they re-join themselves back in the hospital they've essentially created a closed loop of events that will continuously happen over and over again. This can be hard to understand that the events will continuously repeat, however, the rationale is that things that aren't observed again don't actually happen again as part of your reality, so why worry about them. The series of events that put them where they were when the movie continued and eventually ended will happen again, but next time they will happen to their *alternates*.
Causality has other explanations. The Harry Potter example is a bit contrived, since I'm using a fictional account as the basis. I'm not able to go into concepts of relativity of time or quantum states, but you can understand the basics without understanding the details.