Theory of Time

I have a theory of time that underlies how timelines work and how history is changed -- or isn't. This is sort of fundamental to Timelines and I'm kind of surprised I haven't ever written it down before.

1. You cannot change fundamental events. Big, huge, epochal events. You can't stop 9/11. You can't kill Hitler. You can't prevent the Kennedy assassination. And so forth. What will happen, always, is if you go back with intent to make those changes, you find yourself in a situation where you cannot affect that history, e.g., you show up thousands of miles away, or you show up after the event, or both -- the upshot being, you can never get close enough in time or space to make a difference. However, see exception below, following 2).
2. You can change minor events that make no significant difference, because either they are low-probability events that split off a short-lived timeline that folds back into the main timeline, or they are high-probability events that spin off a longer-lived timeline that might survive on its own (supra- or sub-lines).
3. Exception: A fundamental event that has 50/50 (or close to 50/50) probability MAY cause a major timeline shift, to the left or right. Such "split" events create separate timelines that then go on to develop their own future history. E.g.,
   1. Kennedy either is or is not shot (or survives being shot).
   2. Hitler isn't killed but Wilson is. (I love that meme.)
   3. Hitler is killed and someone much worse takes over, which is the current consensus among a lot of strategists.
   4. The 9/11 hijackers are thwarted before they ever get on the planes, or by US Air Force shooting the planes down, or by passengers Flight 73-ing the other three planes, or whatever.
   5. Joe Biden gets a second term and it ends in a major regional nuclear war in East Asia.