Well... My Achilles' heel is science including physics (I'm in year 10 England & I don't recall learning about momentum (maybe I was sick (2 days this school year) so I think I have this ahead of me)) so this theory isn't based on anything 'solid' as it's just my thoughts (very view of them) about time combined in one theory. Well let's say someone stumbled because of my legs position & for example broke leg, my theory is that if I'd gone back in time (in this example) & changed position of my leg, I wouldn't simply change past, but I'd go to another universe (the universe that someone stumbled would be (let's call it)universe 1 & in one he/she/it didn't would be universe 2) that would be different from the one I was in as in the "universe 1" that someone broke leg & let's say that he was a very important businessman, couldn't get on very important meeting wouldn't still be able in "universe 1" as he has broken leg, however the businessman in "universe 2" which doesn't have a broken leg could get onto a meeting. (Ok it seems to me that I'm complicating it more simply put: my theory is that there are infinite number of universes that have differences some of them are very small such as e.g. me eating a yogurt but some have large differences such as lunching a nuke or even bigger. So if I'd go 'back in time' & change anything the I wouldn't be in the same universe in which I was in before. Simpler: think of universes as about different timeliness. Come to think of it, I just realised (or reminded myself) that my theory about time is something that doesn't share much (if any) similarity with the time travel in the show.Anyway I hope this will help.
& in other universe I wouldn't write this reply ;d