When I was about 7 years old, I was obsessed with timetravel (I still am) and my dad recorded Back to the Future when it played on TV at Christmas and I watched it for the first time. I loved the film series then as much as I love it now and it fueled my obsession with timetravel. I think it's a really cool concept to think about. I used to come up with theories and calculations on how I would make a time machine and ways/theories of how it would be possible to time travel. There are 3 main types of timeline structure concepts when it comes to timetravel:
Fixed Timeline:
In a fixed timeline, even when someone travels back in time, the future they left cannot be changed. All events remain as fixed points in time. The actions of the time-traveller when they are in the past have already become part of history.This is known as the Novikov Principle which suggests "if a time traveler's actions in the past would lead to a paradox, then the probability of that event occurring is zero, meaning that time travel paradoxes are impossible." This is demonstrated in Harry Potter and the Prisoner of Azkaban when Harry and Hermione go back in time to save buckbeak and Hermione hits past Harry on the back of the head with a stone which Harry remebers happening meaning it had already happend when that moment in time was their present showing that the timeline is fixed and any events that future Harry and Hermione create in the past won't effect their current reality present and will only lead to how their present is. This conflicts with the timeline demonstrated in Harry Potter and the Cursed Child (not very good for continuity) as in the Cursed Child, when Albus and Scorpius go back in time, the events they change in the past DO alter their future. This concept is known as a Multiverse.
Multiverse:
The foundation concept of the multiverse theory is alternate timelines. This means that when a timetraveller travels back to the past, he or she can do anything and be personally unaffected. For example, if you were to go back in time and stop your grandparents from ever meeting, you wouldn't disappear as you wouldn't have altered YOUR timeline, only created an ALTERNATE timeline where you don't exist. However, you would now no longer be able to travel back to your own timeline when you are alive and subsequently when you return to the present, there would be no trace of your existence anywhere. So yk thats kinda a problem and probably something you should be aware of before deciding to change the course of time in a multiverse reality. Of course this also does open the concept of parallel universes and the probabilty of being able to switch between them which is also a cool concept to think about (another one of my favs) The multiverse concept is demonstrated in Back to the Future 2 where Doc explains that the timeline "skewed into a tangent creating an ALTERNATE 1985" but not in Back to the Future 1(this uses the concept of a Dynamic Timeline). This concept is also demonstrated in the book "Timetravelling with a Hamster" by Ross Welford which was one of my favourite books when i was younger. In this book when Al comes back to the present, his disruption of events erases his existance in his own timeline but as he still exists somewhere else, he himself is not erased from existance and therefore does not disapper (his own timeline remains unaffected).
Dynamic Timeline:
THIS is the timeline demonstrated in Back to the Future part 1. In a Dynamic Timeline, events altered in the past have definite impacts on the present. This is the timeline you need to be aware of most if you are timetravelling as with a multiverse timeline, you still have opportuinity to go back in time and fix your mistakes but with a Dynamic timeline, if you change an event which causes yourself to be erased from existence, if you don't fix it before it's too late, you will be gone and therefore not have the ability to put the past back causing the timeline to be altered permenantly without you in it. When Marty changes the past so *SPOILER WARNING* Loraine falls in love with HIM instead of his dad, he begins disappearing throughout the movie, showing that he is in a Dynamic Timeline. He then only has a certain amount of time before he fades away to get them together to save his own existance. Of course if timetravel were possible, we wouldn't exactly know whether you get this amount of time before you fade away or whether it would be instantaneous meaning you wouldn't be able to do ANYTHING to save your own existance as you would be eliminated as soon as you altered the past. Personally, I think this would make more sense than having a time delay although I guess the time delay in Back to the Future may be explained due to the fact there was still TIME for Loraine and George to get together and Marty would only truly be erased if they didnt kiss on the dance floor at the Enchantment under the sea dance as THAT was the moment Loraine knew she wanted to be with George forever (ik how romantical).
Paradoxes:
Grandfather Paradox:
I'm starting with the Grandfather paradox as it is probably the most well known paradox. The concept is, if you go back in time and kill your own grandfather, you will erase yourself from existance and therefore wouldv'e not been able to go back in time and kill your own grandfather, so your grandfather would live, you would be born, could go back in time, kill your grandfather and so on and so forth. This loop would create a paradox known as the grandfather paradox. If you are in a fixed timeline however, this paradox wouldn't be able to occur. Whatever action you take to kill your grandfather in the past, the timeline will right itself so he isn't dead and you are still born. For example, you kill the man with your grandfather's name but another man takes his idenity and THIS man marrys your grandmother has your mother/father and then they have you, which therefore means there is no paradox.
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