THE ONE DOOR THEORY
If you were able travel back in time does it create a new universe and a new timeline? If yes then the multiverse is real, if no then there's only one universe. Is knowing the multiverse being real necessary? For a regular person no, but not me, I believe that every question deserves an answer but no answer is enough for the questions like this so what we do is theory
Rule 1: The Entrance
When you travel back in time, A door is the entrance, you open the door and you enter (the door is a metaphor it can take any form), the moment you stepped in, the door closes you are stuck there in that timeline and it becomes a loop.
Was it actually a loop? No its not a loop, you can go back to the future but it requires you to be as fast as 99.55% of the speed of light to go back 10 years later which in 2016 it would be almost impossible but because the universe has already written the history theres no record of you going 99.5% the speed of light so the universe stops you
Year: 2026 -> 2016
Rule 2: The Loop
The moment you interact with the surroundings of that timeline the door closes and you can't go back because you become part of that timeline, your life becomes a loop in a never ending cycle, Think of the timeline being a straight line, because you travelled back in time to 10 years earlier that line bends and becomes a shaped like circle and becomes a loop. Do you still die in this timeline? Yes, Then why call it a loop? Because of how the timeline bends,your younger self now travels back in time also sees his younger self and so on and so on. you know you see your younger self but you don't remember seeing yourself in 2016 because the universe prevents you to.
Remember the past is already written and the future is continually being written
Year: 2016 stuck in a loop
Rule 3: The Script
Because you are now part of that timeline, you cannot contradict it. The history is already written, anything you try to change was already what caused it. You fulfill the history but you change it, what ever you do is what just caused the past of the future. If you try to prevent a massive event you are just the one what caused it because the law of universe doesn't let us change what is written. The written history would still continue the event with or without you.
For example: WW2,Covid,Concerts, even small birthday party, you cannot interfere what is written but you're part of it.
Let's say before you travel back in time you opened any online platform for any legit time traveller witnesses or even your face being on that platform in 2016, you found nothing legit so you travelled back in time in 2016 assuming you can change the history, but universe says no, you try your best to show on any online platform or show up in a news but the universe doesn't let you to because of the memory of you browsing in the internet doesn't see you but you exist there.
What if you try to meet your younger self?
The universe doesn't let you meet your younger self why? Because you don't remember meeting someone in the year 2016 but you were there the universe just wouldn't let you to meet no matter how hard you try it just wouldn't let you to because the history is already written and you cannot rewrite it.
But what if you bring a newspaper from 2026 in 2016?
No one would know because the universe has already written the history that a newspaper from 2026 wasn't supposed to be there but you bring one assuming someone would read it, but no, the universe said no one read it because the history is written, people from 2016 would probably see you as crazy or they would just walk off, yes you existed there but the history still plays as what's written
So if the timeline bends like a circle are the other also stuck in a loop?
No you were the only one who's in loop because your Future is being in the past but the others Future are them being in the Future, now that you're in the past you cannot prevent events you just become the one what caused it
Is it true that there's a version of yourself being a doctor in the other universe?
No your future can change depends on what you decide because the future is still being written but the past cannot be rewrite because it's already written.
If you die in 2016 then who made machine?
It's still you, you are stuck inside a loop then your younger self builds a time machine and travel back in time and die in 2016, you are the one writing your future but your in the past right? Yes you're in the past but being in the past is your future and you're stuck there forever now your younger self gets to experience the same way you experienced because both of your future depends on what you decide.
Was the universe like an agency or a group and why it has to follow these rules what makes the universe follow those rules?
Time makes the universe follow it because time and universe need each other to exist but somehow they both need to follow each other because time needs to run at infinite and universe records the history of time running