Scene from Looper, Sony Pictures
You’re not allowed to change things in time travel. Every thing that happens is fated to happen and will happen regardless of what you try to do. Each person has only a single timeline which can “loop” back upon itself but never replace one or the other. So at any given point in time you may have multiple “instances” of a person but looking at the entire timeline, there is only one instance of a person that starts with birth and ends with death.
Let’s take an example:
Collin and Mary are driving along a road happily. Suddenly a deer appears, Mary screams closes her eyes and slams on the brakes but is unable to avoid it. Collin is thrown from the car through the windshield and Mary finds him dead on the highway.
Mary, now grief-stricken continues on in time until she reaches a point at which time-travel is possible. She hops into the time machine to the point prior to the accident and then time-ports Collin our of the car and further into the past. History is changed right?
In time travel theory 1, because past Mary saw and witnessed Collin’s death, Collin has to die at that point in time. Even though future Mary seems to have saved Collin from his fate, events will conspire so that Collin ends up back in the car at the point of death.
In our fictional scenario, future Mary tells saved Collin about his untimely death and how she came back to save him. But now, destiny cannot be thwarted and fate takes over. Like the monkey’s paw, it’s inevitable that future Mary’s well-laid plans will go astray.
Perhaps saved Collin now goes to sleep but inadvertently rolls over into the time-machine and jumps forward back to right before the point of impact and is now in the backseat. He sees past Mary close her eyes, he sees future Mary jump back and jump out the past Collin.
The car hits the deer and from the backseat, saved Collin is thrown through the front window and dies.
When past Mary opens her eyes she sees Collin dead on the highway. She doesn’t realize that this Collin is a few hours older than the Collin that started the road trip with her, to her it’s the only Collin she knows.
Future Mary wakes up and finds saved Collin gone. She rushes to the scene of the accident, fearful of what she knows happened. There’s saved Collin, dead in the highway. No matter how many more times she comes back to try to change the past, she can never prevent the events that she herself witnessed. Fate must be served.
In this theory, we see that each person can only have a single time-line. There is no branching possibilities, if you did something in the past, that thing has to happen. It cannot be altered. You may think you alter it but whatever you are currently experiencing must occur. An individual’s time-line can “loop” back on itself and there are two instances of the same person visible at the same time but taking a macro-view that’s actually only one person. Each person is represented as a single line with an anchored beginning and end. No matter how many times you twist and loop the string it always ends at the same point.
Notice that this doesn’t prevent possible time paradoxes. For example, supposed Mary’s bringing back of time-machine technology is confiscated by the government in the past. They use that technology to develop time-travel in the future. This is the same time-travel technology future Mary uses to go back in time to try to save Collin - thus inventing time-travel.
The Terminator series of movies is an example of this theory. In the future, a sentient computer network called “Skynet” is being defeated by a human named John Connor. “Skynet” decides to send a robot from the future back in time to kill John Connor’s mother before he is born - thus preventing SkyNet’s imminent defeat. Lo and behold, events conspire so that *spolier alert* the very act of trying to alter the timeline ends up creating John Connor.
Scene from Terminator 2: Judgement Day, TriStar Pictures
Later on in the series, we discover that the act of sending future technology into the past ends up creating SkyNet and the Terminators. The future is inalterable and all attempts to change it only serve to make the future more inevitable.