This is the one that has quickly become the most debated in time travel paradoxes. Called the Grandfather Paradox, one person might feel that they want to die. Perhaps they don’t though but they feel that their family was part of a lot of bad things throughout history.
They are willing to selflessly go back in time and kill their grandfather or great(x times you need) grandfather, all to ensure their family does not exist. This will sadly cause them to not exist either but by doing this, they run into a paradox. If they kill their grandfather, then technically they prevent themselves from ever existing.
Therefore, they could not have gone back in time to kill their grandfather. Basically, it means that it would be like they never went back in time at all. Now, another person could kill their grandfather as long as there was no family relation. However, they simply could not. Keep in mind that this paradox is not exclusive to grandparents but several changes one might make in the past when time traveling.
A lot like the Grandfather Paradox, the Let’s Kill Hitler Paradox works pretty much the same way. However, are some differences here too, especially when looking at the timeline theories. In this instance, a person perhaps is tired of living and wants to go back in time to kill themselves.
Apparently, they don’t have the nerve to kill themselves but could easily kill a baby that happens to be them from the past. It could be any younger version of them, however. This does not need to be a baby, technically. In any case, they go back in time but seem to fail every time they try to kill their past self. Why?
They cannot kill themselves, because they would not be alive to have done so. Yet they could cause possible damage, such as markings that they will experience. However, you too will experience them. Let’s say you shoot at your younger self but miss and shoot their arm. You could look at yourself and see a mark left by the bullet in your arm, as you were shot while you were younger by some strange man who looked a lot like you.
The Bootstrap Paradox is thought to be the most respected theory in the connected timeline. Why? It follows all the rules of the constant past concept without breaking those laws. Basically, in this paradox, you experienced something in the past that was always going to happen. It is part of a casual loop.
In this format, one event causes the second event. Yet it was the second event that was actually the cause of the first. A great example of this can be seen in DC Comics’ The Flash character. Reverse Flash goes back in time to kill young Barry Allen but fails, then the Reverse Flash decides to instead kill Barry’s mother.
Barry got himself out of the home but could not save his mother. By going back in time to save his mother, he’d never become The Flash. Therefore, he has to let his mother die to become the hero the world needs. If she never does, he does not become a forensic specialist for the CCPD the night of a Particle Accelerator Explosion, which gave him his powers. Yet if he’s never The Flash, he cannot run back in time to stop himself from being killed either. Which creates a loop of needed actions.
It’s interesting that we bring up The Flash in this scenario of time travel. He actually does go back in time to stop Reverse Flash from killing his mother in the Flashpoint Paradox story-arc. This is a key part of DC Comics that causes a few rifts in the multi-universe itself and ends up causing a Crisis Event.
In any case, when Flash does this he technically creates a new timeline and universe where his mother exists yet he is not The Flash. A lot of things are the same but many things are not. Bruce Wayne is killed instead of his parents, causing his father Thomas to become Batman while his mother actually becomes this universe’s Joker.
Superman does not land in a field but rather, in the center of the Metropolis. Wonder Woman and Aquaman have an affair leading to the death of Mera, the Queen of Atlantis. Thus causing a war that will wipe out the planet. This must be changed, so Flash finds a way to recreate the conditions that gave him powers, nearly killing himself to do so. All to go back in time. Yet this does not stop changes from happening because it still causes a rift where a third universe or timeline comes about.
If we did go back in time, it is most likely that we would end up being connected to the laws of the second timeline theory we proposed. This is the timeline where the past is constant and cannot technically be changed. When it does change, it would then create another universe branched off on its own.
Basically, if we kill Baby Hitler then we will not see the result of this in the timeline we return to. Hitler always existed in our timeline and always will. However, the interesting notion is to consider how we could end up remaining in this Hitlerless universe. One would conclude the only way to do this is to remain in the timeline we’ve gone back to.
Therefore, the changes you make would branch off a different timeline and you’d remain in it. However, you might not have to remain behind. It is interesting to note that if we did use the mirrored parallel universe to go back in time, to begin with, we’d likely be able to visit this new universe we created eventually. If we can access one universe, what stops us from accessing another? We just shouldn’t assume our time travel changes will affect our present timeline.