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Source: https://writers.stackexchange.com/a/42556
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

 **Time travel v. 1** : Back to the Future model. In this model people can go back in time and change the past; however, they must avoid the grandfather paradox or else a "bad thing" will happen like disappearing, which resolves it by force, but may also destroy the universe. This brings a second "dimension" of time into the mix, wherein regardless of how Marty moves in the first dimension he's always going to disappear unless he fixes the loop that will allow him to go back in time.

The problem with this form of time travel is when the "bad thing" does happen. The act of the bad thing happening causes many changes in of it self. Imagine if somebody noticed a Marty disappearing that would set in motion a series of events that would probably have prevented him from ever being able to disappear, which causes more problems. What BttF fails to define is what would happen if Marty disappears, would all of causality keep going in loop where he cannot exist but disappears and thus can exist, or would time in each case go on without him, creating what are essentially two universes. Or would somebody notice Marty disappearing and/or the universes changing into one another thus causing a cascading effect where eventually, everything this side of the event horizon will be in a state of complete uncertainty. Either way the "bad thing" seems to be an overly convoluted way to bring about a simpler result, and leaves a whole lot to be explained. Finally, ignoring the "bad thing" entirely, this form of time travel is hard to keep straight in your head.

**Time Travel v. 2** : The Terminator way. These rules are pretty simple, when somebody goes back in time there is no need to remember the "previous" timeline at all, you can kill your own grandfather or do whatever you want because it doesn't matter, a whole new separate timeline has been created.

There are a couple problems here, the first one is that if all of the world lines in space time can change then what is the point of time anyway? 4D space time is what allows time to be possible. And if it is not necessary wouldn't it be simpler for conscious beings to just live in a 4D reality that changes?  
Although the only thing this paradox defies is Occum's Razor, and universe design best practices, it is a pretty clear problem, while a "bad thing" will always be a little more foggy as to whether or not it may be logically consistent; however, many people will still prefer the Terminator method over something as messy as disappearing.

**Time Travel v. 3** : The Lost method. In Harry Potter and Lost, the future is destiny. Anything that a time traveler thinks they might be changing, is actually just something they never knew happened, and any attempts to change history will ultimately fail. Spacetime is a completely static object, albeit yet highly responsive to our actions, constantly thwarting that which might change things.

The problem with this form of time travel is probably the easiest to spot. So maybe the first time one tries to kill their grandfather it turns out, that he wasn't actually the father of your father. And maybe your gun jams on the second attempt, and on your third attempt he already had sex with your grandma by the time you got him. On your 8th try you might even learn that your dad didn't even do the thing you are trying to prevent him from being born for, but it's principal at this point. Eventually; however, your going to have witnessed nearly every inch of every second surrounding the time your grandparents bang, and the universe will have run out of space to bring up some wacky antic to stop you.

As logically inconsistent as this seems there can ever be any proof that time travel actually works via a self fulfilling destiny. Both Harry Potter and Lost might actually follow the Back to the Future model, we are only seeing things from the point of view after a stable sequence of events have been set in place. In BttF we only see when Marty first goes on his adventure; however, as his t2 goes on his memories of hearing about how his mother and father met by being hit by a car will eventually disappear with the image on the picture. We don't see this (as it is kind of sad), but they will be replaced by the knowledge that he was named after the person who brought his rich parents together after being hit by a car. From his new perspective his adventure will never have changed anything, he was simply fulfilling what had to be done. Likewise, when Harry saves himself he might actually just practicing responsible time travel ship by keeping the timeline consistent. Even though he knows that he would some how survive the death eaters, trying to wait for another savior could easily put him in an endless loop of getting half his soul sucked out, saving himself, only to have himself not save himself, and so forth, or wind up causing a "bad thing".

There are also reasons why each of the other types of time travel might appear to be this one; however, I don't want to spend all day on this.

**Time travel v. 4** : The last one. It might be the case that whenever somebody might use a time machine to cause a Grandfather paradox, or some other kind of problem, the time machine just doesn't work. I cannot think of an example of this one, but I would bet it would lead to tension between characters who cannot ever get the time machine to work, and the characters who have the resolve required to stop themselves from doing whatever these things are.

I cannot think of any paradoxes with this form of time travel, at least none that are unique to it. Nor does the failure of the time machine introduce a hundred more questions like a "bad thing" does; however, it does not give you quite as much creative freedom as the Terminator method does, which could possibly be why I haven't seen it implemented, sacrificing quality for consistency defeats the whole point of consistency. However, I would argue that there is something to be said about originality.

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Original score: 2