Time travel and Alternate Realities - A rant

I just had a long rant to my housemate and thought it would be interesting to just put it out there. If you, dear reader, have any thoughts, please comment below.

The instigator for this rant was me going to see Source Code, as I mentioned in the previous post.
In Source Code, the entire movie is Jake Gyllenhaal in a machine that has him live the last 8 minutes of another person's life before he dies. This is to investigate the bombing of a train and to find the perpetrator before he sets off a dirty bomb in a major city. That's all fine and well.

****SPOILERS****

However, the explanation given by the inventor is that these aren't just memories, they're alternate realities. Jake's actually displacing the man and is, for all intents and purposes, that man in that reality. This usually wouldn't matter since he dies one way or another after 8 minutes. At the ending of the movie, Jake insists on going in one last time before they pull the plug on him in his original reality and he dies. This is to save everyone on the train, including Michelle Monaghan, with whom he's fallen in love. After saving everyone, he sees the clock count down his last few seconds, and he prepares to die. Presumably the original man will reappear, somewhat confused, be a hero and get the girl.

But that's not the end. Oh no, that would be too sad. Instead, to Jake's surprise, he's still alive and goes on to, presumably, live happily ever after as the man he displaced.

BUT WAIT.

He's not himself in this new reality. He's a history teacher. What happened to the original person? He's been completely replaced by Jake. Is he dead? Also, Jake now has to fake being someone else. Someone with different memories, a different personality, different skill set. How can he pass himself off as a stranger? The girl he's in love with is in love with the original person. She'll eventually realize he's essentially a different person from the man she used to know. Etc, etc.

This has been bothering me for the last week.

****END SPOILERS****

I love time travel and alternate realities. I think they're awesome, but I am very wary when they're used as plot devices.

If the entire premise of the TV show/movie/book hinges on time travel/alternate realities then the mechanics and rules should be well thought out. Admittedly, a bit of hand-waving is necessary, but there should be a consistent set of rules with loop holes thought of in advance. For example, in Quantum Leap, Sam Beckett displaces people much like Jake Gyllenhaal does in Source Code. However, they explain that the person who is displaced appears in the "waiting room" in the present day and is returned once Sam leaps out of their life. Quantum Leap was not completely perfect, but gave a reasonable explanation for what happens to the poor sod who's replaced.

Another inconsistency that's been bothering me takes place in my beloved time travel series, Doctor Who. In the revived series, in the first season, they establish an important rule. You may exist in the same time as a previous version of yourself, but can never have physical contact. This creates a paradox that the time-space continuum really hates and creates rips in the fabric of reality. This formed the entire basis of an episode.

Fast-forward to the end of the latest season. As reality is collapsing on itself, Amy Pond is sealed in the Pandorica until approximately 1997, and subsequently interacts with and physically touches the 1997 incarnation of herself. This may be acceptable since the Amy in the Pandorica and the young Amy are not precisely the same person, due to the collapse of reality.

However, the most recent Christmas Special does infringe upon this rule. It's the Doctor Who version of a Christmas Carol and the Doctor is playing with a Scrooge's past selves to make him into a nicer person. It nearly works, but doesn't quite until the Doctor brings the child version of the Scrooge into the present day and horrifies him with the nasty man he will/has become. This finally softens the old Scrooge's heart. And he hugs his past self.

This blatantly disregards the previously established rule. And this is the same individual, not going through the collapse of reality loophole, otherwise the Doctor's scheme would not work.

Basically, time travel and alternate realities are fantastic devices, but my plea to writers is this: Think of the rules and stick to them. No more, no less.

And I didn't even mention Back to the Future