Harry Potter and the Prisoner of Azkaban
In the movie/book the time travel element is introduced by way of Hermione’s hectic class schedule. She uses a device called a “Time Turner” so she can literally be in two classrooms at one time by attending one class all the way through and then turning back time and going to the second class. This method of time travel will immediately raise red flags with all the science buffs because we once again run into the paradox of creating mass out of nothing. Hermione does not time travel back into her previous conciseness, otherwise there would only be one Hermione and there would be no paradox, she just time travels in the exact spot she happens to be in and the original Hermione still exists in her first class. However, if she did time travel back in her own conciseness it would not enable her to be in two places at once, which would defeat the whole purpose of the time travel. Thus creating a giant loop in time where the original Hermione continues to go back and create two of herself, which in the end always evens out because she never jumps forward in time she just lives out time from where she originally jumped back. (Much like Primer) However, unlike Primer, the double doesn’t become a constant problem because once the period of class is over the original Hermione will always time travel back to the beginning of class (go the second class and become second Hermione) and the second Hermione will continue to live as normal until the next class in which (for all intents and purposes) she is now original Hermione. (Attends class, time travels, and becomes second Hermione)
For the most part, all of the consequences associated with this time travel method are minimally consequential. If the main concern is the creation of mass, that is easily tucked away by the fact that THEY DO FREAKING MAGIC! Yes, it is mentioned that the witches and wizards cannot create mass, but Rowling does slip up every now and then with certain spells throughout the series. If the main concern is bumping into your double, that is easily avoided since the doubles only exist in the same time frame for a short span (i.e. until the original reaches the end of the first class and time travels back to take the second class) and one double is aware of the other’s every action within the small time window since it previously lived those actions. Also, because this theoretical time travel effects the portrayed events in the same way it did in Primer (the repercussions from visits to the past are always present even before the viewer is aware of past visits or the time travel itself) even if the doubles did bump into one another in the past the “present” has already felt the repercussions of that event since it did in fact already occur and thus effected the character who has now traveled back into the past. If it effected the previous self, then it already happened and they would already be feeling the repercussions of their future actions that occurred in the past. Which is kind of a freeing thought, ‘Hey we didn’t screw up any past events directly related to ourselves, because we already lived them and we’re fine, no worries’. Because no matter what you went back and did in the past it still brought you to this point in which you time traveled back. This is basically the direct opposite view of time travel that is observed in The Sound of Thunder. In The Sound of Thunder events are seen to have “changed” due to the meddling of the past, but if you looked at time the way you do in the third Harry Potter movie the meddling in the past would just be used to explain the way the future already is. The trip always happened, so nothing would have “changed”, it just always was. Which brings up another issue in the time travel method: freewill and decision-making.
In the movie, the reason Hermione knows to throw the rock through the window is because it already happened. She knew it happened and she knew that was the exact rock lying in front of her. Would she have decided to throw the rock all on her own if she had not been there when it first occurred? Maybe, maybe not. If the original Hermione had not have been in the hut with Harry and Ron when the executioner was coming would she have still decided to throw the rock? Who’s idea was it to throw the rock? That is where this “It has always happened” view gets muddy. If the only reason she threw the rock is because she knew it already happened, well then things get tricky because what sparked the original idea? But it is possible that she was looking around deciding to throw something found the rock and the past knowledge of knowing that was the rock just merely confirmed in her head that it was indeed the right thing to do.
Lastly, I liked the movie. I mean as a reader of the books you kind of have no choice, but to support the movies. It is my second least favorite of the movies in the series, (First being the sixth one, they botched that up real well) mostly because I had read the books and I was extremely pumped for Hermione to punch Draco and the punch was horrible! She was clearly way too far away to actually have hit him that hard, it was just awkward and really obvious that she did not actually punch him. So I was pissed. However, when it came out I was pumped to find who they were going to cast as Sirius Black and I approve their choice even though a lot of people did not. Yep, that’s all I have to say on that.