Time travel is never easy to write – or to read! More than any other kind of fantasy, it defies all the laws that govern our world. It’s far easier to imagine waving a wand to levitate a feather, brewing a potion, or discovering a magical beast than traveling back in time to change the past, because there is simply nothing even close to an equivalent in the real world. The past is untouchable. Set in stone.
Sure, we can think about what we’d do if we could travel in time. Would you take life-saving medicine back to the days of the Bubonic Plague? Would you try to stop Hitler’s rise to power? Would you copy down the winning lottery numbers and go back in time to buy a ticket? Could you do any of those things without risking irreversible damage to time itself? It all seems to depend on which theories you buy into. In some fictional worlds, messing with the past at all is a risky business, while in others, time seems to fall into place around the time travelers as if they’d always been there. Harry Potter and the Prisoner of Azkaban seems to be the latter, but there are subtle hints that it is in fact the former.
In the movie in particular, Harry and Hermione’s actions in the past only seem to explain things that came out of nowhere before. The patronus that saves them is a big one, but smaller scenes are added, such as Hermione imitating a werewolf’s howl and Harry throwing pebbles at their past selves to make them realize they have to leave. However, in the book, Hermione explains how dangerous time turning can be:
“We’re breaking one of the most important wizarding laws! … Professor McGonagall told me what awful things have happened when wizards have meddled with time. … Loads of them ended up killing their past or future selves by mistake!”
If it’s possible to kill your past self, then it sounds like the Grandfather Paradox is in full effect here. The consequences of doing such a thing are not fully explained. However, in Cursed Child, Albus Potter’s actions in the past inadvertently cause his father’s death, erasing him from existence. I’m not sure whether to apply Cursed Child logic to Prisoner of Azkaban, since it was written so long after and the time turners there seem to follow different rules (or at least allow the user to travel back much farther into the past). However, there are hints even in Prisoner of Azkaban that time travel is a messy, dangerous business.
There is still one important moment, though, when it seems that Harry has already changed the past before he goes back in time to do so (which makes time travel seem more stable and less risky than it might be otherwise). During the trip back in time, he casts a patronus, saving his past self. If he hadn’t already been there to cast the patronus, he wouldn’t have lasted long enough to go back in time and do so. The only things they change are things that they didn’t personally witness the first time around, or things that they did see, but didn’t fully understand. Is this simply because Hermione is careful and knows to keep out of sight? Or is that just the way time travel works – and if so, why all the sinister warnings about wizards who ended up murdering their past selves?