system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why couldn't Harry have used Hermione's Time Turner to go back and stop Voldemort and his parents dying? Thus stopping years of tragedy?
gpt
Time travel in Harry Potter doesn't work that way. Rather, it works in causality loops. Harry can't save his parents in the past because they're already dead in the future. This is how the Patronus works in the third book - he knows he can cast one because he already casts one in the past/future.