The point of fiction is to make the impossible plausible and in doing so generate a good story. What is actually impossible is beyond the ken of you or I. As far as we know travel backwards in time is impossible in the first place until someone does it, let alone change things when you get there. It makes your whole comment kind of pointless doesn't it? Taking issue with time paradox is somewhat silly since the premise of the story hinges on "what if it was possible?" Like all good science fiction, it deals with the social, political, and ethical questions of something normally beyond means but now becoming possible.
This was another great SK novel ... however, let me point out that changing the past is totally impossible ... even if we had the ability to travel back in time. Before the hordes devour me, let me explain why:
You are regurgitating a popular time paradox here and all of us have heard it a thousand times before. While I don't see the point in wanting to argue with a fictional (and somewhat magical) process in a story, I'll play a long for a bit. Let me point out that I'm sure there were many great, learned men back in the day who took issue with the whole crazy idea that the world is round or the Earth revolves around the sun claptrap. I'm sure the made idiotic statements like, "therefore as a scientist and a really great Monk I must conclude a round world or the Earth revolving around the sun to be impossible." Do you see where I'm going with this? The more we learn the more we discover how little we know. Rehashing old paradox arguments is about as interesting as listening to people argue about how may actual episodes of Star Trek there are in fact.
"If I could travel back in time & kill someone as the novel describes, I could also kill my own mother before I was born! But then I would never be born so I wouldn't exist to travel back in time to commit the murder. Therefore, as a physicist (no, not Sheldon Cooper) & a fantastic singer, I have to conclude that changing the past is impossible."