Paradoxes in future and past

Rating: 9/10 (5 March 2005)

I liked a lot this movie. The fact they travel to the future and the past to prevent horrible things happen just makes a great movie. However, there were certain things I didn't like:

Jennifer is now played by another actress, so that makes the storyline act with no sense.

These paradoxes appear:

--If Biff traveled to the past and gave himself the almanac, making another 1985, Marty and Doc wouldn't ever have traveled to 2015 because Marty wouldn't have known Doc, because he's in a hospital. If they never traveled to 2015, that would make impossible for Biff to travel to 1955. That is, a paradox in time.
--Now that Biff is rich and put Doc in a hospital he wouldn't be able to create the Time Machine, making impossible for the future Biff to travel. Another paradox.
--If now exists another 1985, with the rich Biff and Marty's dad dead, Jennifer wouldn't be able to see future's Marty's dad, when she's in her future house.
--Jennifer sees her future self. Why the future self is surprised of seeing the 1985 Jennifer? It's obvious that the future Jennifer was sometime the 1985 Jennifer, so she knows that she will find the 1985 Jennifer in her house. But no, she's surprised.
--If Doc and Marty go to the past, and prevent Biff be rich with the almanac, and the normal 1985 returns, then the normal 1985 Marty and Doc wouldn't have a reason to travel because they do not know that the almanac exist. In that reality, Doc's and Marty's travel would end when they returned to the 1985 (that was the alternative one, but since that now is normal, they don't have to return) Then they didn't travel to the past to prevent Biff having the book. That's a great paradox.
But even though all that, is a good movie. I liked it a lot.