After all, reality is entangled and there's no well-defined framework of physical law that allows FTL travel and also reduces to something like relativity as long as you don't turn on your magical engine. Everyone who doesn't have a magical FTL engine would still see your FTL travel as being time travel, and the same questions would have to be answered as you raise below: what changes are allowed, etc.
This is a popular but incorrect idea. If you interpret special relativity via (neo-)Lorenz Ether Theory, you can have perfectly mundane kind of FTL without any problematic time travel, you'd just have a preferred reference frame. It'd be possible to send messages to the past according to some reference frames (in spacelike fashion), but not others, and you'd never get to communicate information in time loops. There is no serious problem with reconciling FTL and special relativity, FTL goes the way of ether for Occam's razor reasons, not because it doesn't make sense conceptually.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: