[loqi]

I haven't read the book, but from what I gather[1], the explanations were not all good. Getting to and from Miller's world should have required absurd amounts of energy. But as extreme luck would have it, there were "intermediate-mass black holes" positioned ever-so-conveniently to make those trips possible (along with some other stuff). Cherry-picking the physics you like and papering over the rest with a series of extremely implausible coincidences isn't good science or good storytelling.

[1]: http://www.scottaaronson.com/blog/?p=2040