Originally Posted by Steve Jordan
"I hate time travel." -Kathryn Janeway
I personally don't consider time travel and traveling to alternate realities Hard SF... and especially when accomplished by ships made out of quartz by Victorians using "Platternite." (Not that the story isn't good, mind you--I haven't read it.) Time travel and alternate realities are wonderful and inventive theories, with obvious attractions, but without any concrete basis... which makes them Soft SF.
Time's Eye includes time travel and pocket universes. So some would not consider it Hard SF. The next book in the series is definitely Hard SF. The last book is more of a hard biscuit covered in mold.