It's a long complicated answer -
http://www.bustle.com/articles/4753...w-to-understand-the-films-complicated-physics
Basically it's outside the realm of time and space. When he's in the bookshelf, he's in a few place where all time is visible. The advanced beings limited it the daughters room because they knew that she would be the one to save humanity. But they couldn't tell what time and timeline would effect the past and allow her to do so
He was able to do that cause he knew his daughter. He knew she'd look at the watch. So he relayed the data that the robot got from falling in the black hole first.
That's why he could exist behind the shelf in the beginning, shake hands in the ship, and still be alive in the moment to witness it.
they actually brought in a scientist from nasa who consulted on the science in the movie. He actually took parts of the movie back to nasa to use as teaching tools
He himself said the "worst" science in the movie is the ice clouds cause nothing could hold them up there. Other than that, he said it was spot on.
Neil Tyson chimed in and said interstellar had the best showing of the theory of relativity ever in a movie