Like Inception before it, Interstellar showed off Nolan's love of time dilation. This time, instead of deepness of dream levels, it was strength of gravity that led for things to go squiffy. When visiting the planet near the black hole, an hour translated into seven years, so it is to be assumed in the singularity itself this effect would be amplified. That seems to be true to a degree. Cooper spends about ten minutes in there meddling with time and is spat out over fifty years later, a speed up that in general terms fits science theory. But with him leaving linear time all together, things don't appear to be that simple. From when he discovers the bookshelves, Cooper has entered into a three-dimensional version of the fifth dimension, meaning that time isn't passing for him at all, at lest not in the conventional sense. It's not really a plot hole, rather a lack of information means the exact mechanics of the singularity are (unlike the wormhole, which got its layman explanation) incredibly muddy. Potential Answer: Cooper is perceiving the fifth dimension, but that doesn't mean he's left three-dimensional space - time is still moving for him slightly, hence why he can make it to his future. Now why did he get deposited as Saturn?
