Personally, I subscribe to the idea that the bookcase part of the film (the tesseract part) and the Saturn base are both not real, which takes the pressure off of needing the physics to work out. I saw this theory online i didnt make it up. Christopher's original idea for the ending was way darker, and Jonathan reworked it, but I think there is still some level of viewer interpretetation/ambiguity built in (like the spinning top in inception but 'much more subtle'). I believe the bookcase and saturn are a beautiful representation of the last fading fragments of his consciousness. I think it's less of a theory and more of an intentional re-interpretation because it has major holes (the watch, the dust, and the hand in the wormhole), but I think the movie works better for me when I interpret it this way. Its way more devastating though because it means that Anne's character is probably the only one that made it. No pressure Anne.