Infact the whole movie is about relativity, time dilation, space travel when space is dimension (warmhole) what if time and space dimensions are represented physically, etc. Its not possible on earth by humans because we cant perceive these two things physically. Movie also considers Gravity being influenced by relativity. It's put by Einstein. That's why Einstein was partially able to prove Newton wrong. Sure, the movie has paradoxes, but what most of the people think ain't possible is perfectly possible somewhere in the universe.