Time travel is also something that only occurs in science fiction. While we can theoretically understand the concept, actually traveling back and forth in time like in 'Back to the Future' is not possible yet. There are so many paradoxes associated with it, like the grandfather paradox. If you go back in time and accidentally prevent your grandfather from meeting your grandmother, then how could you exist in the first place? It's a mind - boggling concept that remains in the realm of science fiction for now.
Interstellar travel at the speed of light or faster. The vast distances between stars are just too great. In science fiction, like in 'Star Wars', spaceships can zip across the galaxy in no time, but in reality, with our current technology, it would take an incredibly long time to reach even the closest star systems.