"What is the relationship between past and future?"
Translation:Jaka jest relacja pomiędzy przeszłością a przyszłością?
"a" establishes a contrast between the things it connects, here "past" and "future".
Do you mean that both are possible but "a" sounds more natural for polish ear?
Btw, are you a polish native speaker?
I'll let myself confirm - yes, both should probably work but 'a', as the contrastive 'and' seems better.