Time travel as depicted in Back To The Future 1 , 2 , 3 or any future reboot is the one bit of tech that we will never see. Ever. We’d know about it if time travel was invented at any point in the history of our civilisation (or those that came and went before and after). If time travel exists at any point in history it exists in all points of history, so unless Doc Brown is hiding his invention over in the States we’re forever out of luck on that score. Which is a shame, because I think I’d rather suit Marty’s body warmer.