When Lucas shot Ford's role in December 1992, he realized the scene opened up the possibility of a film with an older Indiana set in the 1950s. The film could reflect a science fiction 1950s B-movie, with aliens as the plot device.
Ford disliked the new angle, telling Lucas "No way am I being in a Steve Spielberg movie like that." Spielberg himself, who depicted aliens in Close Encounters of the Third Kind and E.T. the Extra-Terrestrial, resisted it.
Lucas convinced Spielberg to use aliens in the plot by saying they were not "extraterrestrials", but "interdimensional", with this concept taking inspiration in the superstring theory.