In the OT we see many people who are old enough to be around during the PT, and they seem to regard the Force as some myth, or "simple tricks and nonsense". Motti and Tarkin are well old enough to remember the heyday of the Republic and the Jedi, yet Motti makes reference to "that ancient religion". To me, something should be more than 10 or 20 years old to be regarded as "ancient". In the PT Jedi are regarded by most as great warriors, and everyone knows that there really is a "force" that they can use for knowledge, defense, and in some cases power. What happens to convince everyone in the galaxy that the Jedi are just tricksters and fakes, and makes everyone stop believing in the Force?