after seeing ROTS, I believe that Sidious and Plagieus knew the secret (given his conversastion in the theatre), but that only one adgearing to the light side can achieve it. In other words, Vader and Palpatine knew the secret but only Vader returns to the light side and thus is able to have immortality as it is called. I BELIEVE that this is backed up by the actual scripted (and cut unfortunately) dialogue between Yoda and Qui-gon at the end of ROTS. I think I remember the mention in this dialogue that the irony is that the Sith have spent thousands of years searching for this ability but that only by using the light side can it truly be achieved.
well i think that. Being able to live forever/comunticat through the force was only possible when you had become one with the force eg somthing yoda obi and Qui gon all learn to do...and to answer the question no one tells him(for one they never see each other until ANH. but as Anakin/Vader is the choosen one and was 'created by the mirclorienins themselves' he is one with the force. (Plus all the other stuff you lot have said which makes sence)
i am of the mind qui gon was so intune with the force that he was gradually trying to rip his way back into the world of the living so he could communicate to his jedi's. You can hear him scream in episode 2, he is still not quite as far as becoming a ghost but maybe qui gon was continually getting stronger to the point were he could appear in ghost form. Than maybe from there he can guide certain other dead jedi into becoming ghosts as well. I am of the mind it is just qui gon is so ube intune with the force he discovered a technique of visiting the living while being dead and taught the skill to others like yoda and obi and anikin
I'm not sure where I read this, but the ability to be preserved in the Force is to do with latching your spirit onto a nearby Jedi when you die. Yoda, Obi and Anakin all attach themselves to Luke, and hence he can see them, as he is there when each of them dies. Qui Gon himself attached himself to Obi, which also explains why Yoda explains to Obi that he will be able to communicate with Qui Gon. Quite how Yoda achieves this communication is beyond me. The reason Sideous didn't latch onto someone because there was no Jedi close enough that he would wish to latch on to. None of this is 100%, but I hope someone can expand on this theory.