After watching the prequels, I was lost when it came to force spirits (e.g. when Obi-Wan dies he disappears and becomes a force spirit). In the films, it was noted that it was not until Qui-Gon discovered how to become one with the force that anyone learned how to live on after death.

However, in contrast, in the EU there are plenty of instances where force spirits, by both the Jedi and the Sith, appear to others -- especially in the Tales of the Jedi series that take place 4,000 years before the birth of Luke. There are plenty of instances where characters encounter these force spirits.

Nonetheless, after pondering this lack of continuity with the films and the EU, I came up with a theory that may explain it. I believe, since there were many instances where the Jedi and the Sith were almost erradicated (e.g. KOTOR: Sith Lords where the Jedi were almost erradicated and also after the battle of Ruusan, the SIth were almost destroyed completely), both the Jedi and the Sith lost the knowledge to become force spirits. They both lost ways to extend themselves after death.

So the knowledge of this was lost for both groups for thousands of years. As a result, both Darth Plagueis and Qui-Gon were looking for ways to continue on -- even though they both went about it in different ways such as Plagueis looking to cheat death in the physical rhelm while Qui-Gon searching for a way to beome one with the force to extend his life in the afterlife.

It also may appear that the knowledge was only kept by the Whills. So it was through them, Qui-Gon was able to find the lost knowledge of how to live on in the force -- as referenced in the EU. The Whills were able to either discover or was able to retain (the one I prefer) the knowledge to become a force spirit.

So what do you all think? Since I am no EU scholar -- this year I began reading the EU -- I respect anyone that may further shed light upon this area. So feel free to chime in.

Have a good one all.