Okay, here's where it gets complicated. It is a truth universally acknowledged in Star Wars that Jedis can become one with the Force and disappear... but not all of them have. Obi-Wan (Alec Guinness) certainly did when Darth Vader struck him down in A New Hope. Yoda (Frank Oz) did when he died in Return of the Jedi. Even Luke (Mark Hamill) vanished at the end of The Last Jedi. Qui-Gon Jinn (Liam Neeson) in Phantom Menace, however, did not — nor did any of the dozens of Jedi who were slaughtered in Revenge of the Sith. A scene that appeared in the script Episode III, and was later placed in the animated series The Clone Wars , explains that Qui-Gon Jinn (Liam Neeson) learned how to become one with the Force after death and taught that skill to Obi-Wan and Yoda as a Force Ghost. We can only presume that Luke passed this knowledge on to his padawans, which is how Ben managed to do so when he died.