Here's a theory: We know that three "Jedi Ghosts" appear at the end of the OT ? Yoda, Obi-Wan and Anakin. What do all three have in common? Well, among other things, they all died in the presence of Luke Skywalker. Obi-Wan on the Death Star, struck down as Luke watched; Yoda on Dagobah; Anakin on the new Death Star. I think it's possible that they sent their spirits into Luke. Luke is the only one who can see or hear them. That's because they only exist within Luke. He becomes an embodiment of all of them. As each becomes part of his body, he power grows. Obi-Wan's spirit gave him the power to blow up the Death Star. Yoda's spirit gave him the power to defeat Vader and the Emporer. I'm not sure what Anakin's spirit gave him the power to do because the movie ended. Perhaps that is what being the "chosen one," or the son of the "chosen one" is all about ? the ability to host the spirits of fallen Jedi.