What's Up with Force Ghosts in Star Wars?

Ever since we first heard Obi-Wan Kenobi calling to Luke Skywalker from beyond the grave, we began to understand just what the old Jedi Master meant when he told his former pupil turned rival, “If you strike me down, I shall become more powerful than you can imagine.” Force Spirits are one of the coolest things about Star Wars. So what’s up with them, anyway?

Decades before people were screaming “Your Snoke Theory Sucks” at each other, one of the greatest mysteries in Star Wars was this: why was it that both Obi-Wan Kenobi and Yoda disappeared at death, but Darth Vader did not? We saw Luke burn his father’s body on Endor just as we would see Qui Gon Jinn’s burned in Episode I. Prepare for spoilers from here on out.