Okay, people have debated since Star Wars Phantom Menace about the “Midichlorians.” Many fans don’t believe in it because they feel it goes against what the Force is in the original Star Wars trilogy. With the new movie coming out, J.J. Abrams stated that they will not be in the new trilogy. So I have come up with a theory about them. It’s a debunked science like when people thought the world was flat or when people thought the Earth was the center of the solar system. But you ask how does this science change between Revenge of the Sith and Star Wars (I don’t call it A New Hope, I call it by their original titles). At the end of Revenge, Yoda said that Qui-Gon had returned from the afterlife becoming one with the Force and will teach Obi-Wan to do the same. I believe, that after Qui-Gon died, he learned how to become one with the Force and learned that the Force is not science or genetic, but mystical. He taught this Obi-Wan and Yoda, and they taught this to Luke, which is why it wasn’t referenced in the original trilogy.
Basically, Midichlorians is a debunked theory that Obi-Wan and Yoda learned from Qui-Gon after he crossed over. That’s my theory and I feel it works so that all 7 movies can be canonical.