Okay, I’ve seen sssooooooo many people talk about this, but they way I’ve seen everyone word it…is…confusing to me. I’m sure I’m just misreading, but allow me to take the time to 1, explain how I’m confused, 2, explain what I think is trying to be said and 3 talk about it a little. Hopefully if anyone else is having the same problems understanding this will clear it up a little

I’ve seen the theory on the force field worded two different ways (these are paraphrased).
1. “The force field Ford and Dipper put up around the Shack is still up, that’s why it’s safe and why Stan and everyone else was in there. This also explains why Bill can’t leave Gravity Falls.”
2. “Stan read about how to create a force field to keep out Bill when he had Ford’s journals,or he copied the journals and read about how to, and he created the force field that is keeping Bill in Gravity Falls. This also explains why the Shack is totally safe from Bill.”

The way I’m reading both of these are “Either Ford’s force field is still up, or Stan put up a force field, and that one force field is both keeping Bill out of Shack, and keeping him in Gravity Falls.”
Again, I think I’m misreading, I think what is actually meant is: “Ford’s force field is still up, and that is what’s protecting the Shack, and why everyone is in the Shack. Stan also knew how to create the force field because he had Ford’s journals, and either he read it before they burned, or he made copies that he looked at. So Stan probably made another force field that’s keeping Bill in” (that’s kinda like combing two theories into one.)

If I am reading these theories correctly, and the theory is in fact that one force field is both keeping Bill out of the Shack and in Gravity Falls, well first then I may have just created a new theory by accident lol, and second, it’s really not possible to have one force field do both of those things, unless the Mystery Shack isn’t in Gravity Falls. Which I suppose is possible, it could maybe be right outside of the city lines of Gravity Falls. But I don’t think that’s likely, I think Gravity Falls is in a valley, and the entire town is in the valley. So there has to be two different force fields, one that Ford and Dipper made in The Last Mabelcorn (which it would make sense if that force field was protecting the shack, because that force field hasn’t come into play. I mean, what was the point of doing an entire episode dedicated to creating the force field if it was never gonna come into play at all?) and the second force field was created by an unknown person at an unknown time. 

And now I want to talk about this theory

Everyone seems to think this “unknown person” is Stan, and I love that theory, and it makes the most logical sense. Stan had plenty of access to the journals, and is really the only other person who had any access to the journals who wasn’t either on screen doing other things, and wasn’t turned into a statue. And we know he copied journal #3, so he probably copied the others, and this would also explain why Celestab- the unicorn whose name I can’t spell was in the Shack with everyone else. 
However, I’m a little nervous that we will be disappointed, and it will not have been Stan who created the force field around Gravity Falls. I can’t think of a lot of examples right now, but I remember a ton of theories about Stan doing something like this, that was him saving the day, and I remember the only one that came true was Stan saving his brother from the portal. The only example coming to mind right now was when we were all wondering why Stan was taking everyone on a roadtrip after such a huge episode like The Last Mabelcorn, and there was a theory that Stan was trying to protect the kids from Ford (this theory came up before TLM aired, so no one knew what was going to happen in that episode, the theory was that Stan would find reason to believe Ford is dangerous in that episode, and that’s why he was taking them on the roadtrip in Roadside Attraction, to get the kids away from Ford), and it ended up being that he just wanted to prank a bunch of people. I remember there being other little theories like that, that were Stan doing something significant to the plot, and ever since Not What He Seems, it seems to me like the writers haven’t had Stan do much for the main, overall plot of Gravity Falls (I mean yes he’s played parts in the plots of episodes, but with the exception of this recent episode, where he was with a group of people and was the chief according to his sash, he doesn’t seem to have played any other part in the main plot since season 2b started, (someone please correct me if I’m wrong. I want to be wrong here)) and I’m worried this will happen again, where Stan ends up having hardly any significance. 
On the other hand, this is the most perfect time for me to be proven wrong. This is going to be one of the most significant points of the plot, and perhaps Stan hasn’t played a big role in the plot since getting his brother out of the portal because the writers were planning on having him be significant in more important ways.