A theoretical physicist walks into a bar….
Oh wait, that’s not right. In theoretical physics, everything happens inside a perfect vacuum.
No, I don’t expect you to have taken college physics (or even high school physics, for that matter) to get something out of this post. All you need to know is this: in physics, a perfect vacuum is a space that doesn’t have any external forces or influences so that when you do some sort of calculation (say, how far a meteor will travel in a given amount of time), you don’t have to take into account things like friction, gravity, or air resistance. That is to say, you can calculate without making any other considerations.