In Schwarzschild spacetime there are a couple of "vacuum" states: the Boulware vacuum, the Unruh vacuum and the Hartle-Hawking vacuum. I actually don't know if there are others.
This is something that puzzles me a little bit. By this discussion it is quite clear to me that distinct, disjoint states, describe actually different quantum systems. So depending on the vacuum we pick we are describing a different system from this perspective.
And here I ask, please correct me if I'm wrong in the interpretation of this.
Now, it is a fact that thinking physically there is actually one system (a scalar KG field for instance, or one Electromagnetic field, or one electron field), and it is in some state.
So there should be some criteria for picking a vacuum.
My question here is: could someone point out references discussing the distinction between these vacua, and how one picks one of them? What I'm looking for is a reference that discusses the issue of having these three vacuas and discusses how one actualy deals with this situation.