The particle content of a given state In Carroll's we read

...The Unruh effect teaches us the most important lesson of Quantum Field Theory (QFT) in curved spacetime, the idea that "vacuum" and "particles" are observer-dependent notions rather than fundamental concepts.

I wonder are we talking about the same observer or two different observers using two different frames? 
Put it another way, can the same observer who observed nothing using a frame detect a thermal spectrum if switched to another frame that is uniformly accelerating with respect to his previous frame?
Does it apply both ways? I mean if an observer detects particles, does this observant-dependent notion of particle content of a given state, implies that he can switch to another frame in the blink of an eye, and detect nothing?
 A: That is exactly the point: if the field is in a vacuum with respect to observer A, and observer B accelerates uniformly with respect to A, then B will observe a field state with nonzero particle content.
It doesn't really matter whether you talk about different observers on different frames of reference, or of a single observer who 'switches' their frame of reference. The notion of 'observer' is not meant to imply a specific person, nor does the experimenter himself have to be at rest or in inertial motion with respect to those coordinates. The terms 'observer' and 'frame of reference' refer to ways of making sense of observations more than actual people.
Finally, note that the Unruh effect can sometimes work the other way  - i.e. if observer A sees a field with particle content, there may exist a second observer B in uniformly-accelerating motion w.r.t. A for whom the field is in vacuum - but this is not guaranteed. What the effect teaches us is that such observers B will observe a different particle content than A, but in general this need not be the vacuum.
