Particle Content of a Given State: Unruh Effect Explained

[victorvmotti]

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?

 

[Matterwave]

If the observer starts to accelerate, where previously he was staying still, then he would, in theory, begin to see the Unruh radiation. "Observer dependent" should probably more clearly read "frame dependent".