Quantum Field Theory In Curved Spacetime: Quant... -

is a linear combination of both the old annihilation and creation operators:

. This approach serves as a robust approximation for environments where gravity is strong but quantum gravitational effects—such as fluctuations of the metric itself—are not yet dominant. 1. The Fundamental Shift: From Particles to Fields Quantum Field Theory in Curved Spacetime: Quant...

: The concept of a "particle" becomes local and observer-dependent. Different observers (e.g., one inertial and one accelerating) may disagree on whether a state contains particles or is a vacuum. is a linear combination of both the old