  We show that the operational definition of non-classicality of a quantum
state depends on the motion of the observer. We use the relativistic
Unruh-DeWitt detector model to witness nonclassicality of the probed field
state. It turns out that the witness based on the properties of the
P-representation of the quantum state depends on the trajectory of the
detector. Inertial and noninertial motion of the device have qualitatively
different impact on the performance of the witness.
