According to QM the ground state of the quantized empty space is not considered to be empty but as a fluctuating sea of virtual particles creating and annihilating continuously. In the case of virtual particles with mass I have a problem with this. According to which reference inertial frame are these supposed to appear? If two inertial observers look at the same region of space (say one in a laboratory at rest with earth, the other one traveling throughout space at nearly the speed of light, but both examining the same vacuum), would they agree? According to QFT do they see the same Lorentz transformed particles appear, or something completely different? Please note that I'm not asking about the Unruh effect, I'm considering only inertial reference frames.