In this thesis I discuss various topics relating to the definition ofparticles and vacuum states in quantum field theory in general, andapply it to non-inertial reference frames in Minkowski spacetime. Theparticle concept in quantum field theory is shown to be ratherambiguous and subjective.
I discuss generally what particles are and how they should be definedin quantum field theories. I then discuss what ambiguities areinherent in such a definition and in particular what ambiguities thereare for observers in different stationary non-inertial referenceframes in Minkowski spacetime. I use this to gain a broaderperspective on the Unruh effect, the effect by which an acceleratedobserver will view the vacuum state of an inertial reference frame asbeing filled by a thermal ensemble of particles. I conclude that theeffect actually depends on how the solutions of the field equation arejoined across the event horizon that is present in hyperboliclyaccelerated reference frames, and that the effect is really moresubjective than is commonly assumed in the literature. Finally, Iinvestigate the behaviour of a model particle detector. I concludethat it does not necessarily reflect the spectrum of particles that ispresent in the reference frame of the detector, because the excitationspectrum of the detector may be distorted by particle states withnegative energy, which are present in many non-inertial referenceframes. The results of detection experiments will therefore notgenerally agree with the usual definition of particles in quantumfield theory.
Along the way I provide explicit calculations of all possible Killingvector fields and stationary trajectories in Minkowski spacetime,discussions of what the corresponding reference frames look like, aswell as solutions of the Klein-Gordon equation in coordinatesappropriate to the different reference frames.