##### Abstract

In this thesis I discuss various topics relating to the definition of

particles and vacuum states in quantum field theory in general, and

apply it to non-inertial reference frames in Minkowski spacetime. The

particle concept in quantum field theory is shown to be rather

ambiguous and subjective.

I discuss generally what particles are and how they should be defined

in quantum field theories. I then discuss what ambiguities are

inherent in such a definition and in particular what ambiguities there

are for observers in different stationary non-inertial reference

frames in Minkowski spacetime. I use this to gain a broader

perspective on the Unruh effect, the effect by which an accelerated

observer will view the vacuum state of an inertial reference frame as

being filled by a thermal ensemble of particles. I conclude that the

effect actually depends on how the solutions of the field equation are

joined across the event horizon that is present in hyperbolicly

accelerated reference frames, and that the effect is really more

subjective than is commonly assumed in the literature. Finally, I

investigate the behaviour of a model particle detector. I conclude

that it does not necessarily reflect the spectrum of particles that is

present in the reference frame of the detector, because the excitation

spectrum of the detector may be distorted by particle states with

negative energy, which are present in many non-inertial reference

frames. The results of detection experiments will therefore not

generally agree with the usual definition of particles in quantum

field theory.

Along the way I provide explicit calculations of all possible Killing

vector fields and stationary trajectories in Minkowski spacetime,

discussions of what the corresponding reference frames look like, as

well as solutions of the Klein-Gordon equation in coordinates

appropriate to the different reference frames.