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.