Non-perturbative unitarity and fictitious ghosts in quantum gravity

TL;DR

This paper explores how apparent ghost states in truncated quantum gravity models are fictitious and do not violate unitarity when the full effective action is considered, emphasizing non-perturbative aspects.

Contribution

It demonstrates that ghost degrees of freedom in truncated models are fictitious and do not imply unitarity violation when the complete effective action is included.

Findings

Fictitious ghosts vanish in the fully-dressed propagator.

Non-perturbative unitarity is preserved in quantum gravity.

Ghost-like poles are artifacts of truncation, not physical states.

Abstract

We discuss aspects of non-perturbative unitarity in quantum field theory. The additional ghost degrees of freedom arising in "truncations" of an effective action at a finite order in derivatives could be fictitious degrees of freedom. Their contributions to the fully-dressed propagator -- the residues of the corresponding ghost-like poles -- vanish once all operators compatible with the symmetry of the theory are included in the effective action. These "fake ghosts" do not indicate a violation of unitarity.