  We study ghosts in multimetric gravity by combining the mini-superspace and
the Hamiltonian constraint analysis. We first revisit bimetric gravity and
explain why it is ghost-free. Then, we apply our method to trimetric gravity
and clarify when the model contains a ghost. More precisely, we prove trimetric
gravity generically contains a ghost. However, if we cut the interaction of a
pair of metrics, trimetric gravity becomes ghost-free. We further extend the
Hamiltonian analysis to general multimetric gravity and calculate the number of
ghosts in various models. Thus, we find multimetric gravity with loop type
interactions never becomes ghost-free.

High Energy Physics - Theory;General Relativity and Quantum Cosmology