  We compute the stress--energy operator for a scalar linear quantum field in
curved space-time, modulo c-numbers. For the associated Hamiltonian operators,
even those generating evolution along timelike vector fields, we find that in
general on locally Fock-like (`Hadamard') representations: (a) The Hamiltonians
cannot be self-adjoint operators; (b) The automorphisms of the field algebra
generated by the evolution cannot be unitarily implemented; (c) The expectation
values of the Hamiltonians are well-defined on a dense family of states; but
(d) These expectation values are unbounded below, even for evolution along
future-directed timelike vector fields and even on Hadamard states. These are
all local, ultraviolet, effects.
