How much negative energy does a wormhole need?

It is known that traversible wormholes require negative energy density. We here argue how much negative energy is needed for wormholes, using a local analysis which does not assume any symmetry, and in particular allows dynam ic (non-stationary) but non-degenerate wormholes. We find that wormholes re quire two constraints on the energy density, given by two independent compo nents of the Einstein equation. (C) 1999 Published by Elsevier Science B.V. All rights reserved.