Description
Wormhole solutions—bridges connecting different regions of spacetime—were proposed early in the history of General Relativity, and it was soon realized that they could potentially act as time machines, violating causality. It was soon shown that traversable wormholes necessarily violate the classical energy conditions, which impose non-negativity constraints on contractions of the stress-energy tensor. Since quantum fields can violate these conditions, it was initially hoped that wormholes might be realizable within semiclassical gravity. However, negative energy densities in quantum field theory are themselves subject to constraints. Quantum energy inequalities and averaged energy conditions place bounds on renormalized negative energies when averaged along geodesics, thereby imposing restrictions on the construction and maintenance of traversable wormholes. Recently, long wormholes—wormholes for which traversal takes longer than traveling between the same endpoints through the exterior spacetime—have been proposed as an alternative. Do quantum energy inequalities constrain long wormholes in the same way? Can long wormholes ever become time machines, or does their extended traversal time prevent causality violation? In this talk, I will review the construction of long wormholes, investigate whether they can be converted into time machines, and discuss the constraints imposed by quantum energy inequalities.