Würzburg Seminar on Quantum Field Theory and Gravity

We explore the behavior of QFT in the spacetime of very compact stars. The trapping of modes around the inner light ring at the 
classical level leads, in the quantum theory, to a vacuum renormalized stress tensor that diverges in the Buchdahl limit and violates 
energy conditions. Thus, some well known classical theorems forbidding compact objects in GR might need revision at the quantum 
level.