The equivalence principle and QFT: Can a particle detector tell if we   live inside a hollow shell?
  We show that a particle detector can distinguish the interior of a hollow shell from flat space for switching times much shorter than the light-crossing time of the shell, even though the local metrics are indistinguishable. This shows that a particle detector can read out information about the non-local structure of spacetime even when switched on for scales much shorter than the characteristic scale of the non-locality. 
Keith K. Ng, Robert B. Mann and Eduardo Martin-Martinez
quant-ph gr-qc hep-th