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SUMMARY:Causality and Microcausaliy in Curved Spacetime - Graham Shore (Sw
 ansea)
DTSTART:20071128T141500Z
DTEND:20071128T153000Z
UID:TALK8082@talks.cam.ac.uk
CONTACT:Daniel Wesley
DESCRIPTION:We consider how causality and micro-causality are realised in 
 QED in curved spacetime. The photon propagator is found to exhibit novel n
 on-analytic behaviour due to vacuum polarization\, which invalidates the K
 ramers-Kronig dispersion relation and calls into question the validity of 
 micro-causality in curved spacetime. This non-analyticity is ultimately re
 lated to the generic focusing nature of congruences of geodesics in curved
  spacetime\, as implied by the null energy condition\, and the existence o
 f conjugate points. These results arise from a calculation of the complete
  non-perturbative frequency dependence of the vacuum polarization tensor i
 n QED\, using novel world-line path integral methods together with the Pen
 rose plane-wave limit of spacetime in the neighbourhood of a null geodesic
 . The refractive index of curved spacetime is shown to exhibit superlumina
 l phase velocities\, dispersion\, absorption (due to \\gamma \\to e^+e^-) 
 and bi-refringence\, but we demonstrate that the wavefront velocity (the h
 igh-frequency limit of the phase velocity) is indeed c\, thereby guarantee
 ing that causality itself is respected.
LOCATION:MR2\, Centre for Mathematical Sciences
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