Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: ``Faster than Light'' Photons in Gravitational Fields -- Causality,
  Anomalies and Horizons   A number of general issues relating to superluminal photon propagation in
gravitational fields are explored. The possibility of superluminal, yet causal,
photon propagation arises because of Equivalence Principle violating
interactions induced by vacuum polarisation in QED in curved spacetime. Two
general theorems are presented: first, a polarisation sum rule which relates
the polarisation averaged velocity shift to the matter energy-momentum tensor
and second, a `horizon theorem' which ensures that the geometric event horizon
for black hole spacetimes remains a true horizon for real photon propagation in
QED. A comparision is made with the equivalent results for electromagnetic
birefringence and possible connections between superluminal photon propagation,
causality and the conformal anomaly are exposed.

29
The categories of this paper are: funct-an
6
The categories of this paper are: astro-ph
11
The categories of this paper are: hep-lat
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
37
The categories of this paper are: supr-con
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th