Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: `Faster than light' photons and rotating black holes   The effective action for QED in curved spacetime includes equivalence
principle violating interactions between the electromagnetic field and the
spacetime curvature. These interactions admit the possibility of superluminal
yet causal photon propagation in gravitational fields. In this paper, we extend
our analysis of photon propagation in gravitational backgrounds to the Kerr
spacetime describing a rotating black hole. The results support two general
theorems -- a polarisation sum rule and a `horizon theorem'. The implications
for the stationary limit surface bounding the ergosphere are also discussed.

2
The categories of this paper are: math
18
The categories of this paper are: econ
6
The categories of this paper are: astro-ph
21
The categories of this paper are: adap-org
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph
24
The categories of this paper are: dg-ga
36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
11
The categories of this paper are: hep-lat
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th