Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The gravitational interaction of light: from weak to strong fields   An explanation is proposed for the fact that pp-waves superpose linearly when
they propagate parallely, while they interact nonlinearly, scatter and form
singularities or Cauchy horizons if they are antiparallel. Parallel pp-waves do
interact, but a generalized gravitoelectric force is exactly cancelled by a
gravitomagnetic force. In an analogy, the interaction of light beams in
linearized general relativity is also revisited and clarified, a new result is
obtained for photon to photon attraction, and a conjecture is proved. Given
equal energy density in the beams, the light-to-light attraction is twice the
matter-to-light attraction and four times the matter-to-matter attraction.

30
The categories of this paper are: q-alg
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex