Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Violation of the action-reaction principle in an asymmetrically excited
  system   Violation of the action-reaction principle is shown to occur in the van der
Waals interaction between two atoms, one of which is excited. It is accompanied
by the transfer of linear momentum to the electromagnetic vacuum. The vacuum
momentum results from the asymmetric interference of the virtual photons
scattered off each atom along the interatomic direction, which is in itself a
manifestation of the optical theorem. This momentum, of equal strength and
opposite direction to the momentum gained by the two-atom system, is ultimately
released through directional spontaneous emission. A quantitative prediction of
this phenomenon is made in a two-alkali atom system. It is conjectured that
action-reaction violation takes place in any asymmetrically excited system.

33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th