Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Lorentz invariant photon number density   A Lorentz invariant positive definite expression for photon number density is
derived as the absolute square of the invariant scalar product of a
polarization sensitive position eigenvector and the photon wave function. It is
found that this scalar product is independent of the form chosen for the wave
function and that the normalized positive frequency vector potential-electric
field pair is a convenient choice of wave function in the presence of matter.
The number amplitude describing a localized state is a delta-function at the
instant at which localization and detection are seen as simultaneous.

1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
4
The categories of this paper are: cond-mat
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
29
The categories of this paper are: funct-an
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
23
The categories of this paper are: solv-int