Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the number and spin of photons in classical electromagnetic field   A relativistically invariant expression for the number of photons in free
classical electromagnetic field through the currents, that created the field,
is derived based on the formula for the total energy--momentum of the field. It
is demonstrated that it corresponds to the classical limit of the photon number
operator known from the quantum electrodynamics. An expression for the total
spin moment of the classical electromagnetic field is derived and it is shown
that it can be interpreted in terms of spin moments of elementary photons.
Similar to the total number of photons the classical spin moment is the limit
of the quantum operator of the total spin. It is revealed that the total number
of photons as well as the total spin moment are related to each other through a
second order tensor, which can be expressed through currents that created the
field.

20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
29
The categories of this paper are: funct-an
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph