Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On Photon Spin and the Electrodynamic Origin of the charge of the
  Electron   We recently performed experiments on the transfer of photon spin to electron
orbital angular momentum. For an interpretation of the experimental results we
used a classical electrodynamic model of the photon as a propagating
electromagnetic solitary wave which is developed in detail here. A linearly
polarized monochromatic photon is considered as a propagating solitary
electromagnetic wave of finite energy hf which carries an angular momentum
h/2pi with the frequency f and Plancks constant h. This model has, apart from
being a tool for an interpretation of our experimental results, far reaching
consequences of fundamental relevance and guides us to an outline to a unified
quantum theory of electromagnetism and gravitation including an explanation of
the electrodynamic origin of the quantized charge of an electron.

5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
7
The categories of this paper are: hep-th
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
19
The categories of this paper are: eess