Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Causality in Quantum Field Theory with Classical Sources - Quantum
  Electrodynamics   In an exact quantum-mechanical framework, we show that expectation values of
the second-quantized electro-magnetic fields in the Coulomb gauge, and in the
presence of classical sources, automatically lead to causal and retarded
electro-magnetic field strengths. The classical $\hbar$-independent Maxwell's
equations naturally emerge from this fundamental quantum-mechanical approach in
terms of expectation values of quantum fields, and are therefore also
consistent with the special theory of relativity. The fundamental difference
between interference phenomena due to the linear nature of the classical
Maxwell theory as, e.g., in classical optics, and interference effects of
quantum states is clarified. The framework outlined also provides for a simple
approach to, e.g., spontaneous photon emission and/or absorption processes as
well as to the classical Vavilov-Cherenkov radiation. The inherent and
necessary quantum fluctuations, limiting a precise space-time knowledge of
expectation values of the quantum fields considered, are, finally, recalled.

26
The categories of this paper are: bayes-an
31
The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
21
The categories of this paper are: adap-org
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
9
The categories of this paper are: nlin