Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Problems about Causality in Fermi's Two-Atom Model and Possible
  Resolutions   In order to check finite propagation speed Fermi, in 1932, had considered two
atoms A and B separated by some distance R. At time t=0, A is in an excited
state, B in its ground state, and no photons are present. Fermi's idea was to
calculate the excitation probability of B. In a model-independent way and with
minimal assumptions - Hilbert space and positive energy only - it is proved,
not just for atoms but for any systems A and B, that the excitation probability
of B is nonzero immediately after t=0. Possible ways out to avoid a
contradiction to finite propagation speed are discussed. The notions of strong
and weak Einstein causality are introduced.

31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
15
The categories of this paper are: nucl-ex
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
19
The categories of this paper are: eess
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph