Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Feynman's i-epsilon prescription, almost real spacetimes, and acceptable
  complex spacetimes   Feynman's i-epsilon prescription for quantum field theoretic propagators has
a quite natural reinterpretation in terms of a slight complex deformation of
the Minkowski spacetime metric. Though originally a strictly flat-space result,
once reinterpreted in this way, these ideas can be naturally extended first to
semi-classical curved-spacetime QFT on a fixed background geometry and then,
(with more work), to fluctuating spacetime geometries. There are intimate
connections with variants of the weak energy condition. We shall take the
Lorentzian signature metric as primary, but note that allowing the complex
deformation to become large leads to a variant of Wick rotation, and more
importantly leads to physically motivated constraints on the configuration
space of acceptable off-shell geometries to include in Feynman's functional
integral when attempting to quantize gravity. Ultimately this observation
allows one to connect the discussion back to recent ideas on "acceptable"
complex metrics, in the Louko-Sorkin and Kontsevich-Segal-Witten sense, with
Lorentzian signature spacetimes occurring exactly on the boundary of the set of
"acceptable" complex metrics. By adopting the tetrad formalism we explicitly
construct the most general set of acceptable complex metrics satisfying the
0-form, 1-form, and 2-form acceptability conditions.

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The categories of this paper are: q-bio
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The categories of this paper are: supr-con
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The categories of this paper are: funct-an
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The categories of this paper are: mtrl-th
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The categories of this paper are: stat
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The categories of this paper are: plasm-ph
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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: physics
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The categories of this paper are: hep-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: hep-ph