Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Classical and Quantum Implications of the Causality Structure of
  Two-Dimensional Spacetimes with Degenerate Metrics   The causality structure of two-dimensional manifolds with degenerate metrics
is analysed in terms of global solutions of the massless wave equation. Certain
novel features emerge. Despite the absence of a traditional Lorentzian Cauchy
surface on manifolds with a Euclidean domain it is possible to uniquely
determine a global solution (if it exists), satisfying well defined matching
conditions at the degeneracy curve, from Cauchy data on certain spacelike
curves in the Lorentzian region. In general, however, no global solution
satisfying such matching conditions will be consistent with this data.
Attention is drawn to a number of obstructions that arise prohibiting the
construction of a bounded operator connecting asymptotic single particle
states. The implications of these results for the existence of a unitary
quantum field theory are discussed.

15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
19
The categories of this paper are: eess
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The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
6
The categories of this paper are: astro-ph
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
16
The categories of this paper are: stat
7
The categories of this paper are: hep-th