Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Time Machines and the Breakdown of Unitarity   We present a generic way of thinking about time machines from the view of a
far away observer. In this model the universe consists of three (or more)
regions: One containing the entrance of the time machine, another the exit and
the remaining one(s) the rest of the universe. In the latter we know ordinary
quantum mechanics to be valid and thus are able to write down a Hamiltonian
describing this generic time machine. We prove the time-evolution operator to
be non-symmetric. Various interpretations of this irreversibility are given.

10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
11
The categories of this paper are: hep-lat
20
The categories of this paper are: acc-phys
26
The categories of this paper are: bayes-an
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex