Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The emergent nature of time and the complex numbers in quantum cosmology   The nature of time in quantum mechanics is closely related to the use of a
complex, rather than say real, Hilbert space. This becomes particularly clear
when considering quantum field theory in time dependent backgrounds, such as in
cosmology, when the notion of positive frequency ceases to be well defined. In
spacetimes lacking time orientation, i.e without the possibility of defining an
arrow of time, one is forced to abandon complex quantum mechanics. One also has
to face this problem in quantum cosmology. I use this to argue that this
suggests that, at a fundamental level, quantum mechanics may be really real
with not one, but a multitude of complex structures. I relate these ideas to
other suggestions that in quantum gravity time evolution may not be unitary,
possibly implemented by a super-scattering matrix, and the status of CPT.

37
The categories of this paper are: supr-con
4
The categories of this paper are: cond-mat
6
The categories of this paper are: astro-ph
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
21
The categories of this paper are: adap-org
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg