Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The quantum physics of chronology protection   This is a brief survey of the current status of Stephen Hawking's
``chronology protection conjecture''. That is: ``Why does nature abhor a time
machine?'' I'll discuss a few examples of spacetimes containing ``time
machines'' (closed causal curves), the sorts of peculiarities that arise, and
the reactions of the physics community. While pointing out other possibilities,
this article concentrates on the possibility of ``chronology protection''. As
Stephen puts it:
  ``It seems that there is a Chronology Protection Agency which prevents the
appearance of closed timelike curves and so makes the universe safe for
historians.''

4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
25
The categories of this paper are: patt-sol
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org