Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Small-Scale Structure of Spacetime: Bounds and Conjectures   This review consists of two parts. The first part establishes certain
astrophysical bounds on the smoothness of classical spacetime. Some of the best
bounds to date are based on the absence of vacuum Cherenkov radiation in
ultrahigh-energy cosmic rays. The second part discusses possible implications
of these bounds for the quantum structure of spacetime. One conjecture is that
the fundamental length scale of quantum spacetime may be different from the
Planck length.

15
The categories of this paper are: nucl-ex
1
The categories of this paper are: cs
25
The categories of this paper are: patt-sol
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin