Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Causal evolution of spin networks   A new approach to quantum gravity is described which joins the loop
representation formulation of the canonical theory to the causal set
formulation of the path integral. The theory assigns quantum amplitudes to
special classes of causal sets, which consist of spin networks representing
quantum states of the gravitational field joined together by labeled null
edges. The theory exists in 3+1, 2+1 and 1+1 dimensional versions, and may also
be interepreted as a theory of labeled timelike surfaces. The dynamics is
specified by a choice of functions of the labelings of d+1 dimensional
simplices,which represent elementary future light cones of events in these
discrete spacetimes. The quantum dynamics thus respects the discrete causal
structure of the causal sets. In the 1+1 dimensional case the theory is closely
related to directed percolation models. In this case, at least, the theory may
have critical behavior associated with percolation, leading to the existence of
a classical limit.

3
The categories of this paper are: physics
29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
21
The categories of this paper are: adap-org
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
14
The categories of this paper are: math-ph