Approaches to Quantum Gravity

278 A. Perez
the Levi–Civita tensor. Similarly the holonomy W p [A] around the boundary of the
plaquette p (see Fig. 15.2)isgivenby
W p [A] =1l + ɛ 2 F p (A) + O(ɛ 2 ). (15.12)
The previous two equations imply that F[N] =lim ɛ→0
∑p Tr[N pW p ], and lead
to the following definition: given s, s ′ ∈ Cyl (think of spin network states) the
physical inner product (15.9)isgivenby
〈s ′ P, s〉 :=lim 〈s ∏ ∫
ɛ→0
p
dN p exp(iTr[N p W p ]), s〉. (15.13)
The partition is chosen so that the links of the underlying spin network graphs
border the plaquettes. One can easily perform the integration over the N p using the
identity (Peter–Weyl theorem)
∫
dN exp(iTr[NW]) = ∑ (2 j + 1) Tr[ (W j )], (15.14)
j
where j (W ) is the spin j unitary irreducible representation of SU(2). Using the
previous equation
〈s ′ P, s〉 :=lim
ɛ→0
n p (ɛ)
∏
p
∑
(2 j p + 1) 〈s ′ Tr[ j p
(W p )]), s〉, (15.15)
j p
where the spin j p is associated with the pth plaquette, and n p (ɛ) is the number of
plaquettes. Since the elements of the set of Wilson loop operators {W p } commute,
the ordering of plaquette-operators in the previous product does not matter. The
limit ɛ → 0 exists and one can give a closed expression for the physical inner
product. That the regulator can be removed follows from the orthonormality of
SU(2) irreducible representations which implies that the two spin sums associated
with the action of two neighboring plaquettes collapses into a single sum over the
action of the fusion of the corresponding plaquettes (see Fig 15.3). One can also
show that it is finite, 4 and satisfies all the properties of an inner product [6].
4 The physical inner product between spin network states satisfies the following inequality
∣ 〈s, s ′ ∣
〉 p ∑ ≤ C j (2 j + 1)2−2g ,
for some positive constant C. The convergence of the sum for genus g ≥ 2 follows directly. The case of the
sphere g = 0 and the torus g = 1 can be treated individually [6].