We can start ``à la Schrödinger'' by expressing quantum states by means of the amplitude of the connection, namely by means of functionals of the (smooth) connection. These functionals form a linear space, which we promote to a Hilbert space by defining a inner product. To define the inner product, we choose a particular set of states, which we denote ``cylindrical states'' and begin by defining the scalar product between these.

Pick a graph
, say with
*n*
links, denoted
, immersed in the manifold
*M*
. For technical reasons, we require the links to be analytic. Let
be the parallel transport operator of the connection
*A*
along
.
is an element of
*SU*
(2). Pick a function
on
. The graph
and the function
*f*
determine a functional of the connection as follows

(These states are called cylindrical states because they were introduced in [14, 15, 16] as cylindrical functions for the definition of a cylindrical measure.) Notice that we can always ``enlarge the graph'', in the sense that if is a subgraph of , we can always write

by simply choosing
*f*
' independent from the
's of the links which are in
but not in
. Thus, given any two cylindrical functions, we can always view
them as having the same graph (formed by the union of the two
graphs). Given this observation, we define the scalar product
between any two cylindrical functions [137,
14,
15,
16] by

where
*dg*
is the Haar measure on
*SU*
(2). This scalar product extends by linearity to finite linear
combinations of cylindrical functions. It is not difficult to
show that (14) defines a well defined scalar product on the space of these
linear combinations. Completing the space of these linear
combinations in the Hilbert norm, we obtain a Hilbert space
. This is the (unconstrained) quantum state space of loop
gravity.
carries a natural unitary representation of the diffeomorphism
group and of the group of the local
*SU*
(2) transformations, obtained transforming the argument of the
functionals. An important property of the scalar product (14) is that it is invariant under both these transformations.

is non-separable. At first sight, this may seem a serious obstacle to its physical interpretation. But we will see below that, after factoring away diffeomorphism invariance, we may obtain a separable Hilbert space (see section 6.8). Also, standard spectral theory holds on , and it turns out that using spin networks (discussed below) one can express as a direct sum over finite dimensional subspaces which have the structure of Hilbert spaces of spin systems; this makes practical calculations very manageable.

Finally, we will use a Dirac notation and write

in the same manner in which one may write in ordinary quantum mechanics. As in that case, however, we should remember that is not a normalizable state.

Loop Quantum Gravity
Carlo Rovelli
http://www.livingreviews.org/lrr-1998-1
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