The reason for searching a mathematical-physics level of
precision is that in quantum gravity one moves on a very
unfamiliar terrain -quantum field theory on manifolds- where the
experience accumulated in conventional quantum field theory is
often useless and sometimes even misleading. Given the
unlikelihood of finding direct experimental corroboration, the
research can only aim, at least for the moment, at the goal of
finding a
*consistent*
theory, with the correct limits in the regimes that we control
experimentally. In these conditions, high mathematical rigor is
the only assurance of the consistency of the theory. In the
development of quantum field theory mathematical rigor could be
very low because extremely accurate empirical verifications
assured the physicists that ``the theory may be mathematically
meaningless, but it is nevertheless physical correct, and
therefore the theory must make sense even if we do not understand
how.'' Here, such an indirect experimental reassurance is lacking
and the claim that the theory is well founded can be based only
on a solid mathematical control of the theory.

One may object that a rigorous definition of quantum gravity is a vain hope, given that we do not even have a rigorous definition of QED, presumably a much simpler theory. The objection is particularly valid from the point of view of a physicist who views gravity ``just as any other field theory like the ones we already understand''. But the (serious) difficulties of QED and of the other conventional field theories are ultraviolet. The physical hope supporting the quantum gravity research program is that the ultraviolet structure of a diffeomorphism invariant quantum field theory is profoundly different from the one of conventional theories. Indeed, recall that in a very precise sense there is no short distance limit in the theory; the theory naturally cuts itself off at the Planck scale, due to the very quantum discreteness of spacetime. Thus the hope that quantum gravity could be defined rigorously may be optimistic, but it is not ill founded.

After these comments, let me briefly mention some of the
structures that have been explored in
. First of all, the spin network states satisfy the Kauffman
axioms of the tangle theoretical version of recoupling theory [130] (in the ``classical'' case
*A*
=-1) at all the points (in 3d space) where they meet. (This fact
is often misunderstood: recoupling theory lives in 2d and is
associated by Kauffman to knot theory by means of the usual
projection of knots from 3d to 2d. Here, the Kauffman axioms are
not satisfied at the intersections created by the 2d projection
of the spin network, but only at the nodes in 3d. See [77] for a detailed discussion.) For instance, consider a 4-valent
node of four links colored
*a*,
*b*,
*c*,
*d*
. The color of the node is determined by expanding the 4-valent
node into a trivalent tree; in this case, we have a single
internal links. The expansion can be done in different ways (by
pairing links differently). These are related to each other by
the recoupling theorem of pg. 60 in Ref.\ [130]

where the quantities
are
*su*
(2) six-j symbols (normalized as in [130]). Equation (29) follows just from the definitions given above. Recoupling
theory provides a powerful computational tool in this
context.

Since spin network states satisfy recoupling theory, they form a Temperley-Lieb algebra [130]. The scalar product (14) in is given also by the Temperley-Lieb trace of the spin networks, or, equivalently by the Kauffman brackets, or, equivalently, by the chromatic evaluation of the spin network. See Ref. [77] for an extensive discussion of these relations.

Next,
admits a rigorous representation as an
space, namely a space of square integrable functions. To obtain
this representation, however, we have to extend the notion of
connection, to a notion of ``distributional connection''. The
space of the distributional connections is the closure of the
space of smooth connection in a certain topology. Thus,
distributional connections can be seen as limits of sequences of
connections, in the same manner in which distributions can be
seen as limits of sequences of functions. Usual distributions are
defined as elements of the topological dual of certain spaces of
functions. Here, there is no natural linear structure in the
space of the connections, but there is a natural duality between
connections and curves in
*M*
: A smooth connection
*A*
assigns a group element
to every segment
. The group elements satisfy certain properties. For instance if
is the composition of the two segments
and
, then
.

A generalized connection
is defined as a map that assigns an element of
*SU*
(2), which we denote as
or
, to each (oriented) curve
in
*M*, satisfying the following requirements: i)
; and, ii)
, where
is obtained from
by reversing its orientation,
denotes the composition of the two curves (obtained by
connecting the end of
with the beginning of
) and
is the composition in
*SU*
(2). The space of such generalized connections is denoted
. The cylindrical functions
, defined in section
6.3
as functions on the space of smooth connections, extend
immediately to generalized connections

We can define a measure on the space of generalized connections by

In fact, one may show that (31) defines (by linearity and continuity) a well-defined absolutely continuous measure on . This is the Ashtekar-Lewandowski (or Ashtekar-Lewandowski-Baez) measure [14, 15, 16, 26]. Then, one can prove that , under the natural isomorphism given by identifying cylindrical functions. It follows immediately that the transformation (19) between the connection representation and the ``old'' loop representation is given by

This is the loop transform formula that was derived heuristically in [184]; here it becomes rigorously defined.

Furthermore,
can be seen as the projective limit of the projective family of
the Hilbert spaces
, associated to each graph
immersed in
*M*
.
is defined as the space
, where
*n*
is the number of links in
. The cylindrical function
is naturally associated to the function
*f*
in
, and the projective structure is given by the natural map (13) [18,
149].

Finally, Ashtekar and Isham [12] have recovered the representation of the loop algebra by using
C*-algebra representation theory: The space
, where
is the group of local
*SU*
(2) transformations (which acts in the obvious way on generalized
connections), is precisely the Gelfand spectrum of the abelian
part of the loop algebra. One can show that this is a suitable
norm closure of the space of smooth
*SU*
(2) connections over physical space, modulo gauge
transformations.

Thus, a number of powerful mathematical tools are at hand for
dealing with nonperturbative quantum gravity. These include:
Penrose's spin network theory,
*SU*
(2) representation theory, Kauffman tangle theoretical recoupling
theory, Temperley-Lieb algebras, Gelfand's
algebra spectral representation theory, infinite dimensional
measure theory and differential geometry over infinite
dimensional spaces.

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