The Frontiers of Fundamental Physics 14 conference again captures my interest this week and I’ve been looking at a paper, ‘ How Many Quanta are there in a Quantum Spacetime?‘

In this paper the authors develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. They show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. So in order to say how many quanta are in a quantum spacetime further information about what has been measured has to be given.

**Introduction**

The electromagnetic field can be viewed as formed by individual photons. This is a consequence of quantum theory. Similarly, quantum theory is likely to imply a granularity of the gravitational field, and therefore a granularity of space.

How many quanta form a macroscopic region of space? This question has implications for the quantum physics of black holes, scattering calculations in non perturbative quantum gravity and quantum cosmology. It is related to the question of the number of nodes representing a macroscopic geometry in a spin network state in loop gravity. In this context, it takes the following form: what is the relation between a state with many nodes and small spins, and a state with few nodes but large spins?

**Quanta of space**

A quanta of space may be a quanta of energy from the excitation of the gravitational field. In loop quantum gravity, each quanta is a quantum polyhedron. The geometry of quantum polyhedron defined by graph. We associate a state or element of Hilbert space for each quanta of space. The basis which spanned this Hilbert space is the spin network basis.

**A quantum tetrahedron and its dual space geometry: the graph **

A graph γ is a finite set N of element n called nodes and a set of L of oriented couples called links l = (n, n’). Each node corresponds to one quantum tetrahedron. Four links pointing out from the node correspond to each triangle of the tetrahedron.

**How many quanta in a field?**

Consider a free scalar field in a finite box, in a classical configuration φ(x,t). The standard quantum-field-theoretical number operator, which sums the number the quanta on each mode, has a well defined classical limit. The number operator is

**where a**_{n} and a^{†}_{n} are the annihilation and creation operators for the mode n of the field and the sum is over the modes, namely the Fourier components, of the field. Since the energy can be expressed as a sum over modes as

where ω_{n} is the angular frequency and E_{n} its energy of the mode n, it follows that the number of particles is given

by

which is a well defined classical expression that can be directly obtained from φ(x,t) by computing the energy in each mode. Therefore each classical configuration defines a total particle-number N and a distribution of these particles over the modes

**Subset graphs**

The state space of loop quantum gravity contains subspaces H_{γ} associated to abstract graphs γ. A graph γ is defined by a finite set N of |N| elements n called nodes and a set L of |L| oriented couples *l* = (n, n’) called links.

A pure state |ψ〉 determines the density matrix ρ_{γ}= |ψ〉〈ψ|. A generic state can be written in the form

In the loop gravity the operators defined on H_{γ} can be interpreted as the description of the geometry of |N| quantum polyhedra connected to one another when there is a link between the corresponding nodes.

Given a graph , define a subset graph Γ which partitions N into subsets N such that each N is a set of nodes connected among themselves by sequences of links entirely formed by nodes in N.

We define the area of the big link by

and the volume of the big node by

where we recall that v is the expression for the classical volume of a polyhedron. The operators A_{L} and V_{N} commute, so they can be diagonalized together. The quantum numbers of the big areas are half integers J_{L} and the quantum numbers of the volume are V_{N}.

**Course graining spin networks**

Coarse-graining the entire graph into a graph Γ formed by a single node N with legs b

A set of small links *l* that are contained in a single large link L.

Any general coarse-graining is a combination of collecting nodes and summing links

**The geometry of the subset graph**

The geometrical interpretation of the coarse grained states in H_{Γ} is that these describe the geometry of connected polyhedra. The partition that defines the subset graph Γ is a coarse-graining of the polyhedra into larger chunks of space. The surfaces that separate these larger chunks of space are labelled by the big links L and are formed by joining the individual faces labelled by the links l in L.

In general, it is clearly not the case that the area A_{L} is equal to the sum of the areas A_{l} of all l in L. However, this is the case if all these faces are parallel and have the same orientation. Similarly, in general, it is clearly not the case that the volume V_{N} is equal to the sum of the volumes V_{n} for the n in N. However, this is true if in gluing n polyhedra one obtains a at polyhedron with flat faces.

The two operators,

provide a good measure of the failure of the geometry that the state associates to Γto be flat.

To have a good visualization of the coarse-grained geometries, it is helpful to consider the classical picture. In the 4-dimensional theory, the graph is defined at the boundary of a 3-dimensional hypersurface, the spin operator on the links is related to the area operator by

Given a 3-valent graph with spins operators J_{la}, J_{lb} , and J_{lc} on each link, the dihedral angle between J_{lb} and J_{lc} can be obtained from the angle operator, defined by

Applying this operator to the spin network state gives the dihedral angle between J_{lb }and J_{lc} on the internal links l_{b} and l_{c}.

The Regge intrinsic curvature of a discretized manifold is given by the deficit angle on the hinges, the (n -2) dimensional simplices of the n-dimensional simplex. Thus, given a loopgraph with n-external links, the deficit angle for a general n-polytope (n-valent loop graph) is:

**Coarse grained area**

The boundary of spacetime is a 3-dimensional space. Triangulation on the boundary is defined using flat polyhedra. Every closed, flat, n polyhedron satisfies the closure relation on the node given by

Consider the net of a polyhedron:

Since the interior of polyhedron is at, the closure relation can be written as

Then the area operator on the base is

and we can define the coarse-grained area as:

Thus, for a 2-dimensional surface, we can always think the coarse grained area A_{L} as the area of the base of a polyhedron, while the total sum of area A_{l} is the area around the hat – the area of n triangle which form the net of the polyhedron.

The differences between the coarse-grained and the fine-grained area gives a good measurement on how the space deviates from being flat. It is possible to obtain the explicit relation between the Regge curvature with these area differences in some special cases.

The Regge curvature for a 2-dimensional surface is defined as 2 minus the sum of all dihedral angle surrounding a point of the triangulation, which is n. The Regge curvature as a function of the coarse-grained and the fine-grained area is:

In the classical limit, it is clear that there can be states where ε = 0 or A_{L} = 0. These correspond to geometries where the normals to the facets forming the large surface L are parallel. However, this is only true in the classical limit, namely disregarding Planck scale effects. If we take Planck-scale effects into account, we have the result that

where n + 1 is the number of facets. Therefore the fine grained area is always strictly larger than the coarse grained area. There is a Planck length square contribution for each additional facet. It is as if there was an irreducible Planck-scale fluctuation in the orientation of the facets.

** Coarse-grained volume**

In the same manner as the surface’s coarse-graining, we triangulate a 3-dimensional chunk of space using n symmetric tetrahedra. The Regge curvature is defined by the dihedral angle on the bones of the tetrahedra. Using the volume of one tetrahedron,

we obtain the fine-grained volume, which is

The coarse-grained volume is the volume of the 3-dimensional base, which is the volume of the n-diamond:

so the Regge curvature is

Notice that this is just a classical example. In the quantum picture, adding two quantum tetrahedra does not gives only a triangular bipyramid, it could give other possible geometries which have 6 facets, i.e., a parallelepiped, or a pentagonal-pyramid.

** Conclusion**

The number of quanta is not an absolute property of a quantum state: it depends on the basis on which the state is expanded. In turn, this depends on the way we are interacting with the system. The quanta of the gravitational field we interact with, are those described by the quantum numbers of coarse-grained operators like A_{L} and V, not the maximally fine-grained ones.

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