Canonical Quantum Gravity, Constructive QFT, and Renormalisation

1. Introduction

The canonical approach to quantum gravity has been initialised long time ago [1–14]. However, the mathematical foundations of the theory remained veiled due to the tremendous non-linearity of the gravitational interaction. This has much changed with the reformulation of general relativity as a Yang–Mills type gauge theory in terms of connection, rather than metric variables [15,16], and has culminated in a research programme now known as loop quantum gravity (LQG) (see e.g., Refs. 17–21 for monographs and recent reviews on the subject). The qualifier ‘loop’ stems from the fact that for gauge theories of Yang–Mills type, it has proved useful to formulate the theory in terms of holonomies of the connection along closed paths (loops) in order to maintain manifest gauge invariance. Such so-called (Wilson) loop variables are widely used, for instance, in (lattice) QCD [22].

LQG has succeeded in providing a rigorous mathematical framework: The representation theory of the canonical commutation relations and the * relations has been studied and a unique representation has been singled out [23–27] that allows for a unitary representation of the spatial diffeomorphism group. Moreover, the generators of temporal diffeomorphisms, sometimes referred to as Wheeler–DeWitt operators, could be rigorously quantised on the corresponding Hilbert space [28–32], and in contrast to the perturbative approach to quantum gravity [33, 34], no ultraviolet divergences were found. It should be emphasised that this was achieved 1) in the continuum, rather than on a lattice, that is, there is no artificial cut-off left over; 2) for the physical Lorentzian signature, rather than unphysical Euclidian one; and 3) non-perturbatively and background independently, that is, one does not perturb around a classical background metric and then quantises the fluctuations which thus manifestly preserves the diffeomorphism covariance of all constructions.

However, the theory is not yet completed: Due to the tremendously non-polynomial nature of the gravitational interaction, the usual factor ordering ambiguity in the quantisation of operator-valued distributions which are non-linear in the fields is much more severe. Thus, the operators defined in Refs. 28–32 suffer from those ambiguities. Moreover, the following problem arises: In the classical theory, the canonical generators of space-time diffeomorphisms (i.e., their Hamiltonian vector fields) form a Lie algebroid (i.e., a Lie algebra except that the structure constants are replaced by structure functions on the phase space) known as the hypersurface algebroid [35]. The structure functions are themselves promoted to operator-valued distributions upon quantization; thus, it becomes even harder to find quantization of those generators such that the algebroid is represented without anomalies than it would be for an honest Lie algebra. Specifically, the commutator between two temporal diffeomorphism generators is supposed to 1) be proportional to a linear combination of spatial diffeomorphism generators with operator-valued distributions as coefficients and 2) in an ordering, such that the following holds: The image of any such commutator of a dense domain of vectors in the Hilbert space must be in the kernel of the space of spatially diffeomorphism-invariant distributions on that domain. In Ref. 37, it is shown that both conditions 1) and 2) hold; however, the coefficients in that linear combination do not qualify as quantisations of their classical counterpart. Thus, while the quantisation of the hypersurface algebroid closes, it does so with the wrong operator-valued distributions as coefficients.

Thus, the status of LQG can be summarised as follows:

As compared to Refs. 1–14, it is now possible to ask and answer precise questions about the mathematical consistency of the whole framework. As compared to the perturbative approach, the framework does not suffer from ultraviolet divergences and one does not have to worry about the convergence of a perturbation series due to the manifestly non-perturbative definition of LQG. However, just as in the perturbative approach, one needs further input in order to draw predictions from the theory, although of a different kind: In the perturbative approach, there are an infinite number of counter terms necessary due to non-perturbative non-renormalisability all of which come with coefficients that have to be measured, but one can argue that only a finite number of them is of interest for processes involving energies not exceeding a certain threshold (effective field theory point of view). In LQG, there are in principle infinitely many quantisation ordering prescriptions possible, each of which comes with definite coefficients in order to yield the correct naive continuum limit, but it is not clear which ordering to choose so that presently one resorts to the principle of least technical complexity.

Various proposals have been made in order to improve the situation. In Ref. 38, one exploits the fact that classically one can always trade a set of first-class constraints by a single weighted sum of their squares (called the master constraint). Since a single constraint always closes with itself and the weights can be chosen such that the master constraint commutes with spatial diffeomorphisms, one can now focus on the quantisation ambiguities involved in the master constraint without having to worry about anomalies. In Ref. 39, the case of general relativity coupled to perfect fluid matter was considered, which allows solving the constraints before quantisation so that the remaining quantisation ambiguity now only rests in the corresponding physical Hamiltonian that drives the time evolution of the physical (i.e., space-time diffeomorphism-invariant) observables. In Refs. 40–42, the constraints are quantised on a suitable space of distributions with respect to a dense domain of the Hilbert space, rather than the Hilbert space itself in order to find a representation of the hypersurface algebroid directly on that space of distributions which would at least partially fix the aforementioned ordering ambiguity.

It transpires that additional input is necessary in order to fix the quantisation ambiguity in the dynamics of LQG and thus to complete the definition of the theory. This would also put additional faith in applications of LQG, for instance to quantum cosmology [43–46] (where the amount of ambiguity is drastically reduced) which are believed to be approximations of LQG by enabling to make the connection between LQG and those approximations precise including an error control (see Refs. 47–53 for recent progress in that respect). In the recent proposal [54–57] which we intend to review in this article, the authors were inspired by Wilson’s observation [54–57] that renormalisation methods help identify among the principally infinitely many interaction terms in Hamiltonians relevant for condensed matter physics the finitely many relevant ones that need to be measured. This insight implies that a theory may be perturbatively non-renomalisable but non-perturbatively renormalisable, also known as asymptotically safe [58]. The asymptotic safety approach to quantum gravity for Euclidian [59–68] and Lorentzian signature [69, 70] precisely rests on that idea and has received much attention recently. In fact, there is much in common between our proposal and asymptotically safe quantum gravity (especially for Lorentzian signature), and we will have the opportunity to spell out more precisely points of contact in the longer version of this article [196].

Also, there is a large body of work on renormalisation [71–75] in the so-called spin foam approach [85–92] and the related group field theory [76–81] and tensor model¹ [82–84] approach to quantum gravity. The spin foam approach is loosely connected to LQG in the following sense: The states of the Hilbert space underlying LQG are labelled by collections of loops, that is, 3D graphs. A spin foam is an operator that maps such states excited on a graph to states excited on another graph. The operator depends on a specific class of 4D cell complex (foam) such that its boundary 3D complex is dual to the union of the two graphs corresponding to the incoming and outgoing Hilbert spaces. The operator is supposed to form the rigging map [93] of LQG, that is, a generalised projector onto the joint kernel of the Wheeler–Dewitt constraints. We say that the connection is loose because the rigging nature of current spin foams in 4D is not confirmed yet. In any case, a spin foam operator can be formulated as a state sum model, and thus, renormalisation ideas apply. (For applications of renormalisation group ideas in the cosmological sector of LQG, see Refs. 94–96.)

Most of the work on renormalisation is either within classical statistical physics (e.g., Ref. 97) or the Euclidian (also called constructive) approach to the quantum field theory [98–100]. In the Euclidian approach, the quantum field, which is an operator-valued distribution on Minkowski space, is replaced by a distribution-valued random variable on Euclidian space. While the dynamics in the Minkowski theory is given by Heisenberg equations, in the Euclidian theory, it is encoded in a measure on the space of random variables. We are then back in the realm of statistical physics because loosely speaking, the measure can be considered as a Gibbs factor for a Hamiltonian (sometimes called Euclidian action) in four spatial dimensions. How then should one use renormalisation ideas for quantum gravity? Quantum gravity is not a quantum field theory on Minkowski space (unless one works in the perturbative regime, but then it is non-renormalisable). Also, while the Minkowski and Euclidian signature of metrics are related by simple analytic rotation in time from the real to the imaginary axis, this does not even work for classical metrics with curvature, not to mention the quantum nature of the metric (in ordinary QFT, the metric is just a non-dynamic background structure). One can, of course, start with Euclidian signature GR and try to build a measure theoretic framework, but then the relation to the Lorentzian signature theory is unclear. Moreover, while as an ansatz for the Euclidian signature measure, we can take the exponential of the Euclidian Einstein–Hilbert action, that action is not bounded from below, and thus, the measure cannot be a probability measure which is one of the assumptions of constructive QFT. Finally, in contrast to constructive QFT, in quantum gravity expectation, values (operator language) or means (measure language) of basic operators (or random variables) such as the metric tensor have no direct physical meaning because coordinate transformations are considered as gauge transformations; hence, none of the basic fields correspond to observables.

In our approach [54–57], we will use the framework [39], that is, we do not consider vacuum GR but GR coupled to matter which acts as a dynamical reference field. This enables us 1) to solve the spatial diffeomorphism and Hamiltonian constraints classically, 2) to work directly on the physical Hilbert space (i.e., the generalised kernel of all constraints equipped with the inner product induced by the rigged Hilbert space structure, 3) to have at our disposal immediately the gauge-invariant degrees of freedom such that the physical Hilbert space is the representation space of a * representation of those observables, and 4) to be equipped with a physical Hamiltonian that drives the physical time evolution of those observables. Concretely and out of mathematical convenience, we use the perfect fluid matter suggested in Refs. 101 and 102, but for what follows, these details are not important. Important is only that it is possible to rephrase GR coupled to matter as a conservative Hamiltonian system and that all the machinery that was developed for LQG can be imported. Now, the quantisation ambiguity rests, of course, in the physical Hamiltonian and it is that object and its renormalisation on which we focus our attention.

As we just explained, we can bring GR coupled to matter somewhat closer to the usual setting of ordinary QFT or statistical physics, but still we cannot apply the usual path integral renormalisation scheme because we work in the canonical (or Hamiltonian) framework. The idea is then to make use of Feynman–Kac–Trotter–Wiener–like ideas in order to generate a Wiener measure theoretic framework from the Hamiltonian setting and vice versa to use Osterwalder–Schrader reconstruction to map the measure theoretic (or path integral) framework to the Hamiltonian one. This way we can map between the two frameworks and thus import path integral renormalisation techniques into the Hamiltonian framework which are strictly equivalent to those employed in path integral renormalisation. In order that this works one needs to check, of course, that the Wiener measure constructed obeys at least a minimal subset [103] of Osterwalder–Schrader axioms [104] in order for the reconstruction to be applicable, most importantly reflection positivity.

This was one of the goals of [54–57], namely, to define a renormalisation group flow directly within the Hamiltonian setting with strict equivalence to the path integral flow. Specifically, the flow is a flow of Osterwalder–Schrader triples $\left(\mathcal{H},H,\text{Ω}\right)$ consisting of a Hilbert space $\mathcal{H}$, a self-adjoint Hamiltonian H thereon bounded from below, and a vacuum vector $\text{Ω}\in \mathcal{H}$ annihilated by H. While physically well-motivated, of course, one does not need to do this. Indeed, renormalisation techniques for Hamiltonians and vacua directly within the Hamiltonian setting were invented before, and we devote the next section for putting our framework into context with schemes closely related to ours. The fact that we have a precise relation between Hamiltonian and path integral renormalisation makes it possible to bring Hamiltonian formulations of quantum gravity such as LQG and path integral formulations, such as asymptotically safe quantum gravity, into closer contact.

The architecture of this article is as follows:

In the second section, we give an incomplete overview over and sketch Hamiltonian renormalisation frameworks closely related to ours and point out differences and similarities.

In the third section, we review how classical general relativity coupled to suitable matter can be brought into the form of a conservative Hamiltonian system and the LQG quantisation thereof. The necessity to remove quantisation ambiguities will be highlighted.

In the fourth section, we recall some background material on constructive QFT, the Feynman–Kac–Trotter–Wiener construction, and Osterwalder–Schrader reconstruction.

In the fifth section, we derive the natural relation between families of cylindrically defined measures, coarse graining, renormalisation group flows, and their fixed points. We then use Osterwalder–Schrader reconstruction to map the flow into the Hamiltonian framework. This section contains new material as compared to [54–57] in the sense that we 1) develop some systematics in the choice of coarse graining maps that are motivated by naturally available structures in the classical theory, 2) clarify the importance of the choice of random variable or stochastic process when performing OS reconstruction, and 3) improve the derivation of the Hamiltonian renormalisation flow by adding the uniqueness of the vacuum as an additional assumption (also made in the OS framework of Euclidian QFT [98–100]) as well as some machinery concerning degenerate contraction semi-groups and associated Kato–Trotter formulae.

In the sixth section, we summarise, spell out implications of the renormalisation programme for the anomaly-free implementation of the hypersurface algebroid, and outline the next steps when trying to apply the framework to interacting QFT and finally canonical quantum gravity such as LQG.

The paper is supplemented by the following appendices:

In Supplementary Appendix A, we prove some properties for a coarse graining scheme appropriate for non-Abelian gauge theories; in Supplementary Appendix B, we prove a lemma on the existence of certain Abelian $C^{\text{*}}-$algebras needed for the construction of stochastic processes during OS reconstruction; in Supplementary Appendix C, we collect some renormalisation terminology for readers more familiar with actions, rather than measures; in Supplementary Appendix D, we give a proof for the Kato–Trotter product formula for semi-groups and projections in the simple case that the semi-group has a bounded generator; and in Supplementary Appendix E, we prove a strong limit identity between projections needed in Section 5.3.

In Supplementary Appendix F, we mention concrete points of contact between the scheme developed here and others in the context of density matrix, entanglement, and projective renormalisation.

In Supplementary Appendix G, we sketch a relation between Hamiltonian renormalisation via Osterwalder–Schrader reconstruction and the functional renormalisation group which is the underlying technique of the asymptotic safety programme. This article is the journal version of Ref. 196 which is organised slightly differently in the sense that Appendices F, G of this article are part of the main text of Ref. 196.

2. Overview Over Related Hamiltonian Renormalisation Schemes

The purpose of this section is not to give a complete scan of the vast literature on the subject of Hamiltonian renormalisation but just to give an overview over those programmes that we believe are closest to ours. Also, we leave out many finer details as we just want to sketch their relation to our framework in broad terms. In sections 6 and 7 of Ref. 196, we will give a few more details on the connection between our approach and the density matrix and functional renormalisation group.

The starting point is, of course, the seminal works by Kadanoff [105] and Wilson [106, 107]. Kadanoff introduced the concept of a block spin transformation in statistical physics, that is, a coarse graining transformation in real space (namely, on the location of the spin degrees of freedom on the lattice), rather than in some more abstract space (e.g., momentum space blocking/suppressing as used, e.g., in the asymptotically safe quantum gravity approach). This kind of real-space coarse graining map is widely used not only in statistical physics but also in the path integral approach to QFT as, for instance, in lattice QCD [108]. On the other hand, Wilson introduced the concept of Hamiltonian diagonalisation to solve the Kondo problem (the low-temperature behaviour of the electrical resistance in metals with impurities). This defines a renormalisation group flow directly on the space of Hamiltonians and its lowest lying energy eigenstates. More precisely, one considers a family of Hamiltonians labelled by an integer-valued cut-off on the momentum mode label of the electron annihilation and creation operators. The renormalisation group flow is defined by diagonalising the Hamiltonian given by a certain cut-off label, and to use the eigenstates so computed to construct the matrix elements of the Hamiltonian at the next cut-off label. To make this practical, Wilson considered a truncation, at each renormalisation step, of the full energy spectrum to the ${10}^3$lowest lying energy levels which was sufficient for the low-temperature Kondo problem. This is in fact nothing but the concrete application of the Rayleigh–Ritz method. The concept of truncation plays an important role also in most other renormalisation schemes, as otherwise the calculations become unmanageable.

The next step was done by Wegner [109, 110] as well as Glazek and Wilson [111] which can be considered as a generalisation of the Hamiltonian methods of Refs. 106 and 107. It could be called perturbative Hamiltonian block diagonalisation and was applied in QFT already (e.g., Refs. 112 and 113 and references therein). Roughly speaking, one introduces a momentum cut-off on the modes of the annihilation and creation operators involved in the free part of the Hamiltonian, then perturbatively (with respect to the coupling constant) constructs unitarities which at least block diagonalise that Hamiltonian with respect to a basis defined by modes that lie below half the cut-off and those that lie between half and the full cut-off, and then projects the Hamiltonian onto the Hilbert space defined by the modes below half of the cut-off to define a new Hamiltonian at half the cut-off. This can be done for each value of the cut-off and thus defines a flow of Hamiltonians (and vacua defined as their ground states). Another branch of work closely related to this is the projective programme due to Kijowski [114, 115]. Here, a flow of Hamiltonians on Hilbert spaces for different resolutions is given by the partial traces of the corresponding density matrices given by minus their exponential (Gibbs factors, assuming that these are trace class). (See also Refs. 116–123 for more recent work on renormalisation building on this programme.)

In these developments, the spectrum of the Hamiltonian was directly used to define the flow. Another proposal was made by White [124] who defined the density matrix renormalisation group. This is a real-space renormalisation group flow which considers the reduced density matrix corresponding to the tensor product split of a vector (e.g., the ground state of a Hamiltonian) of the total Hilbert space into two factors corresponding to a block and the rest (or at least a much larger ‘superblock’). This density matrix is diagonalised, and then, the Hilbert space is truncated by keeping only a certain fixed number of highest lying eigenvalues of the reduced density matrix. Finally, the Hamiltonian corresponding to the block is projected, and then, the resulting structure is considered as the new structure on the coarser lattice resulting from collapsing the blocks to new vertices (we are skipping here some finer details). This method thus makes use of entanglement ideas since the reduced density matrix defines the degree of entanglement via its von Neumann entropy.

A variant of this is the tensor renormalisation group approach due to Levin and Nave [125]. It is based on the fact that each vector in a finite tensor product of finite-dimensional Hilbert spaces can be written as a matrix product state, that is, the coefficients of the vector with respect to the tensor product base can be written as a trace of a product of matrices of which there are, in general, as many as the dimensionality of the Hilbert space. One now performs a real-space renormalisation scheme directly in terms of those matrices which are considered to be located on a lattice with as many vertices as tensor product factors. Importantly, this work connects renormalisation to the powerful numerical machinery of tensor networks [126].

Finally, as observed by Vidal [127] and Evenbly and Vidal [128, 129], one can improve [124, 125] by building in an additional unitary disentanglement step into the tensor network renormalisation scheme. This is quite natural because a tensor network can also be considered as a quantum circuit with the truncation steps involved considered as isometries, but a quantum circuit in quantum computing [130] consists of a network of unitary gates, some of which have a disentangling nature depending on the state that they act upon. The resulting scheme is called multi-scale entanglement renormalisation ansatz (MERA).

As this brief and incomplete discussion reveals, there are numerous proposals in the literature for how to renormalise quantum systems. They crucially differ from each other in the choice of the coarse graining map. There are various aspects that discriminate between these maps, such as the following:

(1) Real space vs. other labels

The degrees of freedom to be coarse grained are labelled by points in space-time or else (momentum, energy, etc.).

(2) Kinematic vs. dynamical

Real-space block spin transformations are an example of a kinematic coarse graining, that is, the form of the action, a Hamiltonian, its vacuum vector, its associated reduced density matrix, and the corresponding degree of entanglement do not play any role. By contrast, Hamiltonian block diagonalisation, density matrix, and entanglement renormalisation take such dynamical information into account.

(3) Truncated vs. exact

In principle, any renormalisation scheme can be performed exactly, for example, in real-space path integral renormalisation, one can just integrate the excess degrees of freedom that live on the finer lattice but not on the coarser, thus obtaining the measure (or effective action) on the coarser lattice from that of the finer one. The same is true, for example, for the procedure followed in asymptotically safe quantum gravity. However, in practice, this may quickly become unmanageable, and thus, one resorts to approximation methods, for example, by truncation in the space of coupling constants, energy eigenstates, or reduced density matrix eigenstates.

For the newcomer to the subject, this plethora of suggestions may appear confusing. Which choice of coarse graining is preferred? Do different choices lead to equivalent physics? What can be said about the convergence of various schemes and what is the meaning of the fixed point(s) if it (they) exist(s)? The physical intuition is that different schemes should give equivalent results if 1) the corresponding fixed point conditions capture necessary and sufficient properties that the theory should have in order to qualify as a continuum theory and 2) when performed exactly. The first condition is obvious: we start from what we believe to be an initial guess for how the theory looks at different resolutions and then formulate a coarse graining flow whose fixed points are such that they qualify to define a continuum theory. The second condition entails that the coarse graining maps just differ in the separation of the total set of degrees of freedom into subsets corresponding to coarse and fine resolution, hence corresponds to choices of coordinate systems which, of course, can be translated into each other. However, when truncations come into play, this equivalence is lost because different schemes truncate different sets of degrees of freedom which are generically no longer in bijection. It is conceivable therefore that dynamically driven truncation schemes perform better at identifying the correct fixed point structure of the theory in the sense that they may converge faster and are less vulnerable to truncation errors or automatically pick the truncation of irrelevant couplings. This seems to be confirmed in spin system examples, but we are not aware of a general proof. Recently, the importance of the kinematic vs. dynamic issue has also been emphasised for the LQG and spin foam approach [131–133].

In our work, we currently are not concerned with issues of computationability, that is, we consider an exact scheme. Next, as far as the coarse graining map is concerned, we currently favour a kinematic scheme. The reason for doing this is that kinematic schemes are naturally suggested by measure theoretic questions, namely, measures on spaces of infinitely many degrees of freedom are never of the type of the exponential of some action times a normalisation constant times Lebesgue measure. Neither of these three ingredients is well defined. What is well defined are integrals of certain probe functions of the field with respect to that measure. These probe functions, in turn, are naturally chosen to depend on test functions that one integrates the field against. Thus, these test functions provide a natural notion of resolution, discretisation, and coarse graining. By integrating the measure against probe functions, one obtains a family of measures labelled by the test functions involved. The relation between test functions at different resolution induces a corresponding relation between members of the family of measures which must hold exactly for a true measure of the continuum QFT. In turn, such consistency relations called cylindrical consistency can be used to define a measure on a space of infinitely many degrees of freedom [134], called a projective limit. The idea is then to formulate measure renormalisation in such a way that its fixed points solve the consistency relations. This approach has been advocated in Refs. 135 and 136 for Euclidian Yang–Mills theory and in Refs. 137 and 138 for spin foams. Note that spin foams, strictly speaking, do not construct measures but rather are supposed to construct a rigging map so that Hamiltonian methods come also into play. Indeed, in Refs. 131–133, it was shown that the cylindrically consistent coarse graining of the rigging map and its underlying space-time lattice, thought of as an anti-linear functional on the kinematical Hilbert space, induce a coarse graining of the spatial lattice on its boundary and thus the Hilbert space thereon, equipping it with a system of consistent embeddings, a structure similar to inductive limits of Hilbert spaces (an inductive structure requires in addition the injections to be isometric). That latter structure underlies the kinematical Hilbert space of LQG, and a renormalisation procedure based on inductive limits was already proposed in Refs. 139–141 due to the similarity of LQG to the lattice gauge theory.

Another reason for why picking real-space coarse graining schemes as compared to, say, momentum space–based ones is their background independence, which is especially important for quantum gravity. In our work, as we consider the version of LQG in which the constraints already have been solved, we will work with probability measures. As we will see, the connection between inductive limits of Hilbert spaces and projective limits of path integral measures can be made crystal clear in this case. The price we pay by using an exact, kinematical scheme is that the fixed point (or renormalised) Hamiltonian becomes spatially non-local at finite resolution. However, in the free QFT examples studied [54–57], which are spatially local in the continuum, by blocking the known fixed point theory from the continuum, one can see that this is natural and must happen for such schemes; hence, it is not a reason for concern but, in fact, physical reality. The degree of spatial non-locality, in fact, decreases as we increase the resolution scale.

When applying the framework to interacting QFT, one will have to resort to some kind of approximation scheme, and possibly, tools from entanglement renormalisation combined with tensor network techniques may prove useful. However, note that QFT of bosonic fields (gravity is an example) deals with infinite-dimensional Hilbert spaces even when the theory depends only on a finite number of degrees of freedom, say, by discretising it on a lattice and confining it to finite volume. Thus, to apply tensor network techniques which, to the best of our knowledge, require the factors in the tensor product to be finite-dimensional Hilbert spaces, one would have to cut off the dimensions of those Hilbert spaces right from the beginning, that is, one would have to work with three cut-offs, rather than two (see, e.g., Refs. 142 and 143 where quantum group representations are used in gauge theories, rather than classical group representations, and perform real-space renormalisation or [144] where one combines both the UV and the dimension cut-off into one by turning the dimension of tensor spaces in tensor models into a finite coarse graining parameter and otherwise performs the asymptotic safety programme which is often formulated in the presence of a cut-off anyway).

Some sort of truncation or approximation has to be made in practice when treating complex systems numerically. The physical insight behind the tensor network and density matrix/entanglement renormalisation developments, namely, the dynamically interesting vectors in a Hilbert space appear to lie in a ‘tiny’ subspace thereof is presumably a profound one, and the truncation of the Hilbert space to the corresponding subspaces appears to be well-motivated by the model (spin) systems studied so far. Still, what one would like to have is some sort of error control or convergence criteria on those truncations. We appreciate that this is a hard task for the future. For the time being, we phrase our framework without incorporating a cut-off on the dimension of Hilbert spaces as we are not yet concerned with numerical investigations; however, we may have to use some of these ideas in the future.

3. Canonical Quantum Gravity Coupled to Reference Matter

The physical idea is quite simple and goes back to Ref. 145: General relativity is a gauge theory, the gauge group being the space-time diffeomorphism group. Thus, the basic tensor and spinor fields in terms of which one writes the Einstein–Hilbert action and the action of the standard model coupled to the metric (or its tetrad) are not observable. However, the value of, say, a scalar field $\text{Φ}$ at that space-time point $X_y$, at which four reference scalar fields ${\varphi }^0,\dots ,{\varphi }^3$ take values $y^0,\mathrm{.}\mathrm{.},y^3$, that is, $\text{Φ}\left(X_y\right);{\varphi }^{\mu }\left(X_y\right)=y^{\mu }$ is space-time diffeomorphism-invariant. For this to work, the relation ${\varphi }^{\mu }\left(X\right)=y^{\mu }$ must, of course, be invertible, in particular the reference scalar fields must not vanish anywhere or anytime. This seems to be a property of dark matter [146].

These kinds of relational observables have been further developed by various authors, in particular [147–153]. When one couples general relativity and such reference matter preserving general covariance, it becomes possible to formulate the theory in a manifestly gauge-invariant way. The form of that gauge-invariant formulation, of course, strongly depends on the type of reference matter used and its Lagrangian. In what follows, we use the concrete model [39] out of mathematical convenience, but we emphasise that the same technique works in a fairly general context. In the next subsection, that model will be introduced and the classical gauge-invariant formulation will be derived. After that, we quantise it using LQG methods which will be introduced in tandem.

3.1. Gaussian Dust Model

The Lagrangian of the theory takes the form

where $L_{EH}$ is the Einstein–Hilbert Lagrangian, $L_{SM}$ is the standard model Lagrangian coupled to GR via the metric, its tetrad or its spin connection, and $L_D$ is the Gaussian dust Lagrangian [101, 102]

where g is the Lorentzian signature metric tensor, ${\nabla }_{\mu }$ its Levi-Civita covariant differential, ${\varphi }^0:=T,{\varphi }^j:=S^j;j=\mathrm{1,2,3}$ are the reference scalar fields introduced above, and $\rho ,W_j$ are additional four scalar fields. The latter four fields appear without derivatives and thus give rise to primary constraints in addition to those present even in vacuum GR. One can easily show that the contribution of $L_D$ to the energy momentum tensor is of perfect fluid type. Further physical properties and motivations are discussed in Refs. 101 and 102. For what follows, it suffices to know that the equations of motion for $T,S^j$, say that ${\nabla }_{\mu }T$ is a time-like geodesic cotangent and that $S^j$ is constant along the geodesic spray. Thus, those geodesics can be interpreted as world lines of dynamically coupled test observers.

The full constraint analysis of Eq. 3.1 is carried out in Ref. 39. There are secondary constraints, and the full set of constraints contains those of the first and second classes (see Ref. 153 for a modern treatment of Dirac’s algorithm [154]). One has to introduce a Dirac bracket and solve the second-class constraints in the course of which the variables $\rho ,W_j$ are eliminated. The remaining constraints are then of first class and read as

Here, C is the Wheeler–DeWitt constraint function (including standard matter) and $C_a,a=\mathrm{1,2,3}$ are the spatial diffeomorphism functions (including standard matter). The Dirac bracket reduces to the Poisson bracket on all the variables involved in Eq. 3.3, and $P,P_j$ are the momenta conjugate to $T,S^j$, for example, $\left\{P\left(x\right),T\left(y\right)\right\}=\delta \left(x,y\right)$. Here, $a=\mathrm{1,2,3}$ are tensorial indices on the spatial hypersurface σ of the Arnowitt–Deser–Misner foliation underlying the Hamiltonian formulation of GR [155] with intrinsic metric tensor $q_{ab}$. For the moment, it is just necessary to know that $C\text{ and }{C}_a$ do not involve the variables $T,P,S^j,\text{ and }{P}_j$.

The constraints (Eq. 3.3) encode the space-time diffeomorphism gauge symmetry in Hamiltonian form, in particular they represent the hypersurface deformation algebra [111]. It is possible to solve these remaining constraints to determine the complete set of gauge-invariant (the so-called Dirac) observables and to determine the physical Hamiltonian H that drives their physical time evolution [39]. Equivalently, we may gauge fix Eq. 3.3. The above interpretation of $T\text{ and }{S}^j$ suggest to use the gauge conditions $G=T-t\text{ and }{G}^a={\delta }_j^a{S}^j-x^a$. The stabilisation of these gauge conditions fixes the Lagrange multipliers $\lambda \text{ and }{\lambda }^a$ in the gauge generator

namely,

which when evaluated at $G=G^a=0$ yields the unique solution $\lambda =1,{\lambda }^a=0$. Likewise, in this gauge, the constraints can be uniquely solved for $P=-C\text{ and}{P}_j=-{\delta }_j^a{C}_a$ while $T\text{ and }{S}^j$ are pure gauge. This shows that the physical degrees of freedom are those not involving $T,P,S^j,\text{ and }{P}_j$.

For any function F independent of these variables, the reduced or physical Hamiltonian is that function on the phase space coordinatised by the physical degrees of freedom which generates the same time evolution as K when the constraints, gauge conditions, and stabilising Lagrange multipliers are installed

which shows that

Thus, the final picture is remarkably simple: The physical phase space is simply coordinatised by all metric and standard matter degrees of freedom (and their conjugate momenta), while the physical Hamiltonian is just the integral of the usual Wheeler–DeWitt constraint. The influence of the reference matter now only reveals itself in the fact that H is not constrained to vanish as it only involves the geometry and standard matter contribution C of $C^{\text{tot}}$ and that the number of physical degrees of freedom has increased by four as compared to the system without reference matter. This phenomenon is, of course, well-known from the electroweak interaction: One can solve the three isospin SU(2) Gauss constraints for three of the four degrees of freedom sitting in the complex-valued Higgs isodublett, leaving a single scalar Higgs field and three massive, rather than massless, vector bosons. (See Refs. 156 and 157 for further discussion.)

We close this subsection with three remarks: First, a complete discussion requires to show that the gauge cut $G=G^a=0$ on the constraint surface of the phase space be reachable from anywhere on the constraint surface. As Eq. 3.5 shows, this requires that $S_{,a}^j$ be invertible. We thus impose this as an anholonomic constraint on the total phase space. One easily verifies from Eq. 3.5 that this condition is gauge-invariant, that is, compatible with the dynamics.

Second, the simplicity of the final picture is due to the particular choice of reference matter. Other reference matter most likely will increase the complexity (see, e.g., Ref. 158), which produces a square root Hamiltonian! One may argue that the dust is a form of cold dark matter [146], but it is unclear whether this is physically viable. Nevertheless, the present model serves as a proof of principle, namely, that GR coupled to standard matter and reference can be cast into the form of a conservative Hamiltonian system.

Third, it should be appreciated that the reference matter helps us accomplish a huge step in the quantum gravity programme: It frees us from quantising and solving the constraints and constructing the physical inner product, the gauge-invariant observables, and their physical time evolution. All of these steps are of tremendous technical difficulty [17–21]. All we are left to do is to quantise the physical degrees of freedom and the physical Hamiltonian.

3.2. Loop Quantum Gravity Quantisation of the Reduced Physical System

In order to keep the technical complexity to a minimum, we consider just the contribution to H coming from the gravitational degrees of freedom (see [17–21, 28–32] for more detail on standard matter coupling). The Hamiltonian directly written in terms of $SU\left(2\right)$ gauge theory variables reads (we drop some numerical coefficients that are not important for our discussion)

Here, A is an SU(2) connection and E an SU(2) non-Abelian electric field that one would encounter also in an SU(2) Yang–Mills theory. However, the geometric interpretation of $A\text{ and }E$ is different, namely, $e_j^a:=E_j^a/\sqrt{\left|\text{det}\left(E\right)\right|}$ is a triad, that is, $q^{ab}={\delta }^{jk}{e}_j^a{e}_k^b$ is the inverse spatial metric. Here, as before, $a,b,c,\mathrm{.}\mathrm{.}=\mathrm{1,2,3}$ denote spatial tensor indices, while now $j,k,l,\mathrm{.}\mathrm{.}=\mathrm{1,2,3}$ denote su(2) Lie algebra indices. Further, let ${\text{Γ}}_a^j$ be the spin connection of $e_j^a$. Then, $K_{ab}:=\left(A_a^j-{\text{Γ}}_a^j\right)e_b^k{\delta }_{jk}$ has the meaning of the extrinsic curvature of the ADM slices [155] on the kernel of the SU(2) Gauss constraint

The important quantity V is recognised as the total volume of the hypersurface σ, and $H_E\text{ and }{H}_L$ are known as the Euclidian and Lorentzian contributions to H. (See Refs. 28–32 for further details.) The Poisson brackets displayed are with respect to the standard symplectic structure

where $\hslash \kappa ={\ell }_P^2$ is the Planck area. The definition of the phase space is completed by the statement that the elementary fields $A\text{ and }E$ are real-valued

The traces involved in 3.2 are carried out by introducing the Lie algebra-valued 1-forms $A=A_a^j{\tau }_{j}d{x}^a$, where $2i{\tau }_j$ are the Pauli matrices and $F=2\left(dA+A\wedge A\right)$ is the curvature of A. The non-polynomiality of GR is hidden in the Poisson brackets that appear in Eq. 3.8. The reason why we use these particular Poisson bracket structure will become clear only later.

To quantise the theory, we start from functions on the phase space that are usually employed in the lattice gauge theory (see, e.g., Ref. 159), namely, non-Abelian magnetic holonomy and electric flux variables

where $\mathcal{P}$ denotes path ordering, c is a piecewise analytic real curve, S is a piecewise real analytic surface, f is an su(2)-valued function, and $\text{*}E={\mathit{ϵ}}_{abc}{E}^{a}d{x}^b\wedge d{x}^c/2$ is the pseudo 2-form corresponding to the su(2)-valued vector density E. Note that $A\left(c\right)$ is SU(2)-valued, while $E_f\left(S\right)$ is su(2)-valued

where $c^{-1}$ is the same curve as c but with the opposite orientation. The simplest non-trivial Poisson brackets are

in case that $S{{\displaystyle \cap }}^{\text{}}c$ is a single point in the interior of both $c\text{ and }S$, see Refs. 23–27 for a complete discussion. The relations (3.13) and (3.14) are the defining relations of a non-commutative abstract $-\text{*}$algebra $\mathcal{U}$ generated by fluxes and complex-valued smooth functions F of a finite number of holonomy variables [23–27]. It is the free algebra generated by them and divided by the two-sided ideal generated by the canonical commutation relations $E_f\left(S\right)A\left(c\right)-A\left(c\right)E_f\left(S\right)=i\hslash \left\{E_f\left(S\right),F\right\}$ and the adjointness relations (Eq. 3.13). (See Refs. 23–27 for more details.)

Interestingly, the physical Hamiltonian H has a large symmetry group, namely, it is invariant under the group $\mathcal{G}=SU{\left(2\right)}_{\text{loc}}\mathrm{⋊}Diff\left(\sigma \right)$, where $SU{\left(2\right)}_{\text{loc}}$ denotes the group of local SU(2)-valued gauge transformations and $Diff\left(\sigma \right)$ denotes the group of (piecewise real analytic) diffeomorphisms of σ. An element of $\mathcal{G}$ is given by a pair $\mathcal{g}=\left(g,\phi \right)$, which acts on the basic variables as

where ${\phi }^{\text{*}}$ denotes the pull-back action of diffeomorphisms on differential forms. This action lifts to the algebra $\mathcal{U}$, specifically

where $b\left(c\right)\text{ and }f\left(c\right)$ denote the beginning and final points of c, and this simple covariant transformation behaviour was part of the reason why the particular ‘smearing’ of A along curves involved in holonomies is used. Note also the different character of the two groups: While we still have to find the gauge-invariant observables with respect to the Gauss constraint, the diffeomorphism constraint is already solved. The diffeomorphisms in $\mathcal{G}$ are thus to be considered as active diffeomorphisms, rather than passive ones.

The mathematical problem in quantising the theory consists in constructing a $\text{*}$ representation of $\mathcal{U}$, that is, a representation $\left(\pi ,\mathcal{H}\right)$ of elements $a\in \mathcal{U}$ as operators $\pi \left(a\right)$ densely defined on a common, invariant domain $\mathcal{D}$ of a Hilbert space $\mathcal{H}$ such that the * relations are implemented as adjointness relations and such that the canonical commutation relations are implemented as commutators between them. Thus, we want, in particular, that

for all $a,b,c\in \mathcal{U},z\in \mathrm{\mathbb{Z}}$ if $ab-ba=c$. In QFT, this problem is known to typically have an uncountably infinite number of unitarily inequivalent solutions; there is no Stone–von Neumann uniqueness theorem when the number of degrees of freedom in infinite. Hence, to make progress, we must use additional physical input. That input can only come from the Hamiltonian. Thus, we require in addition that the representation supports H as a self-adjoint operator (H is real-valued) also densely defined on $\mathcal{D}$ and such that $\mathcal{H}$ carries a unitary representation U of $\mathcal{G}$ (such that its generators are self-adjoint by Stone’s theorem). Using the powerful machinery of the Gel’fand–Naimark–Segal construction [160], the representation property and the unitarity property can be granted if we find a positive linear and $\mathcal{G}$ invariant functional $\omega :\mathcal{U}\to \mathrm{ℂ}$ on $\mathcal{U}$, that is,

In Refs. 23–27, it was found that there is a unique ω satisfying (3.18). While the derivation is somewhat involved, the final result can be described in a compact form. The dense domain $\mathcal{D}$ consists of functions of the form

that is, ${\psi }_{\gamma }$ is complex-valued, smooth functions of a finite number of holonomy variables. The union of the curves of these holonomies forms a finite graph γ, where $E\left(\gamma \right)$ denotes the set of its edges. Note that the elements of $\mathcal{U}$ that just depend on the connection are themselves of the form (Eq. 3.19), and thus, their action by multiplication

is densely defined. The fluxes are densely defined when acting by derivation

which also solves the canonical commutation relations.

To see that the adjointness conditions hold, we need the inner product. To define it, we note that graphs defined by finitely many piecewise analytic curves are partially ordered by set theoretic inclusion, and they are directed in the sense that for any two graphs ${\gamma }_1,{\gamma }_2$, there exists ${\gamma }_3$ with ${\gamma }_1,{\gamma }_2\subset {\gamma }_3$, for instance ${\gamma }_3={\gamma }_1\cup {\gamma }_2$. Then, we can decompose all edges of ${\gamma }_1,{\gamma }_2$ with respect to the edges of ${\gamma }_3$ and use the algebraic relations of the holonomy $A\left(c^{-1}\right)=A{\left(c\right)}^{-1}\text{ and }A\left(c\circ c\prime \right)=A\left(c\right)A\left(c\prime \right)$, where $c\circ c\prime$ is the composition of curves $f\left(c\right)=b\left(c\prime \right)$ in order to write ${\psi }_1\text{ and }{\psi }_2$ excited over ${\gamma }_1\text{ and }{\gamma }_2$, respectively, as functions excited over ${\gamma }_3$. Thus, it is sufficient to know the inner product of functions excited over the same graph γ which is given by

where ${\mu }_H$ is the Haar measure on SU(2). One can check that the adjointness relations are indeed satisfied, in fact $\pi \left(E_f\left(S\right)\right)$ is an unbounded but essentially self-adjoint operator (i.e., a symmetric operator with unique self-adjoint extension).

In fact, Eq. 3.22 defines a cylindrical family of measures ${\mu }_{\gamma }$, one for every graph γ. One has to check that Eq. 3.22 is well-defined because a function excited on γ can be written also as a function excited over any finer graph $\gamma \prime$ by extending it trivially to the additional edges. This is, in fact, the case [23–27]. Then, the Kolmogorov-type extension theorems grant that the family extends to an honest continuum measure μ on the quantum configuration space $\overline{\mathcal{A}}$ of distributional connections. We will not go into the details here which can be found in Refs. 23–27 but just mentioning for the interested reader that this space coincides with the so-called Gel’fand spectrum of the Abelian C* algebra that one obtains by completing the space of functions (Eq. 3.19) in the sup norm. It follows that the Hilbert space is given by $\mathcal{H}=L_2\left(\overline{\mathcal{A}},d\mu \right)$.

By construction, the Hilbert space $\mathcal{H}$ carries a unitary representation U of $\mathcal{G}$ given by

To check this, one uses the properties of the Haar measure (translation invariance) and the diffeomorphism invariance of Eq. 3.22 which does not care about the location and shape of the curves involved.

The Hilbert space comes equipped with an explicitly known orthonormal basis called spin network functions (SNWFs). This makes use of harmonic analysis on compact groups G [161], in particular the Peter and Weyl theorem which states that the matrix element functions of the irreducible representations of G, which are all finite-dimensional and unitary without loss of generality, are mutually orthogonal, unless equivalent, with respect to the inner product defined by the Haar measure on G; moreover, they span the whole Hilbert space. As the irreducible representations of SU(2) are labelled by spin quantum numbers, the name SNWF comes at no surprise. More in detail, an SNWF $T_{\gamma ,j,\iota }$ is labelled by a graph γ, a tuple $j={\left\{j_c\right\}}_{c\in E\left(\gamma \right)}$ of spin quantum numbers decorating the edges, and a tuple $\iota ={\left\{{\iota }_v\right\}}_{v\in V\left(\gamma \right)}$ of intertwiners decorating the vertices v in the vertex set $V\left(\gamma \right)$ of γ. Here, an intertwiner ${\iota }_v$ projects the tensor product of irreducible representations corresponding to the edges incident at v onto one of the irreducible representations appearing in its decomposition into irreducibles (Clebsch–Gordan theory). Besides providing an ONB convenient for concrete calculations, SNWFs make it easy to solve the Gauss constraint: A detailed analysis [17–21] shows that Eq. 3.9 can be quantised in the given representation and just imposes that the space of intertwiners be restricted to those projecting on the trivial (spin zero) representation. We call such intertwiners gauge-invariant. Hence, the joint kernel of the Gauss constraints is a closed subspace of $\mathcal{H}$ which is explicitly known. We will abuse the notation and will not distinguish between that subspace and $\mathcal{H}$ and henceforth consider the Gauss constraint as solved. All operators considered in what follows are manifestly gauge invariant and preserve that subspace.

As a historical remark, solutions of the Gauss constraint are excited on closed graphs since there is no non-trivial intertwiner between the trivial representation and a single irreducible one; hence, open ends are forbidden. For closed graphs, one can alternatively label SNWFs by homotopically independent closed paths (loops) with a common starting point (vertex) on that graph. Originally, one used loops as labels, hence the name loop quantum gravity (LQG).

One of the many unfamiliar features of $\mathcal{H}$ is that it is not separable which easily follows from the uncountable cardinality of the set of graphs. This is a direct consequence of the diffeomorphism invariance of the inner product: Two graphs that are arbitrarily close but disjoint are simultaneously also arbitrarily far apart under the inner product. Thus, if the measure clusters for far apart support of the smearing functions (here the graphs), then the orthogonality of the corresponding spin network functions comes at no surprise. A direct consequence of this is that the diffeomorphism operators $U\left(\phi \right)$ do not act (strongly) continuously; hence, a generator of infinitesimal diffeomorphisms generated by the integral curves of vector fields cannot exist. Yet another direct consequence is that the connection operator A itself does not exist; only its holonomies do.

The remaining task is to quantise the Hamiltonian, and it is at this point where the aforementioned quantisation ambiguities arise. The strategy followed in Refs. 28–32 is as follows: It turns out that the volume operator appearing in Eq. 3.2 can be quantised on $\mathcal{H}$ as an essentially self-adjoint operator whose spectrum is pure point (discrete) [162–164]. It is densely defined on the span of the SNWF, and it acts vertex-wise, with no contribution from gauge-(in)variant vertices that are not at least three (four) valent or from vertices whose incident edges have tangents in a common two-dimensional or one-dimensional space. Next, the holonomy along an open curve c can be expanded as $A\left(c\right)=1_2+{\displaystyle {\int }_{c}A+\dots }$ and along a closed curve α as $A\left(\alpha \right)=1_2+{\displaystyle {\int }_{S,\partial S=\alpha }F}$ so that the functions $A\text{ and }F$ that appear in Eq. 3.8 can be approximated by suitable holonomies where the approximation is in terms of the ‘length’ of the curves involved which are matched with the coordinate volume assigned by the Lebesgue measure $d^{3}x$ appearing in Eq. 3.8, approximating the integral by a Riemann sum (this is a regularisation step). Suppose then that somehow a well-defined operator $H_E$ can be defined by replacing the classical functions by operators and the Poisson brackets by commutator times $\hslash$. Then, the same argument can be applied to the Lorentzian piece. As a final piece of information, one uses the observation that a spatially diffeomorphism-invariant operator, densely defined on the span of SNWF, cannot have non-trivial matrix elements between SNWF excited over different graphs [36]. This has the following consequence: Let ${\mathcal{H}}_{\gamma }$ be the closed linear span of SNWF excited precisely over γ. Then, if H is supposed to preserve its classical diffeomorphism invariance upon quantisation, we necessarily have

where each $H_{\gamma }$ is self-adjoint on ${\mathcal{H}}_{\gamma }$, in particular it preserves this space. Let now $P_{\gamma }:\mathcal{H}\to {\mathcal{H}}_{\gamma }$ be the orthogonal projection. Then, the following concrete expression for H can be given [39] (again we drop some numerical coefficients and set $\hslash =1$)

The sum is over vertices of γ and triples of edges incident at them (taken with outgoing orientation). For each vertex v and pairs of edges $c,c\prime$ outgoing from v, one defines ${\alpha }_{\gamma ,v,c,c\prime }$ as that loop within γ starting at v along c and ending at v along ${\left(c\prime \right)}^{-1}$ with the minimal number of elements of $E\left(\gamma \right)$ used (if that loop is not unique, we average over them). It has been shown that the concrete expression (Eq. 3.25) has the correct semi-classical limit in terms of expectation values with respect to semi-classical coherent states [165–168] on sufficiently fine graphs of cubic topology [166–172].

Remarkably, Eq. 3.25 defines an essentially self-adjoint, diffeomorphism-invariant, continuum Hamiltonian operator for Lorentzian quantum gravity in four space-time dimensions, densely defined on the physical continuum Hilbert space $\mathcal{H}$which is manifestly free of ultraviolet divergences, that is, while for each given graph γ, the theory looks like a lattice gauge theory on γ; the theory is defined on all lattices simultaneously, which makes it a continuum theory. Moreover, note that the vector $\text{Ω}=1$ has norm unity and that $H\text{Ω}=0$.

Yet, one cannot be satisfied with Eq. 3.25 for the following reasons:

1. While it is true that one can give a better motivated derivation than we could sketch here for reasons of space, there are some ad hoc steps involved.

2. There are several ordering ambiguities involved in Eq. 3.25: Not only could we have written the factors in different orders but instead of using the fundamental representation to approximate connections in terms of holonomies, we could have used higher spin representations [173] or an average over several of them, and in each case, we would have different coefficients appearing in front of these terms.

3. Of particular concern is definition of the minimal loop. While this gives good semi-classical results on sufficiently fine lattices, the theory lives on all lattices, also those which are very coarse, and on those, expression Eq. 3.25 is doubtful because the Riemann approximation mentioned above would suggest to use a much finer loop. In fact, one is supposed to take the regulator (i.e., the coordinate volume ϵ of the Riemann approximants) away, and in that limit, the loop would shrink to zero. One can justify that this does not happen by using a sufficiently weak operator topology [28–32], namely, there exist diffeomorphism-invariant distributions (linear functionals) l on the dense span of SNWF ψ [36], and we define an operator $O_{\mathit{ϵ}}$ to converge to an operator O in that topology if $l\left(\left[O_{\mathit{ϵ}}-O\right]\psi \right)\to 0$ for all $l,\psi$. Now, due to diffeomorphism invariance, we can deform for any ϵ the small loop to any diffeomorphic one as long as we do not cross other edges of the graph, in particular we can deform it as close as we want to the minimal one. Then, the result mentioned above about the matrix elements of diffeomorphism-invariant operators, in fact, forces us to choose that loop precisely, not only approximately. Of course, while the diffeomorphism symmetry of H makes the space of diffeomorphism-invariant distributions a natural space to consider, it is still not perfectly justified to use it in order to define a topology.

4. The naive dequantisation of Eq. 3.25 will perform poorly on very coarse graphs and will be far from the continuum expression Eq. 3.8, but one could argue that that vectors supported on coarse graphs simply do not qualify as good semi-classical states.

5. Using the same argument as in (3), there is nothing sacred about the minimal loop, and one could take again other loops and/or average of over them with certain weights. However, then the locality of Eq. 3.25 is lost.

6. The block diagonal or superselection structure (Eq. 3.24) which is forced on us by the non-separability of the Hilbert space and its spatial diffeomorphism covariance appears unphysical, and one would expect that the Hamiltonian creates also new excitations.

It transpires that we must improve Eq. 3.25, and the discussion has indicated a possible solution: Blocking free QFT from the continuum (i.e., restricting the Hilbert space to vectors of finite spatial resolution) with respect to a kinematic real-space coarse graining scheme exactly produces such a high degree of non-locality at finite resolution even if the continuum measure or the continuum Hamiltonian is local [54–57, 71–75, 108]. This bears the chance that what we see in Eq. 3.25 is nothing but a naive guess of a continuum Hamiltonian which is blocked from the continuum but whose off-block diagonal form we cannot determine with the technology used so far. Accordingly, this calls for shifting our strategy which was already started in Refs. 169–172, 174 (in the sense that the block diagonal structure was dropped, but only one infinite graph was kept):

We take the above speculation serious and consider the operators $H_{\gamma }$ as projections onto the subspaces ${\mathcal{H}}_{\gamma }$ of $\mathcal{H}$ of a continuum Hamiltonian H, but we will drop the unphysical block diagonal structure 3.24 which arises from the non-separability of $\mathcal{H}$. Rather the relation between $H_{\gamma }$ is to be imposed by a renormalisation scheme induced by the path integral renormalisation scheme adopted in quantum statistical physics. To do this, we must first derive a path integral measure ${\mu }_{\gamma }$ from the OS data, ${\mathcal{H}}_{\gamma },H_{\gamma },{\text{ and Ω}}_{\gamma }$ where ${\text{Ω}}_{\gamma }$ is the vacuum of $H_{\gamma }$ by the usual Feynman–Kac–Trotter–Wiener formalism. Then, we can compute the flow of ${\mu }_{\gamma }$in the usual way and then translate into a flow of OS data by OS reconstructing them from the measures. The fixed points of the flow will then define the possible continuum theories, and these may be ‘phases’ quite different from Eq. 3.25. The details of this programme will be the subject of the following sections.

4. Constructive QFT, Feynman–Kac–Trotter–Wiener construction and Osterwalder–Schrader reconstruction

The purpose of this section is to provide some background information on constructive QFT and related topics such as the Feynman–Kac–Trotter–Wiener construction of measures (path integrals) from a Hamiltonian formulation (operator formulation) and vice versa the Osterwalder–Schrader reconstruction of a Hamiltonian framework from a measure. Our description will be minimal. The prime textbook references are [98–100, 175].

4.1. Measure Theoretic Glossary

Let S be a set. A collection B of the so-called measurable subsets of S is called a $\sigma -$algebra if i. it is closed under taking complements with respect to S, ii. closed under taking countable unions, and iii. B contains the empty set $\mathrm{\varnothing }$. The pair $\left(S,B\right)$ is called a measurable space. A measure space is a triple $\left(S,B,\mu \right)$, where $\left(S,B\right)$ is a measure space and μ is a positive set function $\mu :B\to {\mathrm{ℝ}}_0^{+}{{\displaystyle \cup }}^{\text{}}\left\{+\infty \right\}s\mapsto \mu \left(s\right)$ which is $\sigma -$additive, that is, for any pairwise disjoint $s_I{{\displaystyle \cap }}^{\text{}}{s}_J=\varnothing ,I\ne J;I,J=\in \mathrm{\mathbb{N}}$, we have

The measure μ is called a probability measure if $\mu \left(S\right)=1$. One uses the notation

where ${\chi }_s\left(p\right)=1$ if $p\in s$ and ${\chi }_s\left(p\right)=0$, else is called the characteristic function of $s\in B$.

Consider now a second measurable space $\left(\tilde{S},\tilde{B}\right)$. A function $X:S\to \tilde{S}$ is called measurable or a random variable if the pre-images $X^{-1}\left(\tilde{s}\right)=\left\{p\in S;f\left(p\right)\in \tilde{s}\right\}$ of measurable sets $\tilde{s}\subset \tilde{S}$ are measurable in S. Let $\mathrm{ℱ}$ be the set of random variables $X:S\to \tilde{s}$; then for $X\in \mathrm{ℱ}$, the set function

defines also a probability measure called the distribution of X. We consider real-valued functions $f:\tilde{S}\to \mathrm{ℝ}$ of the simple form

where the sum is over at most finitely many terms and define their integral as

One can show that this identity extends from simple functions to Borel functions that is, measurable functions $f:\tilde{S}\to \mathrm{ℝ}$, where $\mathrm{ℝ}$ is equipped with the Borel $\sigma -$algebra (the smallest σ algebra containing all open intervals). We can then also extend it to those complex functions whose real and imaginary parts are Borel by linearity.

A stochastic process indexed by an index set $\mathrm{ℐ}$ is a family ${\left\{X_i\right\}}_{i\in \mathrm{ℐ}}$of random variables $X_i:S\to \tilde{S}$. For any finite subset $I=\left\{i_1,\mathrm{.}\mathrm{.},i_N\right\}\subset \mathrm{ℐ}$, we have the joint distribution

The probability measures $\mu {\prime }_I$ are called cylinder measures. For any complex-valued Borel function $f:{\tilde{S}}^N\to \mathrm{ℂ}$, we have similarly as in Eq. 4.4

Functions on S of the form $f_I\left(p\right)=f\left({\left\{X_{i_k}\left(p\right)\right\}}_{k=1}^N\right)$ are called cylinder functions.

In what follows, we assume that for each $N\in {\mathrm{\mathbb{N}}}_0$, there exists a distinguished system ${\mathcal{W}}_N$ of complex-valued, bounded elementary functions W on N copies of $\tilde{S}$ such that the corresponding cylinder functions enjoy the following properties:

(1) They generate an Abelian $\text{*}$ algebra, that is, for all $I,I\prime \in \mathrm{ℐ}$, the product $W_{I}W{\prime }_{I\prime }$ is a finite, complex linear combination of suitable $W_{I^{\prime \prime }}^{\prime \prime },I^{\prime \prime }\in \mathrm{ℐ},{\mathcal{W}}^{\prime \prime }\in {\mathcal{W}}_{\left|{\mathrm{ℐ}}^{\prime \prime }\right|}$ and also $\overline{W_I}$ is of that form.

(2) ${\mathcal{W}}_N$ contains the constant function.

(3) For each $I\in \mathrm{ℐ}$, the moments $\mu \left(W_I\right),W\in {\mathcal{W}}_{\left|I\right|}$ determine ${\tilde{\mu }}_I$ uniquely.

(4) These properties show that $W_I$ are $L_2\left(d{\tilde{\mu }}_I,{\tilde{S}}^{\left|I\right|}\right)$ functions. We require their span to be dense.

(5) We saw that a probability measure μ together with a stochastic process gives rise to a family of cylindrical probability measures ${\left({\tilde{\mu }}_I\right)}_{I\in \mathrm{\Im }}$ on ${\tilde{S}}^{\left|I\right|}$. The converse question is under which circumstances a cylindrical family of cylinder probability measures determines a measure μ. A necessary criterion is as follows: The set $\mathrm{ℐ}$ is partially ordered and directed by inclusion, that is, for each $I,J\in \mathrm{ℐ}$, we find $K\in \mathrm{ℐ}$ such that $I,J\subset K$ (for instance, $K=I\cup J$). Suppose that $I\subset J$. Then,

where $X_I={\left\{X_i\right\}}_{i\in I},{\tilde{s}}_I\subset {\tilde{B}}^{\left|I\right|}$. Furthermore, for any permutation π on $N=\left|I\right|$ elements set, $\pi \cdot I=\left\{i_{\pi \left(1\right)},\mathrm{.}\mathrm{.},i_{\pi \left(N\right)}\right\}$ and $\pi \cdot {\tilde{s}}_I=\left\{\left({\tilde{p}}_{\pi \left(1\right)},\mathrm{.}\mathrm{.},{\tilde{p}}_{\pi \left(N\right)}\right);\left({\tilde{p}}_1,\mathrm{.}\mathrm{.},{\tilde{p}}_N\right)\in {\tilde{s}}_I\right\}$. Then,

Even more generally, a partial order on the set $\mathrm{\Im }$ of finite subsets I of $\mathrm{ℐ}$ is a transitive, reflexive, and antisymmetric relation, that is, $I<J\wedge J<K\Rightarrow I<K$ and $I<I$ and $I<J\wedge J<I\Rightarrow I=J$ for all $I,J,K\in \mathrm{\Im }$. The set $\mathrm{\Im }$ is called directed with respect to $<$, provided that for all $I,J\in \mathrm{\Im }$, we find $K\in \mathrm{\Im }$ such that $I,J<K$. For $I<J$, we may have surjective maps $P_{JI}:{\tilde{S}}^{\left|J\right|}\to {\tilde{S}}^{\left|I\right|}$ such that $X_I\left(p\right)=P_{JI}\left(X_J\left(p\right)\right)$ and such that for $I<J<K$, we have $P_{JI}\circ P_{KJ}=P_{KI}$. Then, similar as in Eq. 4.8, we necessarily must have for $I<J$

It turns out that these two conditions, Eqs 4.8, 4.9, or 4.10 is also sufficient in fortunate cases (for instance, if $\tilde{S}=\mathrm{ℝ}$, which is the classical Kolmogorov theorem, see Ref. 134), that is, we can then reconstruct the measure space $\left(S,B,\mu \right)$ and a stochastic process ${\left\{X_i\right\}}_{i\in \mathrm{ℐ}}$ such that ${\tilde{\mu }}_I$ are the cylinder measures of μ. It follows that the $W_I\in {\mathcal{W}}_{\left|I\right|},I\in \mathrm{ℐ}$ lie dense in $L_2\left(S,d\mu \right)$.

Physical meaning: We consider the elements $p\in S$ to be space-time fields $\text{Φ}$ or spatial fields ϕ, respectively. The index set $\mathrm{ℐ}$ will have the meaning of a set of test functions or more generally distributions whose elements i label the random variable $X_i$. These map the fields smeared with test functions to a finite-dimensional manifold (usually copies of $\mathrm{ℝ}$ or more generally of a Lie group). For instance, for a scalar field $\text{Φ}$, we may consider the random variable $X_F\left(\text{Φ}\right)=\text{exp}\left(i\underset{\mathrm{ℝ}\times \sigma }{{\displaystyle \int }}{d}^{4}xF\left(x\right)\text{Φ}\left(x\right)\right)$ which takes values in $\tilde{S}=U\left(1\right)$. It is also customary to consider the field $p=\text{Φ}$ itself as a random variable indexed by the same index set or to simply write $X_i\left(p\right)=p\left(i\right)$ as an abbreviation.

4.2. Constructive QFT

The application of interest of the previous subsection is a stochastic process indexed by either $\mathrm{ℝ}\times L$ or just by L, where the label set L is a certain set of distributions on the spatial manifold. We distinguish between random variables $\text{Φ}$ indexed by a pair $\left(t,f\right)\in \mathrm{ℝ}\times L$ and random variables ϕ indexed by $f\in L$. Some examples are as follows:

Real quantum scalar fields with smooth smearing:

Consider $L=\mathcal{S}\left({\mathrm{ℝ}}^3\right)$, the space of smooth test functions of rapid decrease and $\tilde{S}=\mathrm{ℝ}$ equipped with the Borel $\sigma -$algebra. Then, $\varphi \left(f\right)=\langle f,\varphi \rangle =\underset{\sigma }{{\displaystyle \int }}{d}^{3}xf\left(x\right)\varphi \left(x\right)$ and $\text{Φ}\left(t,f\right)=\langle f,\text{Φ}\left(t,.\right)\rangle$. Given $F:=\left(f_1,\mathrm{.}\mathrm{.},f_N\right)\in L^N$, consider $\varphi \left(F\right)=(\varphi \left(f_1\right),\mathrm{.}\mathrm{.},\varphi \left(f_N\right)\in {\mathrm{ℝ}}^N$. The space ${\mathcal{W}}_N$ of elementary functions on N copies of $\mathrm{ℝ}$ can be chosen to be generated by the exponentials

with $r_1,\mathrm{.}\mathrm{.},r_N\in \mathrm{ℝ}$ labelling the (necessarily one-dimensional) unitary irreducible representations of $U\left(1\right)$.

In fact, since in this case, the space L is a vector space, it is sufficient to consider the functions $w\left(\varphi \left(f\right)\right)=\text{exp}\left(i\varphi \left(f\right)\right),f\in L$. Analogously, the space of elementary functions for the time-dependent fields can be chosen as ($w_k\in {\mathcal{W}}_{N_k}$)

which, of course, reduces to

for certain $f{\prime }_k\in L$. Obviously, the Abelian $-\text{*}$algebra and boundedness conditions are satisfied. That these elementary functions suffice to determine the cylindrical measures requires a more involved argument (Bochner’s theorem, [134]).

(2) Real quantum scalar fields with distributional smearing:

Consider a subset $L=\subset \mathcal{S}\prime \left({\mathrm{ℝ}}^3\right)$ of the tempered distributions and $\tilde{S}=U\left(1\right)$ equipped with the Borel $\sigma -$algebra. In applications to scalar fields coupled to general relativity elements, $f\in L$ are typically $\delta -$distributions supported at a single point.

Then, $\varphi \left(f\right):=\text{exp}\left(i\langle f,\varphi \rangle \right)$ and $\text{Φ}\left(t,f\right)=\text{exp}\left(i\langle f,\text{Φ}\left(t,.\right)\rangle \right)$, where $\langle f,\rangle >$ is the evaluation of $f\in L$ on ϕ. Given $F:=\left(f_1,\mathrm{.}\mathrm{.},f_N\right)\in L^N$, consider $\varphi \left(F\right)=(\varphi \left(f_1\right),\mathrm{.}\mathrm{.},\varphi \left(f_N\right)\in U{\left(1\right)}^N$. The space ${\mathcal{W}}_N$ of elementary functions on N copies of $U\left(1\right)$ can be chosen to be generated by the exponentials

with $r_1,\mathrm{.}\mathrm{.},r_N\in \mathrm{ℝ}$ labelling the (necessarily one-dimensional) unitary irreducible representations of $U\left(1\right)$. Analogously, the space of elementary functions for the time-dependent fields can be chosen as ($w_k\in {\mathcal{W}}_{N_k}$)

In this case, we could still equip L with the structure of a real vector space if we extend L to the finite real linear combinations $\tilde{L}$ of its generating set L. Since this is no longer possible for the non-Abelian gauge theory example below, we will refrain from doing this, in order to highlight the structural similarity between the examples.

(3) Non-Abelian gauge fields for compact gauge groups G:

A form factor is a distribution

where c is a one-dimensional path in σ. We take $\tilde{S}=G$ equipped with the natural Borel $\sigma -algebra$ and

where we have identified ϕ as a G connection and $\mathcal{P}$ denotes path ordering. Thus, Eq. 4.17 is the direct analogue of the scalar field construction (note that the Lie generators are anti–self-adjoint since G is compact so that Eq. 4.17 is unitary) and $\varphi \left(f_c\right)$ is simply the holonomy of ϕ along c. Likewise,

Note that the form factors do not form a vector space; in general, they cannot be added (unless two curves share a boundary point), and they can never be multiplied by a non-integer real scalar (there is a certain groupoid structure behind this [17–21]). Accordingly, our space of generating set of elementary functions ${\mathcal{W}}_N$ on N copies of G need to be more sophisticated. We consider the space L of form factors, and for each $F=\left(f_{c_1},\mathrm{.}\mathrm{.},f_{c_N}\right)\in L^N$, the ‘pairing’ $\varphi \left(F\right)=\left(\varphi \left(c_1\right),\mathrm{.}\mathrm{.},\varphi \left(c_N\right)\right)\in G^N$. Then, a possible choice of generating set ${\mathcal{W}}_N$ of elementary functions is

with $\delta :=\left\{\left(j_1,m_1,n_1\right),\mathrm{.}\mathrm{.},\left(j_N,m_N,n_N\right)\right\}$. In fact, it is sufficient to consider mutually disjoint (up to end points), piecewise real analytic curves $c_k$. Here, j labels an irreducible representation ${\pi }_j$ of G of dimension $d_j$ and ${\left[{\pi }_j\left(g\right)\right]}_{m,n};m,n=1,\mathrm{.}\mathrm{.},d_j$ its matrix element functions. By the Peter and Weyl theorem, these functions suffice to determine the cylindrical measures uniquely at least if they are absolutely continuous with respect to the product Haar measure. Likewise, we consider the elementary functions

The fact that these functions satisfy all requirements is the statement of Clebsch–Gordan decomposition theory together with the properties of the holonomy to factorise along segments of a curve (note the piecewise analyticity condition).

This ends our list of examples. We will denote the measure related to the stochastic process $\left\{\text{Φ}\left(t,f\right)\right\}$ by μ and the measure related to the stochastic process $\left\{\varphi \left(f\right)\right\}$ by ν. As the notation suggests, $\text{Φ}$ is a field defined on space-time $M=\mathrm{ℝ}\times \sigma$, while ϕ is a field defined on space σ. Note that $M=\mathrm{ℝ}\times \sigma$ with σ any 3D manifold is a consequence of the requirement of global hyperbolicity [176, 177].

The measures μ underlying a relativistic QFT are not only probability measures. In addition, they need to satisfy a set of axioms [98–100, 104] called Osterwalder–Schrader axioms which, however, are tailored to $M={\mathrm{ℝ}}^4$, stochastic processes with L being a vector space and with an Euclidean background metric at one’s disposal. In quantum gravity and more generally in non-Abelian gauge theories, one typically must or may want to drop some of these structures. As a consequence, we will only keep those axioms that can also be applied in this more general context.

Some of them generalise to stochastic processes not indexed by a vector space, and some do not. Some generalise from the manifold ${\mathrm{ℝ}}^4$ to the general space-time manifold $\mathrm{ℝ}\times \sigma$ allowed by global hyperbolicity, and some do not. Fortunately, those that do generalise are sufficient for the reconstruction process [103]. We call them the minimal OS axioms, and we call a probability measure that satisfies them an OS measure.

An important remark is that the measures for gauge theories (such as general relativity) are to be formulated in terms of observable (gauge-invariant) fields which are typically composites of the elementary fields. That is why we work in a manifestly gauge (diffeomorphism)-invariant (equivalently, gauge-fixed) context as outlined in Section 3. In fact, in Ref. 39, we find an explicit formula that relates the observable composite fields to the elementary ones. The crucial condition is that the algebra of those observable fields is under sufficient mathematical control in order that Hilbert space representations can be found. This is the case for the construction sketched in Section 3.

The minimal set of OS axioms can be phrased as follows:

Let $\theta \left(t,x\right):=\left(-t,x\right)$ and $T_s\left(t,x\right):=\left(t+s,x\right)$ denote time reflection and time translation, respectively. Let $w_k\in {\mathcal{W}}_{N_k},k=1,\mathrm{.}\mathrm{.},T,F_k\in L^{N_k},t_k\in \mathrm{ℝ}$ and

Then, we have the following conditions on the generating functional

I. Time reflection invariance: