Generalized Holographic Principle, Gauge Invariance and the Emergence of Gravity à la Wilczek
- 1Center for Theoretical Physics, College of Physics Science and Technology, Sichuan University, Chengdu, China
- 2INFN Sezione Roma Tor Vergata, Rome, Italy
- 3Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taipei, Taiwan
- 4Department of Physics, National Taiwan University, Taipei, Taiwan
- 5Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA, United States
- 6College of Design and Innovation, Tongji University, Shanghai, China
- 7Caunes Minervois, Paris, France
- 8Institut de Chimie Radicalaire, Aix-Marseille University, Marseille, France
- 9Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology, Shenzhen, China
- 10Department of Physics and Center for Field Theory and Particle Physics, Fudan University, Shanghai, China
- 11INFN Laboratori Nazionali di Frascati, Frascati, Italy
- 12Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan, Sweden
We show that a generalized version of the holographic principle can be derived from the Hamiltonian description of information flow within a quantum system that maintains a separable state. We then show that this generalized holographic principle entails a general principle of gauge invariance. When this is realized in an ambient Lorentzian space-time, gauge invariance under the Poincaré group is immediately achieved. We apply this pathway to retrieve the action of gravity. The latter is cast à la Wilczek through a similar formulation derived by MacDowell and Mansouri, which involves the representation theory of the Lie groups SO
Almost one hundred years of attempts to quantize gravity suggest that physical perspective may be responsible for this failure (Garay, 1995). While continuing to seek an UV-complete theory of either General Relativity (GR) or one of its possible extensions (Polchinski, 1998; Rovelli, 2004; Modesto, 2012; Modesto and Rachwal, 2014), an alternative option is to look at gravity as an emergent phenomenon (Jacobson, 1995; Barcelo et al., 2005; Van Raamsdonk, 2010; Verlinde, 2011; Swingle and Van Raamsdonk, 2014; Chiang et al., 2016; Oh et al., 2018). Among many possible instantiations of this simple idea stands a paradigm of emergence that aims at recovering gravity via its analogical similarity with Yang-Mills gauge theories. As remarked by Chen-Ning Yang, while electromagnetism is evidently a gauge theory, and the fact that gravity can be seen as such a theory is universally accepted, how this exactly happens to be the case must be still clarified. Notable explorations along these lines have been provided in the past by Weyl (1918), and more recently by MacDowell and Mansouri (1977), and Chamseddine et al. (1977), with subsequent improvements by Stelle and West (1979).
At the same time, we heuristically note that gravity may naturally encode principles of information theory. Such consideration naturally follows pondering that gravity is the field that is involved in the very definition of both masses and spacetime distances, and that specifies the propagation velocities of point-like particles, and hence of information, through the geodesic equations. Thus it is reasonable to pursue a fundamental theory of gravity from this perspective. Indeed, the underlying graph-structure of information networks is a set of nodes and links—this is reminiscent of the basis of the states in Loop Quantum Gravity (Rovelli, 2004).
There have been huge achievements in the direction of a quantum-information based theory of gravity, with several different attempts developed so far—see e.g. Faulkner et al. (2014). More generally, deep links between quantum information theory and an “emergent” quantum theory of observable physical systems have been developed by many studies (Chiribella et al., 2016; Hamma and Markopoulou, 2011; Flammia et al., 2009). It is not within the present scope to summarize this vast literature. Instead, we focus on a specific alternative approach: we show that when the holographic principle is reformulated from a semi-classical to a fully general, quantum-theoretic principle, gravity emerges as a gauge theory along the lines of the gauge formulation of gravity, as proposed by Wilczek (1998).
We start by showing in Section 2 that a generalized holographic principle (GHP) characterizes information transfer within any finite quantum system in a separable state. The HP is recovered from this more general, purely-quantum principle by requiring covariance. We then show in Section 3 that compliance with the GHP entails gauge invariance under the Poincaré group in an ambient Lorentzian space-time. Hence the gauge principle has purely quantum-theoretic roots and characterizes all finite systems in separable states. We use this to retrieve the action of gravity in Section 4. In Section 5, we provide, as an example, an emergent theory of gravity, a theory of Yang-Mills gauge fields and Higgs pentaplets that is cast à la Wilczek. This is a formulation similar to a previous one envisaged by MacDowell and Mansouri, which involves the representation theory of the Lie group SO
2 Generalized Holographic Principle for Finite Quantum Systems
2.1 Historical Remarks on the Genesis of the Holographic Principle
Probably the most direct way to summarize the Holographic Principle (HP) is via its original statement by ’t Hooft (1993):
“given any closed surface, we can represent all that happens inside it by degrees of freedom on this surface itself.”
The path that led to the formulation of the HP can be traced from the Bekenstein’s area law (Bekenstein, 2004) for black holes (BH),
where S denotes the thermodynamic entropy of a BH and A its horizon area in Planck units. Bekenstein conjectured the existence of an upper bound, S itself, to the entropy of any physical system contained within a bounded volume:
“the entropy contained in any spatial region will not exceed the area of the region’s boundary.”
Historically, this conjecture was first instantiated by Susskind (1995), who implemented a mapping from volume to surface degrees of freedom for a general closed system. This was based on the assumption that all light rays that are normal to any element within the volume are also normal to the surface. Bousso (2002) then showed that it is actually covariance that induces the holographic limit on information transfer by light; he further provided several counterexamples showing the failure of a straightforward interpretation of the HP as a spacelike entropy bound. Instead, Bousso formulated a covariant entropy bound:
We note that both (2.1) and (2.2) are semiclassical. The limits on the entropy S that they impose are “quantum” only in their reliance on Planck units and hence a finite value of
“The inside metric could be so much curved that an entire universe could be squeezed inside our closed surface, regardless how small it is. Now we see that this possibility will not add to the number of allowed states at all.”
It bears emphasis that “allowed states” in this context are thermodynamic states, i.e. states that can be counted by measuring energy transfer between the system and its external environment. As made fully explicit by Rovelli in the case of BH (Rovelli, 2017; Rovelli, 2019), states that are effectively isolated (e.g. isolated for some time interval much larger than relevant interaction times) from the external environment do not contribute to
While the demonstration by Maldacena (1998) of a formal duality acting as an equivalence, at the level of the encoded information, between string quantum gravity on d-dimensional anti-de Sitter (AdS) spacetime and conformal quantum field theory (CFT) on its
“an apparent law of physics that stands by itself, both uncontradicted and unexplained by existing theories that may still prove incorrect or merely accidental, signifying no deeper origin.”
Our goal in the next section is to place the HP on a much deeper intuitive footing, by generalizing it from a semi-classical to a fully quantum principle, one that is entirely independent of geometric considerations.
2.2 Information Transfer in Finite, Separable Systems
We now assume
for every finite
that maintain the separability of
Let us now consider an interval
Theorem 1. Given any finite-dimensional quantum system
Proof. The information
Note that if the assumption of separability is dropped and two subsystems cannot be distinguished, Eq. 2.6 fails, the von Neumann entropy of
2.3 The HP Is a Special Case of the Generalized Holographic Principle
Theorem 1 places a principled restriction on information transfer within any separable quantum system; as noted above, the notion of information transfer within a non-separable (i.e. entangled) quantum system is meaningless. The HP is a principled restriction on information transfer within a semiclassical system that is separable by definition. Hence the two should be related. This relation can be made explicit by stating:
Generalized Holographic Principle (GHP): Given any finite-dimensional quantum system
We note that this GHP is formulated entirely independently of geometric assumptions; in particular, it is prior to any assumption of general covariance.
To make the physical meaning of the GHP clear, let us consider a specific example. Suppose A and B interact by alternately preparing and measuring the states of N shared, non-interacting qubits as shown in Figure 1. We can consider that, in a time interval τ, A prepares the N qubits in her choice of basis, i.e. using her
FIGURE 1. Systems A and B exchange bits via an ancillary array of non-interacting qubits. Bit values are preserved if a quantum reference frame (here, a z axis) is shared a priori. Adapted from Fields and Marcianò (2020); CC-BY license.
The qubit-mediated interaction shown in Figure 1 still makes no geometric assumptions. If we now imagine, however, that the array of qubits is embedded at maximal density in an ancillary real 2-dimensional surface
We note that the GHP provides, when
Both Theorem 1 and the GHP above are formulated for fixed N. Generalizing to the case of N varying slowly, i.e. remaining piecewise constant in time for intervals
3 Poincaré Symmetries and Gauge Invariance
3.1 The Generalized Holographic Principle Requires Gauge Invariance for Finite, Separable Systems
Theorem 1 and hence the GHP restricts access to information, and so states an invariance: the information
Theorem 2. In any
Proof. The situation is completely symmetrical, so considering either
Note that gauge invariance here depends explictly on separability: if
Theorem 2, like Theorem 1 and the GHP, involves no assumptions about geometry. We introduce these below, with QED as an initial example.
3.2 QED and the Consequences of the Generalized Holographic Principle
As a specific example, consider a finite system
We can make the presentation more precise at the mathematical level to illustrate the independence of observable results from coordinate (i.e. basis) transformations, even in the presence of the ancillary space-time geometry with points labelled by x. The local gauge freedom for the choice of the vector field
The quantization of the theory can be achieved following the standard path integral procedure. The partition function for the system A, namely the
The redundancy due to the gauge transformations, i.e. choices of
where as customary
Having all set up, we can easily show the invariance of the path-integral:
where the normalization functions
The perspective provided by the GHP allows us to place a novel physical interpretation on the action of
Further, it appears to be essential for this arrangement of the things introduced in physics that, at a specific time, these things claim an existence independent of one another, insofar as these things “lie in different parts of space.”
“Claiming an existence independent of one another” obviously requires separability.
It is well known that the gauge condition can be cast in a more general form, employing an arbitrary function f. In this latter case, the gauge functional:
actually introduces a family of gauge-fixing terms. The independence of the physical observables on the gauge fixing is recovered through a process of average that is realized by integrating over f the gauge fixing terms weighted with the factor exp
This invariance of the partition function under different choices of the gauge fixing condition, i.e. different choices of f, percolates into the gauge invariance of the expectation value of any observable
where f and g are two different gauge-fixings.
We may take into account now the other interaction partner in QED, the system B composed by Dirac matter fields, for simplicity electrons. The path integral formulation of the system, composed by only one fermionic species ψ, then casts:
The observable quantities
As previously done for the bosonic system A, also for the system B we can introduce a local gauge transformation having the meaning of a transformation among observers. Of course, this transformation cannot change the values of
where q stands for the charge parameter. This is a
3.3 Extension to Gravity and Local Lorentz Invariance
So far we have first considered generic quantum systems with finite number of degrees of freedom, and stated the GHP within these simplified but completely general contexts, which do not necessarily require geometric concepts. In this sense, these notions shall be considered as pre-geometric. We have then extended our focus to continuous systems with an infinite number of degrees of freedom, focusing specifically on the paradigmatic example of QED, which is embedded on a flat Minkowski space-time. This embedding requires the addition of ancillary coordinates x into the description of the system, which are necessary to specify its evolution and fully capture the dynamics as it is observed by spatially-separated observers.
Let us now include gravity in this construction, extending the arguments previously exposed. Our joint system
A time-like unit vector n must be then defined that is orthogonal to any tangent vector v on
Given this framework, denoted as ADM (Arnowitt et al., 1959) in the literature, we can now introduce a time coordinate
which allows to define the symplectic structure of the system, namely:
with vanishing momenta conjugated to
The Hamiltonian density of the gravitational system, which can be calculated by the usual Legendre transform
For simplicity, we assumed the hyperspace
The first term of the Hamiltonian represents the Hamiltonian constraint, which generalizes time reparametrization, while the second term is the space-diffeomorphism constraint, respectively:
Involving the continuous version of the Poisson brackets for the phase-space variables of the system, namely:
we may recover the algebra of constraints for the gravitational system, known as Dirac algebra, namely:
The scalar and the vector constraints entering the total Hamiltonian can be cast in terms of the Einstein tensor components, contracted with the normal
while the spatial components
4 Emergent Poincaré Symmetries From an Emergent Gauge Theory
There is a deep similarity among gauge symmetries and diffeomorphisms, which becomes manifest as soon as both the gauge theories and gravity are formulated as principle bundle theories. This turns space-time symmetry into an emergent concept, similarly to what has been discussed in the previous section, while considering the consequences of the GHP. A celebrated framework in which gravity, and thus the Poincaré symmetries, are shown to be emergent from a gauge structure was provided by MacDowell and Mansouri. Nonetheless, the gauge symmetry is explicitly broken in this model. We briefly review here this theoretical framework, as a propedeutic element to the next section, where we review a model, due by Wilczek, in which gravity is emergent from a fully gauge-invariant theory.
4.1 Einstein–Hilbert Action
Before introducing MacDowell–Mansouri gravity, it is useful to remind the Palatini formulation of gravity in the Einstein–Hilbert action. This casts in terms of the metric
with R being the Ricci scalar. The Ricci scalar, encoding non-linearly first order derivatives and linearly second order ones, is defined as the contraction of the Riemann tensor, namely:
with the Riemann tensor expressed as:
A first-order formulation of the Einstein–Hilbert action of gravity is admitted in terms of the
which can be recast as
A new topological invariant can be added to the action of gravity, without affecting the classical equation of motions. The Holst term can be added to the Einstein–Hilbert action, then leading to the new action that involves a real (Barbero–Immirzi) parameter γ, i.e.:
the phase-space of which retains the symplectic form:
4.2 BF Formulation of Gravity
The Einstein–Hilbert–Holst action admits a formulation within the BF framework, as a deviation from the topological theory. The BF theory is defined as a G-principle bundle on a D-dimensional base manifold
which specialized to the case of
4.3 MacDowell–Mansouri Action
Switching now to the MacDowell–Mansouri action, we introduce an extended (anti-de Sitter) group
According to this decomposition, the connection casts as:
and correspondently the curvature 2-form, with indices contracted with the structure constants of the
decomposes into the
The MacDowell–Mansouri action deploys this extended formalism, but with the crucial underlying assumption of (explicit) symmetry breaking:
The Einstein–Hilbert action of gravity can then be encoded in a general framework, moving from the action:
The action then entails the Einstein–Hilbert action, with the cosmological term, and some 4D Euler characteristic:
The equations of motion read:
The MacDowell–Mansouri theory admits a straightforward
the equations of motion of which imply that: i) varying in
Without entering into further details, we notice that the MacDowell–Mansouri theory can be cast in a similar fashion in terms of an internal de Sitter group
5 Wilczek Gravity
Frank Wilczek proposed in Wilczek (1998) a model that is reminiscent of the theory formulated by MacDowell and Mansouri, with internal
The Lagrangian considered in Wilczek (1998) is:
Two novel terms with respect to the MacDowell–Mansouri action, were introduced in the Wilczek model:
1. The interaction potential of
By varying with respect to
2. A term that constrains the determinant of the metric in the spontaneous symmetry broken phase:
J denoting the determinant of the metric.
The total Lagrangian proposed by Wilczek then reads:
In order to unveil the emergence of the gravity, we may instantiate the spontaneous symmetry breaking Eq. 4.13 directly on the Lagrangian in Eq. 5.1, using the decompositions recovered in Eqs. (4.11) and (4.12), and then finding:
This equation corresponds to the Einstein–Hilbert action, introduced in Eq. 4.5, plus the cosmological constant term. The unimodular term is not essential for our arguments, and we can neglect it here.
We can recast the main term of the Lagrangian density proposed by Wilczek, rearranging as:
works as a simplicity constraint, which here drags the Wilczek model away from its topological phase.
In the Higgs condensate phase, the
6 Conclusions and Outlooks
We have shown here that a generalized version of the holographic principle can be derived from fundamental considerations of quantum information theory, in particular, the imposition of separability on a joint state. This GHP entails gauge invariance. We emphasized that as soon as this is instantiated in an ambient Lorentzian space-time, gauge invariance under the Poincaré group automatically follows. Indeed, following this pathway we can recover the action of gravity. We summarize several gauge-invariant models for gravity, including gravity cast à la Wilczek. This is a formulation of the Einstein theory of gravity similar to the one proposed by MacDowell and Mansouri, which involves the representation theory of the Lie groups
As the GHP provides a natural and completely general distinction between “bulk” and “boundary” degrees of freedom, one that is independent of geometry, it would be worth to investigate whether the AdS/CFT and dS/CFT correspondences could fit within this framework. This would require complementing the GHP with the concept of the renormalization group flow. Indeed, group renormalization flow techniques might be actually considered to connect the fully symmetric
Data Availability Statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.
Ideation by CF and AM. Development by all the authors.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The handling editor declared a past co-authorship with one of the authors AM.
1Here the efficiency relates to the thermodynamic transformations triggering the exchange of information bits among the two subsystems A and B.
2Notice that irreversibility is connected to the efficiency bound
3Here we denote with
Chiang, H.-W., Hu, Y.-C., and Chen, P. (2016). Quantization of Spacetime Based on a Spacetime Interval Operator. Phys. Rev. D 93 (no.8), 084043. arXiv:1512.03157 [hep-th]]. doi:10.1103/PhysRevD.93.084043
Chiribella, G., Perinotti, G. M., and Pernotti, P. (2016). “Quantum from Principles,” in Quantum Theory: Informational Foundations and Foils (Fundamental Theories of Physics 181). Editors G. Chiribella, and R. W. Spekkens (Dordrecht: Springer), 171–221. doi:10.1007/978-94-017-7303-4_6
Faulkner, T., Guica, M., Hartman, T., Myers, R. C., and Van Raamsdonk, M. (2014). Search for Physics beyond the Standard Model in Multilepton Final States in Proton-Proton Collisions at S√s = 13 TeV. JHEP 03, 051. [arXiv:1312.7856 [hep-th]]. doi:10.1007/JHEP03
Flammia, S. T., Hamma, A., Hughes, T. L., and Wen, X. G. (2009). Topological Entanglement Rényi Entropy and Reduced Density Matrix Structure. Phys. Rev. Lett. 103, 261601. [arXiv:0909.3305 [cond-mat.str-el]]. doi:10.1103/PhysRevLett.103.261601
Oh, E., Park, I. Y., and Sin, S.-J. (2018). Complete Einstein Equations from the Generalized First Law of Entanglement. Phys. Rev. D 98 (no. 2), 026020. arXiv:1709.05752 [hep-th]]. doi:10.1103/PhysRevD.98.026020
Keywords: Wilczek gravity, black hole information loss problem, emergent gravity, gauge invariance, holographic principle
Citation: Addazi A, Chen P, Fabrocini F, Fields C, Greco E, Lulli M, Marcianò A and Pasechnik R (2021) Generalized Holographic Principle, Gauge Invariance and the Emergence of Gravity à la Wilczek. Front. Astron. Space Sci. 8:563450. doi: 10.3389/fspas.2021.563450
Received: 18 May 2020; Accepted: 04 January 2021;
Published: 07 June 2021.
Edited by:Mohamed Chabab, Cadi Ayyad University, Morocco
Reviewed by:Ignazio Licata, Institute for Scientific Methodology (ISEM), Italy
Mohammed Daoud, Ibn Tofail University, Morocco
Copyright © 2021 Addazi, Chen, Fabrocini, Fields, Greco, Lulli, Marcianò and Pasechnik. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Antonino Marciano, firstname.lastname@example.org
†ORCID: Pisin Chenc, orcid.org/0000-0001-5251-7210; Filippo Fabrocini, orcid.org/0000-0002-6972-1020; Chris Fields, orcid.org/0000-0002-4812-0744; Enrico Greco, orcid.org/0000-0003-1564-4661; Matteo Lulli, orcid.org/0000-0002-6172-0197; Antonino Marciano, orcid.org/0000-0003-4719-110X; Roman Pasechnik, orcid.org/0000-0003-4231-0149