Emergent Gravity :: August 25-29, 2008 / Massachusetts Institute of Technology / Center for Theoretical Physics (6-304)
Registration Program Participants Travel Contact

Miles Blencowe

A microwave cavity-embedded DC SQUID analogue of the event horizon

Robert T. Bluhm

Spontaneous Lorentz Violation and Gravity

Spontaneous Lorentz violation occurs when a vector or tensor field acquires a nonzero vacuum expectation value. The effects of this symmetry breaking are examined in the context of gravity. They include the generation of massless Nambu-Goldstone modes and massive Higgs modes, as well as the possibility of a Higgs mechanism. For the case of a vector theory with a vacuum value, a range of solutions is found, including a solution equivalent to Einstein-Maxwell theory as well as alternative solutions with modified gravitational interactions.


David Finkelstein


Zhengcheng Gu

A lattice bosonic model as a quantum theory of gravity

A local quantum bosonic model on a lattice is constructed whose low energy excitations are gravitons described by linearized Einstein action. Thus the bosonic model is a quantum theory of gravity, at least at the linear level. We find that the compactification and the discretization of metric tenor are crucial in obtaining a quantum theory of gravity.


Alioscia Hamma

Lieb-Robinson bounds and the speed of light from topological order

We apply the Lieb-Robinson bounds technique to find the maximum speed of interaction in a spin model with topological order whose low-energy effective theory describes light [see X.-G.~Wen, PRB 68, 115413 (2003)]. The maximum speed of interactions is found in two dimensions to be exactly the speed of emerging light. This result does not rely on mean field theoretic methods. In higher spatial dimensions, the Lieb-Robinson speed is conjectured to increase linearly with the dimension itself. Implications for the horizon problem in cosmology are discussed.


Bei-Lok Hu

Nonlocality in Quantum Mechanics and Emergent Gravity


Mihaela Iftime

Observables and characteristic classes for the gravitational field

The group Diff(M) of spacetime diffeomorphisms plays an important role in classical gravitation theory, however its role in quantum theory depends strongly on the approach is taken. One can get some insight by looking at certain aspects of the diffeomorphism invariance in classical general relativity. One serious consequence of the invariance under the group Diff(M) of global diffeomorphisms is that spacetime points (and spacetime fields) have no direct physical significance, and the only things that have direct physical meaning are the Diff(M) -invariant objects. These gauge-invariant objects represent the true observables in classical gravitation theory, and they are uniquely determined by the physical situation under consideration.


Sergei Kopeikin

Gravitational Field of Magnets: theory and prospects for measurement.

General relativity predicts that gravity originates from both energy and stresses of matter. Experimental testing of the gravitational field induced by magnetic stresses is essential for deeper theoretical exploration of the nature of the long-range interaction between the gauge fields. We compute the gravity field produced by the Maxwell stresses of high-power magnets and demonstrate that it is in the reach of the current technology of torsion balances.


Seth Lloyd

Quantizing the Global Positioning System.


Fotini Markopoulou

Quantum Graphity: A condensed matter model of emergent geometry.

Quantum graphity is a background independent condensed matter model for emergent locality, spatial geometry and matter in quantum gravity. The states of the system are given by bosonic degrees of freedom on a dynamical graph on N vertices. At high energy, the graph is the complete graph on N vertices and the physics is invariant under the full symmetric group acting on the vertices and highly non-local.

We give the conditions undeer which the model has a phase of emergent space: the ground state breaks the permutation symmetry to translations and rotations. In this phase the system is ordered, low-dimensional and local. This model gives rise to an emergent U(1) gauge theory in the ground state by the string-net condensation mechanism of Levin and Wen. After reviewing the model, we discuss how such a model of emergent space may be tested.

Time permitting, I will briefly present a variation of the model in which the evolution is unitary.


Daniele Oriti

On the emergence of spacetime and matter in Group Field Theory

This talk is divided in two parts. In the first part, we introduce the Group Field Theory formalism for Quantum Gravity, and we present the arguments for considering it a promising candidate description of the microstructure of space. We also recapitulate the general strategy for investigating the emergence of a continuum spacetime from the GFT microstructures. In the second part we present the results of some work in progress showing how an effective field theory of matter on a non-commutative flat spacetime (of the ''Deformed Special relativity'' type) emerges in fact from a simple GFT, with a procedure similar to that leading to effective field theories for quasi-particles on an emergent geometry in condensed matter systems.


Federico Piazza

Which space-time actually emerges? The world as seen from inside a spin system.

We take the example of a spin system to address some questions regarding localization in quantum field theory. The spin system is choosen to be a generic one with first neighbours interactions. We compare two different perspectives. The first is the one of the experimental physicist (Samantha) living in the laboratory where the spin system is assumed to be concretely built. She has independent and full access to the spin degrees of freedom, which therefore she calls "local". If the dynamics of the spin system is rich enough (such as e.g. a lattice version of the Standard Model) we can imagine that some time-dependent excitations of the system correspond to "measurements" inside the system itself. The second point of view is therefore that of an observer (Vincent) described -- and made of -- the excitations internal to the spin system. His access to the system's degrees of freedom is conditional to the dynamics within the system itself. The degrees of freedom that Vincent associates to "his" regions of space are not the ones that Samantha defines as "local". We attempt to discuss the implications that this may have on semi-classical gravity.


Jorge Pullin

Emergent diffeomorphisms

We show how to construct, in a 1+1 dimensional situation, a discrete quantum theory such that in the continuum limit diffeomorphism invariance is recovered. The theory is constructed by applying the "uniform discretization" procedure, a technique for discretizing generally covariant theories, to a model of spherically symmetric "gravity without dynamics" a la Husain-Kuchar.


Subir Sachdev

Quantum criticality and black holes

Many recent experiments have studied antiferromagnets and superconductors near quantum phase transitions. The simplest theoretical models of these transitions are strongly-coupled conformal field theories (CFTs). There is a remarkable connection between the finite temperature dynamics of CFTs and the holographic description of Hawking radiation from black holes. I will review how insights from this connection have shed light on experiments on the cuprate superconductors and other "correlated electron" materials.


Lorenzo Sindoni

Emergent Finsler geometry

The analysis of several analogue models for gravity has shown that Finsler geometry, a generalization of Riemannian geometry, is the most natural geometrical structure which emerges from the microphysics of the various models. After reviewing the main features of Finsler geometry, as well as its relationships with emergent gravity scenarios, I will discuss some applications and open problems.


Lee Smolin

Observational tests of the hypothesis that spacetime is emergent.

If spacetime and its classical geometry are emergent then there should be observable consequences, ie phenomena that would not be expected if the metric of spacetime and the continuum are fundamental. I discuss three possible consqeuences of emergent spacetime that have consequences for cosmological observations.


Thomas P. Sotiriou

6+1 lessons from f(R) gravity

There has been a recent stimulus in the study of alternative theories of gravity lately, mostly triggered form combined motivation coming from cosmology/astrophysics and high energy physics. Among the proposed theories, one that has attracted much attention is f(R) gravity. It is certainly debatable whether such a simplistic modification of General Relativity can constitute a viable alternative theory of gravitation. However, it is quite straightforward to see the merits of such a theory when viewed as a toy theory whose role is to help us understand the implications and difficulties of beyond-Einstein gravity. Under this perspective, I review some of the main lessons we seem to have learned though the study of f(R) gravity in the recent past.


Harold Steinacker

Emergent space and gravity from Yang-Mills matrix models

A mechanism for gravity emerging from Yang-Mills-type matrix models is exhibited. The matrix model desribes general noncommutative spaces, which in the semi-classical limit aquire an effective metric depending on the dynamical Poisson structure and an embedding metric. This leads to an emergent gravity intimately related to noncommutativity of space, absorbing the "would-be U(1) gauge fields". The induced gravitational action captures the UV/IR mixing of NC gauge theory. The quantization is discussed qualitatively, which singles out the IKKT model as a prime candidate for a quantum theory of gravity coupled to matter. A mechanism for avoiding the cosmological constant problem is identified.


Yidun Wan

Braided matter in Quantum Gravity: A status report

Recently there has been a substantial amount of work on emergent matter in Quantum Gravity as braids formed by edges of spin networks embedded in topological 3-manifolds. In this talk we will report on the current status along this research line, in particular in the case of 4-valent spin networks. We will show the dynamics, namely interactions and propagations of the so-called 3-strand braids, after an introduction of the basics. We derive conservation laws from interactions of braid-like excitations of embedded framed spin networks in Quantum Gravity. We also demonstrate that the set of stable braid-like excitations almost form a non-commutative algebra under braid interaction, in which the set of actively-interacting braids is a sub-algebra. We show that four-valent braids allow seven and only seven discrete transformations. These transformations can be uniquely mapped to C, P, T, and their products. Each CPT multiplet of actively-interacting braids is found to be uniquely characterized by a non-negative integer. Finally, braid interactions turn out to be invariant under C, P, and T. Some future directions will be pointed out at the end.


Silke Weinfurtner

Signature change events: A challenge for quantum gravity?

Within the framework of either Euclidean quantum gravity or canonical general relativity the signature of the manifold is a priori unconstrained. Furthermore, recent developments in the emergent spacetime programme have led to a physically feasible implementation of signature change events. This suggests that it is time to revisit the controversial topic of signature change in general relativity. More specifically, we shall focus on the behavior of a quantum field theory subjected to a manifold containing regions of different signature. We emphasize that, regardless of the underlying classical theory, there are severe problems associated with any quantum field theory residing on a signature changing background. (Such as the production of what is naively an infinite number of particles with an infinite quantity of energy.) From the viewpoint of quantum gravity phenomenology, we discuss possible consequences of an effective Lorentz symmetry breaking scale. To more fully understand the physics of quantum fields exposed to finite regions of Euclidean-signature (Riemannian) geometry, we show its similarities with the barrier penetration problem, and the super-Hubble horizon modes encountered in cosmology. Finally we raise the question as to whether signature change transitions could be fully understood and dynamically generated within (modified) classical general relativity, or whether they require the knowledge of a full theory of quantum gravity.


Xiao-Gang Wen

Partial quantum freeze and the emergence of photons

A phenomenon -- quantum freeze -- is introduce where a collective mode develops an energy gap due to strong quantum fluctuations. Such a phenomenon is applied to a rotor model to show the emergence of photons.


Cenke Xu

Bose liquid with soft-graviton excitations

There are two standard ground states of bosonic systems in dimensions higher than 1: Mott insulator and superfluid. The gaplessness of bosonic system is almost always related to global U(1) symmetry breaking. A gapless bosonic system without symmetry breaking is called the bose liquid. In the well-known bose liquid, an emergent U(1) gauge symmetry is what protects the low energy excitations from perturbations. In this talk, we will provide another example of bose liquid, whose stable low energy excitation happens to enjoy graviton-like gauge symmetry. We will also establish a general relation between bose liquid and self-dual gauge field structure.


Jan Zaanen

AdS/CFT from a condensed matter perspective.

Is string theory finally good for something? As a condensed matter physicists wih a consumer knowledge of string theory I percieve it as a miracle that by throwing around colorful black holes in a strangely curved space time one reproduces behaviors of the real life quantum condensed matter phenomena we cherish most in 2008. AdS/CFT appears to be a most convenient form of mathematics to explore the vicinity of quantum critical states [1], but up to now it has only produced things we already knew in condensed matter or don’t desire to know. I will present a condense matter wish list of desired answers that all revolve about the devious fermion minus signs [2].
[1] HYPERLINK "http://www.lorentz.leidenuniv.nl/~jan/NVblackhole.pdf" http://www.lorentz.leidenuniv.nl/~jan/NVblackhole.pdf
[2] HYPERLINK "http://www.lorentz.leidenuniv.nl/~jan/perspquantumcrit.pdf" http://www.lorentz.leidenuniv.nl/~jan/perspquantumcrit.pdf


Home / Registration / Program / Participants / Travel / Contact © Massachusetts Institute of Technology
Link: RLE Link: MIT