arXiv subjects

Lee Smolin

Publications and source records attributed to Lee Smolin.

At least 91 records · Page 5Linked to original sources

arXiv2009

Conservative solutions to the black hole information problem

We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence.

gr-qc↗

arXiv2009

The weak anthropic principle and the landscape of string theory

We note that there is an exception to the general arguments that no falsifiable predictions can be made, on the basis of of presently available data, by applying the weak anthropic principle (WAP) to the landscape of string theory. If there are infinitely more vacua in the landscape for one sign of a parameter than the other, within an anthropicaly allowed range, then under very weak assumptions about the probability measure one gets a firm prediction favoring that sign of that parameter. It is interesting to note that while the understanding of the string landscape is evolving, present evidence on the nature of the landscape allows such an argument to be made, leading to the conclusion that the WAP favors a negative value for the cosmological constant, $Λ$, in contradiction to the result of astronomical observations. The viability of applying the WAP to string theory then requires that either there are found an infinite discretum of anthropically allowed vacua for $Λ>0$, or the recently found infinite discretum of solutions for $Λ<0$ be reduced to a finite value.

hep-th↗

arXiv2008

Could deformed special relativity naturally arise from the semiclassical limit of quantum gravity?

A argument is described for how deformed or doubly special relativity may arise in the semiclassical limit of a quantum theory of gravity. We consider a generic quantum theory of gravity coupled to matter, from which we use only the assumption that a Hamiltonian constraint is imposed. We study circumstances in which Lambda, G and hbar all may be separately neglected, but there may arise terms in the ratio of particle energies to the Planck mass which are small but measurable. Such cases include probes of an energy dependent speed of light such as are possible in experiments such as MAGIC and GLAST. We show that in such cases the leading order effect of quantum gravity will, if certain scaling relations are satisfied, be to deform the metric in the effective Hamiltonian of the matter quantum field theory by terms linear in energies. As the Hamiltonian constraint has been imposed there can be no preferred time coordinate or frame of reference, hence this is a modification rather than a breaking of special relativity.

hep-th↗

arXiv2008

The Plebanski action extended to a unification of gravity and Yang-Mills theory

We study a unification of gravity with Yang-Mills fields based on a simple extension of the Plebanski action to a Lie group G which contains the local lorentz group. The Coleman-Mandula theorem is avoided because the theory has no global spacetime symmetry. This may be applied to Lisi's proposal of an E8 unified theory, giving a fully E8 invariant action. The extended form of the Plebanski action suggests a new class of spin foam models.

hep-th↗

arXiv2008

Anomalous CMB polarization and gravitational chirality

We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation.

astro-ph↗

arXiv2008

Particle Identifications from Symmetries of Braided Ribbon Network Invariants

We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge.

hep-th↗

arXiv2008

Matrix universality of gauge and gravitational dynamics

A simple cubic matrix model is presented, which has truncations that, it is argued, lead at the classical level to a variety of theories of gauge fields and gravity. These include Chern-Simons theory in d=3, and BF theory and general relativity in d=4. General relativity coupled to Yang-mills theory for any SU(N) may also arise from quantum corrections. On the basis of these results we conjecture that there are large universality classes of cut-off gauge and gravity theories, connected by transformations that mix up local and spacetime symmetries. If our universe is described by one of these theories then the question of the choice of the laws of physics is to a large extent subsumed in the problem of the choice of initial conditions in cosmology.

hep-th↗

arXiv2007

Propagation and interaction of chiral states in quantum gravity

We study the stability, propagation and interactions of braid states in models of quantum gravity in which the states are four-valent spin networks embedded in a topological three manifold and the evolution moves are given by the dual Pachner moves. There are results for both the framed and unframed case. We study simple braids made up of two nodes which share three edges, which are possibly braided and twisted. We find three classes of such braids, those which both interact and propagate, those that only propagate, and the majority that do neither.

hep-th↗

arXiv2007

Disordered locality in loop quantum gravity states

We show that loop quantum gravity suffers from a potential problem with non-locality, coming from a mismatch between micro-locality, as defined by the combinatorial structures of their microscopic states, and macro-locality, defined by the metric which emerges from the low energy limit. As a result, the low energy limit may suffer from a disordered locality characterized by identifications of far away points. We argue that if such defects in locality are rare enough they will be difficult to detect.

gr-qc↗

arXiv2007

Quantum gravity and the standard model

We show that a class of background independent models of quantum spacetime have local excitations that can be mapped to the first generation fermions of the standard model of particle physics. These states propagate coherently as they can be shown to be noiseless subsystems of the microscopic quantum dynamics. These are identified in terms of certain patterns of braiding of graphs, thus giving a quantum gravitational foundation for the topological preon model proposed by one of us. These results apply to a large class of theories in which the Hilbert space has a basis of states given by ribbon graphs embedded in a three-dimensional manifold up to diffeomorphisms, and the dynamics is given by local moves on the graphs, such as arise in the representation theory of quantum groups. For such models, matter appears to be already included in the microscopic kinematics and dynamics.

hep-th↗

arXiv2006

The status of cosmological natural selection

The problem of making predictions from theories that have landscapes of possible low energy parameters is reviewed. Conditions for such a theory to yield falsifiable predictions for doable experiments are given. It is shown that the hypothesis of cosmological natural selection satisfies these conditions, thus showing that it is possible to continue to do physics on a landscape without invoking the anthropic principle. In particular, this is true whether or not the ensemble of universes generated by black holes bouncing is a sub-ensemble of a larger ensemble that might be generated by a random process such as eternal inflation. A recent criticism of cosmological natural selection made by Vilenkin in hep-th/0610051 is discussed. It is shown to rely on assumptions about both the infrared and ultraviolet behavior of quantum gravity that are very unlikely to be true.

hep-th↗

arXiv2006

Holography and the scale-invariance of density fluctuations

We study a scenario for the very early universe in which there is a fast phase transition from a non-geometric, high temperature phase to a low temperature, geometric phase described by a classical solution to the Einstein equations. In spite of the absence of a classical metric, the thermodynamics of the high temperature phase may be described by making use of the holographic principle. The thermal spectrum of fluctuations in the high temperature phase manifest themselves after the phase transition as a scale invariant spectrum of fluctuations. A simple model of the phase transition confirms that the near scale invariance of the fluctuations is natural; but the model also withstands detailed comparison with the data.

astro-ph↗

arXiv2006

Quantum Graphity

We introduce a new model of background independent physics in which the degrees of freedom live on a complete graph and the physics is invariant under the permutations of all the points. We argue that the model has a low energy phase in which physics on a low dimensional lattice emerges and the permutation symmetry is broken to the translation group of that lattice. In the high temperature, or disordered, phase the permutation symmetry is respected and the average distance between degrees of freedom is small. This may serve as a tractable model for the emergence of classical geometry in background independent models of spacetime. We use this model to argue for a cosmological scenario in which the universe underwent a transition from the high to the low temperature phase, thus avoiding the horizon problem.

hep-th↗

arXiv2006

Could quantum mechanics be an approximation to another theory?

We consider the hypothesis that quantum mechanics is an approximation to another, cosmological theory, accurate only for the description of subsystems of the universe. Quantum theory is then to be derived from the cosmological theory by averaging over variables which are not internal to the subsystem, which may be considered non-local hidden variables. We find conditions for arriving at quantum mechanics through such a procedure. The key lesson is that the effect of the coupling to the external degrees of freedom introduces noise into the evolution of the system degrees of freedom, while preserving a notion of averaged conserved energy and time reversal invariance. These conditions imply that the effective description of the subsystem is Nelson's stochastic formulation of quantum theory. We show that Nelson's formulation is not, by itself, a classical stochastic theory as the conserved averaged energy is not a linear function of the probability density. We also investigate an argument of Wallstrom posed against the equivalence of Nelson's stochastic mechanics and quantum mechanics and show that, at least for a simple case, it is in error.

quant-ph↗

arXiv2006

Generic predictions of quantum theories of gravity

I discuss generic consequences (sometimes called "soft predictions") of a class of background independent quantum theories of spacetime called causal spin network theories. These are theories whose kinematics and dynamics is based on the evolution of labeled graphs, by local moves, such as in loop quantum gravity and spin foam models. Some generic consequences are well known, including the discreteness of quantum geometry, the elimination of spacetime singularities, the entropy of black hole and cosmological horizons and the fact that positive cosmological constant spacetimes are hot. Within the last few years three possible generic consequences have come to light. These are 1) Deformed special relativity as the symmetry of the ground state, 2) Elementary particles as coherent excitations of quantum geometry, 3) Locality is disordered. I discuss some possible experimental consequences of each.

hep-th↗

arXiv2005

Symmetry and entropy of black hole horizons

We argue, using methods taken from the theory of noiseless subsystems in quantum information theory, that the quantum states associated with a Schwarzchild black hole live in the restricted subspace of the Hilbert space of horizon boundary states in which all punctures are equal. Consequently, one value of the Immirzi parameter matches both the Hawking value for the entropy and the quasi normal mode spectrum of the Schwarzchild black hole. The method of noiseless subsystems thus allows us to understand, in this example and more generally, how symmetries, which take physical states to physical states, can emerge from a diffeomorphism invariant formulation of quantum gravity.

hep-th↗

arXiv2005

The case for background independence

The aim of this paper is to explain carefully the arguments behind the assertion that the correct quantum theory of gravity must be background independent. We begin by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background independent theory. The leading background independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity and dynamical triangulations and their main achievements are summarized along with the problems that remain open. Some first attempts to cast string/M theory into a background independent formulation are also mentioned. The relational/absolute debate has implications also for other issues such as unification and how the parameters of the standard models of physics and cosmology are to be explained. The recent issues concerning the string theory landscape are reviewed and it is argued that they can only be resolved within the context of a background independent formulation. Finally, we review some recent proposals to make quantum theory more relational.

hep-th↗

arXiv2005

An invitation to loop quantum gravity

We describe the basic assumptions and key results of loop quantum gravity, which is a background independent approach to quantum gravity. The emphasis is on the basic physical principles and how one deduces predictions from them, at a level suitable for physicsts in other areas such as string theory, cosmology, particle physics, astrophysics and condensed matter physics. No details are given, but references are provided to guide the interested reader to the literature. The present state of knowledge is summarized in a list of 35 key results on topics including the hamiltonian and path integral quantizations, coupling to matter, extensions to supergravity and higher dimensional theories, as well as applications to black holes, cosmology and Plank scale phenomenology. We describe the near term prospects for observational tests of quantum theories of gravity and the expectations that loop quantum gravity may provide predictions for their outcomes. Finally, we provide answers to frequently asked questions and a list of key open problems.

hep-th↗