9,199 research outputs found
Spacetime as a quantum many-body system
Quantum gravity has become a fertile interface between gravitational physics
and quantum many-body physics, with its double goal of identifying the
microscopic constituents of the universe and their fundamental dynamics, and of
understanding their collective properties and how spacetime and geometry
themselves emerge from them at macroscopic scales. In this brief contribution,
we outline the problem of quantum gravity from this emergent spacetime
perspective, and discuss some examples in which ideas and methods from quantum
many-body systems have found a central role in quantum gravity research.Comment: 15 pages; invited contribution to "Many-body approaches at different
scales: A tribute to Norman H. March on the occasion of his 90th birthday",
edited by G. G. N. Angilella and C. Amovilli (New York, Springer, 2017 - to
appear
Spacetime-Free Approach to Quantum Theory and Effective Spacetime Structure
Motivated by hints of the effective emergent nature of spacetime structure,
we formulate a spacetime-free algebraic framework for quantum theory, in which
no a priori background geometric structure is required. Such a framework is
necessary in order to study the emergence of effective spacetime structure in a
consistent manner, without assuming a background geometry from the outset.
Instead, the background geometry is conjectured to arise as an effective
structure of the algebraic and dynamical relations between observables that are
imposed by the background statistics of the system. Namely, we suggest that
quantum reference states on an extended observable algebra, the free algebra
generated by the observables, may give rise to effective spacetime structures.
Accordingly, perturbations of the reference state lead to perturbations of the
induced effective spacetime geometry. We initiate the study of these
perturbations, and their relation to gravitational phenomena
Decay of Spatial Correlations in Thermal States
We study the cluster properties of thermal equilibrium states in theories
with a maximal propagation velocity (such as relativistic QFT). Our analysis,
carried out in the setting of algebraic quantum field theory, shows that there
is a tight relation between spectral properties of the generator of time
translations and the decay of spatial correlations in thermal equilibrium
states, in complete analogy to the well understood case of the vacuum state.Comment: plain tex, 13 pages. To appear in Ann. Inst. H. Poinc. (Phys. Theor.
Statistical equilibrium of tetrahedra from maximum entropy principle
Discrete formulations of (quantum) gravity in four spacetime dimensions build
space out of tetrahedra. We investigate a statistical mechanical system of
tetrahedra from a many-body point of view based on non-local, combinatorial
gluing constraints that are modelled as multi-particle interactions. We focus
on Gibbs equilibrium states, constructed using Jaynes' principle of constrained
maximisation of entropy, which has been shown recently to play an important
role in characterising equilibrium in background independent systems. We apply
this principle first to classical systems of many tetrahedra using different
examples of geometrically motivated constraints. Then for a system of quantum
tetrahedra, we show that the quantum statistical partition function of a Gibbs
state with respect to some constraint operator can be reinterpreted as a
partition function for a quantum field theory of tetrahedra, taking the form of
a group field theory.Comment: v3 published version; v2 18 pages, 4 figures, improved text in
sections IIIC & IVB, minor changes elsewher
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