33,509 research outputs found
M(atrix) Theory: Matrix Quantum Mechanics as a Fundamental Theory
A self-contained review is given of the matrix model of M-theory. The
introductory part of the review is intended to be accessible to the general
reader. M-theory is an eleven-dimensional quantum theory of gravity which is
believed to underlie all superstring theories. This is the only candidate at
present for a theory of fundamental physics which reconciles gravity and
quantum field theory in a potentially realistic fashion. Evidence for the
existence of M-theory is still only circumstantial---no complete
background-independent formulation of the theory yet exists. Matrix theory was
first developed as a regularized theory of a supersymmetric quantum membrane.
More recently, the theory appeared in a different guise as the discrete
light-cone quantization of M-theory in flat space. These two approaches to
matrix theory are described in detail and compared. It is shown that matrix
theory is a well-defined quantum theory which reduces to a supersymmetric
theory of gravity at low energies. Although the fundamental degrees of freedom
of matrix theory are essentially pointlike, it is shown that higher-dimensional
fluctuating objects (branes) arise through the nonabelian structure of the
matrix degrees of freedom. The problem of formulating matrix theory in a
general space-time background is discussed, and the connections between matrix
theory and other related models are reviewed.Comment: 56 pages, 3 figures, LaTeX, revtex style; v2: references adde
Causal Dynamical Triangulations and the Quest for Quantum Gravity
Quantum Gravity by Causal Dynamical Triangulation has over the last few years
emerged as a serious contender for a nonperturbative description of the theory.
It is a nonperturbative implementation of the sum-over-histories, which relies
on few ingredients and initial assumptions, has few free parameters and -
crucially - is amenable to numerical simulations. It is the only approach to
have demonstrated that a classical universe can be generated dynamically from
Planckian quantum fluctuations. At the same time, it allows for the explicit
evaluation of expectation values of invariants characterizing the highly
nonclassical, short-distance behaviour of spacetime. As an added bonus, we have
learned important lessons on which aspects of spacetime need to be fixed a
priori as part of the background structure and which can be expected to emerge
dynamically.Comment: To appear in "Foundations of Space and Time", Cambridge Univ. Press,
eds. G. Ellis, J. Murugan, A Weltma
Macroscopic Quantum Phenomena from the Correlation, Coupling and Criticality Perspectives
In this sequel paper we explore how macroscopic quantum phenomena can be
measured or understood from the behavior of quantum correlations which exist in
a quantum system of many particles or components and how the interaction
strengths change with energy or scale, under ordinary situations and when the
system is near its critical point. We use the nPI (master) effective action
related to the Boltzmann-BBGKY / Schwinger-Dyson hierarchy of equations as a
tool for systemizing the contributions of higher order correlation functions to
the dynamics of lower order correlation functions. Together with the large N
expansion discussed in our first paper(MQP1) we explore 1) the conditions
whereby an H-theorem is obtained, which can be viewed as a signifier of the
emergence of macroscopic behavior in the system. We give two more examples from
past work: 2) the nonequilibrium dynamics of N atoms in an optical lattice
under the large (field components), 2PI and second order perturbative
expansions, illustrating how N and enter in these three aspects of
quantum correlations, coherence and coupling strength. 3) the behavior of an
interacting quantum system near its critical point, the effects of quantum and
thermal fluctuations and the conditions under which the system manifests
infrared dimensional reduction. We also discuss how the effective field theory
concept bears on macroscopic quantum phenomena: the running of the coupling
parameters with energy or scale imparts a dynamical-dependent and an
interaction-sensitive definition of `macroscopia'.Comment: For IARD 2010 meeting, Hualien, Taiwan. Proceedings to appear in J.
Physics (Conf. Series
On the Infrared Behavior of Landau Gauge Yang-Mills Theory with a Fundamentally Charged Scalar Field
Recently it has been shown that infrared singularities of Landau gauge QCD
can confine static quarks via a linearly rising potential. We show that the
same mechanism can also provide a confining interaction between charged scalar
fields in the fundamental representation. This confirms that within this
scenario static confinement is a universal property of the gauge sector even
though it is formally represented in the functional equations of the matter
sector. The simplifications compared to the fermionic case make the scalar
system an ideal laboratory for a detailed analysis of the confinement mechanism
in numerical studies of the functional equations as well as in gauge-fixed
lattice simulations.Comment: 8 pages, PDFLaTe
Quantum Gravity: General Introduction and Recent Developments
I briefly review the current status of quantum gravity. After giving some
general motivations for the need of such a theory, I discuss the main
approaches in quantizing general relativity: Covariant approaches (perturbation
theory, effective theory, and path integrals) and canonical approaches (quantum
geometrodynamics, loop quantum gravity). I then address quantum gravitational
aspects of string theory. This is followed by a discussion of black holes and
quantum cosmology. I end with some remarks on the observational status of
quantum gravity.Comment: 21 pages, 6 figures, invited contribution for "Annalen der Physik",
v2: minor corrections, additional reference
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