8,987 research outputs found

    Gravitational theories coupled to matter as invariant theories under Kac-Moody algebras

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    Many recent researches indicate that several gravitational D-dimensional theories suitably coupled to some matter fields (including in particular pure gravity in D dimensions, the low energy effective actions of the bosonic string and the bosonic sector of M-theory) would be characterized by infinite dimensional Kac-Moody algebras G^{++} and G^{+++}. The possible existence of these extended symmetries motivates a development of a new description of gravitational theories based on these symmetries. The importance of Kac-Moody algebras and the link between the G^{+++}-invariant theories and the uncompactified space-time covariant theories are discussed.Comment: 7 pages, 1 figure, Contribution to the proceedings of the RTN project 'Constituents, Fundamental Forces and Symmetries of the Universe' conference in Napoli, October 9-13, 200

    Primordial Inflation

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    A macroscopic universe may emerge naturally from a Planck cell fluctuation by unfolding through a stage of exponential expansion towards a homogeneous cosmological background. Such primordial inflation requires a large and presumably infinite degeneracy at the Planck scale, rooted in the unbounded negative gravitational energy stored in conformal classes. This complex Planck structure is consistent with a quantum tunneling description of the transition from the Planck scale to the inflationary era and implies, in the limit of vanishing Planck size, the Hartle-Hawking no-time boundary condition. On the other hand, string theory give credence to the holographic principle and the concomitant depletion of states at the Planck scale. The apparent incompatibility of primordial inflation with holography either invalidates one of these two notions or relegates the nature of the Planck size outside the realm of quantum physics, as we know it.Comment: LaTeX, 24 pages, 4 figures, presented at the Erice International School on Basics and Highlights in Fundamental Physics, August 199

    On the black hole unitarity issue

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    I discuss features required for preserving unitarity in black hole decay and concepts underlying such a perspective. Unitarity requires that correlations extend on the scale of the horizon. I show, in a toy model inspired by string theories, that such correlations can indeed arise. The model suggests that, after a time of order 4M ln M following the onset of Hawking radiation, quantum effects could maintain throughout the decay a collapsing star within a Planck distance of its Schwarzschild radius. In this way information loss would be avoided. The concept of black hole ``complementarity'', which could reconcile these macroscopic departures from classical physics with the equivalence principle, is interpreted in terms of weak values of quantum operators.Comment: 14 pages, latex, no figures. Talk presented at the Puri workshop (December 1996

    Broken symmetry and Yang-Mills theory

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    From its inception in statistical physics to its role in the construction and in the development of the asymmetric Yang-Mills phase in quantum field theory, the notion of spontaneous broken symmetry permeates contemporary physics. This is reviewed with particular emphasis on the conceptual issues.Comment: Latex 30 pages, 9 figures. Typo corrected. Contribution to "Fifty years of Yang Mills theory", editor G. 't Hooft, to be published by World Scientifi

    A Brief Course in Spontaneous Symmetry Breaking II. Modern Times: The BEH Mechanism

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    The theory of symmetry breaking in presence of gauge fields is presented, following the historical track. Particular emphasis is placed upon the underlying concepts.Comment: LaTeX file, 25 pages 9 figures. Presented at the 2001 Corfu Summer Institute on Elementary Particle Physic


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    In conventional field theories, the emission of Hawking radiation in the background of a collapsing star requires transplanckian energy fluctuations. These fluctuations are encoded in the weak values of the energy-momentum operator constructed from matrix elements between both -in and -out states. It is argued that taming of these weak values by back-reaction may lead to geometrical backgrounds which are also build from weak values of the gravitational field operators. This leads to different causal histories of the black hole as reconstructed by observers crossing the horizon at different times but reduces, in accordance with the equivalence principle, to the classical description of the collapse for the proper history of the star as recorded by an observer comoving with it. For observers never crossing the horizon, the evaporation would be interpreted within a topologically trivial ``achronon geometry" void of horizon and singularity: after the initial ignition of the radiation from pair creation out of the vacuum of the collapsing star of mass M, as in the conventional theory, the source of the thermal radiation would shift gradually to the star itself in a time at least of order 4Mln⁥2M4M\ln 2M. The burning of the star could be consistent with a quantum unitary evolution along the lines suggested by 't Hooft. A provisional formal expression of general black hole complementarity is proposed and its possible relevance for testing features of a theory of quantum gravity is suggested.Comment: presented at the Oskar Klein Centenary Symposium (September 1994); 29 pages, phyzzx, no figure

    Life, Logic, and the Pursuit of Purity

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    In the *Science of Logic*, Hegel states unequivocally that the category of “life” is a strictly logical, or pure, form of thinking. His treatment of actual life – i.e., that which empirically constitutes nature – arises first in his *Philosophy of Nature* when the logic is applied under the conditions of space and time. Nevertheless, many commentators find Hegel’s development of this category as a purely logical one especially difficult to accept. Indeed, they find this development only comprehensible as long as one simultaneously assumes that Hegel breaks his promise to let the logic do the leading. However, if Hegel were to in fact allow the logical development to be led by biological analogies at this point, problems would ensue. Not only would it contradict his own speculative method, which should secure the necessity of the categories, but it would also endanger the ontological generality of the category of life itself. Beyond undermining his method and the logical integrity of the category, however, I will argue that such a reading makes the transition to the next category of “cognition” unintelligible and problematic. My aim in the first part of this paper is to argue how logical life can be read as a pure category. I then argue in the second part how my reconstruction makes the transition to cognition intelligible without resorting to profane or supernatural interpretations