It is shown that the currently studied "string-inspired" model for gravity on a line can be formulated as a gauge invariant theory based on the Poincaré group with central extension -a formulation that complements and simplifies H. Verlinde's construction based on the unextended Poincaré group.

We revisit the quantization of matter-coupled, two-dimensional dilaton gravity. At the classical level and with a cosmological term, a series of field transformations leads to a set of free fields of indefinite signature. Without matter the system is represented by two scalar fields of opposite signature. With a particular quantization for the scalar with negative kinetic energy, the system has zero central charge and we find some physical states satisfying all the Virasoro conditions. With matter, the constraints cannot be solved because of the Virasoro anomaly. We discuss two avenues for consistent quantization: modification of the constraints, and BRST quantization. The first avenue appears to lead to very few physical states. The second, which roughly corresponds to satisfying half of the Virasoro conditions, results in a rich spectrum of physical states. This spectrum, however, differs significantly from that of free matter fields propagating on flat two-dimensional space-time.

We investigate the effective action of 2+1 dimensional charged spin 1/2 fermions and spin 0 bosons in the presence of a U (1) gauge field. We evaluate terms in an expansion up to second order in derivatives of the field strength, but exactly in the mass parameter and in the magnitude of the nonvanishing constant field strength. We find that in a strong uniform magnetic field background, space-derivative terms lower the energy, and there arises an instability toward inhomogeneous magnetic fields. PACS numbers: 12.20.-m, 12.20.Ds, 71.20.Ad UCLA/94/TEP/35 hep-th/9409113 UCONN-94-5

dimensional gravity is derived and compared with the ADM definition of energy. expression for energy in a gauge theoretical formulation of the string-inspired 1+1 the I S O(2, 1) gauge theoretical formulation of Einstein gravity. In addition, an dimensions, expressions are obtained for energy and angular momentum arising in the gravitational Einstein-Hilbert action is derived and discussed in detail. In 2+1 procedure in 3+1 dimensions, a symmetric energy-momentum (pseudo) tensor for symmetry. Using Noether's theorem and a generalized Belinfante symmetrizationWe discuss general properties of the conservation law associated with a local ABSTRACT P@¤B19959

We determine the boundary state for both the NS-NS and R-R sectors of superstring theory. We show how they are modified under a boost. The boosted boundary state is then used for computing the interaction of two D-branes moving with constant velocity reproducing with a completely different method a recent calculation by Bachas.Comment: 10 pages, LaTeX, no figures; Eqs. (47) and (48) correcte