829 research outputs found
Classical and Quantized Tensionless Strings
{}From the ordinary tensile string we derive a geometric action for the
tensionless () string and discuss its symmetries and field equations. The
Weyl symmetry of the usual string is shown to be replaced by a global
space-time conformal symmetry in the limit. We present the explicit
expressions for the generators of this group in the light-cone gauge. Using
these, we quantize the theory in an operator form and require the conformal
symmetry to remain a symmetry of the quantum theory. Modulo details concerning
zero-modes that are discussed in the paper, this leads to the stringent
restriction that the physical states should be singlets under space-time
diffeomorphisms, hinting at a topological theory. We present the details of the
calculation that leads to this conclusion.Comment: 34 pages, Latex, USITP 93-1
Schild's Null Strings in Flat and Curved Backgrounds
Schild's null (tensionless) strings are discussed in certain flat and curved
backgrounds. We find closed, stationary, null strings as natural configurations
existing on the horizons of spacetimes which possess such null hypersurfaces.
Examples of these are obtained in Schwarzschild and Rindler spacetimes. A
dynamic null string is also identified in Rindler spacetime. Furthermore, a
general prescription (with explicit examples) is outlined by means of which
null string configurations can be obtained in a large class of cosmological
backgrounds.Comment: RevTex 3.0, 14 Pages, no figure
Null Branes in Curved Backgrounds
We consider null bosonic p-branes in curved space-times. Some exact solutions
of the classical equations of motion and of the constraints for the null
membrane in general stationary, axially symmetrical, four dimensional, gravity
background are found.Comment: 19 pages, LaTeX, no figures. Extended version. To appear in Phys.
Rev.
Tree-Level Unitarity Constraints on the Gravitational Couplings of Higher-Spin Massive Fields
We analyse the high-energy behavior of tree-level graviton Compton amplitudes
for particles of mass m and arbitrary spin, concentrating on a combination of
forward amplitudes that will be unaffected by eventual cross- couplings to
other, higher spins. We first show that for any spin larger than 2, tree-level
unitarity is already violated at energies well below the Planck scale M, if m
<< M. We then restore unitarity to this amplitude up to M by adding non-minimal
couplings that depend on the curvature and its derivatives, and modify the
minimal description - including particle gravitational quadrupole moments - at
scales O(1/m).Comment: 12 pages (Latex file, needs FEYNMAN macros), IASSNS-HEP-94/63,
NYU-TH-94/05/01, CERN-TH.7388/9
Space-Time Symmetries of Quantized Tensionless Strings
The tensionless limit of the free bosonic string is space-time conformally
symmetric classically. Requiring invariance of the quantum theory in the light
cone gauge tests the reparametrization symmetry needed to fix this gauge. The
full conformal symmetry gives stronger constraints than the Poincar\'e
subalgebra. We find that the symmetry may be preserved in any space-time
dimension, but only if the spectrum is drastically reduced (part of this
reduction is natural in a zero tension limit of the ordinary string spectrum).
The quantum states are required to be symmetric ({\it i.e.} singlets) under
space-time diffeomorphisms, except for the centre of mass wave function.Comment: 15pp, plain latex, USITP-92-
String Tension and the Generation of the Conformal Anomaly
The origin of the string conformal anomaly is studied in detail. We use a
reformulated string Lagrangian which allows to consider the string tension
as a small perturbation. The expansion parameter is the worldsheet
speed of light c, which is proportional to . We examine carefully the
interplay between a null (tensionless) string and a tensionful string which
includes orders and higher. The conformal algebra generated by the
constraints is considered. At the quantum level the normal ordering provides a
central charge proportional to . Thus it is clear that quantum null
strings respect conformal invariance and it is the string tension which
generates the conformal anomaly.Comment: More references are included. Final version, to appear in Phys.Rev.D.
6 pages, LaTex, no figure
Null Branes in String Theory Backgrounds
We consider null bosonic p-branes moving in curved space-times and develop a
method for solving their equations of motion and constraints, which is suitable
for string theory backgrounds. As an application, we give an exact solution for
such background in ten dimensions.Comment: 11 pages, LaTeX. Final version, to appear in Phys. Rev.
Small Instantons and Tensionless Non-critical Strings
T-duality is used to extract information on an instanton of zero size in the
heterotic string. We discuss the possibility of the appearance
of a tensionless anti-self-dual non-critical string through an implementation
of the mechanism suggested by Strominger of two coincident 5-branes. It is
argued that when an instanton shrinks to zero size a tensionless non-critical
string appears at the core of the instanton. It is further conjectured that
appearance of tensionless strings in the spectrum leads to new phase
transitions in six dimensions in much the same way as massless particles do in
four dimensions.Comment: 22 pages, Latex, two figures added, reference added, We also added a
discussion on new phase transitions in six dimensions. The appearance of
tensionless strings in the spectrum leads to new phase transitions in six
dimensions similarly to the appearance of massless particles at special
points in the moduli space in four dimensional theorie
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