12 research outputs found
String Form Factors
We compute the cross section for scattering of light string probes by
randomly excited closed strings. For high energy probes, the cross section
factorizes and can be used to define effective form factors for the excited
targets. These form factors are well defined without the need for infinite
subtractions and contain information about the shape and size of typical
strings. For highly excited strings the elastic form factor can be written in
terms of the `plasma dispersion function', which describes charge screening in
high temperature plasmas.Comment: 18 pages, 3 figures. Typos corrected, 1 footnote (in Section 4) and 1
reference adde
Quantum Gravitational Corrections to the Nonrelativistic Scattering Potential of Two Masses
We treat general relativity as an effective field theory, obtaining the full
nonanalytic component of the scattering matrix potential to one-loop order. The
lowest order vertex rules for the resulting effective field theory are
presented and the one-loop diagrams which yield the leading nonrelativistic
post-Newtonian and quantum corrections to the gravitational scattering
amplitude to second order in G are calculated in detail. The Fourier
transformed amplitudes yield a nonrelativistic potential and our result is
discussed in relation to previous calculations. The definition of a potential
is discussed as well and we show how the ambiguity of the potential under
coordinate changes is resolved.Comment: 27 pages, 17 figure
Exact Gravitational Shockwaves and Planckian Scattering on Branes
We obtain a solution describing a gravitational shockwave propagating along a
Randall-Sundrum brane. The interest of such a solution is twofold: on the one
hand, it is the first exact solution for a localized source on a
Randall-Sundrum three-brane. On the other hand, one can use it to study forward
scattering at Planckian energies, including the effects of the continuum of
Kaluza-Klein modes. We map out the different regimes for the scattering
obtained by varying the center-of-mass energy and the impact parameter. We also
discuss exact shockwaves in ADD scenarios with compact extra dimensions.Comment: 19 pages, 3 figures. v2: references added, minor improvements and
small errors correcte
Is string theory a theory of quantum gravity?
Some problems in finding a complete quantum theory incorporating gravity are
discussed. One is that of giving a consistent unitary description of
high-energy scattering. Another is that of giving a consistent quantum
description of cosmology, with appropriate observables. While string theory
addresses some problems of quantum gravity, its ability to resolve these
remains unclear. Answers may require new mechanisms and constructs, whether
within string theory, or in another framework.Comment: Invited contribution for "Forty Years of String Theory: Reflecting on
the Foundations," a special issue of Found. Phys., ed. by G 't Hooft, E.
Verlinde, D. Dieks, S. de Haro. 32 pages, 5 figs., harvmac. v2: final version
to appear in journal (small revisions
Instability of Spacelike and Null Orbifold Singularities
Time dependent orbifolds with spacelike or null singularities have recently
been studied as simple models of cosmological singularities. We show that their
apparent simplicity is an illusion: the introduction of a single particle
causes the spacetime to collapse to a strong curvature singularity (a Big
Crunch), even in regions arbitrarily far from the particle.Comment: 16 pages. References and comments added. Discussion of Milne with
shift correcte
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
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Bremsstrahlung radiation in high energy particle collisions in the visible region
Some features of bremsstrahlung radiation are reviewed for visible region photons with possible applications to ISA luminosity measurements. (auth