1,391 research outputs found
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
Some solutions of linearized 5-d gravity with brane
We consider linearized 5-d gravity in the Randall-Sundrum brane world. The
class of static solutions for linearized Einstein equations is found. Also we
obtaine wave solutions describing radiation from an imaginary point source
located at the Planck distance from the brane. We analyze the fields asymptotic
behavior and peculiarities of matter sources.Comment: Latex, 8 page
Warped brane-world compactification with Gauss-Bonnet term
In the Randall-Sundrum (RS) brane-world model a singular delta-function
source is matched by the second derivative of the warp factor. So one should
take possible curvature corrections in the effective action of the RS models in
a Gauss-Bonnet (GB) form. We present a linearized treatment of gravity in the
RS brane-world with the Gauss-Bonnet modification to Einstein gravity. We give
explicit expressions for the Neumann propagator in arbitrary D dimensions and
show that a bulk GB term gives, along with a tower of Kaluza-Klein modes in the
bulk, a massless graviton on the brane, as in the standard RS model. Moreover,
a non-trivial GB coupling can allow a new branch of solutions with finite
Planck scale and no naked bulk singularity, which might be useful to avoid some
of the previously known ``no--go theorems'' for RS brane-world
compactifications.Comment: 23 pages, typos in Secs. 5 & 6 corrected, expanded/published version
(IJMPA
Nonlocality vs. complementarity: a conservative approach to the information problem
A proposal for resolution of the information paradox is that "nice slice"
states, which have been viewed as providing a sharp argument for information
loss, do not in fact do so as they do not give a fully accurate description of
the quantum state of a black hole. This however leaves an information
*problem*, which is to provide a consistent description of how information
escapes when a black hole evaporates. While a rather extreme form of
nonlocality has been advocated in the form of complementarity, this paper
argues that is not necessary, and more modest nonlocality could solve the
information problem. One possible distinguishing characteristic of scenarios is
the information retention time. The question of whether such nonlocality
implies acausality, and particularly inconsistency, is briefly addressed. The
need for such nonlocality, and its apparent tension with our empirical
observations of local quantum field theory, may be a critical missing piece in
understanding the principles of quantum gravity.Comment: 11 pages of text and figures, + references. v2 minor text. v3 small
revisions to match final journal versio
Entropy in Black Hole Pair Production
Pair production of Reissner-Nordstrom black holes in a magnetic field can be
described by a euclidean instanton. It is shown that the instanton amplitude
contains an explicit factor of , where is the area of the event
horizon. This is consistent with the hypothesis that measures the
number of black hole states.Comment: 24 pages (harvmac l mode
Is there the radion in the RS2 model ?
We analyse the physical boundary conditions at infinity for metric
fluctuations and gauge functions in the RS2 model with matter on the brane. We
argue that due to these boundary conditions the radion field cannot be gauged
out in this case. Thus, it represents a physical degree of freedom of the
model.Comment: 9 page
Comments on information loss and remnants
The information loss and remnant proposals for resolving the black hole
information paradox are reconsidered. It is argued that in typical cases
information loss implies energy loss, and thus can be thought of in terms of
coupling to a spectrum of ``fictitious'' remnants. This suggests proposals for
information loss that do not imply planckian energy fluctuations in the low
energy world. However, if consistency of gravity prevents energy
non-conservation, these remnants must then be considered to be real. In either
case, the catastrophe corresponding to infinite pair production remains a
potential problem. Using Reissner-Nordstrom black holes as a paradigm for a
theory of remnants, it is argued that couplings in such a theory may give
finite production despite an infinite spectrum. Evidence for this is found in
analyzing the instanton for Schwinger production; fluctuations from the
infinite number of states lead to a divergent stress tensor, spoiling the
instanton calculation. Therefore naive arguements for infinite production fail.Comment: 30 pages (harvmac l mode) UCSBTH-93-35 (minor reference and typo
corrections
Multiphase flow and boiling heat transfer modelling of nanofluids in horizontal tubes embedded in a metal foam
© 2019 Elsevier Masson SAS The aim of this numerical study is to evaluate the boiling process of nanofluid in horizontal tubes in the presence of a metal foam as porous medium and represent the experimental work of Zhao et al. in a numerical aspect with a different range of dependent variables. High conductive metal foams are employed to increase the rate of heat transfer and enhance the boiling performance in the domain. Two-phase mixture model is used to simulate the characteristics of nanofluid and solve the governing equations in a two-phase flow and boiling heat transfer problem. R134a and ZnO are considered as the base-fluid and nanoparticles, respectively. The characteristics of metal foam including the porosity and pore density as well as operating conditions including the fluid flow including the velocity, induced heat flux and concentration of nanoparticles on the pressure drop, vapour volume fraction and heat transfer coefficient are examined. The results show the positive effect of the metal foam on vapour production and overall heat transfer coefficient of the nanofluid in the pipe outlet; however, due to the flow resistance as a result of porous medium addition, a higher pressure drop is achieved. For the heat flux of 19 kW/m2 and inlet velocity of 0.05 m/s, by using a metal foam with the porosity of 70% and pore density of 20PPI, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 7.1%, 9.4% and 82.7%, respectively, compared with the case of without metal foam. However, by using the porosity of 90%, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 1.6%, 3.5%, and 7.0%, respectively. Consequently, according to the developed results in this paper, a system with a moderate to low porosity with a high to moderate pore density is recommended which is finally determined based on the required vapour production and allowed pressure drop
UV-Completion by Classicalization
We suggest a novel approach to UV-completion of a class of non-renormalizable
theories, according to which the high-energy scattering amplitudes get
unitarized by production of extended classical objects (classicalons), playing
a role analogous to black holes, in the case of non-gravitational theories. The
key property of classicalization is the existence of a classicalizer field that
couples to energy-momentum sources. Such localized sources are excited in
high-energy scattering processes and lead to the formation of classicalons. Two
kinds of natural classicalizers are Nambu-Goldstone bosons (or, equivalently,
longitudinal polarizations of massive gauge fields) and scalars coupled to
energy-momentum type sources. Classicalization has interesting phenomenological
applications for the UV-completion of the Standard Model both with or without
the Higgs. In the Higgless Standard Model the high-energy scattering amplitudes
of longitudinal -bosons self-unitarize via classicalization, without the
help of any new weakly-coupled physics. Alternatively, in the presence of a
Higgs boson, classicalization could explain the stabilization of the hierarchy.
In both scenarios the high-energy scatterings are dominated by the formation of
classicalons, which subsequently decay into many particle states. The
experimental signatures at the LHC are quite distinctive, with sharp
differences in the two cases.Comment: 37 page
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