1,683 research outputs found
On the Multiple Deaths of Whitehead's Theory of Gravity
Whitehead's 1922 theory of gravitation continues to attract the attention of
philosophers, despite evidence presented in 1971 that it violates experiment.
We demonstrate that the theory strongly fails five quite different experimental
tests, and conclude that, notwithstanding its meritorious philosophical
underpinnings, Whitehead's theory is truly dead.Comment: 22 pages; to be submitted to Studies In History And Philosophy Of
Modern Physic
Brane World Sum Rules
A set of consistency conditions is derived from Einstein equations for brane
world scenarios with a spatially periodic internal space. In particular, the
sum of the total tension of the flat branes and the non-negative integral of
the gradient energy of the bulk scalars must vanish. This constraint allows us
to make a simple consistency check of several models. We show that the
two-brane Randall-Sundrum model satisfies this constraint, but it does not
allow a generalization with smooth branes (domain walls), independently of the
issue of supersymmetry. The Goldberger-Wise model of brane stabilization has to
include the backreaction on the metric and the fine tuning of the cosmological
constant to satisfy the constraints. We check that this is achieved in the
DeWolfe-Freedman-Gubser-Karch scenario. Our constraints are automatically
satisfied in supersymmetric brane world models.Comment: 13 pages, JHEP, references adde
Nonlinear perturbations of the Kaluza-Klein monopole
We consider the nonlinear stability of the Kaluza-Klein monopole viewed as
the static solution of the five dimensional vacuum Einstein equations. Using
both numerical and analytical methods we give evidence that the Kaluza-Klein
monopole is asymptotically stable within the cohomogeneity-two biaxial Bianchi
IX ansatz recently introduced in \cite{bcs}. We also show that for sufficiently
large perturbations the Kaluza-Klein monopole loses stability and collapses to
a Kaluza-Klein black hole. The relevance of our results for the stability of
BPS states in M/String theory is briefly discussed.Comment: 4 pages, 4 figure
The Physics of 2-d Stringy Spacetimes
We examine the two-dimensional spacetimes that emerge from string theory. We
find all the solutions with no tachyons, and show that the only non-trivial
solution is the black hole spacetime. We examine the role of duality in this
picture. We then explore the thermodynamics of these solutions which is
complicated by the fact that only in two spacetime dimensions is it impossible
to redefine the dilaton field in terms of a canonical scalar field. Finally, we
extend our analysis to the heterotic string, and briefly comment on exact, as
opposed to perturbative, solutions
Discrete Newtonian Cosmology
In this paper we lay down the foundations for a purely Newtonian theory of
cosmology, valid at scales small compared with the Hubble radius, using only
Newtonian point particles acted on by gravity and a possible cosmological term.
We describe the cosmological background which is given by an exact solution of
the equations of motion in which the particles expand homothetically with their
comoving positions constituting a central configuration. We point out, using
previous work, that an important class of central configurations are
homogeneous and isotropic, thus justifying the usual assumptions of elementary
treatments. The scale factor is shown to satisfy the standard Raychaudhuri and
Friedmann equations without making any fluid dynamic or continuum
approximations. Since we make no commitment as to the identity of the point
particles, our results are valid for cold dark matter, galaxies, or clusters of
galaxies. In future publications we plan to discuss perturbations of our
cosmological background from the point particle viewpoint laid down in this
paper and show consistency with much standard theory usually obtained by more
complicated and conceptually less clear continuum methods. Apart from its
potential use in large scale structure studies, we believe that out approach
has great pedagogic advantages over existing elementary treatments of the
expanding universe, since it requires no use of general relativity or continuum
mechanics but concentrates on the basic physics: Newton's laws for
gravitationally interacting particles.Comment: 33 pages; typos fixed, references added, some clarification
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