162 research outputs found
What can we learn about neutron stars from gravity-wave observations?
In the next few years, the first detections of gravity-wave signals using
Earth-based interferometric detectors will begin to provide precious new
information about the structure and dynamics of compact bodies such as neutron
stars. The intrinsic weakness of gravity-wave signals requires a proactive
approach to modeling the prospective sources and anticipating the shape of the
signals that we seek to detect. Full-blown 3-D numerical simulations of the
sources are playing and will play an important role in planning the
gravity-wave data-analysis effort. I review some recent analytical and
numerical work on neutron stars as sources of gravity waves.Comment: Revtex 4, 3 EPS figures. To appear in the proceedings of the 25th J.
Hopkins Workshop on Current Problems in Particle Theory; 2001: A Relativistic
Spacetime Odyssey, Florence, Sep. 3--5, 200
Ephemeral point-events: is there a last remnant of physical objectivity?
For the past two decades, Einstein's Hole Argument (which deals with the
apparent indeterminateness of general relativity due to the general covariance
of the field equations) and its resolution in terms of Leibniz equivalence (the
statement that Riemannian geometries related by active diffeomorphisms
represent the same physical solution) have been the starting point for a lively
philosophical debate on the objectivity of the point-events of space-time. It
seems that Leibniz equivalence makes it impossible to consider the points of
the space-time manifold as physically individuated without recourse to
dynamical individuating fields. Various authors have posited that the metric
field itself can be used in this way, but nobody so far has considered the
problem of explicitly distilling the metrical fingerprint of point-events from
the gauge-dependent components of the metric field. Working in the Hamiltonian
formulation of general relativity, and building on the results of Lusanna and
Pauri (2002), we show how Bergmann and Komar's intrinsic pseudo-coordinates
(based on the value of curvature invariants) can be used to provide a physical
individuation of point-events in terms of the true degrees of freedom (the
Dirac observables) of the gravitational field, and we suggest how this
conceptual individuation could in principle be implemented with a well-defined
empirical procedure. We argue from these results that point-events retain a
significant kind of physical objectivity.Comment: LaTeX, natbib, 34 pages. Final journal versio
Marzke-Wheeler coordinates for accelerated observers in special relativity
In special relativity, the definition of coordinate systems adapted to
generic accelerated observers is a long-standing problem, which has found
unequivocal solutions only for the simplest motions. We show that the
Marzke-Wheeler construction, an extension of the Einstein synchronization
convention, produces accelerated systems of coordinates with desirable
properties: (a) they reduce to Lorentz coordinates in a neighborhood of the
observers' world-lines; (b) they index continuously and completely the causal
envelope of the world-line (that is, the intersection of its causal past and
its causal future: for well-behaved world-lines, the entire space-time). In
particular, Marzke-Wheeler coordinates provide a smooth and consistent
foliation of the causal envelope of any accelerated observer into space-like
surfaces.
We compare the Marzke-Wheeler procedure with other definitions of accelerated
coordinates; we examine it in the special case of stationary motions, and we
provide explicit coordinate transformations for uniformly accelerated and
uniformly rotating observers. Finally, we employ the notion of Marzke-Wheeler
simultaneity to clarify the relativistic paradox of the twins, by pinpointing
the local origin of differential aging.Comment: AmsLaTeX, 22 pages, 8 eps figures; revised, references added. To
appear in Foundations of Physics Letters, October 200
Bayesian inference for pulsar timing models
The extremely regular, periodic radio emission from millisecond pulsars makes
them useful tools for studying neutron star astrophysics, general relativity,
and low-frequency gravitational waves. These studies require that the observed
pulse times of arrival be fit to complex timing models that describe numerous
effects such as the astrometry of the source, the evolution of the pulsar's
spin, the presence of a binary companion, and the propagation of the pulses
through the interstellar medium. In this paper, we discuss the benefits of
using Bayesian inference to obtain pulsar timing solutions. These benefits
include the validation of linearized least-squares model fits when they are
correct, and the proper characterization of parameter uncertainties when they
are not; the incorporation of prior parameter information and of models of
correlated noise; and the Bayesian comparison of alternative timing models. We
describe our computational setup, which combines the timing models of Tempo2
with the nested-sampling integrator MultiNest. We compare the timing solutions
generated using Bayesian inference and linearized least-squares for three
pulsars: B1953+29, J2317+1439, and J1640+2224, which demonstrate a variety of
the benefits that we posit.Comment: 13 pages, 4 figures, RevTeX 4.1. Revised in response to referee's
suggestions; contains a broader discussion of model comparison, revised Monte
Carlo runs, improved figure
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