1,822 research outputs found
The Long-Term Future of Space Travel
The fact that we apparently live in an accelerating universe places
limitations on where humans might visit. If the current energy density of the
universe is dominated by a cosmological constant, a rocket could reach a galaxy
observed today at a redshift of 1.7 on a one-way journey or merely 0.65 on a
round trip. Unfortunately these maximal trips are impractical as they require
an infinite proper time to traverse. However, calculating the rocket trajectory
in detail shows that a rocketeer could nearly reach such galaxies within a
lifetime (a long lifetime admittedly -- about 100 years). For less negative
values of the maximal redshift increases becoming infinite for .Comment: 5 pages, 3 figures, minor changes to reflect version accepted to PR
The damping of gravitational waves in dust
We examine a simple model of interaction of gravitational waves with matter
(primarily represented by dust). The aim is to investigate a possible damping
effect on the intensity of gravitational wave when passing through media. This
might be important for gravitational wave astronomy when the sources are
obscured by dust or molecular clouds.Comment: 7 pages, accepted to Phys. Sc
On the Clock Paradox in the case of circular motion of the moving clock
In this paper we deal analytically with a version of the so called clock
paradox in which the moving clock performs a circular motion of constant
radius. The rest clock is denoted as (1), the rotating clock is (2), the
inertial frame in which (1) is at rest and (2) moves is I and, finally, the
accelerated frame in which (2) is at rest and (1) rotates is A. By using the
General Theory of Relativity in order to describe the motion of (1) as seen in
A we will show the following features. I) A differential aging between (1) and
(2) occurs at their reunion and it has an absolute character, i.e. the proper
time interval measured by a given clock is the same both in I and in A. II)
From a quantitative point of view, the magnitude of the differential aging
between (1) and (2) does depend on the kind of rotational motion performed by
A. Indeed, if it is uniform there is no any tangential force in the direction
of motion of (2) but only normal to it. In this case, the proper time interval
reckoned by (2) does depend only on its constant velocity v=romega. On the
contrary, if the rotational motion is uniformly accelerated, i.e. a constant
force acts tangentially along the direction of motion, the proper time
intervals on the angular acceleration alpha. III) Finally, in
regard to the sign of the aging, the moving clock (2) measures always a
interval of proper time with respect to (1).Comment: LaTex2e, 9 pages, no figures, no tables. It is the follow-on of the
paper physics/040503
Yang-Mills Magneto-Fluid Unification
We generalize the hybrid magneto-fluid model of a charged fluid interacting
with an electromagnetic field to the dynamics of a relativistic hot fluid
interacting with a non-Abelian field. The fluid itself is endowed with a
non-Abelian charge and the consequences of this generalization are worked out.
Applications of this formalism to the Quark Gluon Plasma are suggested.Comment: 6 pages, RevTex
The Algebra of the Energy-Momentum Tensor and the Noether Currents in Classical Non-Linear Sigma Models
The recently derived current algebra of classical non-linear sigma models on
arbitrary Riemannian manifolds is extended to include the energy-momentum
tensor. It is found that in two dimensions the energy-momentum tensor
, the Noether current associated with the global
symmetry of the theory and the composite field appearing as the coefficient
of the Schwinger term in the current algebra, together with the derivatives of
and , generate a closed algebra. The subalgebra generated by the
light-cone components of the energy-momentum tensor consists of two commuting
copies of the Virasoro algebra, with central charge , reflecting
the classical conformal invariance of the theory, but the current algebra part
and the semidirect product structure are quite different from the usual
Kac-Moody / Sugawara type construction.Comment: 10 pages, THEP 92/2
Non-equilibrium dynamics of a Bose-Einstein condensate in an optical lattice
The dynamical evolution of a Bose-Einstein condensate trapped in a
one-dimensional lattice potential is investigated theoretically in the
framework of the Bose-Hubbard model. The emphasis is set on the
far-from-equilibrium evolution in a case where the gas is strongly interacting.
This is realized by an appropriate choice of the parameters in the Hamiltonian,
and by starting with an initial state, where one lattice well contains a
Bose-Einstein condensate while all other wells are empty. Oscillations of the
condensate as well as non-condensate fractions of the gas between the different
sites of the lattice are found to be damped as a consequence of the collisional
interactions between the atoms. Functional integral techniques involving
self-consistently determined mean fields as well as two-point correlation
functions are used to derive the two-particle-irreducible (2PI) effective
action. The action is expanded in inverse powers of the number of field
components N, and the dynamic equations are derived from it to next-to-leading
order in this expansion. This approach reaches considerably beyond the
Hartree-Fock-Bogoliubov mean-field theory, and its results are compared to the
exact quantum dynamics obtained by A.M. Rey et al., Phys. Rev. A 69, 033610
(2004) for small atom numbers.Comment: 9 pages RevTeX, 3 figure
New Geometric Formalism for Gravity Equation in Empty Space
In this paper, a complex daor field which can be regarded as the square root
of space-time metric is proposed to represent gravity. The locally complexified
geometry is set up, and the complex spin connection constructs a bridge between
gravity and SU(1,3) gauge field. Daor field equations in empty space are
acquired, which are one-order differential equations and not conflict with
Einstein's gravity theory.Comment: 20 pages, to appear in Int. J. Mod. Phys.
Initial data for gravity coupled to scalar, electromagnetic and Yang-Mills fields
We give ansatze for solving classically the initial value constraints of
general relativity minimally coupled to a scalar field, electromagnetism or
Yang-Mills theory. The results include both time-symmetric and asymmetric data.
The time-asymmetric examples are used to test Penrose's cosmic censorship
inequality. We find that the inequality can be violated if only the weak energy
condition holds.Comment: 16 pages, RevTeX, references added, presentational changes, version
to appear in Phys Rev.
Supersymmetric quantum cosmological billiards
D=11 Supergravity near a space-like singularity admits a cosmological
billiard description based on the hyperbolic Kac-Moody group E10. The
quantization of this system via the supersymmetry constraint is shown to lead
to wavefunctions involving automorphic (Maass wave) forms under the modular
group W^+(E10)=PSL(2,O) with Dirichlet boundary conditions on the billiard
domain. A general inequality for the Laplace eigenvalues of these automorphic
forms implies that the wave function of the universe is generically complex and
always tends to zero when approaching the initial singularity. We discuss
possible implications of this result for the question of singularity resolution
in quantum cosmology and comment on the differences with other approaches.Comment: 4 pages. v2: Added ref. Version to be published in PR
Schwarzschild horizon and the gravitational redshift formula
The gravitational redshift formula is usually derived in the geometric optics
approximation. In this note we consider an exact formulation of the problem in
the Schwarzschild space-time, with the intention to clarify under what
conditions this redshift law is valid. It is shown that in the case of shocks
the radial component of the Poynting vector can scale according to the redshift
formula, under a suitable condition. If that condition is not satisfied, then
the effect of the backscattering can lead to significant modifications. The
obtained results imply that the energy flux of the short wavelength radiation
obeys the standard gravitational redshift formula while the energy flux of long
waves can scale differently, with redshifts being dependent on the frequency.Comment: Revtex, 5 p. Rewritten Sec. II, minor changes in Secs III - VII. To
appear in the Classical and Quantum Gravit
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