25,176 research outputs found
Gravitational Theory with a Dynamical Time
A gravitational theory involving a vector field , whose zero
component has the properties of a dynamical time, is studied. The variation of
the action with respect to gives the covariant conservation of an
energy momentum tensor . Studying the theory in a
background which has killing vectors and killing tensors we find appropriate
shift symmetries of the field which lead to conservation laws. The
energy momentum that is the source of gravity is different
but related to and the covariant conservation of determines in general the vector field . When is chosen to be proportional to the metric, the theory
coincides with the Two Measures Theory, which has been studied before in
relation to the Cosmological Constant Problem. When the matter model consists
of point particles, or strings, the form of , solutions for
are found. For the case of a string gas cosmology, we find that
the Milne Universe can be a solution, where the gas of strings does not curve
the spacetime since although , , as a model for the early universe, this solution is also free
of the horizon problem. There may be also an application to the "time problem"
of quantum cosmology.Comment: 21 pages, discussions extended, some more explicit proofs included,
more references include
Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables
Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables
(SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together
with a sample of Miras in this field a comparison is made with a sample of
field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity
& period-colour) with the PG3 Miras, in which the SRVs are the short period
extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o
relation, while this is not the case for the field and PG3 SRVs. Both the PG3
SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et
al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode
and have metallicities spanning the range from intermediate to approximately
solar.Comment: 14 pages LaTeX (2 tables, 8 figures), to appear in A&A 338 (1998);
minor modifications in tex
Phase diagrams of XXZ model on depleted square lattice
Using quantum Monte Carlo (QMC) simulations and a mean field (MF) theory, we
investigate the spin-1/2 XXZ model with nearest neighbor interactions on a
periodic depleted square lattice. In particular, we present results for 1/4
depleted lattice in an applied magnetic field and investigate the effect of
depletion on the ground state. The ground state phase diagram is found to
include an antiferromagnetic (AF) phase of magnetization and an
in-plane ferromagnetic (FM) phase with finite spin stiffness. The agreement
between the QMC simulations and the mean field theory based on resonating
trimers suggests the AF phase and in-plane FM phase can be interpreted as a
Mott insulator and superfluid of trimer states respectively. While the thermal
transitions of the in-plane FM phase are well described by the
Kosterlitz-Thouless transition, the quantum phase transition from the AF phase
to in-plane FM phase undergo a direct second order insulator-superfluid
transition upon increasing magnetic field.Comment: 7 pages, 8 figures. Revised version, accepted by PRB
A Turbo-Detection Aided Serially Concatenated MPEG-4/TCM Videophone Transceiver
A Turbo-detection aided serially concatenated inner Trellis Coded Modulation (TCM) scheme is combined with four different outer codes, namely with a Reversible Variable Length Code (RVLC), a Non-Systematic Convolutional (NSC) code a Recursive Systematic Convolutional (RSC) code or a Low Density Parity Check (LDPC) code. These four outer constituent codes are comparatively studied in the context of an MPEG4 videophone transceiver. These serially concatenated schemes are also compared to a stand-alone LDPC coded MPEG4 videophone system at the same effective overall coding rate. The performance of the proposed schemes is evaluated when communicating over uncorrelated Rayleigh fading channels. It was found that the serially concatenated TCM-NSC scheme was the most attractive one in terms of coding gain and decoding complexity among all the schemes considered in the context of the MPEG4 videophone transceiver. By contrast, the serially concatenated TCM-RSC scheme was found to attain the highest iteration gain among the schemes considered
From computation to black holes and space-time foam
We show that quantum mechanics and general relativity limit the speed
of a simple computer (such as a black hole) and its memory space
to \tilde{\nu}^2 I^{-1} \lsim t_P^{-2}, where is the Planck time.
We also show that the life-time of a simple clock and its precision are
similarly limited. These bounds and the holographic bound originate from the
same physics that governs the quantum fluctuations of space-time. We further
show that these physical bounds are realized for black holes, yielding the
correct Hawking black hole lifetime, and that space-time undergoes much larger
quantum fluctuations than conventional wisdom claims -- almost within range of
detection with modern gravitational-wave interferometers.Comment: A misidentification of computer speeds is corrected. Our results for
black hole computation now agree with those given by S. Lloyd. All other
conclusions remain unchange
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