1,476 research outputs found

### The Height of a Giraffe

A minor modification of the arguments of Press and Lightman leads to an
estimate of the height of the tallest running, breathing organism on a
habitable planet as the Bohr radius multiplied by the three-tenths power of the
ratio of the electrical to gravitational forces between two protons (rather
than the one-quarter power that Press got for the largest animal that would not
break in falling over, after making an assumption of unreasonable brittleness).
My new estimate gives a height of about 3.6 meters rather than Press's original
estimate of about 2.6 cm. It also implies that the number of atoms in the
tallest runner is very roughly of the order of the nine-tenths power of the
ratio of the electrical to gravitational forces between two protons, which is
about 3 x 10^32.Comment: 12 pages, LaTe

### Numerical computation of real or complex elliptic integrals

Algorithms for numerical computation of symmetric elliptic integrals of all
three kinds are improved in several ways and extended to complex values of the
variables (with some restrictions in the case of the integral of the third
kind). Numerical check values, consistency checks, and relations to Legendre's
integrals and Bulirsch's integrals are included

### Matrix product representation of gauge invariant states in a Z_2 lattice gauge theory

The Gauss law needs to be imposed on quantum states to guarantee gauge
invariance when one studies gauge theory in hamiltonian formalism. In this
work, we propose an efficient variational method based on the matrix product
ansatz for a Z_2 lattice gauge theory on a spatial ladder chain. Gauge
invariant low-lying states are identified by evaluating expectation values of
the Gauss law operator after numerical diagonalization of the gauge
hamiltonian.Comment: 15 pages, 6 figures, minor corrections, accepted for publication in
JHE

Recommended from our members

### Customized design of hearing aids using statistical shape learning

3D shape modeling is a crucial component of rapid prototyping systems
that customize shapes of implants and prosthetic devices to a patientâs
anatomy. In this paper, we present a solution to the problem of customized 3D
shape modeling using a statistical shape analysis framework. We design a novel
method to learn the relationship between two classes of shapes, which are related
by certain operations or transformation. The two associated shape classes are
represented in a lower dimensional manifold, and the reduced set of parameters
obtained in this subspace is utilized in an estimation, which is exemplified by a
multivariate regression in this paper.We demonstrate our method with a felicitous
application to estimation of customized hearing aid devices

### Superradiant instabilities of rotating black branes and strings

Black branes and strings are generally unstable against a certain sector of
gravitational perturbations. This is known as the Gregory-Laflamme instability.
It has been recently argued that there exists another general instability
affecting many rotating extended black objects. This instability is in a sense
universal, in that it is triggered by any massless field, and not just
gravitational perturbations. Here we investigate this novel mechanism in
detail. For this instability to work, two ingredients are necessary: (i) an
ergo-region, which gives rise to superradiant amplification of waves, and (ii)
``bound'' states in the effective potential governing the evolution of the
particular mode under study. We show that the black brane Kerr_4 x R^p is
unstable against this mechanism, and we present numerical results for
instability timescales for this case. On the other hand, and quite
surprisingly, black branes of the form Kerr_d x R^p are all stable against this
mechanism for d>4. This is quite an unexpected result, and it stems from the
fact that there are no stable circular orbits in higher dimensional black hole
spacetimes, or in a wave picture, that there are no bound states in the
effective potential. We also show that it is quite easy to simulate this
instability in the laboratory with acoustic black branes.Comment: 19 pages, 10 figures. v2: Enlarged discussion on the necessary
conditions for the existence of instabilit

### Critical Exponents and Stability at the Black Hole Threshold for a Complex Scalar Field

This paper continues a study on Choptuik scaling in gravitational collapse of
a complex scalar field at the threshold for black hole formation. We perform a
linear perturbation analysis of the previously derived complex critical
solution, and calculate the critical exponent for black hole mass, $\gamma
\approx 0.387106$. We also show that this critical solution is unstable via a
growing oscillatory mode.Comment: 15 pages of latex/revtex; added details of numerics, in press in Phys
Rev D; 1 figure included, or available by anonymous ftp to
ftp://ftp.itp.ucsb.edu/figures/nsf-itp-95-58.ep

### Towards the realistic fermion masses with a single family in extra dimensions

In a class of multidimensional models, topology of a thick brane provides
three chiral fermionic families with hierarchical masses and mixings in the
effective four-dimensional theory, while the full model contains a single
vector-like generation. We carry out numerical simulations and reproduce all
known Standard Model fermion masses and mixings in one of these models.Comment: 12 pages, 2 figures, uses JHEP3.cls. Some minor corrections are mad

### Is the present expansion of the universe really accelerating?

The current observations are usually explained by an accelerating expansion
of the present universe. However, with the present quality of the supernovae Ia
data, the allowed parameter space is wide enough to accommodate the
decelerating models as well. This is shown by considering a particular example
of the dark energy equation-of-state $w_\phi\equiv p_\phi/\rho_\phi=-1/3$,
which is equivalent to modifying the \emph{geometrical curvature} index $k$ of
the standard cosmology by shifting it to $(k-\alpha)$ where $\alpha$ is a
constant. The resulting decelerating model is consistent with the recent CMB
observations made by WMAP, as well as, with the high redshift supernovae Ia
data including SN 1997ff at $z= 1.755$. It is also consistent with the newly
discovered supernovae SN 2002dc at $z=0.475$ and SN 2002dd at $z=0.95$ which
have a general tendency to improve the fit.Comment: Replaced with the accepted version to appear in MNRA

### Choosing integration points for QCD calculations by numerical integration

I discuss how to sample the space of parton momenta in order to best perform
the numerical integrations that lead to a calculation of three jet cross
sections and similar observables in electron-positron annihilation.Comment: 25 pages with 8 figure

### Instabilities of Twisted Strings

A linear stability analysis of twisted flux-tubes (strings) in an SU(2)
semilocal theory -- an Abelian-Higgs model with two charged scalar fields with
a global SU(2) symmetry -- is carried out. Here the twist refers to a relative
phase between the two complex scalars (with linear dependence on, say, the $z$
coordinate), and importantly it leads to a global current flowing along the the
string. Such twisted strings bifurcate with the Abrikosov-Nielsen-Olesen (ANO)
solution embedded in the semilocal theory. Our numerical investigations of the
small fluctuation spectrum confirm previous results that twisted strings
exhibit instabilities whose amplitudes grow exponentially in time. More
precisely twisted strings with a single magnetic flux quantum admit a
continuous family of unstable eigenmodes with harmonic $z$ dependence, indexed
by a wavenumber $k\in[-k_{\rm m},k_{\rm m}]$. Carrying out a perturbative
semi-analytic analysis of the bifurcation, it is found that the purely
numerical results are very well reproduced. This way one obtains not only a
good qualitative description of the twisted solutions themselves as well as of
their instabilities, but also a quantitative description of the numerical
results. Our semi-analytic results indicate that in close analogy to the known
instability of the embedded ANO vortex a twisted string is also likely to
expand in size caused by the spreading out of its magnetic flux.Comment: 27 pages, 18 figures. Typos corrected, references adde

- âŠ