695,803 research outputs found
Top-Down Causation and the Rise of Information in the Emergence of Life
abstract: Biological systems represent a unique class of physical systems in how they process and manage information. This suggests that changes in the flow and distribution of information played a prominent role in the origin of life. Here I review and expand on an emerging conceptual framework suggesting that the origin of life may be identified as a transition in causal structure and information flow, and detail some of the implications for understanding the early stages chemical evolution
Strong-Pinning Effects in Low-Temperature Creep: Charge-Density Waves in TaS_3
Nonlinear conduction in the quasi-one dimensional conductor o-TaS_3 has been
studied in the low-temperature region down to 30 mK. It was found that at
temperatures below a few Kelvins the current-voltage (I-V) characteristics
consist of several branches. The temperature evolution of the I-V curve
proceeds through sequential freezing-out of the branches. The origin of each
branch is attributed to a particular strong pinning impurity type.
Similar behavior is expected for other physical systems with collective
transport (spin-density waves, Wigner crystals, vortex lattices in type-II
superconductors etc.) in the presence of strong pinning centers.Comment: 11 pages, 3 ps figures, Revtex, To be published in Phys. Rev. Letters
(1997
A disk census for the nearest group of young stars: Mid-infrared observations of the TW Hydrae Association
A group of young, active stars in the vicinity of TW Hydrae has recently been
identified as a possible physical association with a common origin. Given its
proximity (50 pc), age (10 Myr) and abundance of binary systems,
the TW Hya Association is ideally suited to studies of diversity and evolution
of circumstellar disks. Here we present mid-infrared observations of 15
candidate members of the group, 11 of which have no previous flux measurements
at wavelengths longer than 2m. We report the discovery of a possible
10m excess in CD -337795, which may be due to a circumstellar
disk or a faint, as yet undetected binary companion. Of the other stars, only
TW Hya, HD 98800, Hen 3-600A, and HR 4796A -- all of which were detected by
IRAS -- show excess thermal emission. Our 10m flux measurements for the
remaining members of the Association are consistent with photospheric emission,
allowing us to rule out dusty inner disks. In light of these findings, we
discuss the origin and age of the TW Hya Association as well as implications
for disk evolution timescales.Comment: 10 pages and 1 PostScript figure, accepted for publication in The
Astrophysical Journal Letter
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I - Evolution of Structural and Dynamical Properties
We present deep near-infrared spectra for a sample of 24 quiescent galaxies
in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at
the W. M. Keck Observatory. In conjunction with a similar dataset we obtained
in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic
and structural properties for 80 quiescent galaxies, the largest
homogeneously-selected sample to date spanning 3 Gyr of early cosmic history.
Analysis of our Keck spectra together with measurements derived from associated
HST images reveals increasingly larger stellar velocity dispersions and smaller
sizes to redshifts beyond z~2. By classifying our sample according to Sersic
indices, we find that among disk-like systems the flatter ones show a higher
dynamical to stellar mass ratio compared to their rounder counterparts which we
interpret as evidence for a significant contribution of rotational motion. For
this subset of disk-like systems, we estimate that V/sigma, the ratio of the
circular velocity to the intrinsic velocity dispersion, is a factor of two
larger than for present-day disky quiescent galaxies. We use the velocity
dispersion measurements also to explore the redshift evolution of the dynamical
to stellar mass ratio, and to measure for the first time the physical size
growth rate of individual systems over two distinct redshift ranges, finding a
faster evolution at earlier times. We discuss the physical origin of this
time-dependent growth in size in the context of the associated reduction of the
systematic rotation.Comment: Updated to match the published versio
Interferences in quantum eraser reveal geometric phases in modular and weak values
In this letter, we present a new procedure to determine completely the
complex modular values of arbitrary observables of pre- and post-selected
ensembles, which works experimentally for all measurement strengths and all
post-selected states. This procedure allows us to discuss the physics of
modular and weak values in interferometric experiments involving a qubit meter.
We determine both the modulus and the argument of the modular value for any
measurement strength in a single step, by controlling simultaneously the
visibility and the phase in a quantum eraser interference experiment. Modular
and weak values are closely related. Using entangled qubits for the probed and
meter systems, we show that the phase of the modular and weak values has a
topological origin. This phase is completely defined by the intrinsic physical
properties of the probed system and its time evolution. The physical
significance of this phase can thus be used to evaluate the quantumness of weak
values
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