27,416 research outputs found
Structure and screening in molecular and metallic hydrogen at high pressure
A variational wavefunction is used to express the (spin restricted) Hartree-Fock energy as reciprocal lattice sums for static lattice FCC monatomic hydrogen and diatomic Pa3 molecular hydrogen. In the monatomic phase the hydrogenic orbital range closely parallels the inverse Thomas-Fermi wavevector; the corresponding energy E has a minimum of -0.929 Ryd/electron at r sub s = 1.67. For the diatomic phase E(r sub s) is similar, but the constituent energies, screening, and bond length reflect a qualitative change in the nature of the solid at r sub s = 2.8. This change is interpreted in terms of a transition from protons as structural units (at high density) to weakly interacting models (at low density). Insensitivity of the total energy to a rapid fall in the bond length suggests association with the rotational transition where the rapid molecular orientations characteristic of high pressures disappear and the molecules rotate freely at low pressure
Are Vision Transformers More Data Hungry Than Newborn Visual Systems?
Vision transformers (ViTs) are top performing models on many computer vision
benchmarks and can accurately predict human behavior on object recognition
tasks. However, researchers question the value of using ViTs as models of
biological learning because ViTs are thought to be more data hungry than
brains, with ViTs requiring more training data to reach similar levels of
performance. To test this assumption, we directly compared the learning
abilities of ViTs and animals, by performing parallel controlled rearing
experiments on ViTs and newborn chicks. We first raised chicks in impoverished
visual environments containing a single object, then simulated the training
data available in those environments by building virtual animal chambers in a
video game engine. We recorded the first-person images acquired by agents
moving through the virtual chambers and used those images to train self
supervised ViTs that leverage time as a teaching signal, akin to biological
visual systems. When ViTs were trained through the eyes of newborn chicks, the
ViTs solved the same view invariant object recognition tasks as the chicks.
Thus, ViTs were not more data hungry than newborn visual systems: both learned
view invariant object representations in impoverished visual environments. The
flexible and generic attention based learning mechanism in ViTs combined with
the embodied data streams available to newborn animals appears sufficient to
drive the development of animal-like object recognition.Comment: Accepted in Thirty-seventh Conference on Neural Information
Processing Systems (NeurIPS 2023
Photometry of SN 2002ic and Implications for the Progenitor Mass-Loss History
We present new pre-maximum and late-time optical photometry of the Type
Ia/IIn supernova 2002ic. These observations are combined with the published
V-band magnitudes of Hamuy et al. (2003) and the VLT spectrophotometry of Wang
et al. (2004) to construct the most extensive light curve to date of this
unusual supernova. The observed flux at late time is significantly higher
relative to the flux at maximum than that of any other observed Type Ia
supernova and continues to fade very slowly a year after explosion. Our
analysis of the light curve suggests that a non-Type Ia supernova component
becomes prominent days after explosion. Modeling of the non-Type Ia
supernova component as heating from the shock interaction of the supernova
ejecta with pre-existing circumstellar material suggests the presence of a
cm gap or trough between the progenitor system and the
surrounding circumstellar material. This gap could be due to significantly
lower mass-loss years prior to explosion or
evacuation of the circumstellar material by a low-density fast wind. The latter
is consistent with observed properties of proto-planetary nebulae and with
models of white-dwarf + asymptotic giant branch star progenitor systems with
the asymptotic giant branch star in the proto-planetary nebula phase.Comment: accepted for publication in Ap
A newborn embodied Turing test for view-invariant object recognition
Recent progress in artificial intelligence has renewed interest in building
machines that learn like animals. Almost all of the work comparing learning
across biological and artificial systems comes from studies where animals and
machines received different training data, obscuring whether differences
between animals and machines emerged from differences in learning mechanisms
versus training data. We present an experimental approach-a "newborn embodied
Turing Test"-that allows newborn animals and machines to be raised in the same
environments and tested with the same tasks, permitting direct comparison of
their learning abilities. To make this platform, we first collected
controlled-rearing data from newborn chicks, then performed "digital twin"
experiments in which machines were raised in virtual environments that mimicked
the rearing conditions of the chicks. We found that (1) machines (deep
reinforcement learning agents with intrinsic motivation) can spontaneously
develop visually guided preference behavior, akin to imprinting in newborn
chicks, and (2) machines are still far from newborn-level performance on object
recognition tasks. Almost all of the chicks developed view-invariant object
recognition, whereas the machines tended to develop view-dependent recognition.
The learning outcomes were also far more constrained in the chicks versus
machines. Ultimately, we anticipate that this approach will help researchers
develop embodied AI systems that learn like newborn animals.Comment: 7 Pages. 4 figures, 1 table. This paper was accepted to the CogSci
2023 Conference. (https://cognitivesciencesociety.org/
Light Nuclei as Quantized Skyrmions
We consider the rigid body quantization of Skyrmions with topological charges
1 to 8, as approximated by the rational map ansatz. Novel, general expressions
for the elements of the inertia tensors, in terms of the approximating rational
map, are presented and are used to determine the kinetic energy contribution to
the total energy of the ground and excited states of the quantized Skyrmions.
Our results are compared to the experimentally determined energy levels of the
corresponding nuclei, and the energies and spins of a few as yet unobserved
states are predicted.Comment: 33 pages, 16 figures, Section 13 replace
Feasibility investigation of integrated optics Fourier transform devices
The possibility of producing an integrated optics data processing device based upon Fourier transformations or other parallel processing techniques, and the ways in which such techniques may be used to upgrade the performance of present and projected NASA systems were investigated. Activities toward this goal include; (1) production of near-diffraction-limited geodesic lenses in glass waveguides; (2) development of grinding and polishing techniques for the production of geodesic lenses in LiNbO3 waveguides; (3) development of a characterization technique for waveguide lenses; and (4) development of a theory for corrected aspheric geodesic lenses. A holographic subtraction system was devised which should be capable of rapid on-board preprocessing of a large number of parallel data channels. The principle involved is validated in three demonstrations
X-ray Light Curves and Accretion Disk Structure of EX Hydrae
We present X-ray light curves for the cataclysmic variable EX Hydrae obtained
with the Chandra High Energy Transmission Grating Spectrometer and the Extreme
Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the
shape and amplitude of the binary light curve and discuss a new feature: the
phase of the minimum in the binary light curve, associated with absorption by
the bulge on the accretion disk, increases with wavelength. We discuss several
scenarios that could account for this trend and conclude that, most likely, the
ionization state of the bulge gas is not constant, but rather decreases with
binary phase. We also conclude that photoionization of the bulge by radiation
originating from the white dwarf is not the main source of ionization, but that
it is heated by shocks originating from the interaction between the inflowing
material from the companion and the accretion disk. The findings in this paper
provide a strong test for accretion disk models in close binary systems.Comment: 19 pages, 4 figures, accepted for publication in the Ap
Anisotropic charge transport in non-polar GaN QW: polarization induced charge and interface roughness scattering
Charge transport in GaN quantum well (QW) devices grown in non-polar
direction has been theoretically investigated . Emergence of anisotropic line
charge scattering mechanism originating as a result of anisotropic rough
surface morphology in conjunction with in-plane built-in polarization has been
proposed. It has shown that in-plane growth anisotropy leads to large
anisotropic carrier transport at low temperatures. At high temperatures, this
anisotropy in charge transport is partially washed out by strong isotropic
optical phonon scattering in GaN QW.Comment: 4 pages, 4 figure
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