234 research outputs found
Sophisticated Inference.
Active inference offers a first principle account of sentient behavior, from which special and important cases-for example, reinforcement learning, active learning, Bayes optimal inference, Bayes optimal design-can be derived. Active inference finesses the exploitation-exploration dilemma in relation to prior preferences by placing information gain on the same footing as reward or value. In brief, active inference replaces value functions with functionals of (Bayesian) beliefs, in the form of an expected (variational) free energy. In this letter, we consider a sophisticated kind of active inference using a recursive form of expected free energy. Sophistication describes the degree to which an agent has beliefs about beliefs. We consider agents with beliefs about the counterfactual consequences of action for states of affairs and beliefs about those latent states. In other words, we move from simply considering beliefs about "what would happen if I did that" to "what I would believe about what would happen if I did that." The recursive form of the free energy functional effectively implements a deep tree search over actions and outcomes in the future. Crucially, this search is over sequences of belief states as opposed to states per se. We illustrate the competence of this scheme using numerical simulations of deep decision problems
Observational signatures of disk and jet misalignment in images of accreting black holes
Black hole accretion is one of nature's most efficient energy extraction processes. When gas falls in, a significant fraction of its gravitational binding energy is either converted into radiation or flows outwards in the form of black hole-driven jets and disk-driven winds. Recently, the Event Horizon Telescope (EHT), an Earth-size sub-millimetre radio interferometer, captured the first images of M87's black hole. These images were analysed and interpreted using general-relativistic magnetohydrodynamics (GRMHD) models of accretion disks with rotation axes aligned with the black hole spin axis. However, since infalling gas is often insensitive to the black hole spin direction, misalignment between accretion disk and black hole spin may be a common occurrence in nature. In this work, we use the general-relativistic radiative transfer (GRRT) code \texttt{BHOSS} to calculate the first synthetic radio images of (highly) tilted disk/jet models generated by our GPU-accelerated GRMHD code \texttt{HAMR}. While the tilt does not have a noticeable effect on the system dynamics beyond a few tens of gravitational radii from the black hole, the warping of the disk and jet can imprint observable signatures in EHT images on smaller scales. Comparing the images from our GRMHD models to the 43 GHz and 230 GHz EHT images of M87, we find that M87 may feature a tilted disk/jet system. Further, tilted disks and jets display significant time variability in the 230 GHz flux that can be further tested by longer-duration EHT observations of M87
H-AMR: A New GPU-accelerated GRMHD Code for Exascale Computing With 3D Adaptive Mesh Refinement and Local Adaptive Time-stepping
General-relativistic magnetohydrodynamic (GRMHD) simulations have
revolutionized our understanding of black-hole accretion. Here, we present a
GPU-accelerated GRMHD code H-AMR with multi-faceted optimizations that,
collectively, accelerate computation by 2-5 orders of magnitude for a wide
range of applications. Firstly, it involves a novel implementation of a
spherical-polar grid with 3D adaptive mesh refinement that operates in each of
the 3 dimensions independently. This allows us to circumvent the Courant
condition near the polar singularity, which otherwise cripples high-res
computational performance. Secondly, we demonstrate that local adaptive
time-stepping (LAT) on a logarithmic spherical-polar grid accelerates
computation by a factor of compared to traditional hierarchical
time-stepping approaches. Jointly, these unique features lead to an effective
speed of zone-cycles-per-second-per-node on 5,400 NVIDIA V100 GPUs
(i.e., 900 nodes of the OLCF Summit supercomputer). We demonstrate its
computational performance by presenting the first GRMHD simulation of a tilted
thin accretion disk threaded by a toroidal magnetic field around a rapidly
spinning black hole. With an effective resolution of
,,, cells, and a total of billion
cells and timesteps, it is among the largest astrophysical
simulations ever performed. We find that frame-dragging by the black hole tears
up the disk into two independently precessing sub-disks. The innermost sub-disk
rotation axis intermittently aligns with the black hole spin, demonstrating for
the first time that such long-sought alignment is possible in the absence of
large-scale poloidal magnetic fields.Comment: 10 pages, 5 figures, submitted to MNRAS, for the YouTube playlist,
see https://youtu.be/rIOjKUfzcv
From Generative Models to Generative Passages: A Computational Approach to (Neuro) Phenomenology
This paper presents a version of neurophenomenology based on generative modelling techniques developed in computational neuroscience and biology. Our approach can be described as computational phenomenology because it applies methods originally developed in computational modelling to provide a formal model of the descriptions of lived experience in the phenomenological tradition of philosophy (e.g., the work of Edmund Husserl, Maurice Merleau-Ponty, etc.). The first section presents a brief review of the overall project to naturalize phenomenology. The second section presents and evaluates philosophical objections to that project and situates our version of computational phenomenology with respect to these projects. The third section reviews the generative modelling framework. The final section presents our approach in detail. We conclude by discussing how our approach differs from previous attempts to use generative modelling to help understand consciousness. In summary, we describe a version of computational phenomenology which uses generative modelling to construct a computational model of the inferential or interpretive processes that best explain this or that kind of lived experience
Templated Quasicrystalline Molecular Ordering
Quasicrystals are materials with long-range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on 5-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently separated Fe atoms on icosahedral Al−Cu−Fe to form a unique quasicrystalline lattice, whereas further C60 molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at 10-fold symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag−In−Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a templat
Corridors of barchan dunes: stability and size selection
Barchans are crescentic dunes propagating on a solid ground. They form dune
fields in the shape of elongated corridors in which the size and spacing
between dunes are rather well selected. We show that even very realistic models
for solitary dunes do not reproduce these corridors. Instead, two instabilities
take place. First, barchans receive a sand flux at their back proportional to
their width while the sand escapes only from their horns. Large dunes
proportionally capture more than they loose sand, while the situation is
reversed for small ones: therefore, solitary dunes cannot remain in a steady
state. Second, the propagation speed of dunes decreases with the size of the
dune: this leads -- through the collision process -- to a coarsening of barchan
fields. We show that these phenomena are not specific to the model, but result
from general and robust mechanisms. The length scales needed for these
instabilities to develop are derived and discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further
- yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E.
Pictures of better quality available on reques
The structural analysis of Cu(111)-Te (√3 × √3) R30° and (2√3 × 2√3)R30° surface phases by quantitative LEED and DFT,
The chemisorption of tellurium on atomically clean Cu(111) surface has been studied under ultra-high vacuum conditions. At room temperature, the initial stage of growth was an ordered 23×23R30° phase (0.08 ML). An ordered 3×3R30° phase is formed at 0.33 ML coverage of Te. The adsorption sites of the Te atoms on the Cu(111) surface at 0.08 ML and 0.33 ML coverages are explored by quantitative low energy electron diffraction (LEED) and density functional theory (DFT). Our results indicate that substitutional surface alloy formation starts at very low coverages
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