6,510 research outputs found
Empedocles and the Other Physiologists in Aristotle’s Physics II 8
In this paper I propose to show: 1) that in Phys. II 8 Aristotle takes Empedocles as a paradigm for a theoretical position common to all philosophers who preceded him: the view that materialism implies a mechanistic explanation of natural becoming; and 2) that, since Empedocles is regarded as a philosopher who clearly expresses the position of all mechanistic materialists, Aristotle builds his teleological arguments precisely to refute him. Indeed, Aristotle believes that refuting the arguments of Empedocles – the champion of mechanism – means refuting the mechanistic theory itself. In order to illustrate this point, I will discuss some passages from Phys. II 8, while also turning to consider the Neoplatonic commentators on Aristotle’s Physics. I will then endeavour to explain why in 198b19 ff. Aristotle formulates the argument of rain, which has attracted so much attention from scholars of the Physics: I will consider whether Aristotle believes that rain serves a purpose, contrary to what he claims with regard to meteorological phenomena in Meteorologica
Non-linear optics with two trapped atoms
We show theoretically that two atomic dipoles in a resonator constitute a
non-linear medium, whose properties can be controlled through the relative
position of the atoms inside the cavity and the detuning and intensity of the
driving laser. We identify the parameter regime where the system operates as a
parametric amplifier, based on the cascade emission of the collective dipole of
the atoms, and determine the corresponding spectrum of squeezing of the field
at the cavity output. This dynamics could be observed as a result of
self-organization of laser-cooled atoms in resonators.Comment: 11 pages, 8 figure
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The unfolded protein response in neurodegenerative disorders - therapeutic modulation of the PERK pathway.
The unfolded protein response (UPR) is a highly conserved protein quality control mechanism, activated in response to Endoplasmic Reticulum (ER) stress. Signalling is mediated through three branches, PERK, IRE1, and ATF6, respectively, that together provide a coordinated response that contributes to overcoming disrupted proteostasis. PERK branch activation predominantly causes a rapid reduction in global rates of translation, while the IRE1 and ATF6 branch signalling induce a transcriptional response resulting in expression of chaperones and components of the protein degradation machinery. Protein misfolding neurodegenerative diseases show disruption of proteostasis as a biochemical feature. In the brains of animal models of disease and in human post mortem tissue from many of these disorders, markers of UPR induction, particularly, the PERK pathway can be observed in close association with disease progression. Recent research has revealed dysregulated UPR signalling to be a major pathogenic mechanism in neurodegeneration, and that genetic and pharmacological modulation of the PERK pathway results in potent neuroprotection. Targeting aberrant UPR signalling is the focus of new therapeutic strategies, which importantly could be beneficial across the broad spectrum of neurodegenerative diseases
Therapy for prion diseases Insights from the use of RNA interference
Insights into the molecular basis and the temporal evolution of neurotoxicity in prion disease are increasing, and recent work in mice leads to new avenues for targeting treatment of these disorders. Using lentivirally mediated RNA interference (RNAi) against native prion protein (PrP), White et al. report the first therapeutic intervention that results in neuronal rescue, prevents symptoms and increases survival in mice with established prion disease.(1) Both the target and the timing of treatment here are crucial to the effectiveness of this strategy: the formation of the neurotoxic prion agent is prevented at a point when diseased neurons can still be saved from death. But the data also give new insights into the timing of treatment in the context of the pattern of spread of prion infection throughout the brain, with implications for developing the most effective treatments
Quantum quenches of ion Coulomb crystals across structural instabilities
Quenches in an ion chain can create coherent superpositions of motional
states across the linear-zigzag structural transition. The procedure has been
described in [Phys. Rev. A 84, 063821 (2011)] and makes use of spin-dependent
forces, so that a coherent superposition of the electronic states of one ion
evolves into an entangled state between the chain's internal and external
degrees of freedom. The properties of the crystalline state so generated are
theoretically studied by means of Ramsey interferometry on one ion of the
chain. An analytical expression for the visibility of the interferometric
measurement is obtained for a chain of arbitrary number of ions and as a
function of the time elapsed after the quench. Sufficiently close to the
linear-zigzag instability the visibility decays very fast, but exhibits
revivals at the period of oscillation of the mode that drives the structural
instability. These revivals have a periodicity that is independent of the
crystal size, and they signal the creation of entanglement by the quantum
quench.Comment: 14 pages, 8 figures; added a paragraph in the introduction providing
more background, added paragraph at the end of Sec. IV discussing
experimental parameter
High-pressure behavior of high silica ferrierite
Ferrierite (FER framework topology) is a well-known
aluminosilicate zeolite mineral. An understanding of the structure and
properties of FER remains important because of its role as a catalyst in
commercial reactions. For example, it is important in the petrochemical
industry, where it has been used as a shape selective catalyst for
the production of isobutene. The thermal behavior of this phase (in
its high silica form) was recently studied by Bull et al [1], while its
compressibilty has never been investigated before.
The high pressure (HP) behavior of synthetic high silica zeolite
ferrierite (FER) was investigated by means of in-situ synchrotron X-ray
powder diffraction, with the aim to understand the P-induced deformation
mechanism. The microporous material was sinthesized starting from
pure silica and pyridine and propyl-amine as structure directing agents.
Here we report the preliminary results on the compressibility of the as
synthesized phase. The study of the compressibility of the calcined one
will be carried out in the following steps of the project.The crystal structure of ferrierite is built up of rings of fivecornershared
SiO4 tetrahedra (known as five-membered ringsor 5MRs)
building units, which form layers in the ab plane.The layers are
connected to form a matrix of 10MR channels running parallel to the
c axis, which are intersected by 8MR channels running parallel to
the b axis. Six-membered rings connect the 10MRs alongthe c axis
direction.
The HP diffraction experiments were performed at BM01a beamline
(ESRF), at the fixed wavelength of 0.71 Å, using a modified Merril-
Basset DAC and a mixture of methanol- ethanol and water (16:3:1)
as penetrating P-transmitting medium. The powder patterns were
collected from Pamb to 6.2 GPa. Some patterns were also measured
upon pressure release up to Pamb, to check the reversibility of the
compression effects. The unit cell parameters were refined by means
of Rietveld method.
The main results of this study are:
1) No complete X-ray amorphization is observed up to about 6.6
GPa;
2) No abrupt change of the elastic behavior is observed in the whole
pressure range. Between Pamb and 6.2 GPa the reduction of the
cell parameter are 4%, 5% and 6% for a, b and c respectively,
accounting for a volume reduction of about 14 %.
3) The bulk modulus obtained using a second order Birch-Murnaghan
equation of state and data weighted by the uncertainties in P and
V was K0 = 30.1(3) GPa. This compressibility determined in
m.e.w. is one of the highest when compared with other natural and
synthetic zeolites studied with “penetrating” aqueous media [2],
[3].
4) The P-induced effects on as-syntehsized ferrierite structure are
completely reversible
Compressibility behaviour of as-synthesized high-silica Ferrierite.
Ferrierite (FER framework topology) is a well-known aluminosilicate zeolite
mineral. An understanding of the structure and properties of FER remains
important because of its role as a catalyst in commercial reactions. For example,
it is important in the petrochemical industry, where it has been used as a shape
selective catalyst for the production of isobutene. The thermal behavior of this
phase (in its high silica form) was recently studied by Bull et al [1], while its
compressibilty has never been investigated before.
The high pressure (HP) behavior of synthetic high silica zeolite ferrierite (FER)
was investigated by means of in-situ synchrotron X-ray powder diffraction, with
the aim to understand the P-induced deformation mechanism. The microporous
material was synthesized starting from pure silica and pyridine and propyl-amine
as structure directing agents. Here we report the preliminary results on the
compressibility of the as-synthesized phase. The study of the compressibility of
the calcinated one will be carried out in the following steps of the project.
The crystal structure of ferrierite is built up of rings of fivecorner-shared SiO4
tetrahedra (known as five-membered ringsor 5MRs) building units, which form
layers in the ab plane. The layers are connected to form a matrix of 10MR
channels running parallel to the c axis, which are intersected by 8MR channels
running parallel to the b axis. Six-membered rings connect the 10MRs alongthe c
axis direction.
The HP diffraction experiments were performed at BM01a beamline (ESRF), at
the fixed wavelength of 0.71 Å, using a modified Merril-Basset DAC and a
mixture of methanol:ethanol:water (16:3:1) as P-transmitting medium. The
powder patterns were collected from Pamb to 6.2 GPa. Some patterns were also
measured upon pressure release up to Pamb, to check the reversibility of the
compression effects. The unit cell parameters were refined by means of Rietveld
method.
The main results of this study are:
1) No complete X-ray amorphization is observed up to about 6.6 GPa;
2) No abrupt change of the elastic behavior is observed in the whole pressure
range. Between Pamb and 6.2 GPa the reduction of the cell parameter are 4%, 5%
and 6% for a, b and c respectively, accounting for a volume reduction of about
14 %.
3) The P-induced effects on the as-synthesized Si-ferrierite cell parameters are
completely reversible.
4) The bulk modulus obtained using a second order Birch-Murnaghan equation
of state and data weighted by the uncertainties in P and V was K0 = 30.1(3) GPa.
This compressibility is one of the highest when compared with the other natural
and synthetic zeolites studied with “penetrating” aqueous media [2, 3] and is very
similar to that of SAPO-34 [4], another microporous material studied at HP in
its as-synthesized form containing the organic template
High-resolution radio observations of SNR 1987A at high frequencies
We present new imaging observations of the remnant of Supernova (SN) 1987A at
44 GHz, performed in 2011 with the Australia Telescope Compact Array (ATCA).
The 0\farcs35\times0\farcs23 resolution of the diffraction-limited image is
the highest achieved to date in high-dynamic range. We also present a new ATCA
image at 18 GHz derived from 2011 observations, which is super-resolved to
0\farcs25. The flux density is 402 mJy at 44 GHz and 816 mJy at 18
GHz. At both frequencies, the remnant exhibits a ring-like emission with two
prominent lobes, and an east-west brightness asymmetry that peaks on the
eastern lobe. A central feature of fainter emission appears at 44 GHz. A
comparison with previous ATCA observations at 18 and 36 GHz highlights higher
expansion velocities of the remnant eastern side. The 18-44 GHz spectral index
is (). The spectral index map
suggests slightly steeper values at the brightest sites on the eastern lobe,
whereas flatter values are associated with the inner regions. The remnant
morphology at 44 GHz generally matches the structure seen with contemporaneous
X-ray and H observations. Unlike the H emission, both the radio
and X-ray emission peaks on the eastern lobe. The regions of flatter spectral
index align and partially overlap with the optically-visible ejecta. Simple
free-free absorption models suggest that emission from a pulsar wind nebula or
a compact source inside the remnant may now be detectable at high frequencies,
or at low frequencies if there are holes in the ionised component of the
ejecta.Comment: References updated. High resolution version may be found at
http://ict.icrar.org/store/staff/gio/Papers/Zanardo_2013.pd
Prions: generation and spread versus neurotoxicity.
Neurodegenerative diseases are characterized by the aggregation of misfolded proteins in the brain. Among these disorders are the prion diseases, which are transmissible, and in which the misfolded proteins ("prions") are also the infectious agent. Increasingly, it appears that misfolded proteins in Alzheimer and Parkinson diseases and the tauopathies also propagate in a "prion-like" manner. However, the association between prion formation, spread, and neurotoxicity is not clear. Recently, we showed that in prion disease, protein misfolding leads to neurodegeneration through dysregulation of generic proteostatic mechanisms, specifically, the unfolded protein response. Genetic and pharmacological manipulation of the unfolded protein response was neuroprotective despite continuing prion replication, hence dissociating this from neurotoxicity. The data have clear implications for treatment across the spectrum of these disorders, targeting pathogenic processes downstream of protein misfolding
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