413 research outputs found
Massless interacting particles
We show that classical electrodynamics of massless charged particles and the
Yang--Mills theory of massless quarks do not experience rearranging their
initial degrees of freedom into dressed particles and radiation. Massless
particles do not radiate. We consider a version of the direct interparticle
action theory for these systems following the general strategy of Wheeler and
Feynman.Comment: LaTeX; 20 pages; V4: discussion is slightly modified to clarify some
important points, relevant references are adde
The fundamental cycle of concept construction underlying various theoretical frameworks
In this paper, the development of mathematical concepts over time is considered. Particular reference is given to the shifting of attention from step-by-step procedures that are performed in time, to symbolism that can be manipulated as mental entities on paper and in the mind. The development is analysed using different theoretical perspectives, including the SOLO model and various theories of concept construction to reveal a fundamental cycle underlying the building of concepts that features widely in different ways of thinking that occurs throughout mathematical learning
Structure and regulation of mammalian S-adenosylmethionine decarboxylase
In order to understand the structure and regulation of S-adenosylmethionine decarboxylase, cDNA clones encoding this enzyme have been isolated from rat prostate and human fibroblast cDNA libraries. The authenticity of the cDNAs was verified by: (a) transfecting the Chinese hamster ovary cells with the human cDNA in the pcD vector which resulted in a transient 10-20-fold increase in S-adenosylmethionine decarboxylase activity in recipient cells; and (b) translating the mRNA formed by transcription of the cDNA insert in a reticulocyte lysate and recording an increase in S-adenosylmethionine decarboxylase activity. The amino acid sequences deduced from the cDNAs indicate that the human proenzyme for this protein contains 334 amino acids and has a molecular weight of 38,331 whereas the rat proenzyme contains 333 amino acid residues. The human and rat enzymes are very similar having only 11 amino acid differences and the cDNAs are also closely related showing over 90% homology in the 1617-nucleotide overlap which was sequenced. A further indication of the highly conserved nature of mammalian S-adenosylmethionine decarboxylases is that the amino acid sequences deduced from the human and the rat cDNAs contained peptide sequences identical to those previously reported for the purified bovine enzyme. In vitro transcription/translation experiments showed that the proenzyme is converted to two polypeptides of molecular weights about 32,000 and 6,000 in a processing reaction which generates the prosthetic pyruvate group and that the final enzyme contains both polypeptides. Two forms of S-adenosylmethionine decarboxylase mRNA (2.1 and about 3.4-3.6 kilobases) are present in human and rodent tissues and may originate from the utilization of two different polyadenylation signals. Southern blots of rat genomic DNA indicated that the S-adenosylmethionine decarboxylase gene belongs to a multigene family. Depletion of cellular polyamines by inhibitors or ornithine decarboxylase or the aminopropyltransferases led to an increase in the content of S-adenosylmethionine decarboxylase protein and mRNA, but the elevation in the mRNA was not sufficient to account for all of the change in the enzyme level, particularly in cells in which spermine was depleted
Evolutionary Multi-Objective Design of SARS-CoV-2 Protease Inhibitor Candidates
Computational drug design based on artificial intelligence is an emerging
research area. At the time of writing this paper, the world suffers from an
outbreak of the coronavirus SARS-CoV-2. A promising way to stop the virus
replication is via protease inhibition. We propose an evolutionary
multi-objective algorithm (EMOA) to design potential protease inhibitors for
SARS-CoV-2's main protease. Based on the SELFIES representation the EMOA
maximizes the binding of candidate ligands to the protein using the docking
tool QuickVina 2, while at the same time taking into account further objectives
like drug-likeliness or the fulfillment of filter constraints. The experimental
part analyzes the evolutionary process and discusses the inhibitor candidates.Comment: 15 pages, 7 figures, submitted to PPSN 202
The Uncertainty Relation in "Which-Way" Experiments: How to Observe Directly the Momentum Transfer using Weak Values
A which-way measurement destroys the twin-slit interference pattern. Bohr
argued that distinguishing between two slits a distance s apart gives the
particle a random momentum transfer \wp of order h/s. This was accepted for
more than 60 years, until Scully, Englert and Walther (SEW) proposed a
which-way scheme that, they claimed, entailed no momentum transfer. Storey,
Tan, Collett and Walls (STCW) in turn proved a theorem that, they claimed,
showed that Bohr was right. This work reviews and extends a recent proposal
[Wiseman, Phys. Lett. A 311, 285 (2003)] to resolve the issue using a
weak-valued probability distribution for momentum transfer, P_wv(\wp). We show
that P_wv(\wp) must be wider than h/6s. However, its moments can still be zero
because P_wv(\wp) is not necessarily positive definite. Nevertheless, it is
measurable in a way understandable to a classical physicist. We introduce a new
measure of spread for P_wv(\wp): half of the unit-confidence interval, and
conjecture that it is never less than h/4s. For an idealized example with
infinitely narrow slits, the moments of P_wv(\wp) and of the momentum
distributions are undefined unless a process of apodization is used. We show
that by considering successively smoother initial wave functions, successively
more moments of both P_wv(\wp) and the momentum distributions become defined.
For this example the moments of P_wv(\wp) are zero, and these are equal to the
changes in the moments of the momentum distribution. We prove that this
relation holds for schemes in which the moments of P_wv(\wp) are non-zero, but
only for the first two moments. We also compare these moments to those of two
other momentum-transfer distributions and \hat{p}_f-\hat{p}_i. We find
agreement between all of these, but again only for the first two moments.Comment: 14 pages, 6 figures, submitted to J. Opt.
Factorizations and Physical Representations
A Hilbert space in M dimensions is shown explicitly to accommodate
representations that reflect the prime numbers decomposition of M.
Representations that exhibit the factorization of M into two relatively prime
numbers: the kq representation (J. Zak, Phys. Today, {\bf 23} (2), 51 (1970)),
and related representations termed representations (together with
their conjugates) are analysed, as well as a representation that exhibits the
complete factorization of M. In this latter representation each quantum number
varies in a subspace that is associated with one of the prime numbers that make
up M
Industrial work placement in higher education: a study of civil engineering student engagement
For civil engineering undergraduates, the opportunity to spend a period of time in formal industrial work placement provides an invaluable learning experience. This paper reviews student engagement with short-term industrial placement and provides analysis of questionnaires (n=174) returned by undergraduates studying civil engineering at four Higher Education Institutes (HEIās) in the West of Scotland. The data captures industrial placement statistics, employability skill-sets and presents brief testimonies from students. Whilst the journey to becoming a professional civil engineer is undoubtedly enhanced by short-term placement clear opportunities exist for HEIās to affect and change existing pedagogical discourse. Commentary is likely to resonate beyond civil engineering and serve as a timely reminder of the need to re-invigorate academia / industry curriculum partnerships
Quantum state engineering assisted by entanglement
We suggest a general scheme for quantum state engineering based on
conditional measurements carried out on entangled twin-beam of radiation.
Realistic detection schemes such as {\sc on/off} photodetection, homodyne
detection and joint measurement of two-mode quadratures are analyzed in
details. Imperfections of the apparatuses, such as nonunit quantum efficiency
and finite resolution, are taken into account. We show that conditional {\sc
on/off} photodetection provides a reliable scheme to verify nonclassicality,
whereas conditional homodyning represents a tunable and robust source of
squeezed light. We also describe optical teleportation as a conditional
measurement, and evaluate the degrading effects of finite amount of
entanglement, decoherence due to losses, and nonunit quantum efficiency.Comment: Some pics with low resolution. Originals at http://www.qubit.i
Polyamine metabolism is involved in adipogenesis of 3T3-L1 cells
Polyamines spermidine and spermine are known to be required for mammalian cell proliferation and for embryonic development. Alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase (ODC) a limiting enzyme of polyamine biosynthesis, depleted the cellular polyamines and prevented triglyceride accumulation and differentiation in 3T3-L1 cells. In this study, to explore the function of polyamines in adipogenesis, we examined the effect of polyamine biosynthesis inhibitors on adipocyte differentiation and lipid accumulation of 3T3-L1 cells. The spermidine synthase inhibitor trans-4-methylcyclohexylamine (MCHA) increased spermine/spermidine ratios, whereas the spermine synthase inhibitor N-(3-aminopropyl)-cyclohexylamine (APCHA) decreased the ratios in the cells. MCHA was found to decrease lipid accumulation and GPDH activity during differentiation, while APCHA increased lipid accumulation and GPDH activity indicating the enhancement of differentiation. The polyamine-acetylating enzyme, spermidine/spermine N1-acetyltransferase (SSAT) activity was increased within a few hours after stimulus for differentiation, and was found to be elevated by APCHA. In mature adipocytes APCHA decreased lipid accumulation while MCHA had the opposite effect. An acetylpolyamine oxidase and spermine oxidase inhibitor MDL72527 or an antioxidant N-acetylcysteine prevented the promoting effect of APCHA on adipogenesis. These results suggest that not only spermine/spermidine ratios but also polyamine catabolic enzyme activity may contribute to adipogenesis
Preparation and control of a cavity-field state through atom-driven field interaction: towards long-lived mesoscopic states
The preparation of mesoscopic states of the radiation and matter fields
through atom-field interactions has been achieved in recent years and employed
for a range of striking applications in quantum optics. Here we present a
technique for the preparation and control of a cavity mode which, besides
interacting with a two-level atom, is simultaneously submitted to linear and
parametric amplification processes. The role of the amplification-controlling
fields in the achievement of real mesoscopic states, is to produce
highly-squeezed field states and, consequently, to increase both: i) the
distance in phase space between the components of the prepared superpositions
and ii) the mean photon number of such superpositions. When submitting the
squeezed superposition states to the action of similarly squeezed reservoirs,
we demonstrate that under specific conditions the decoherence time of the
states becomes independent of both the distance in phase space between their
components and their mean photon number. An explanation is presented to support
this remarkable result, together with a discussion on the experimental
implementation of our proposal. We also show how to produce number states with
fidelities higher than those derived as circular states
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