On “Conformal spinor geometry”: An attempt to “Understand” internal symmetry

The natural homomorphism of pure spinors corresponding to a given Clifford algebraC _2n to polarized isotropicn-planes of complex Euclidean spaceE _2n ^c is taken as a starting point for the construction of a geometry called spinor geometry where pure spinors are the only elements out of which all tensors have to be constructed (analytically as bilinear polynomials of the components of a pure spinor).C ₄ andC ₆ spinor geometry are analyzed, but it seems that C₈ spinor geometry is necessary to construct Minkowski spaceM ^3,1.C ₆ spinor field equations give rise in Minkowski space to a pair of Dirac equations (for conformal semispinors) presenting ansu(2) internal symmetry algebra. Mass is generated by breaking spontaneously the originalO(4,2) symmetry of the spinor equation.Invited talk presented at the International Symposium Selected Topics in Quantum Field Theory and Mathematical Physics, Bechyn, Czechoslovakia, June 14–19, 1981.     

A class of 6D conformal field theories: relation to 4D superconformal Yang-Mills theories?

We describe a class of six-dimensional conformal field theories that possibly are related to the tensionless string theories. They have an ADE classification, but no other discrete or continuous parameters, with the A(N-1) theory arising by factoring out the collective "center of mass" degrees of freedom from N noninteracting chiral two-forms. The Hilbert space carries an irreducible representation of the same Heisenberg group that appears in the tensionless string theories, and the "Wilson surface" observables obey the same superselection rules. When compactified on a two-torus, our theories have the same behavior under S duality as N = 4 super Yang-Mills theories.     

The BRST cohomology of the NSR string: Vanishing and “no-ghost” theorems

We investigate the BRST cohomology of the NSR string. We prove vanishing theorems for the full and relative (sub)complexes generalizing the work of Frenkel, Garland and Zuckerman for the bosonic string. Using these results we give simple proofs of the no-ghost theorems for both sectors.     

“Buzz-saw” noise: Prediction of the rotor-alone pressure field

Public expectations of lower environmental noise levels, and increasingly stringent legislative limits on aircraft noise, result in noise being a critical technical issue in the development of jet engines. Noise at take-off, when the engines are at high-power operating conditions, is a key reference level for engine noise certification. “Buzz-saw” noise is the dominant fan tone noise from modern high-bypass-ratio turbofan aircraft engines during take-off. Rotor-alone tones are the key component of buzz-saw noise. The rotor-alone pressure field is cut-off at subsonic fan tip speeds; buzz-saw noise is associated with supersonic fan tip speeds, or equivalently, high power engine operating conditions. A recent series of papers has described new work concerning the prediction of buzz-saw noise. The prediction method is based on modelling the nonlinear propagation of one-dimensional sawtooth waveforms. A sawtooth waveform is a simplified representation of the rotor-alone pressure field. Previous validation of the prediction method focussed entirely on reproducing the spectral characteristics of buzz-saw noise; this was dictated at that time by the availability of spectral data only for comparison between measurement and prediction. In this paper, full validation of the method by comparing measurement and prediction of the rotor-alone pressure field is published for the first time. It is shown that results from the modelling based on a one-dimensional sawtooth waveform capture the essential features of the rotor-alone pressure field as it propagates upstream inside a hard-walled inlet duct. This verifies that predictions of the buzz-saw noise spectrum, which are in good agreement with the measured data, are based on a model which reproduces the key physics of the noise generation process. Validation results for the rotor-alone pressure field in an acoustically lined inlet duct are also shown. Comparisons of the measured and predicted rotor-alone pressure field are more difficult to interpret because the acoustic lining significantly modifies the sawtooth waveform, but there remains good agreement with the measured spectral data. The buzz-saw noise prediction code used to generate the simulations in this paper has been used by the Rolls–Royce Noise Department since 2004.     

An X-ray absorption study of the electric field effect mechanism in “123” cuprates

X-ray absorption spectroscopy in the presence of an electric field has been used to study the doping mechanism of the CuO₂ planes in Nd_1+xBa_2-xCu₃O₇ compounds. The electric field effect doping is well-known as a method to modify the electrical properties of a thin film using an external gate voltage and in the copper based high critical temperature superconductors (HTS), it has been used to shift the critical temperature and even to induce phase transitions. Field effect experiments in ultra-thin HTS are usually interpreted by supposing that the induced charges develop into carriers in the CuO₂ conducting planes, thus changing the filling of the Zhang-Rice (ZR) band. Here we show that the polarization charges in both insulating and superconducting films, are mainly confined in the charge reservoir, and in particular in the CuO chains. The characteristics of the charge reservoir layer determine the doping of the CuO₂ planes, achieved by transfer of a fraction of the total injected charges. Moreover we found that holes injection in the CuO₂ planes is reduced in oxygen deficient NdBCO films.     

An Invariant Formulation of Special Relativity, or the “True Transformations Relativity,” and Comparison with Experiments

Different formulations of special relativity (SR) are briefly theoretically discussed. In the first formulation SR is understood as the theory of a four-dimensional (4D) spacetime with the pseudo-Euclidean geometry. It is an invariant formulation of SR, which we call the true transformations (TT) relativity. There a physical quantity in the 4D spacetime is mathematically represented either by a true tensor (when no basis has been introduced) or equivalently by a coordinate-based geometric quantity comprising both components and a basis (when some basis has been introduced). This invariant formulation is compared with the usual covariant formulation, which mainly deals with the basis components of tensors in a specific, i.e., Einstein's coordinatization of the chosen inertial frame of reference. The third formulation is the usual noncovariant approach to SR in which some quantities are not tensor quantities, but rather quantities from 3+1 space and time, e.g., the synchronously determined spatial length. This formulation is called the apparent transformations (AT) relativity. Some of the well-known experiments: the muon experiment, the Michelson–Morley type experiments, the Kennedy–Thorndike type experiments, and the Ives–Stilwell type experiments are analyzed using the nonrelativistic theory and the mentioned different formulations of SR. It is shown that all the experiments (when they are complete from the TT relativity viewpoint) are in agreement with the TT relativity but not always with the AT relativity.     

An introduction to the statistical physics of active matter: motility-induced phase separation and the “generic instability” of active gels

In this work we review some statistical physics techniques to coarse grain active matter systems, writing down a set of continuum fields which track the evolution of macroscopic fields such as density, momentum, etc. While the method can be applied in general, we will focus here on two simple and by now well-studied, active matter examples. First, we will consider motility-induced phase separation, the phenomenon by which a concentrated suspension of self-propelled particles spontaneously separates into a dense and a dilute phase. Second, we will review the so-called “generic instability” of active gels, which refers to the nonequilibrium phase transition between a quiescent and a spontaneously flowing phase in a concentrated suspension of rodlike active particles. For both these cases, we also outline recent developments in the literature.     

Energetics: An emerging frontier in cellular mechanosensing: Reply to comments on “Cellular mechanosensing of the biophysical microenvironment: A review of mathematical models of biophysical regulation of cell responses”

How do cells can sense the substrate stiffness? Our recent review highlighted a range of theoretical models and simulations that have been proposed to answer this important question. In response to this review, three leading groups in the field noted some important omissions not only from our review itself but also from the field. These groups noted, correctly, that much of our understanding of cellular mechanosensing arises from models that take advantage of equilibrium thermodynamics, and that this is inappropriate because living cells are never in thermodynamic equilibrium. In this response, we highlight some promising research aimed at resolving this conundrum.     

Erratum to: “The influence of higher spatial harmonics of atomic polarization on the saturated absorption resonance upon excitation of open dipole transitions by a field of counterpropagating waves”

Optics and Spectroscopy -