Rebuttal on “anti-gravitation” by S. Hossenfelder

Hossenfelder proposes in [S. Hossenfelder, Phys. Lett. B 636 (2006) 119] and [S. Hossenfelder, gr-qc/0605083] a symmetry between gravitating and anti-gravitating particles by superposing a negative charge to classical relativity. This Letter explicitly shows that the latter proposal does not address any of the difficulties that arise in such context. On the contrary, the “world-lines” of the anti-gravitating particles are defined as observer-dependent entities and no attempt whatsoever to dynamically restore local boost invariance is made. Both papers simply focus on the erroneous construction of a new bundle attached to a preferred timelike vector field, although the author gives the impression that no such preference is needed.     

“Teleparallel” dark energy

Using the “teleparallel” equivalent of General Relativity as the gravitational sector, which is based on torsion instead of curvature, we add a canonical scalar field, allowing for a nonminimal coupling with gravity. Although the minimal case is completely equivalent to standard quintessence, the nonminimal scenario has a richer structure, exhibiting quintessence-like or phantom-like behavior, or experiencing the phantom-divide crossing. The richer structure is manifested in the absence of a conformal transformation to an equivalent minimally-coupled model.     

Photorefractive “camera obscura”

We demonstrate a novel scheme for lensless image formation which combines the properties of an amplifying dynamic hologram and a pinhole camera. The scheme is realized on the base of a SPS:Sb1% photorefractive crystal working at 633 nm.     

On “Comment on Supersymmetry, PT-symmetry and spectral bifurcation”

In “Comment on Supersymmetry, PT-symmetry and spectral bifurcation” [1], Bagchi and Quesne correctly show the presence of a class of states for the complex Scarf-II potential in the unbroken PT-symmetry regime, which were absent in [2]. However, in the spontaneously broken PT-symmetry case, their argument is incorrect since it fails to implement the condition for the potential to be PT-symmetric: C^PT[2(A − B) + α] = 0. It needs to be emphasized that in the models considered in [2], PT is spontaneously broken, implying that the potential is PT-symmetric, whereas the ground state is not. Furthermore, our supersymmetry (SUSY)-based ‘spectral bifurcation’ holds independent of the sl(2) symmetry consideration for a large class of PT-symmetric potentials.     

“q-hat” and jet quenching

We discuss bremsstrahlung rates for a weakly coupled fast jet propagating in dense nuclear matter. A time-dependent radiation rate which smoothly merges with previous limits is introduced, exhibiting a clear transition between two regimes after a traversed length of order one to several fermis. The different physical processes probed in the two regimes are discussed.     

Composition of the “GaAs” quantum dot,grown by droplet epitaxy

Self-assembled strain-free quantum dot (QD) structures were grown on AlGaAs surface by the droplet epitaxal method. The QDs were developed from pure Ga droplets under As pressure. The QDs were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both techniques show that the QDs are very uniform in size and their distribution on the surface is also homogeneous. The high resolution cross-sectional TEM investigation shows perfect lattice matching between the QD and the substrate, and also the faceting of the side walls of QD can be identified exactly by lattice planes. Analytical TEM (elemental mapping by EELS) unambiguously identifies the presence of Al in the QD.     

The “sources” of plasma physics

This paper is the text of the Plasma Science and Applications Committee Prize address given in Santa Fe, NM, on June 7, 1994. The principal thesis is that major advances in the development of the science of plasmas have frequently been triggered by the invention of a new plasma source. Examples are given from the work of many colleagues in basic plasma research. A retrospective of the author's experiments on basic plasma physics, magnetic fusion, and inertial fusion is given, many of these sharing the common theme of transverse electric fields. The author's present and future work concern new plasma sources that are needed for the application of plasma technology to materials processing     

Erratum to “Nonlinear refraction measurements of materials using the moiré deflectometry” [K. Jamshidi-Ghaleh, N. Mansour, Opt. Commun. 234 (2004) 419]

This is an erratum to the paper [K. Jamshidi-Ghaleh, N. Mansour, Opt. Commun. 234 (2004) 419], in which a new method for measuring the nonlinear refractive index of materials using the moiré deflectometry is reported. The paper was published with an error in Eq. (4) and it has propagated through the paper.     

A comment on the paper “Stochastic feedback, nonlinear families of Markov processes, and nonlinear Fokker-Planck equations” by T.D. Frank

The purpose of this comment is to correct mistaken assumptions and claims made in the paper “Stochastic feedback, nonlinear families of Markov processes, and nonlinear Fokker-Planck equations” by T. D. Frank [T.D. Frank, Stochastic feedback, non-linear families of Markov processes, and nonlinear Fokker-Planck equations, Physica A 331 (2004) 391]. Our comment centers on the claims of a “non-linear Markov process” and a “non-linear Fokker-Planck equation.” First, memory in transition densities is misidentified as a Markov process. Second, the paper assumes that one can derive a Fokker-Planck equation from a Chapman-Kolmogorov equation, but no proof was offered that a Chapman-Kolmogorov equation exists for the memory-dependent processes considered. A “non-linear Markov process” is claimed on the basis of a non-linear diffusion pde for a 1-point probability density. We show that, regardless of which initial value problem one may solve for the 1-point density, the resulting stochastic process, defined necessarily by the conditional probabilities (the transition probabilities), is either an ordinary linearly generated Markovian one, or else is a linearly generated non-Markovian process with memory. We provide explicit examples of diffusion coefficients that reflect both the Markovian and the memory-dependent cases. So there is neither a “non-linear Markov process”, nor a “non-linear Fokker-Planck equation” for a conditional probability density. The confusion rampant in the literature arises in part from labeling a non-linear diffusion equation for a 1-point probability density as “non-linear Fokker-Planck,” whereas neither a 1-point density nor an equation of motion for a 1-point density can define a stochastic process. In a closely related context, we point out that Borland misidentified a translation invariant 1-point probability density derived from a non-linear diffusion equation as a conditional probability density. Finally, in the Appendix A we present the theory of Fokker-Planck pdes and Chapman-Kolmogorov equations for stochastic processes with finite memory.     

“Step-up” versus “step-down” scattering asymmetry in the neutralization of H on free-electron vicinal metal surfaces

The charge transfer between H⁻ and a free-electron vicinal metallic surface is studied using a wave-packet propagation method. We apply a statistical Thomas-Fermi-von Weizsäcker model with a local density approximation for the exchange-correlation energy to compute the ground-state electronic structure of the substrate. The long-range image charge effects in the electron transfer are included on a phenomenological level. We obtain the ion-survival probability from a rate equation for a set of realistic scattering trajectories of projectiles that are incident with a kinetic energy of 50 eV. Our calculations reveal a pronounced substrate orientation dependence of the charge transfer dynamics expressed in a “left-right” (or “step-up-step-down”) scattering asymmetry in the final ion-survival probability, which is caused by an enhancement of electron loss on the outgoing part of those ion trajectories which approach steps from below.     

Response to “Comment on paper ‘The bulk modulus and Poisson's ratio of “incompressible” materials’”

The incorrectness of the common assumption that rubbery polymers are incompressible does not preclude its yielding accurate determinations of the elastic modulus for nonlinear deformations.     

Fullerenes in molecular liquids. Solutions in “good” solvents: Another view

Brief review and update information concerning the state of “bare” (unmodified) fullerenes in different solvents, including organosols and hydrosols, is given. The hydrophobic nature of fullerene dispersions in aqueous media is discussed. The possibility of the existence of thermodynamic equilibrium in (fullerene + non-polar solvent) system is questioned. The modern data allow returning to the consideration of C₆₀ (C₇₀, etc.) molecules as colloidal (or sub-colloidal) species, inclined to aggregation. Recent publications support the idea of the solvophobic solvation of fullerene molecules even in “good” solvents. Hence, the solvophobic effect, in concert with the van-der-Waals attraction, seems to be driving forces of permanent (though sometimes very slow) aggregate formation, analogous to coagulation of nano-sized particles of common solvophobic colloidal systems.     

Comments on “Scale-free networks without growth”

In [Y.-B. Xie, T. Zhou, B.-H. Wang, Scale-free networks without growth, Physica A 387 (2008) 1683-1688], a nongrowing scale-free network model has been introduced, which shows that the degree distribution of the model varies from the power-law form to the Poisson form as the free parameter α increases, and indicates that the growth may not be necessary for a scale-free network structure to emerge. However, the model implicitly assumes that self-loops and multiple-links are allowed in the model and counted in the degree distribution. In many real-life networks, such an assumption may not be reasonable. We showed here that the degree distribution of the emergent network does not obey a power-law form if self-loops and multiple-links are allowed in the model but not counted in the degree distribution. We also observed the same result when self-loops and multiple-links are not allowed in the model. Furthermore, we showed that the effect of self-loops and multiple-links on the degree distribution weakens as α increases and even becomes negligible when α is sufficiently large.