Role Of Gravity Research Articles

Role of gravity in the collapse of a wave function: A probe into the Diósi-Penrose model

We investigate the Di\'osi-Penrose (DP) proposal for connecting the collapse of the wave function to gravity. The DP model needs a free parameter, acting as a cutoff to regularize the dynamics, and the predictions of the model highly depend on the value of this cutoff. The Compton wavelength of a nucleon seems to be the most reasonable cutoff value since it justifies the nonrelativistic approach. However, with this value, the DP model predicts an unrealistically high rate of energy increase. Thus, either one is forced to choose a much larger cutoff, which is not physically justified and totally arbitrary, or one needs to include dissipative effects in order to tame the energy increase. Taking the analogy with dissipative collisional decoherence seriously, we develop a dissipative generalization of the DP model. We show that even with dissipative effects, the DP model contradicts known physical facts, unless either the cutoff is kept artificially large or one limits the applicability of the model to massive systems. We also provide an estimation for the mass range of this applicability.

Computational analysis of microbubble flows in bifurcating airways: role of gravity, inertia, and surface tension.

Although mechanical ventilation is a life-saving therapy for patients with severe lung disorders, the microbubble flows generated during ventilation generate hydrodynamic stresses, including pressure and shear stress gradients, which damage the pulmonary epithelium. In this study, we used computational fluid dynamics to investigate how gravity, inertia, and surface tension influence both microbubble flow patterns in bifurcating airways and the magnitude/distribution of hydrodynamic stresses on the airway wall. Direct interface tracking and finite element techniques were used to simulate bubble propagation in a two-dimensional (2D) liquid-filled bifurcating airway. Computational solutions of the full incompressible Navier-Stokes equation were used to investigate how inertia, gravity, and surface tension forces as characterized by the Reynolds (Re), Bond (Bo), and Capillary (Ca) numbers influence pressure and shear stress gradients at the airway wall. Gravity had a significant impact on flow patterns and hydrodynamic stress magnitudes where Bo > 1 led to dramatic changes in bubble shape and increased pressure and shear stress gradients in the upper daughter airway. Interestingly, increased pressure gradients near the bifurcation point (i.e., carina) were only elevated during asymmetric bubble splitting. Although changes in pressure gradient magnitudes were generally more sensitive to Ca, under large Re conditions, both Re and Ca significantly altered the pressure gradient magnitude. We conclude that inertia, gravity, and surface tension can all have a significant impact on microbubble flow patterns and hydrodynamic stresses in bifurcating airways.

The Role of Gravity in Periorbital and Midfacial Aging.

With respect to the pathogenesis of periorbital and midfacial aging, gravity may play a greater role than volume loss. The authors determined the effect of shifting from the upright to the supine position on specific attributes of facial appearance and ascertained whether facial appearance in the supine position bore any resemblance to its appearance in youth. Participants who showed signs of midface aging were positioned in the upright and supine positions, and photographs were obtained during smiling and repose. For each photograph, examiners graded the following anatomic parameters, using a standardized scale: brow position, tear trough length and depth, steatoblepharon, cheek volume, malar bags/festoons, and nasolabial folds. Some participants provided photographs of themselves taken 10 to 15 years earlier; these were compared with the study images. Interobserver correlation was strong. When participants were transferred from upright to supine, all anatomic parameters examined became more youthful in appearance; findings were statistically significant. The grading of anatomic parameters of the earlier photographs most closely matched that of current supine photographs of the subjects smiling. In the supine position, as opposed to the upright position, participants with signs of midface aging appear to have much more volume in the periorbita and midface. For the subset of participants who provided photographs obtained 10 to 15 years earlier, the appearance of facial volume was similar between those images and the current supine photographs. This suggests that volume displacement due to gravitational forces plays an integral role in the morphogenesis of midface aging.

Commentary on: The Role of Gravity in Periorbital and Midfacial Aging.

Mally et al1 have examined subjects in an upright and supine position, smiling and in repose. Observing that many of the signs of aging—cheek flattening, prominent nasolabial folds, malar bags, tear troughs, and steatoblepharon—were reduced by lying supine (recumbent) and smiling ( P = .0001), they conclude that the effects of gravity must therefore have caused these aging signs. They contend that because altering the direction of gravity’s pull by 90 (not 180) degrees makes the face look improved, the force of gravity must have created these changes of aging. If the question at hand is the effect of gravity and its reversal on the soft tissues of the midface, then one would expect to compare the upright face with the upside-down face. If so, there is a clear gravitational opposite, a model that might shed some light or at least some informed discussion on the question. By placing the patient in a recumbent position, the force of gravity pulls posteriorly on the face. This is an altogether different proposition. The soft tissues of the face are quite mobile2; a finger can push the skin of the face in ovals of various sizes, regardless of age. In an upright position, the face generally sits at its lowest limits of excursion, except for over the jowl, where there might be a few millimeters of give. When recumbent, the soft tissues slide posteriorly, much as a breast will slump laterally when supine.3 It is no accident that the authors found that the smile as well as recumbency had similar effects on the face. By the vagaries of evolution, the smile muscles have …

Gravity wave signatures in the dip equatorial ionosphere‐thermosphere system during the annular solar eclipse of 15 January 2010

The present work pertains to the eclipse‐induced gravity wave modulations in the ionosphere‐thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde‐derived foF1 and foF2 parameters are used to analyze the characteristics of the eclipse‐induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30–100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave‐induced winds in generating blanketing Es over the equator is also examined.

Gravitational decoherence, alternative quantum theories and semiclassical gravity

In this report we discuss three aspects: 1) Semiclassical gravity theory (SCG): 4 levels of theories describing the interaction of quantum matter with classical gravity. 2) Alternative Quantum Theories: Discerning those which are derivable from general relativity (GR) plus quantum field theory (QFT) from those which are not 3) Gravitational Decoherence: derivation of a master equation and examination of the assumptions which led to the claims of observational possibilities. We list three sets of corresponding problems worthy of pursuit: a) Newton-Schrödinger Equations in relation to SCG; b) Master equation of gravity-induced effects serving as discriminator of 2); and c) Role of gravity in macroscopic quantum phenomena.

Dual role of gravity on the Faraday threshold for immiscible viscous layers

This work discusses the role of gravity on the Faraday instability, and the differences one can expect to observe in a low-gravity experiment when compared to an earth-based system. These differences are discussed in the context of the viscous linear theory for laterally infinite systems, and a surprising result of the analysis is the existence of a crossover frequency where an interface in low gravity switches from being less to more stable than an earth-based system. We propose this crossover exists in all Faraday systems, and the frequency at which it occurs is shown to be strongly influenced by layer height. In presenting these results physical explanations are provided for the behavior of the predicted forcing amplitude thresholds and wave number selection.

Numerical Simulation on Soil Pressure Distribution Characteristics of Gravity Retaining Wall

The anchor plays a very important role in gravity retaining wall. The displacement of retaining and soil pressure distribution with anchor is different from that without anchor. The numerical simulation software FLAC3D is used to analysis the soil pressure distribution characteristics of gravity retaining wall. The results show that the anchor plays a supporting role in gravity retaining wall. There is a critical length in the anchor in gravity retaining wall. The soil pressure distribution of gravity retaining wall with anchor does not conform to the classical Coulomb linear distribution theory and more research is needed for the soil pressure distribution theory. The research has important guiding significance on the design and construction development of gravity retaining wall.

Gel transition in adhesive hard-sphere colloidal dispersions: the role of gravitational effects.

The role of gravity in gelation of adhesive hard spheres is studied and a critical criterion developed for homogeneous gelation within the gas-liquid binodal. We hypothesize that gelation by Brownian diffusion competes with phase separation enhanced by gravitational settling. This competition is characterized by the gravitational Péclet number Pe(g), which is a function of particle size, volume fraction, and gravitational acceleration. Through a systematic variation of the parameters, we observe the critical Pe(g) of ∼ 0.01 can predict the stability of gels composed of adhesive hard spheres.

Does gravity induce wavefunction collapse? An examination of Penrose's conjecture

According to Penrose, the fundamental conflict between the superposition principle of quantum mechanics and the principle of general covariance of general relativity entails the existence of wavefunction collapse, e.g. a quantum superposition of two different space–time geometries will collapse to one of them due to the ill-definedness of the time-translation operator for the superposition. In this paper, we argue that Penrose's conjecture on gravity's role in wavefunction collapse is debatable. First of all, it is still a controversial issue what the exact nature of the conflict is and how to resolve it. Secondly, Penrose's argument by analogy is too weak to establish a necessary connection between wavefunction collapse and the conflict as understood by him. Thirdly, the conflict does not necessarily lead to wavefunction collapse. The reason is that the conflict or the problem of ill-definedness for a superposition of different space–time geometries also needs to be solved before the collapse of the superposition finishes, and once the conflict has been resolved, the wavefunction collapse will lose its physical basis relating to the conflict. In addition, we argue that Penrose's suggestions for the collapse time formula and the preferred basis are also problematic.

A New Approach to the Modeling of a Simple Re-infiltration Gravity Drainage Process in Naturally Fractured Reservoirs

One of the most important concerns regarding prediction of production performance in naturally fractured reservoirs is the issue of re-infiltration phenomena. In this study, the modeling of a simple re-infiltration process with no effect of capillary continuity between blocks is presented by extending the gravity drainage mechanism for a single block. First, a qualitative analysis of the gravity drainage process through porous media was conducted and the role of gravity and capillary forces was investigated. Then a model for one-dimensional gravity drainage in a single block was developed in dimensionless form, a modified version of which can be found in the literature. Then, using the method of separation of variables, the corresponding partial differential equation was solved for a single block with certain boundary and initial conditions. The upper boundary is a no-feed boundary and at the lower boundary the gas saturation is always zero. At the initial condition, the gas saturation is equal to zero throughout the block. In order to model the simple re-infiltration process, the above-mentioned problem for concept was used and, based on the fact that the blocks of a stack governed by simple re-infiltration process act as single block, using a MATLAB program (The MathWorks, Inc., Natick, MA), the rate of drainage and cumulative production as function of time were obtained. The results are in very good agreement with the nature of the process in an actual case. This method can be extended to solve the problem of a stack of N blocks.

Gravity, strings, modular and quasimodular forms

Modular and quasimodular forms have played an important role in gravity and string theory. Eisenstein series have appeared systematically in the determination of spectrums and partition functions, in the description of non-perturbative effects, in higher-order corrections of scalar-field spaces, . . . The latter often appear as gravitational instantons i.e. as special solutions of Einstein’s equations. In the present lecture notes we present a class of such solutions in four dimensions, obtained by requiring (conformal) self-duality and Bianchi IX homogeneity. In this case, a vast range of configurations exist, which exhibit interesting modular properties. Examples of other Einstein spaces, without Bianchi IX symmetry, but with similar features are also given. Finally we discuss the emergence and the role of Eisenstein series in the framework of field and string theory perturbative expansions, and motivate the need for unravelling novel modular structures.

Reaching while standing in microgravity: a new postural solution to oversimplify movement control

Many studies showed that both arm movements and postural control are characterized by strong invariants. Besides, when a movement requires simultaneous control of the hand trajectory and balance maintenance, these two movement components are highly coordinated. It is well known that the focal and postural invariants are individually tightly linked to gravity, much less is known about the role of gravity in their coordination. It is not clear whether the effect of gravity on different movement components is such as to keep a strong movement-posture coordination even in different gravitational conditions or whether gravitational information is necessary for maintaining motor synergism. We thus set out to analyze the movements of eleven standing subjects reaching for a target in front of them beyond arm's length in normal conditions and in microgravity. The results showed that subjects quickly adapted to microgravity and were able to successfully accomplish the task. In contrast to the hand trajectory, the postural strategy was strongly affected by microgravity, so to become incompatible with normo-gravity balance constraints. The distinct effects of gravity on the focal and postural components determined a significant decrease in their reciprocal coordination. This finding suggests that movement-posture coupling is affected by gravity, and thus, it does not represent a unique hardwired and invariant mode of control. Additional kinematic and dynamic analyses suggest that the new motor strategy corresponds to a global oversimplification of movement control, fulfilling the mechanical and sensory constraints of the microgravity environment.

A nanoscale experiment measuring gravity's role in breaking the unitarity of quantum dynamics

Modern, state-of-the-art nanomechanical devices are capable of creating spatial superpositions that are massive enough to begin to experimentally access the quantum to classical crossover, and thus force us to consider the possible ways in which the usual quantum dynamics may be affected. One recent theoretical proposal describes the crossover from unitary quantum mechanics to classical dynamics as a form of spontaneous symmetry breaking. Here, we propose a specific experimental set-up capable of identifying the source of unitarity breaking in such a mechanism. The experiment is aimed specifically at clarifying the role played by gravity, and distinguishes the resulting dynamics from that suggested by alternative scenarios for the quantum to classical crossover. We give both a theoretical description of the expected dynamics, and a discussion of the involved experimental parameter values and the proposed experimental protocol.

Simulation of Two-Dimensional Transverse Laser Cooling of Cesium Beam from Pyramidal Magneto-Optical Trap Atom Funnel

Simulation of two-dimensional (2D) transverse laser cooling of Cs atomic beam from pyramidal magneto-optical trap atom funnel (PMOTAF) conceived for atom lithography is presented. The results show that both the minimum full width at half maximum (FWHM) and the maximum peak value of the spatial profile of the atomic beam occur at the frequency detuning of optical molasses equals to -0.5 Г. Moreover, for each frequency detuning, an increase in the intensity of the optical molasses leads to smaller FWHM and higher peak value. The not negligible role of gravity on the atomic beam of sub-thermal longitudinal velocity along the horizontal direction is that every atomic trajectory possesses a parabolic motion either before or after laser cooling which leads to a noticeable displacement of the peak value at the observation plane with respect to the starting point.

The role of gravity in formation of deep structure of shear zones

Deformation structures which occur in rock masses in horizontal shear zones are studied with application of theoretical tectonophysical methods of analysis, including mathematical simulation of stresses. The paper presents results of such studies for cases with the rheology of geomedium represented by an elasto­cataclastic body. Taking into account that above the yield stress (in this case, not true plastic dislocation, but cataclastic, i.e. fracturing), results of deformation and the morphology of fractures are dependent on loading modes, in the present study it is proposed to consider the gravity stress state (hereafter GSS) as the initial state of stresses, maintaining deviatory components. Equations based on criteria of the theory of plasticity are presented; they provide for calculation of depths of GSS transition from pure elastic deformation to elastocataclastic deformation. It is shown that for hard and consolidated rocks of the upper crust outside fault zones, the creeping mechanism is associated with cataclastic flow, rather than with dislocations in crystals and grains; this predetermines the dependence of deviatory stresses on isotropic pressure and maintenance of such stresses at a specific level in the rock mass. For the object under study, fracturing occurs at the initial phase of loading under the impact of GSS. Fractures continue to develop during horizontal displacement which is quazi­homogeneous through depth and laterally. The formation of the structural ensemble of fractures is finalized after completion of a long­term phase of displacement of blocks of the crystalline base, i.e. after the phase of localized displacement. By theoretical analyses of the evolution of the state of stresses and the morphology of deformation structures, it is revealed that numerous fractures with shear component are present deep in the middle part of the rock mass; such fractures occur both at the initial, gravity loading phase and during phases of uniform and localized horizontal displacement. Fractures with shear components are mainly formed in the upper part and close to the axial, deep part of the profile. The analysis presented in this publication is necessary for correct reconstruction of loading mechanisms of geological objects. It can also be applicable in exploration geology for forecasting of high fracturing areas of specific morphology.

Variability of interrill erosion at low slopes

AbstractNumerous models and risk assessments have been developed in order to estimate soil erosion from agricultural land, with some including estimates of nutrient and contaminant transfer. Many of these models have a slope term as a control over particle transfer, with increased transfer associated with increased slopes. This is based on data collected over a wide range of slopes and using relatively small soil flumes and physical principals, i.e. the role of gravity in splash transport and flow. This study uses laboratory rainfall simulation on a large soil flume to investigate interrill soil erosion of a silt loam under a rainfall intensity of 47 mm h−1 on 3%, 6% and 9% slopes, which are representative of agricultural land in much of northwest Europe. The results show: (1) wide variation in runoff and sediment concentration data from replicate experiments, which indicates the complexities in interrill soil erosion processes; and (2) that at low slopes processes related to surface area connectivity, soil saturation, flow patterns and water depth may dominant over those related to gravity. Consequently, this questions the use of risk assessments and soil erosion models with a dominant slope term when assessing soil erosion from agricultural land at low slopes. Copyright © 2010 John Wiley & Sons, Ltd.

ISM properties in hydrodynamic galaxy simulations: turbulence cascades, cloud formation, role of gravity and feedback

We study the properties of ISM substructure and turbulence in hydrodynamic (AMR) galaxy simulations with resolutions up to 0.8 pc and 5x10^3 Msun. We analyse the power spectrum of the density distribution, and various components of the velocity field. We show that the disk thickness is about the average Jeans scale length, and is mainly regulated by gravitational instabilities. From this scale of energy injection, a turbulence cascade towards small-scale is observed, with almost isotropic small-scale motions. On scales larger than the disk thickness, density waves are observed, but there is also a full range of substructures with chaotic and strongly non-isotropic gas velocity dispersions. The power spectrum of vorticity in an LMC-sized model suggests that an inverse cascade of turbulence might be present, although energy input over a wide range of scales in the coupled gaseous+stellar fluid could also explain this quasi-2D regime on scales larger than the disk scale height. Similar regimes of gas turbulence are also found in massive high-redshift disks with high gas fractions. Disk properties and ISM turbulence appear to be mainly regulated by gravitational processes, both on large scales and inside dense clouds. Star formation feedback is however essential to maintain the ISM in a steady state by balancing a systematic gas dissipation into dense and small clumps. Our galaxy simulations employ a thermal model based on a barotropic Equation of State (EoS) aimed at modelling the equilibrium of gas between various heating and cooling processes. Denser gas is typically colder in this approach, which is shown to correctly reproduce the density structures of a star-forming, turbulent, unstable and cloudy ISM down to scales of a few parsecs.

A Trick of Gravity

It's both surprising and rewarding when an old, standard problem reveals a subtlety that expands its pedagogic value. I realized recently that the role of gravity in the range equation for a projectile is not so simple as first appears. This realization may be completely obvious to others but was quite new to me.

On the role of gravity and shear on inertial particle accelerations in near-wall turbulence

Recent experiments in a turbulent boundary layer by Gerashchenko et al. (J. Fluid Mech., vol. 617, 2008, pp. 255–281) showed that the variance of inertial particle accelerations in the near-wall region increased with increasing particle inertia, contrary to the trend found in homogeneous and isotropic turbulence. This behaviour was attributed to the non-trivial interaction of the inertial particles with both the mean shear and gravity. To investigate this issue, we perform direct numerical simulations of channel flow with suspended inertial particles that are tracked in the Lagrangian frame of reference. Three simulations have been carried out considering (i) fluid particles, (ii) inertial particles with gravity and (iii) inertial particles without gravity. For each set of simulations, three particle response times were examined, corresponding to particle Stokes numbers (in wall units) of 0.9, 1.8 and 11.8. Mean and r.m.s. profiles of particle acceleration computed in the simulation are in qualitative (and in several cases quantitative) agreement with the experimental results, supporting the assumptions made in the simulations. Furthermore, by comparing results from simulations with and without gravity, we are able to isolate and quantify the significant effect of gravitational settling on the phenomenon.