SEARCH · PubMed Health

Results for “GRAVITATION”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6Linked to original sources

PubMed1980

[Effect of weightlessness and artificial gravitation on morphological manifestations of the adrenal cortex reaction in rats after space flight on board the biosatellite "Cosmos-936"].

Comparing the changes revealed in the adrenals of rats subjected to weightlessness and artificial gravitation, it has been stated that rotation of the animals in the centrifuge, while flying (with an acceleration of lg), prevents development of morphological stress reaction as a response to landing. After the effect of weightlessness, return to normal gravitation is accompanied with definite signs of an acute stress-reaction in the rat adrenal cortex. Differences in the adrenal reaction of the animals of the groups compared speak in favour of the fact that application of artificial gravitation in an orbital space flight contributes to level the changes resulting from weightlessness and, hence, increase the organism's stability to stress effects, while returning to the Earth.

Adrenal Cortex↗

PubMed1977

[Stimulation of post-traumatic regeneration of rat spleen under conditions of gravitational stress].

Post-traumatic regeneration of the rat spleen was studied after resection of half the organ, under gravitation overloading (11 units) using spleen tissue extract prepared by Filatov's method. Gravitation overloading caused a decrease in the size of the spleen nodules, smooths their contours, increases the red pulp infiltration by the lymphocytes, reduces the number of labeled cells and the intensity of the label in the reactive centers of the spleen nodules, decelerates the capsule formation in the resection area. Application of the stimulant normalized the structure of white pulp, increased the number of labeled cells, and accelerated the capsule formation. The tissue extract used in gravitation overloading brought the restoration process nearer to the usual course of the posttraumatic spleen regeneration (by the character and periods of tissue differentiation development).

Animals↗

PubMed1997

Gravitational Settling in the Electrophoresis of Spheroids with Small Potentials in Quiescent Fluid

Electrophoretic motion is analyzed for non-neutrally buoyant, spheroidal particles settling in an unbounded fluid otherwise at rest. The double-layer thickness is arbitrary, and the surface potential of the spheroid is assumed to be small so that the deformation of the double-layer is neglected. Examples are presented for spheroids when the applied electric field is perpendicular to the gravity. The electrophoretic motions are determined for spheroids of arbitrary aspect ratios at arbitrary orientations. Depending on their orientations the gravitational drift can contribute up to 10% of the observed particle velocity in the direction of the electric field. The gravitational contribution is maximum when the angle between the axisymmetric axis of spheroids and the gravity is pi/4. However, for Brownian spheroids with a uniform orientation distribution the sedimentation effects are averaged out, thus there is no net gravitational drift in the direction of the applied electric field.

Journal Article↗

PubMed1998

Simultaneous Determination of Two-Phase Relative Permeability and Capillary Pressure of Porous Rocks from Steady-State Flow Experiments: Accounting for Gravitational Forces and Fluid Compressibility.

A methodology for simultaneously measuring two-phase relative permeability and capillary pressure of porous rock samples from steady-state flow experiments, reported by Virnovsky et al. in 1995, is considered. The method requires measuring pressure drops in individual fluid phases across a rock sample. This paper studies the role of gravitational forces, neglected in the original derivation of the method. Direct numerical simulations of steady-state vertical flow experiments, performed over a typical range of rocks and flow parameters, show that the error caused by neglecting gravitational forces in the original method, is acceptable even for extremely high ( approximately 10(-10) m2) permeability samples. Thus, if multiphase flow measurements are performed on vertical samples, the gravitational force can be neglected for estimating relative permeability and capillary pressure. An extension of this method to the case of compressible fluids is presented. Copyright 1998 Academic Press.

Journal Article↗

PubMed2001

Nonsingular van der Waals Potential and Its Contributions to Gravitational Coagulation.

The coagulation and the stability of dilute colloids at high Péclet number were studied in consideration of the universal nonsingular van der Waals interactions recently developed by J. X. Lu and W. H. Marlow (1995, Phys. Rev. Lett. 74, 1724). The capture efficiency of gravitational coagulation of uncharged colloids was found to be diminished due to the effect of finite molecular size (EFMS). The gravitational coagulation stability diagram of charged colloids was also found to be shifted when the gravitational convection was much stronger than the Brownian diffusion. Copyright 2001 Academic Press.

Journal Article↗

PubMed2005

Gravitational shunt management of long-standing overt ventriculomegaly in adult (LOVA) hydrocephalus.

OBJECTIVES: Recently a new subtype of chronic hydrocephalus was described: long-standing overt ventriculomegaly in adults (LOVA). Experience to date has indicated that shunt therapy was contraindicated, due to over-drainage. Therefore we investigated whether this problem could be overcome using gravitational shunts. MATERIALS AND METHODS: Thirty macrocephalic adults (17-72 years of age), suffering from progressive hydrocephalus were managed with two different gravitational shunts. The post-operative observation period was 5-87 months. RESULTS: Only two patients developed hygromas, and only one of these required surgical shunt revision. Eighty-seven percent of patients had a long-lasting clinical improvement. Ventricular size was only slightly reduced in 29 patients. There was no correlation between reduction in ventricular size and clinical improvement. CONCLUSION: Contrary to clinical guidelines issued to date, we demonstrate that LOVA can be treated reliably with gravitational shunts, making them a genuine alternative to endoscopic third ventriculostomy (ETV).

Adolescent↗

PubMed2005

The plasticity of gravitational reference frame and the subjective vertical: peripheral visual information affects the oblique effect.

The experiment examined the human visual perception of orientations and the nature of reference frame in which the oblique effect (lower performance in oblique orientations than in vertical or horizontal ones) was defined. Previous research [M. Luyat, E. Gentaz, Body tilt effect on the reproduction of orientations: studies on the visual oblique effect and subjective orientations, J. Exp. Psychol. Hum. Percept. Perform. 28 (2002) 1002-1011. M. Luyat, E. Gentaz, T.R. Corte, M. Guerraz, Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect, Percept. Psychophys. 63 (2001) 541-554], using head tilt paradigm to uncouple the gravitational, egocentric and subjective reference frames, showed that the oblique effect was mapped in a subjective gravitational reference frame with the subjective vertical as a cardinal orientation. However, the subjective vertical is not only affected by the tilt of head but also by the tilt of visual context. Then, the tilt of visual oriented cues is another paradigm permitting to evidence the role of the subjective gravitational reference frame. Sixteen participants were asked to reproduce five different orientations of a luminous line: horizontal (0 degrees ), 45 degrees (oblique), 90 degrees (vertical), 135 degrees (oblique) and the subjective vertical. These orientations were reproduced with no visual contextual cues and with tilted visual contextual cues tilted 15 degrees either to the left or to the right. The results showed that the oblique effect decreased with tilted visual contexts but was not completely suppressed. These results proved that this oblique effect is defined in a multimodal reference frame which integrates not only vestibular and proprioceptive cues but also peripheral visual information.

Adult↗

PubMed2001

Features of vestibuloocular reflex modulations induced by altered gravitational forces in tadpoles (Xenopus laevis).

In Xenopus laevis tadpoles, we studied the static vestibuloocular reflex (rVOR) in relation to modifications of the gravitational environment to find basic mechanisms of how altered gravitational forces (AGF) affect this reflex. Animals were exposed to microgravity during space flight or hypergravity (3g) for 4 to 12 days. Basic observations were that (1)the development of the rVOR is significantly affected by altered gravitational conditions, (2) the duration of 1g-readaptation depends on the strength of the test stimulus, (3) microgravity induces malformations of the body which are related to the rVOR depression. Future studies are based on the hypotheses (1) that the vestibular nuclei play a key roll in the adaptation to AGF conditions, (2) that the stimulus transducing systems in the sense organ are affected by AGF conditions, and (3) that fertilized eggs will be converted to normal adults guided by physiological and morphological set points representing the genetic programs. Developmental retardation or acceleration, or otherwise occurring deviations from standard development during embryonic and postembryonic life will activate genes that direct the developmental processes towards normality.

Adaptation, Physiological↗

PubMed2003

A gravitationally lensed quasar with quadruple images separated by 14.62 arcseconds.

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

Journal Article↗

PubMed1992

Configurations of adsorbed hard spheres after diffusion in a gravitational field.

The deposition and adhesion of particles on a solid surface are governed by a great number of interplaying forces. In this paper we analyze, by means of computer simulations, the influence of (i) the short-range repulsive forces, modeled by hard sphere interactions, (ii) the gravitational forces, and (iii) the diffusion process of the particles in the fluid on the structure of the surface covered by the particles. In particular, the evolution of the limiting coverage, [symbol, see text] infinity (where [symbol, see text] is the reduced relative surface coverage), and the radial distribution, g(r), at the jamming limit, are determined as a function of the gravitational forces. These forces play an important role in many experiments performed on latex beads. Our results should stimulate new experiments in this field and, thus, be directly experimentally tested. It is shown, for example, that for polystyrene particles [symbol, see text] infinity is constant and equal to the random sequential adsorption jamming limit value for radii R not larger than 1 micron. It increases for 1 </= R </= 3 micron and tends, for higher R, to a plateau, whose value is approximately equal to 0.61. The tendency to a closer packing when R is large, and thus large gravitational forces, is confirmed by the shape of the radial distribution function. This phenomenon occurs not only for jammed surfaces but also for unsaturated surfaces.

Journal Article↗

PubMed1995

Gravitational lensing of active galactic nuclei.

Most of the known cases of strong gravitational lensing involve multiple imaging of an active galactic nucleus. The properties of lensed active galactic nuclei make them promising systems for astrophysical applications of gravitational lensing; in particular, they show structure on scales of milliseconds of arc to tens of seconds of arc, they are variable, and they are polarized. More than 20 cases of strong gravitational lenses are now known, and about half of them are radio sources. High-resolution radio imaging is making possible the development of well-constrained lens models. Variability studies at radio and optical wavelengths are beginning to yield results of astrophysical interest, such as an independent measure of the distance scale and limits on source sizes.

Journal Article↗

PubMed1998

A gravitational diffusion model without dark matter.

In this model, without dark matter, the flat rotation curves of galaxies and the mass-to-light ratios of clusters of galaxies are described quantitatively. The hypothesis is that the agent of gravitational force is propagated as if it were scattered with a mean free path of approximately 5 kiloparsecs. As a result, the force between moderately distant masses, separated by more than the mean free path, diminishes as the inverse first power of the distance, following diffusion equations, and describes the flat rotation curves of galaxies. The force between masses separated by <1 kiloparsec diminishes as the inverse square of distance. The excess gravitational force (ratio of 1/r:1/r2) increases with the scale of structures from galaxies to clusters of galaxies. However, there is reduced force at great distances because of the approximately 12 billion years that has been available for diffusion to occur. This model with a mean free path of approximately 5 kiloparsecs predicts a maximum excess force of a few hundredfold for objects the size of galactic clusters a few megaparsecs in size. With only a single free parameter, the predicted curve for excess gravitational force vs. size of structures fits reasonably well with observations from those for dwarf galaxies through galactic clusters. Under the diffusion model, no matter is proposed in addition to the observed baryons plus radiation and thus the proposed density of the universe is only a few percent of that required for closure.

Journal Article↗

PubMed2001

Rotation-induced phase transition in a spherical gravitating system.

Due to the infinite range and singularity of the gravitational force, it is difficult to directly apply the standard methods of statistical physics to self-gravitating systems, e.g., interstellar grains, globular clusters, galaxies, etc. Unusual phenomena can occur, such as a negative heat capacity, unbounded mass, or the gravothermal catastrophe where the equilibrium state is fully collapsed and the entropy is unbounded. Using mean field theory, we investigate the influence of rotation on a purely spherical gravitational system. Although spherical symmetry nullifies the total angular momentum, its square is finite and conserved. Here we study the case where each particle has specific angular momentum of the same magnitude l. We rigorously prove the existence of an upper bound on the entropy and a lower bound for the energy. We demonstrate that, in the microcanonical and canonical ensembles, a phase transition occurs when l falls below a critical value. We characterize the properties of each phase and construct the coexistence curve for each ensemble. Possible applications to astrophysics are considered.

Journal Article↗

PubMed2002

Oblique electrostatic modes in self-gravitating dusty plasmas.

The propagation of oblique and perpendicular electrostatic modes in dusty self-gravitational magnetized plasmas is treated with due care for the small gravitational effects between charged dust particles and for the correct balance between electrostatic and gravitational forces. At purely oblique propagation, generalizations of the dust-cyclotron and dust-acoustic modes are found, where the latter can be subject to a Jeans instability. For strictly perpendicular propagation, only a mixed dust-acoustic and dust lower-hybrid mode occur at low frequencies. A Jeans collapse can be avoided by strong enough magnetic fields, the criterion for which is given.

Journal Article↗

PubMed2003

Dynamics of solid growth under a gravitational field: influence of the formation of a diffusive layer.

We discuss the gravitational sedimentation of particles in terms of a stochastic model considering, in view of experimental evidence, that the aggregation to the growing surface (deposit) is mediated by the formation of a layer of suspended particles subject to gravitational forces, thermal agitation, as well as aggregation (contact) forces. The aggregation of such partially buoyant particles is ruled by the rates of occurrence of the different stochastic events: incorporation to the layer of suspended particles, sedimentation, and gravitationally biased diffusion. The model introduces bridges across different standard solid on solid deposition models which can be considered as limit cases of the present one. Analytical and numerical results show that for finite (realistic) deposits there are different regimes of aggregation including situations in which the deposit is grown completely during the transient time of the system.

Journal Article↗

PubMed2004

Anomalous diffusion and collapse of self-gravitating Langevin particles in D dimensions.

We address the generalized thermodynamics and the collapse of a system of self-gravitating Langevin particles exhibiting anomalous diffusion in a space of dimension D. This is a basic model of stochastic particles in interaction. The equilibrium states correspond to polytropic configurations similar to stellar polytropes and polytropic stars. The index n of the polytrope is related to the exponent of anomalous diffusion. We consider a high-friction limit and reduce the problem to the study of the nonlinear Smoluchowacute;ski-Poisson system. We show that the associated Lyapunov functional is the Tsallis free energy. We discuss in detail the equilibrium phase diagram of self-gravitating polytropes as a function of D and n, and determine their stability by using turning point arguments and analytical methods. When no equilibrium state exists, we investigate self-similar solutions of the nonlinear Smoluchowski-Poisson system describing the collapse. Our stability analysis of polytropic spheres can be used to settle the generalized thermodynamical stability of self-gravitating Langevin particles as well as the nonlinear dynamical stability of stellar polytropes, polytropic stars and polytropic vortices. Our study also has applications concerning the chemotactic aggregation of bacterial populations.

Journal Article↗

PubMed2004

Statistical mechanics and thermodynamic limit of self-gravitating fermions in D dimensions.

We discuss the statistical mechanics of a system of self-gravitating fermions in a space of dimension D. We plot the caloric curves of the self-gravitating Fermi gas giving the temperature as a function of energy and investigate the nature of phase transitions as a function of the dimension of space. We consider stable states (global entropy maxima) as well as metastable states (local entropy maxima). We show that for D> or =4, there exists a critical temperature (for sufficiently large systems) and a critical energy below which the system cannot be found in statistical equilibrium. Therefore, for D> or =4, quantum mechanics cannot stabilize matter against gravitational collapse. This is similar to a result found by Ehrenfest (1917) at the atomic level for Coulomb forces. This makes the dimension D=3 of our Universe very particular with possible implications regarding the anthropic principle. Our study joins a long tradition of scientific and philosophical papers that examined how the dimension of space affects the laws of physics.

Journal Article↗

PubMed2004

Three-body dynamics in a (1+1) -dimensional relativistic self-gravitating system.

The results of our study of the motion of a three particle, self-gravitating system in general relativistic lineal gravity is presented for an arbitrary ratio of the particle masses. We derive a canonical expression for the Hamiltonian of the system and discuss the numerical solution of the resulting equations of motion. This solution is compared to the corresponding nonrelativistic and post-Newtonian approximation solutions so that the dynamics of the fully relativistic system can be interpreted as a correction to the one-dimensional Newtonian self-gravitating system. We find that the structure of the phase space of each of these systems yields a large variety of interesting dynamics that can be divided into three distinct regions: annulus, pretzel, and chaotic; the first two being regions of quasiperiodicity while the latter is a region of chaos. By changing the relative masses of the three particles we find that the relative sizes of these three phase space regions changes, and that this deformation can be interpreted physically in terms of the gravitational interactions of the particles. Furthermore, we find that many of the interesting characteristics found in the case where all of the particles share the same mass also appear in our more general study. We find that there are additional regions of chaos in the unequal mass system which are not present in the equal mass case. We compare these results to those found in similar systems.

Journal Article↗