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Results for “GRAVITATION”

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At least 19 recordsLinked to original sources
PubMed2004

A gravitational hypothesis of essential hypertension as a natural adaptation to increased gravitational stress caused by regular, prolonged sitting typical of modern life.

BACKGROUND: Spaceflight studies have demonstrated that adaptation to increased gravitational stress after prolonged microgravity includes sympathetic activation, water retention, and arterial pressure increase, i.e. is very similar to essential hypertension, which looks like an advanced stage of adaptation to a further increase in gravitational stress. MATERIAL/METHODS: Theoretical analysis and literature review were performed to develop an hypothesis of essential hypertension as adaptation to increased gravitational stress associated with prolonged sitting. RESULTS: Gravitation shifts blood downwards, and gravitational stress (GS) on the cardiovascular system in an upright position may be defined as the amount of work necessary to return the blood upwards and maintain adequate upper-body circulation calculated as the product of the gravitational potential (Ugr=g*h) and the mass of blood moved: GS =Ugr*Mshift. In a complex vascular network, this blood shift is actually a function of time, estimated in a first approximation as Mshift(t)=(Ugr*t)/Rd (Rd: the resistance to downward blood flow). Thus, gravitational stress is proportional to the time spent upright GS=Ugr*Mshift(t)=(Ugr2*t)/Rd=(g2*h2*t)/Rd. From this analysis, regular, prolonged sitting typical of modern life should cause a significant increase in gravitational stress on the cardiovascular system, requiring advanced antigravitational response with sympathetic hyperactivity, vasoconstriction, volume overload, and arterial hypertension. The hypertensive effect of prolonged sitting has been demonstrated in clinical studies. CONCLUSIONS: Essential hypertension may be explained as adaptation to increased gravitational stress caused by regular, prolonged sitting. This gravitational hypothesis of essential hypertension, supported by clinical data, offers a way to its complete healing through elimination of the primary factor of prolonged sitting.

Adaptation, Physiological↗
PubMed2002

Phase transitions in self-gravitating systems: self-gravitating fermions and hard-sphere models.

We discuss the nature of phase transitions in self-gravitating systems both in the microcanonical and in the canonical ensemble. We avoid the divergence of the gravitational potential at short distances by considering the case of self-gravitating fermions and hard-sphere models. Depending on the values of the parameters, three kinds of phase transitions (of zero, first, and second order) are evidenced. They separate a "gaseous" phase with a smoothly varying distribution of matter from a "condensed" phase with a core-halo structure. We propose a simple analytical model to describe these phase transitions. We determine the value of energy (in the microcanonical ensemble) and temperature (in the canonical ensemble) at the transition point and we study their dependence on the degeneracy parameter (for fermions) or on the size of the particles (for a hard-sphere gas). Scaling laws are obtained analytically in the asymptotic limit of a small short distance cutoff. Our analytical model captures the essential physics of the problem and compares remarkably well with the full numerical solutions. We also stress some analogies with the liquid-gas transition and with the Blume-Emery-Griffiths model with infinite range interactions. In particular, our system presents two tricritical points at which the transition passes from first order to second order.

Journal Article↗
PubMed2002

Gravitational eccentric correction optics (GECO): an optical-gravitational device to compensate for flexures in astronomical spectrographs.

Mechanical flexure is a source of major failures in astronomical spectrographs, for which the reimaging of a focal-plane pinhole has to be maintained in position within a fraction of a CCD pixel that has dimensions of the order of 15 microm. The d.o.lo.res. (an acronym for device optimized for low resolution) spectrograph for the Italian national telescope, Galileo, showed displacements of the image of the pinhole more than 10 times greater than expected. The mechanical failure was overcome by the insertion of a passive optical wedge that can add an out-of-phase circle to the flexure ellipse. The results encourage the use of the gravitational eccentric correction optics (GECO) optical-gravitational device in all astronomical observations made with the d.o.lo.res. spectrograph.

Journal Article↗
PubMed2005

Gravitational-wave emission from rotating gravitational collapse in three dimensions.

We present the first three-dimensional (3D) calculations of the gravitational-wave emission in the collapse of uniformly rotating stars to black holes. The initial models are polytropes which are dynamically unstable and near the mass-shedding limit. The waveforms have been extracted using a gauge-invariant approach and reflect the properties of both the initial stellar models and of newly produced black holes, being in good qualitative agreement with those computed in previous 2D simulations. The wave amplitudes, however, are about 1 order of magnitude smaller, giving, for a source at 10 kpc, a signal-to-noise ratio S/N approximately 0.25 for LIGO-VIRGO and S/N less than or approximately equal 4 for LIGO II.

Journal Article↗