Position In Field Research Articles

Evaluation of 12-lead electrocardiogram at 0.55T for improved cardiac monitoring in magnetic resonance imaging

The 12-lead electrocardiogram (ECG) is a standard diagnostic tool for monitoring cardiac ischemia and heart rhythm during cardiac interventional procedures and stress testing. These procedures can benefit from magnetic resonance imaging (MRI) information; however, the MRI scanner magnetic field leads to ECG distortion that limits ECG interpretation. This study evaluated the potential for improved ECG interpretation in a "low field" 0.55T MRI scanner. The 12-lead ECGs were recorded inside 0.55T, 1.5T, and 3T MRI scanners, as well as at scanner table "home" position in the fringe field and outside the scanner room (seven pigs). To assess interpretation of ischemic ECG changes in a 0.55T MRI scanner, ECGs were recorded before and after coronary artery occlusion (seven pigs). ECGs was also recorded for five healthy human volunteers in the 0.55T scanner. ECG error and variation were assessed over 2-minute recordings for ECG features relevant to clinical interpretation: the PR interval, QRS interval, J point, and ST segment. ECG error was lower at 0.55T compared to higher field scanners. Only at 0.55T table home position, did the error approach the guideline recommended 0.025mV ceiling for ECG distortion (median 0.03mV). At scanner isocenter, only in the 0.55T scanner did J point error fall within the 0.1mV threshold for detecting myocardial ischemia (median 0.03mV in pigs and 0.06mV in healthy volunteers). Correlation of J point deviation inside versus outside the 0.55T scanner following coronary artery occlusion was excellent at scanner table home position (r2 =0.97), and strong at scanner isocenter (r2 =0.92). ECG distortion is improved in 0.55T compared to 1.5T and 3T MRI scanners. At scanner home position, ECG distortion at 0.55T is low enough that clinical interpretation appears feasible without need for more cumbersome patient repositioning. At 0.55T scanner isocenter, ST segment changes during coronary artery occlusion appear detectable but distortion is enough to obscure subtle ST segment changes that could be clinically relevant. Reduced ECG distortion in 0.55T scanners may simplify the problem of suppressing residual distortion by ECG cable positioning, averaging, and filtering and could reduce current restrictions on ECG monitoring during interventional MRI procedures.

Satire Meets Critical Realism in the European Semiperiphery: A Study on Ibsen and Caragiale

Henrik Ibsen and Ion Luca Caragiale seem to have nothing in common but that each is considered the “national playwright” of Norway and Romania, respectively. The present contribution seeks to draw some parallels between the two (semi)peripheral authors from the late 19th and early 20th century based on their ideological positions in the literary fields of their countries. After tracing the relationship between their dramatic output and the idea of revolution, i.e., how revolution in several of its forms is reflected in their works, I work my way towards their common preoccupation for “legalizing” the world—either through bureaucracy or finance—and how this preoccupation finds its most suitable expression. Finally, I discuss how form plays a crucial role in officializing content and how Ibsen’s seriousness reflects the more prosperous, more established Norwegian bourgeoisie, whereas Caragiale’s mockery, satire, and absurdism lays bare the “frivolousness” of the new Romanian middleclass, as aspirational as it is morally precarious.

Chromatic processing and receptive-field structure in neurons of the anterior optic tract of the honeybee brain.

Color vision in honeybees is a well-documented perceptual phenomenon including multiple behavioral tests of trichromaticity and color opponency. Data on the combined color/space properties of high order visual neurons in the bee brain is however limited. Here we fill this gap by analyzing the activity of neurons in the anterior optic tract (AOT), a high order brain region suggested to be involved in chromatic processing. The spectral response properties of 72 units were measured using UV, blue and green light stimuli presented in 266 positions of the visual field. The majority of these units comprise combined chromatic-spatial processing properties. We found eight different neuron categories in terms of their spectral, spatial and temporal response properties. Color-opponent neurons, the most abundant neural category in the AOT, present large receptive fields and activity patterns that were typically opponent between UV and blue or green, particularly during the on-tonic response phase. Receptive field shapes and activity patterns of these color processing neurons are more similar between blue and green, than between UV and blue or green. We also identified intricate spatial antagonism and double spectral opponency in some receptive fields of color-opponent units. Stimulation protocols with different color combinations applied to 21 AOT units allowed us to uncover additional levels of spectral antagonism and hidden inhibitory inputs, even in some units that were initially classified as broad-band neurons based in their responses to single spectral lights. The results are discussed in the context of floral color discrimination and celestial spectral gradients.

Effect of juggling expertise on pointing performance in peripheral vision.

Juggling is a very complex activity requiring motor, visual and coordination skills. Expert jugglers experience a "third eye" monitoring leftward and rightward ball zenith positions alternately, in the upper visual fields, while maintaining their gaze straight-ahead. This "third eye" reduces their motor noise (improved body stability and decrease in hand movement variability) as it avoids the numerous head and eye movements that add noise into the system and make trajectories more uncertain. Neuroimaging studies have shown that learning to juggle induces white and grey matter hypertrophy at the posterior intraparietal sulcus. Damage to this brain region leads to optic ataxia, a clinical condition characterised by peripheral pointing bias toward gaze position. We predicted that expert jugglers would, conversely, present better accuracy in a peripheral pointing task. The mean pointing accuracy of expert jugglers was better for peripheral pointing within the upper visual field, compatible with their subjective experience of the "third eye". Further analyses showed that experts exhibited much less between-subject variability than beginners, reinforcing the interpretation of a vertically asymmetrical calibration of peripheral space, characteristic of juggling and homogenous in the expert group. On the contrary, individual pointing variability did not differ between groups neither globally nor in any sector of space, showing that the reduced motor noise of experts in juggling did not transfer to pointing. It is concluded that the plasticity of the posterior intraparietal sulcus related to juggling expertise does not consist of globally improved visual-to-motor ability. It rather consists of peripheral space calibration by practicing horizontal covert shifts of the attentional spotlight within the upper visual field, between left and right ball zenith positions.

Numerical simulation of magnetic field influence on plasma and electron extraction of electron cyclotron resonance neutralizer

Electron cyclotron resonance (ECR) neutralizer is a key component of electron cyclotron resonance ion thruster (ECRIT) with a diameter of 10 cm, which plays an important role in maintaining the spacecraft potential balance and neutralizing the ions in the plume region. Optimizing magnetic field distribution is an important way to improve the performance of neutralizer. At the same time, the uniformity of the magnetic field and the position of the magnetic trap can affect the magnetic field characteristics, plasma performance, electron extraction process, and beam current. Previous experimental researches showed that the beam current extraction performances of the two ECR neutralizers with different magnetic field uniformity and different magnetic trap locations are significantly different. However, it is difficult to reveal the physical phenomena and causes only through experiments, so numerical simulation is needed. Therefore PIC/MCC codes for the ECR neutralizers with different uniformity of magnetic field and different positions of magnetic trap are established. Under the given electron extraction potential, numerical simulations are accomplished to study electron extraction procedure and analyze its influence on the performance of the neutralizer. The simulation results show that when the magnetic field uniformity is low and the magnetic trap is located upstream of extraction orifice, the migration of electrons from the magnetic trap to the outlet is limited by the magnetic field and the electric field, thus a higher potential energy is needed to extract the electrons. Otherwise, when the magnetic field uniformity is high and the magnetic trap is located at the downstream of extraction orifice, electrons will be more likely to migrate towards the magnet trap. After the electrons reach the magnetic trap, under the action of the anode potential, the external potential is higher, and the external weak magnetic field almost fails to hold these electrons, Therefore a large number of electrons can be extracted at low extraction potential. This research will lay an important foundation for the development of high-performance ECR neutralizer.

Extrinsic chirality tailors Stokes parameters in simple asymmetric metasurfaces.

Metasurfaces tailor electromagnetic confinement at the nanoscale and can be appropriately designed for polarization-dependent light-matter interactions. Adding the asymmetry degree to the desing allows for circular polarizations of opposite handedness to be differently absorbed or emitted, which is of interest in fields spanning from chiral sensing to flat optics. Here, we show that simple, low-cost asymmetric metasurfaces can control Stokes parameters in the transmitted far-field. With only 50 nm of asymmetric plasmonic shells on self-assembled polystyrene nanospheres, our metasurfaces allow for great spectral and incident angle tunability. We first investigated broadband extrinsic chirality in metasurfaces with asymmetric plasmonic semishells; we found high extinction circular dichroism (CD) in the near-infrared range. We then excited it with linear polarization and performed hyperspectral Stokes polarimetry on the transmitted field. We showed that the S3 parameter follows the behavior of CD in extinction, and that the output field position on the Poincaré sphere can be widely controlled by using the incidence angle and wavelength. Furthermore, simulations agreed well with the experiments and showed how the near-field chiro-optical response influences the extrinsic chiral behavior in absorption and the polarization state of the transmitted field.

Globalization, Citizenship and Subjectivity: An analysis of the changes in the last five generations in my family

Human Subjectivities are shaped by one’s social position and practices in the social field. Subjectivities may be durable, but they are changeable. The subjectivities one carries may differ from generation to generation. What I perceive about myself is different from what my parents perceived and what my grandparents perceived and how they developed their subjectivities. We have to see these human subjectivities across time and space. This paper explores the interplay of local, national and global social fields in forming subjectivity and in analyzing the changes in the last five generations in my family. I began my study by looking at the genealogy of Gyawali family. I approached the emergence of such changes through the theoretical lens of Bourdieu, Foucault, and Anderson. As the research problem I wanted to investigate was of a historical nature, the methods that I could choose were already conditioned by that. I picked up oral history research, and biographical interviews as two basic research methods. The changes in the way of life in my family due to such fields can be seen directly in the economy, use of technology, way of life and ultimately in the culture. When we entered into the globalization process the traditional norms and values of the family have transformed with the global way of life. We are becoming more self-reliant and empowered but at the same time, it has also created mental anxiety and stress. New subjectivities and identities have emerged in the last five generations in my family.

Mechanistic modelling of drug dissolution for additively manufactured polymeric tablets: A novel layer-by-layer approach

Additive manufacturing has been consolidating its position in various fields over the last few decades. The development of drug delivery systems like oral tablets have been successfully accomplished using this ingenious technology. The effectiveness of an oral tablet can be analysed by its drug release behaviour and this study aims to predict this release by developing a model for a 3D printed tablet dissolution. A polymer-dissolution based mathematical model have been developed by considering a novel layer-by-layer approach that is found during additive manufacturing procedure. The shape and volume of the tablet have been found to be crucial in predicting the drug dissolution for a given time. Further, the model was verified against the drug dissolution trend obtained from a Fused Deposition Modeling (FDM)-printed polyvinyl alcohol (PVA) tablet having benidipine hydrochloride drug substance. Four different shapes have been considered to test the robustness of the model. Further, the sensitivity test have been conducted on the model and it revealed detachment rate and layer thickness as significant process parameters.

Law Enforcement Against The Criminal Act of Forgery of Documents By a Notary In The Perspective of Law No. 30 of 2004 Concerning The Implementation of Administrative Sanctions or The Notary's Code of Ethics

The existence of a notary as a witness or even as a suspect, if it is related to the existence of his position in the legal field which is intended to support the smooth running of a law enforcement process, including the judicial process, would not be a problem, in other words, it is only natural for a notary to act as a witness or even as a suspect in a judicial process. On the other hand, a notary in carrying out his position as a public official, apart from being related to a position regulation, is also related to the oath of office which he takes when appointed as a notary where the notary is obliged to keep the contents of the deed and information he obtains confidential as regulated in Article 4 Paragraph ( 2) Law Number 30 of 2004 concerning the Position of Notary which states that, I will keep confidential the contents of deeds and information obtained in the performance of my office. Further Article 54 Paragraph (1) of Law Number 2 of 2014 concerning Amendments to the Law -Law Number 30 of 2004 concerning the Position of Notary Public explains that a notary can only give, show, or notify the contents of the deed, grosses deed, copy of the deed or excerpt from the deed, to people who have a direct interest in the deed, heirs, or people who have acquired rights, unless otherwise determined by statutory regulations. The exercise of the right to keep secrets related to one's position is also regulated in criminal procedural law, civil law, and the Criminal Code. Article 170 Paragraph (1) of the Criminal Procedure Code states that those who because of their work, dignity, or position are required to keep secrets, can ask to be released from exercising their right to provide information as a witness, namely about matters entrusted to them. This type of research is Normative research. The approaches used are a statutory approach and a conceptual approach. The data source used is secondary data. Data analysis was carried out descriptively and qualitatively. Concluding is carried out using a deductive method from general to specific, especially those related to the research topic, namely law enforcement against the crime of document falsification by notaries in the perspective of Law no. 30 of 2004. This research resulted in the findings of Law Number 30 of 2004 in conjunction with Law Number 2 of 2014 concerning the Position of Notaries which regulates that when a Notary in carrying out his official duties has committed a violation that causes a deviation from the law, the Notary can be subject to sanctions in the form of sanctions. Civil, Administrative, Code of Ethics for Notary Positions. These sanctions have been regulated in such a way both previously and now in the Law on the Position of Notaries regarding the Code of Ethics for the Notary Position profession where there is no description of criminal sanctions but rather the organization of the Notary Supervisory Council which has the authority to impose punishments on notaries. Notaries in carrying out their official duties do not close the possibility of being in contact with legal issues, even though carrying out the duties has been done carefully and by statutory provisions

Ilmar Talve, Emigré Ethnologist from Estonia

The article focuses on Ilmar Talve (1919–2007) and his exile in Sweden, 1945–1959. The complex nature of an émigré position in a scholarly field is analysed from several angles. I explore how Talve adapted to the Swedish academic field while he was working at the Institute of Folklife Research, led by Sigurd Erixon. I am also interested in how his understanding of ethnology evolved in Esonia as a student of the WWII era and in Sweden after the war. Talve’s efforts to pursue and develop Estonian ethnology while in exile are then examined in more detail. On the one hand, it shows the influence of the contemporary national discourse on research. In some sense, it was an unrewarding dead end, but even as such, it describes the political and societal conditionality of pursuing science at the time. On the other hand, it raises the question of the influence of Erixon’s theoretical views on Talve, and therefore, on Finnish ethnology. Talve implemented his plans in Finland as a professor at the University of Turku. The article also explores the important role played by Finnish scholars for both exile ethnologists in Sweden and Estonian ethnographers in the Soviet Union, and it reflects on Talve’s place in this relationship.

Modeling approaches to AI integration into public relations in Russia as per comparative research of foreign countries’ experience

Artificial intelligence (AI) technologies are one of the most significant solutions that can fundamentally change both public relations and the sphere of public legal relations. The goal of the present research is to conduct a comparative analysis of existing approaches to the AI integration into the sphere of public legal relations of foreign countries and Russia, with the subsequent determination of the most effective constructive approaches to the formation of a regulatory policy regarding AI in Russia. This goal is achieved through the study of the following issues: Strategic goal-setting in relation to AI in Russia and other countries; Tactical implementation of AI integration into public legal relations and possible solutions in order to ensure AI decision-making transparency; Model compensatory measures that ensure AI safe integration into public relations in Russia. The object of the study comprises regulatory sources, strategies, and other documents regulating the AI integration into the sphere of public legal relations in Russia and foreign countries, judicial practice examples, and academic publications on the issues under study. The research methodology incorporates a complex of modern philosophical, general scientific, special scientific methods of cognition, including dialectical, systemic, structural-functional, hermeneutic, comparative legal, formal legal (dogmatic), legal modeling methods, and some others. The present research lays special emphasis on the implementation of a comparative legal study of the approaches to and regulation of AI in the public sphere regarding various states experience. The general scientific dialectical method has made it possible to consider the AI position in the legal field from the point of view of the AI regulation variability and trends in the subsequent development thereof, and to substantiate the advantages and disadvantages of various approaches to solve the respective issues, as well. Additional emphasis is also laid on addressing the issues of the applied nature of the AI use in the field of public relations in Russia, taking into account the legal modeling and development of compensatory measures within the authorities’ regulatory approaches.

Analysis of the Principle and State-of-art Facilities of Mobile Atom Interferometry Gravity

With over 20 years of development, atomic interferometry gravimeters have distinguished themselves in gravity measurements by their high precision and sensitivity. To cater to the demands of field absolute gravity measurements for marine and aerial gravity field measurements, resource exploration, inertial navigation, and positioning, the miniaturization, mobility, and engineering of atomic interferometry gravimeters have become crucial areas of development. On this basis, this article elucidates the principles of atomic interferometry gravimeters, encompassing various laser cooling techniques, magneto-optical traps, and Raman light interference. As a matter of fact, it provides an overview of the principles and achieved precision benchmarks of portable, vehicular, airborne, and shipborne atomic interferometry gravimeters. Additionally, it analyzes the experimental content and results of maritime, terrestrial, and aerial experiments based on portable atomic interferometry gravimeters. The limitations of portable atomic interferometry gravimeters are discussed, along with an exploration of their application prospects. Overall, these results shed light on guiding further exploration of mobile atom interferometry gravity.

Optomechanical effects caused by non-zero field quantities in multiple evanescent waves.

Evanescent waves, with their high energy density, intricate local momentum, and spatial distribution of spins, have been the subject of extensive recent study. These waves offer promising applications in near-field particle manipulation. Consequently, it becomes imperative to gain a deeper understanding of the impacts of scattering and gradient forces on particles in evanescent waves to enhance and refine the manipulation capabilities. In this study, we employ the multipole expansion theory to present analytical expressions for the scattering and gradient forces exerted on an isotropic sphere of any size and composition in multiple evanescent waves. The investigation of these forces reveals several unusual optomechanical phenomena. It is well known that the scattering force does not exist in counter-propagating homogeneous plane waves. Surprisingly, in multiple pairs of counter-propagating evanescent waves, the scattering force can arise due to the nonzero orbital momentum (OM) density and/or the curl part of the imaginary Poynting momentum (IPM) density. More importantly, it is found that the optical scattering force can be switched on and off by simply tuning the polarization. Furthermore, optical forces typically vary with spatial position in an interference field. However, in the interference field generated by evanescent waves, the gradient force becomes a spatial constant in the propagating plane as the particle's radius increases. This is attributed to the decisive role of the non-interference term of the electromagnetic energy density gradient. Our study establishes a comprehensive and rigorous theoretical foundation, propelling the advancement and optimization of optical manipulation techniques harnessed through multiple evanescent waves. Specifically, these insights hold promise in elevating trapping efficiency through precise control and manipulation of optical scattering and gradient forces, stimulating further explorations.

The Business Model of Twitter in Social Media

Nowadays, the competition between enterprises is not the competition of products but the competition between business models. For Internet enterprises that mainly provide virtual products and services, the importance of the business model is even more self-evident. In the mobile Internet environment, the future development trend of the Internet is social network media. Twitter, as an application platform of microblogging technology, occupies an essential position in the new media field with its characteristics of rapid release of information and extensive dissemination. This paper combines the investigation method and literature method from the characteristics of Twitter, user characteristics, and business data analysis of a comprehensive understanding of Twitter, a business model based on the analysis of the deep high-income user market, consolidate B2B market share, develop sex users, optimize the shopping system, in order to consolidate its industry-leading position in the field of new media.

The Variance of German University Governance: Exploring the Effects of Organizational Field Positions

The Variance of German University Governance: Exploring the Effects of Organizational Field Positions

Influences of voltage allocation and magnetic field introduced position on the dust removal performance of multi-electric field ESP

Improving the removal efficiency of PM2.5 particles is significant for the limitation of particle emissions from coal-fired power plants. This work reports a multiple electric field electrostatic precipitator (ESP) enhanced by magnetic confinement effect to efficiently remove PM2.5 particle from flue gas. Firstly, the electromagnetic dust removal mechanism of PM2.5 capture in multi-electric field ESPs is revealed, and the theoretical and numerical models of the multifield coupling under the effect of magnetic field are established. Secondly, the potential distribution law under different working voltages and the dust removal performance for PM2.5 are analyzed, and the magnetic field effect under the optimal voltage allocation is numerically simulated. Finally, the best magnetic field introduction position under the optimal voltage allocation is investigated. The numerical results show that the optimal voltage allocation and the best magnetic field position are determined to achieve the full effect of magnetic field to improve the PM2.5 dust removal efficiency in the multi-electric field ESP under the premise of ensuring the decrease of voltage load. The relevant results can provide ideas and references for greatly reducing PM2.5 emissions and voltage power consumption in thermal power plants.

A configural context signal simultaneously but separably drives positioning and orientation of hippocampal place fields.

Effective self-localization requires that the brain can resolve ambiguities in incoming sensory information arising from self-similarities (symmetries) in the environment structure. We investigated how place cells use environmental cues to resolve the ambiguity of a rotationally symmetric environment, by recording from hippocampal CA1 in rats exploring a "2-box." This apparatus comprises two adjacent rectangular compartments, identical but with directionally opposed layouts (cue card at one end and central connecting doorway) and distinguished by their odor contexts (lemon vs. vanilla). Despite the structural and visual rotational symmetry of the boxes, no place cells rotated their place fields. The majority changed their firing fields (remapped) between boxes but some repeated them, maintaining a translational symmetry and thus adopting a relationship to the layout that was conditional on the odor. In general, the place field ensemble maintained a stable relationship to environment orientation as defined by the odors, but sometimes the whole ensemble rotated its firing en bloc, decoupling from the odor context cues. While the individual elements of these observations-odor remapping, place field repetition, ensemble rotation, and decoupling from context-have been reported in isolation, the combination in the one experiment is incompletely explained within current models. We redress this by proposing a model in which odor cues enter into a three-way association with layout cues and head direction, creating a configural context signal that facilitates two separate processes: place field orientation and place field positioning. This configuration can subsequently still function in the absence of one of its components, explaining the ensemble decoupling from odor. We speculate that these interactions occur in retrosplenial cortex, because it has previously been implicated in context processing, and all the relevant signals converge here.

Background-oriented schlieren sensitivity in terms of geometrical parameters of measurement setup

Background-oriented schlieren imaging is a recently proposed noninvasive optical method for imaging of full ultrasound fields. In this work, the impact of uncertainty in geometrical parameters of a background-oriented schlieren measurement setup for imaging of full ultrasound fields is studied using numerical simulations. The studied parameters are focal length of the camera and positions and orientations of the camera, water tank, and ultrasound field. The results demonstrate that the most sensitive parameters affecting the accuracy of the reconstructed ultrasound fields are the orientations of the camera that change the direction of an effective optical axis. Other sensitive parameters are the focal length of the camera and the position of the ultrasound field in perpendicular directions of an optical axis. This synthetic study demonstrates the accuracy requirements for calibrating the geometrical parameters of a measurement setup that would be required to achieve accuracy comparable to that of hydrophone measurements using the background-oriented schlieren imaging. Explicitly, limits of the variation ranges of the geometrical parameters resulting in relative error ranges of 5% and 10% are given. The results of this study may contribute to help design future background-oriented schlieren measurement setups intended for measurement of full ultrasound fields.

Theoretical study on the pressure expansion mechanism of diffused self-pumping hydrodynamic mechanical seal

The pressure expansion performance is the key and basis for the diffused self-pumping hydrodynamic mechanical seal to achieve its good cooling performance, self-cleaning performance, and sealing performance. Using the moment of momentum theorem and Stodola's formula, the pressurization effect of the spiral groove on the fluid was analyzed, and the energy head equation of the work done by the spiral groove on the fluid is established. According to the principle of conservation of energy, the energy equation and Bernoulli equation of the work done by the spiral groove on the fluid are derived, the mathematical expression of the conversion of fluid kinetic energy into hydrostatic pressure in the diffuser groove was established, the energy change and energy distribution problems of the fluid after the work of the spiral groove are clarified, and the pressure expansion mechanism is revealed. Through numerical simulation, the relationship between the fluid pressure at the sealing interface, the position and size of the high-pressure field, the opening force, and the leakage rate under different rotational speeds and the structural parameters of the diffuser groove were explored. Finally, the pressure expansion performance of the ordinary self-pumping hydrodynamic mechanical seal and diffused self-pumping hydrodynamic mechanical seal is compared. The results show that the diffuser groove can effectively convert part of the fluid kinetic energy into pressure energy, improve the opening force of the sealing interface, and have a good pressurization effect on the sealing end face. With the widening of the diffuser groove, the pressure peak of the sealing interface increases, the high-pressure field area continues to expand and tends to expand toward the outer diameter of the seal ring, and the opening force also increases significantly; increasing the depth of the diffuser groove will cause the pressure peak of the sealing interface to become smaller, and the area of the high-pressure field will also decrease rapidly, which is not conducive to improving the opening force of the sealing end face.

Application of the Method of Fast Expansions to Construction of a Trajectory of Movement of a Body with Variable Mass from Its Initial Position in an Achieved Final Position in a Gravitational Field

Application of the Method of Fast Expansions to Construction of a Trajectory of Movement of a Body with Variable Mass from Its Initial Position in an Achieved Final Position in a Gravitational Field