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Mechanical explanations of gravitation
The mechanical theories or explanations of gravitation are attempts to explain the law of gravity by aid of basic mechanical processes, such as pushes, and without the use of any action at a distance. These theories were developed from the 16th until the 19th century in connection with the aether. However, such models are no longer regarded as viable theories within the mainstream scientific community and the standard model to describe gravitation without the use of actions at a distance is general relativity.
Due to philosophical considerations René Descartes proposed in 1644 that no empty space can exist and that space must consequently be fulfilled with matter. The parts of this matter tend to move in straight paths, but because they lie close together, they can't move freely, which according to Descartes implies that every motion is circular, so the aether is filled with vortices. Descartes also distinguishes between different forms and sizes of matter in which rough matter resists the circular movement more strongly than fine matter. Due some sort of centrifugal force matter tends towards the outer edges of the vortex, which causes a condensation of this matter there. The rough matter cannot follow this movement due to their greater inertia - so due to the pressure of the condensed outer matter those parts will be pushed into the center of the vortex. This inward pressure is nothing else than gravity (at least according to Descartes). He compared this mechanism with the fact that if a rotating, liquid filled vessel is stopped, the liquid goes on to rotate. Now, if one drops small pieces of light matter (e.g. wood) into the vessel, the pieces move to the middle of the vessel.   
Following the basic premises of Descartes, Christiaan Huygens between 1669 and 1690 designed a much more exact vortex model. This model was the first theory of gravitation which was worked out mathematically. He assumed that the aether particles are moving in every direction, but were thrown back at the outer borders of the vortex and this causes (as in the case of Descartes) a greater concentration of fine matter at the outer borders. So also in his model the fine matter presses the rough matter into the center of the vortex. Huygens also found out that the centrifugal force is equal to the force, which acts in the direction of the center of the vortex (centripetal force). He also posited that bodies must consist mostly of empty space so that the aether can penetrate the bodies easily, which is necessary for mass proportionality. He further concluded that the aether moves much faster than the falling bodies. At this time, Newton developed his theory of gravitation which is based on attraction and although Huygens agreed with the mathematical formalism, he said the model is insufficient due to the lack of a mechanical explanation of the force law. Newton's discovery that gravity obeys the inverse square law surprised Huygens and he tried to take this into account by assuming that the speed of the aether is smaller in greater distance.   
Newton objected to the theory that drag must lead to noticeable deviations of the orbits which weren't observed.  Another problem was that moons often move in different directions, against the direction of the vortex motion. Also, Huygens' explanation of the inverse square law destroys itself, because this means that the aether obeys Kepler's third law. But a theory of gravitation has to explain those laws and must not presuppose them.  
This theory was for the first time developed by Nicolas Fatio de Duillier in 1690 and was re-invented and criticised among others by Georges-Louis Le Sage (1748), Lord Kelvin (1872), James Clerk Maxwell (1875), Hendrik Lorentz (1900) and Henri Poincaré (1908)
The theory posits that the force of gravity is the result of tiny particles moving at high speed in all directions, throughout the universe. The intensity of the flux of particles is assumed to be the same in all directions, so an isolated object A is struck equally from all sides, resulting in only an inward-directed pressure but no net directional force. With a second object B present, however, a fraction of the particles that would otherwise have struck A from the direction of B is intercepted, so B works as a shield, i.e. from the direction of B, A will be struck by fewer particles than from the opposite direction. Likewise B will be struck by fewer particles from the direction of A than from the opposite direction. One can say that A and B are "shadowing" each other, and the two bodies are pushed toward each other by the resulting imbalance of forces.
This theory was declined primarily for thermodynamic reasons because a shadow only appears in this model if the particles or waves are at least partly absorbed, which should lead to an enormous heating of the bodies. Just like in the aether vortex theory drag is a great problem too. This drag problem can be solved by assuming superluminal speeds, but this solution increases the thermal problems. 
In 1675 Isaac Newton assumed that the gravitational aether is some sort of fluid which condenses at the surface of matter. Therefore a stream arises which pushes all bodies proportional to 1/r² to each other. 
Similar to Newton, but mathematically in greater detail, Bernhard Riemann assumed in 1853 that the gravitational aether is an incompressible fluid and normal matter represents sinks in this aether. So if the aether is destroyed or absorbed proportionally to the masses within the bodies, a stream arises and carries all surrounding bodies into the direction of the central muss. Riemann speculated that the absorbed aether is transferred into another world or dimension. 
Another attempt to solve the energy problem was made by Ivan Osipovich Yarkovsky in 1888. Based on his aether stream model which was similar to that of Riemann, he argued that the absorbed aether might be converted into new matter, leading to a mass increase of the celestial bodies. 
As in the case of Le Sage's theory the disappearance of energy without explanation violates the energy conservation law, also some drag must arise. Also any process which leads to a creation of matter is unknown.
Unlike his first explanation, in 1717 Newton proposed a static aether which gets thinner and thinner nearby the celestial bodies. In analogy to the lift (force) a force arises, which pushes all bodies to the central mass. ( Newton didn't give any reason why the density should decrease near the masses). He minimized drag by stating an extremely low density of the gravitational aether.  However, Newton later has dissociated himself from all mechanical explanations because of his famous statement Hypotheses non fingo and possibly according to the testimony of some of his friends like Nicolas Fatio de Duillier or David Gregory he thought that gravitation is based directly on the will of God. 
Like Newton, Leonhard Euler presupposed in 1760 that the gravitational aether loses density in accordance to the inverse square law, but he also gave no reason for this loss of density. Similar to others Euler also assumed that to maintain mass proportionality matter consists mostly of empty space. 
As admitted by Newton and Euler, there is no reason why the density of the aether should change. Furthermore James Clerk Maxwell pointed out that in this "hydrostatic" model "the state of stress....which we must suppose to exist in the invisible medium, is 3000 times greater than that which the strongest steel could support". 
Robert Hooke speculated in 1671 that gravitation is the result that all bodies emit waves in all directions through the aether. Other bodies, which interchange with these waves, move in the direction of the source of the waves. Hooke saw an analogy to the fact that small objects on a disturbed surface of water move to the center of the disturbance. 
A similar theory was worked out mathematically by James Challis from 1859 to 1876. He calculated that the case attraction occurs if the wavelength is large in comparison with the distance between the gravitating bodies. If the wavelength is small, the bodies repel each other. By a combination of these effects he also tried to explain all other forces. 
Maxwell objected that this theory requires a steady production of waves, which must be accompanied by an infinite consumption of energy.  Challis himself admitted, that he hasn't reached a definite result due to the complexity of the processes. 
Lord Kelvin (1871) and C.A. Bjerknes (1871-1880) assumed that all bodies pulsate in the aether. This was in analogy to the fact, that if the pulsation of two spheres in a fluid is in phase, they will attract each other; and if the pulsation of two spheres is not in phase, they will repel each other. This mechanísm was also used for explaining the nature of electric charges. Among others this hypothesis has also been examined by George Gabriel Stokes and Woldemar Voigt. 
To explain universal gravitation, one is forced to assume that all pulsations in the universe are in phase - which appears very implausible. In addition, the aether should be incompressible to ensure that attraction also arises on greater distances.  And Maxwell argued that this process must be accompanied by a permanent new production and destruction of aether. 
Other historical speculations
In 1690 Pierre Varignon assumed that all bodies are exposed to pushes by aether particles from all directions. Now, he assumed that there is some sort of limitation at a certain distance to earths surface, which cannot passed by the particles. Now according to Varignon bodies fall to earth if the distance between earth's surface and the body is shorter than the distance between body and the limitation. Because this implies in his opinion that the pushes at the top side of the bodies are stronger than at the bottom of the bodies. 
In 1748 Mikhail Lomonosov assumed that the effect of the aether is proportional to the complete surface of the elementary components, of which matter consists (similar to Huygens and Fatio before him). He also assumed an enormous penetrability of the bodies. However, no clear description has been indicated by him how exactly the aether interchanges with matter so that the law of gravitation arises. 
In 1821 John Herapath tried to apply his co-developed model of the kinetic theory of gases on gravitation. He assumed that the aether is heated by the bodies and loses density so that other bodies are pushed to these regions of lower density.  However, it was shown by Taylor that the decreased density due to thermal expansion is compensated by the increased speed of the heated particles, therefore no attraction arises. 
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mechanical_explanations_of_gravitation". A list of authors is available in Wikipedia.|