Mechanical explanations of gravitation (or kinetic theories of gravitation) are attempts to explain the action of gravity by aid of basic mechanical processes, such as pressure forces caused by pushes, 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 general relativity is now the standard model to describe gravitation without the use of actions at a distance. Modern "quantum gravity" hypotheses also attempt to describe gravity by more fundamental processes such as particle fields, but they are not based on classical mechanics.
The theory posits that the force of gravity is the result of tiny particles or waves 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, so-to-speak—that is, 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.
P5: Permeability, attenuation and mass proportionality
This shadow obeys the inverse square law, because the imbalance of momentum flow over an entire spherical surface enclosing the object is independent of the size of the enclosing sphere, whereas the surface area of the sphere increases in proportion to the square of the radius. To satisfy the need for mass proportionality, the theory posits that a) the basic elements of matter are very small so that gross matter consists mostly of empty space, and b) that the particles are so small, that only a small fraction of them would be intercepted by gross matter. The result is, that the "shadow" of each body is proportional to the surface of every single element of matter.
Criticism: 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. Also drag, i.e. the resistance of the particle streams in the direction of motion, is a great problem too. This problem can be solved by assuming superluminal speeds, but this solution largely increases the thermal problems and contradicts special relativity.[2][3]
Vortex
Aether vortices around celestial bodies
Because of his philosophical beliefs, René Descartes proposed in 1644 that no empty space can exist and that space must consequently be filled with matter. The parts of this matter tend to move in straight paths, but because they lie close together, they cannot 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 to 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 its greater inertia—so due to the pressure of the condensed outer matter those parts will be pushed into the center of the vortex. According to Descartes, this inward pressure is nothing other than gravity. 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.[4][5][6] This idea on the formation of the cosmos by vortices of matter was preceded by the ancientpre-SocraticatomistsLeucippus and Democritus.[7]
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 that 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 was 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.[6][8][9]
Criticism: Newton objected to the theory because drag must lead to noticeable deviations of the orbits which were not observed.[10] 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 is circular, 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.[6][10]
Several British physicists developed vortex theory of the atom in the late nineteenth century. However, the physicist, William Thomson, 1st Baron Kelvin, developed a quite distinct approach. Whereas Descartes had outlined three species of matter – each linked respectively to the emission, transmission, and reflection of light – Thomson developed a theory based on a unitary continuum.[11]
Streams
In a 1675 letter to Henry Oldenburg, and later to Robert Boyle, Newton wrote the following: [Gravity is the result of] “a condensation causing a flow of ether with a corresponding thinning of the ether density associated with the increased velocity of flow.” He also asserted that such a process was consistent with all his other work and Kepler's Laws of Motion.[12] Newtons' idea of a pressure drop associated with increased velocity of flow was mathematically formalised as Bernoulli's principle published in Daniel Bernoulli's book Hydrodynamica in 1738.
However, although he later proposed a second explanation (see section below), Newton's comments to that question remained ambiguous. In the third letter to Bentley in 1692 he wrote:[13]
It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter, without mutual contact, as it must do if gravitation in the sense of Epicurus be essential and inherent in it. And this is one reason why I desired you would not ascribe 'innate gravity' to me. That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers.
On the other hand, Newton is also well known for the phrase Hypotheses non fingo, written in 1713:[14]
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction.
And according to the testimony of some of his friends, such as Nicolas Fatio de Duillier or David Gregory, Newton thought that gravitation is based directly on divine influence.[9]
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 mass. Riemann speculated that the absorbed aether is transferred into another world or dimension.[15]
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.[16]
Criticism: 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, and no process which leads to a creation of matter is known.
Static pressure
Newton updated the second edition of Optics (1717) with another mechanical-ether theory of gravity. Unlike his first explanation (1675 – see Streams), he proposed a stationary aether which gets thinner and thinner nearby the celestial bodies. On the analogy of the lift, a force arises, which pushes all bodies to the central mass. He minimized drag by stating an extremely low density of the gravitational aether.
Like Newton, Leonhard Euler presupposed in 1760 that the gravitational aether loses density in accordance with the inverse square law. Similarly to others, Euler also assumed that to maintain mass proportionality, matter consists mostly of empty space.[17]
Criticism: Both Newton and Euler gave no reason why the density of that static 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".[18]
Waves
Robert Hooke speculated in 1671 that gravitation is the result of all bodies emitting waves in all directions through the aether. Other bodies, which interact 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.[19]
A similar theory was worked out mathematically by James Challis from 1859 to 1876. He calculated that the case of 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.[20]
Criticism: Maxwell objected that this theory requires a steady production of waves, which must be accompanied by an infinite consumption of energy.[21] Challis himself admitted, that he hadn't reached a definite result due to the complexity of the processes.[19]
Pulsation
Lord Kelvin (1871) and Carl Anton Bjerknes (1871) 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 mechanism 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.[22]
Criticism: 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 at greater distances.[22] And Maxwell argued that this process must be accompanied by a permanent new production and destruction of aether.[18]
Other historical speculations
In 1690, Pierre Varignon assumed that all bodies are exposed to pushes by aether particles from all directions, and that there is some sort of limitation at a certain distance from the Earth's surface which cannot be passed by the particles. He assumed that if a body is closer to the Earth than to the limitation boundary, then the body would experience a greater push from above than from below, causing it to fall toward the Earth.[23]
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 was given by him as to how exactly the aether interacts with matter so that the law of gravitation arises.[24]
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.[25] However, it was shown by Taylor that the decreased density due to thermal expansion is compensated for by the increased speed of the heated particles; therefore, no attraction arises.[19]
Recent theorizing
These mechanical explanations for gravity never gained widespread acceptance, although such ideas continued to be studied occasionally by physicists until the beginning of the twentieth century, by which time it was generally considered to be conclusively discredited. However, some researchers outside the scientific mainstream still try to work out some consequences of those theories.
Le Sage's theory was studied by Radzievskii and Kagalnikova (1960),[26] Shneiderov (1961),[27] Buonomano and Engels (1976),[28] Adamut (1982),[29] and Edwards (2014).[30]
Gravity due to static pressure was recently studied by Arminjon.[31][32]
In physics, the fundamental interactions or fundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist:
Physics is a branch of science whose primary objects of study are matter and energy. Discoveries of physics find applications throughout the natural sciences and in technology. Historically, physics emerged from the scientific revolution of the 17th century, grew rapidly in the 19th century, then was transformed by a series of discoveries in the 20th century. Physics today may be divided loosely into classical physics and modern physics.
Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz. The visible band sits adjacent to the infrared and the ultraviolet, called collectively optical radiation.
Luminiferous aether or ether was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do. The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.
Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows:
A body remains at rest, or in motion at a constant speed in a straight line, except insofar as it is acted upon by a force.
At any instant of time, the net force on a body is equal to the body's acceleration multiplied by its mass or, equivalently, the rate at which the body's momentum is changing with time.
If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.
Causality is the relationship between causes and effects. While causality is also a topic studied from the perspectives of philosophy and physics, it is operationalized so that causes of an event must be in the past light cone of the event and ultimately reducible to fundamental interactions. Similarly, a cause cannot have an effect outside its future light cone.
Newton's law of universal gravitation states that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers. Separated objects attract and are attracted as if all their mass were concentrated at their centers. The publication of the law has become known as the "first great unification", as it marked the unification of the previously described phenomena of gravity on Earth with known astronomical behaviors.
Le Sage's theory of gravitation is a kinetic theory of gravity originally proposed by Nicolas Fatio de Duillier in 1690 and later by Georges-Louis Le Sage in 1748. The theory proposed a mechanical explanation for Newton's gravitational force in terms of streams of tiny unseen particles impacting all material objects from all directions. According to this model, any two material bodies partially shield each other from the impinging corpuscles, resulting in a net imbalance in the pressure exerted by the impact of corpuscles on the bodies, tending to drive the bodies together. This mechanical explanation for gravity never gained widespread acceptance.
Action at a distance is the concept in physics that an object's motion can be affected by another object without the two being in physical contact; that is, it is the concept of the non-local interaction of objects that are separated in space. Coulomb's law and Newton's law of universal gravitation are based on action at a distance.
Nicolas Fatio de Duillier was a mathematician, natural philosopher, astronomer, inventor, and religious campaigner. Born in Basel, Switzerland, Fatio mostly grew up in the then-independent Republic of Geneva, of which he was a citizen, before spending much of his adult life in England and Holland. Fatio is known for his collaboration with Giovanni Domenico Cassini on the correct explanation of the astronomical phenomenon of zodiacal light, for inventing the "push" or "shadow" theory of gravitation, for his close association with both Christiaan Huygens and Isaac Newton, and for his role in the Leibniz–Newton calculus controversy. He also invented and developed the first method for fabricating jewel bearings for mechanical watches and clocks.
Georges-Louis Le Sage was a Genevan physicist and is most known for his theory of gravitation, for his invention of an electric telegraph and his anticipation of the kinetic theory of gases. He was a contributor to Diderot's Encyclopédie.
The deductive-nomological model of scientific explanation, also known as Hempel's model, the Hempel–Oppenheim model, the Popper–Hempel model, or the covering law model, is a formal view of scientifically answering questions asking, "Why...?". The DN model poses scientific explanation as a deductive structure, one where truth of its premises entails truth of its conclusion, hinged on accurate prediction or postdiction of the phenomenon to be explained.
According to ancient and medieval science, aether, also known as the fifth element or quintessence, is the material that fills the region of the universe beyond the terrestrial sphere. The concept of aether was used in several theories to explain several natural phenomena, such as the propagation of light and gravity. In the late 19th century, physicists postulated that aether permeated space, providing a medium through which light could travel in a vacuum, but evidence for the presence of such a medium was not found in the Michelson–Morley experiment, and this result has been interpreted to mean that no luminiferous aether exists.
In the history of physics, aether theories proposed the existence of a medium, a space-filling substance or field as a transmission medium for the propagation of electromagnetic or gravitational forces. Since the development of special relativity, theories using a substantial aether fell out of use in modern physics, and are now replaced by more abstract models.
What is now often called Lorentz ether theory (LET) has its roots in Hendrik Lorentz's "theory of electrons", which marked the end of the development of the classical aether theories at the end of the 19th and at the beginning of the 20th century.
In optics, the corpuscular theory of light states that light is made up of small discrete particles called "corpuscles" which travel in a straight line with a finite velocity and possess impetus. This notion was based on an alternate description of atomism of the time period.
In physics, theories of gravitation postulate mechanisms of interaction governing the movements of bodies with mass. There have been numerous theories of gravitation since ancient times. The first extant sources discussing such theories are found in ancient Greek philosophy. This work was furthered through the Middle Ages by Indian, Islamic, and European scientists, before gaining great strides during the Renaissance and Scientific Revolution—culminating in the formulation of Newton's law of gravity. This was superseded by Albert Einstein's theory of relativity in the early 20th century.
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain, and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena.
Treatise on Light: In Which Are Explained the Causes of That Which Occurs in Reflection & Refraction is a book written by Dutch polymath Christiaan Huygens that was published in French in 1690. The book describes Huygens's conception of the nature of light propagation which makes it possible to explain the laws of geometrical optics shown in Descartes's Dioptrique, which Huygens aimed to replace.
In the history of physics, the concept of fields had its origins in the 18th century in a mathematical formulation of Newton's law of universal gravitation, but it was seen as deficient as it implied action at a distance. In 1852, Michael Faraday treated the magnetic field as a physical object, reasoning about lines of force. James Clerk Maxwell used Faraday's conceptualisation to help formulate his unification of electricity and magnetism in his field theory of electromagnetism.
↑ Riemann, B. (1876), Dedekind, R.; Weber, W. (eds.), "Neue mathematische Prinzipien der Naturphilosophie", Bernhard Riemanns Werke und Gesammelter Nachlass, Leipzig: 528–538
↑ Yarkovsky, I. O. (1888), Hypothese cinetique de la Gravitation universelle et connexion avec la formation des elements chimiques, Moscow{{citation}}: CS1 maint: location missing publisher (link)
↑ Radzievskii, V.V. & Kagalnikova, I.I. (1960), "The nature of gravitation", Vsesoyuz. Astronom.-Geodezich. Obsch. Byull., 26 (33): 3–14 A rough English translation appeared in a U.S. government technical report: FTD TT64 323; TT 64 11801 (1964), Foreign Tech. Div., Air Force Systems Command, Wright-Patterson AFB, Ohio (reprinted in Pushing Gravity)
↑ Shneiderov, A. J. (1961), "On the internal temperature of the earth", Bollettino di Geofisica Teorica ed Applicata, 3: 137–159
↑ Adamut, I. A. (1982), "The screen effect of the earth in the TETG. Theory of a screening experiment of a sample body at the equator using the earth as a screen", Nuovo Cimento C, 5 (2): 189–208, Bibcode:1982NCimC...5..189A, doi:10.1007/BF02509010, S2CID117039637
Wikisource has several original texts related to Kinetic gravity.
Aiton, E.J. (1969), "Newton's Aether-Stream Hypothesis and the Inverse Square Law of Gravitation", Annals of Science, 25 (3): 255–260, doi:10.1080/00033796900200151
Van Lunteren, F. (2002), "Nicolas Fatio de Duillier on the mechanical cause of Gravitation", in Edwards, M.R. (ed.), Pushing Gravity: New Perspectives on Le Sage's Theory of Gravitation, Montreal: C. Roy Keys Inc., pp.41–59
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