Mechanical explanations of gravity

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The mechanical explanations of gravity (also kinetic theories of gravity , or pressure theories or impact theories of gravity ) are attempts to explain the law of gravity with the aid of the simple mechanical phenomena of pressure , impact and friction . A long-range effect , which was originally considered a concept not belonging to mechanics, should be avoided. These theories, which were mainly developed between the 16th and 19th centuries in connection with the ether , are no longer regarded as useful explanations of gravitation. The current standard model for describing gravity is general relativity .

shielding

This probably best-known mechanistic explanatory model was first developed by Nicolas Fatio de Duillier (1690) and later by Georges-Louis Le Sage (1748), Lord Kelvin (1872), Hendrik Antoon Lorentz (1900) and by James Clerk Maxwell (1875) and Henri Poincaré (1908) criticized.

The basic assumption of the theory is the existence of a space that is largely isotropically filled by a radiation field consisting of various particles (corpuscles) or waves . These move in a straight line in all possible directions at a constant, very high speed. When a particle hits a body, it transmits some momentum to it. If there is only one body A, it is exposed to an even pressure , ie it is in an equilibrium of forces due to the impacts acting in all directions and will not move. However, if there is a second body B, it acts like a screen, because A is hit by fewer particles from direction B than from the other side, and the same applies vice versa. A and B shade each other and this creates a vacuum on the sides facing each other. This creates an apparently attractive force that works exactly in the direction of the other body.

Penetration, weakening and proportionality to the mass

The law of distance was explained as follows: If one imagines a spherical surface (sphere) around a body, which must be traversed by both the reflected and the incoming particles, it becomes clear that the size of the sphere increases proportionally to the square of the distance. The number of particles in question in these growing sections remains the same, however, and their density therefore decreases. And in order to achieve the proportionality to the mass , it was assumed that the matter consists largely of empty space and that the particles, assumed to be very small, can easily penetrate the body. This means that the particles penetrate the body, interact with all components of matter, are partially shielded or absorbed and then exit again weakened.

criticism

This theory was rejected mainly for thermodynamic reasons, because since shadow formation only occurs when the particles or waves are at least partially absorbed, enormous unobserved warming would have to occur. As in the ether vortex theory, the resistance in the direction of movement is a big problem, which can be solved by assuming much higher speeds of the particles than that of the light , but this makes the thermal problem worse.

whirl

Vortices of liquid celestial matter - commonly referred to as ether - around fixed stars and planets according to René Descartes

On the basis of philosophical considerations, René Descartes explained in 1644 that no empty space (space = physical volume-containing part of the universe or the environment separated from the outside world) could exist and consequently the space had to be filled with matter . In principle, the parts of this matter move in a straight line, but because they are close together, they cannot move freely, and Descartes concludes from this that all movement is basically circular or vortex-shaped . Descartes further differentiates between different shapes and sizes of matter, with matter consisting of coarser parts resisting circular movement more strongly than finer matter. Due to a kind of centrifugal force , the finer matter now tends to move further and further away from the center. This leads to a compression of this matter at the outer edges of the vortices. The coarser matter not only cannot follow this movement due to its inertia , but is pressed into the center of the vortex by the pressure of the compressed finer matter located on the outer edges of the vortex. According to Descartes, this pressure into the center is nothing other than gravity. Descartes compared this mechanism of liquid celestial matter with the fact that if the liquid is rotated in a vessel filled with water and small pieces of light matter (e.g. wood) are dropped into the vessel, this will be in the center of the Will collect the vessel.

Following the basic premises of Descartes, Christiaan Huygens designed a much more precise vortex model between 1669 and 1690, or the first mathematically worked out theory of gravity. He assumed that the etheric matter moves evenly in all directions, but is thrown back at the outer borders of the vortex and occurs there in greater concentration or density, as with Descartes. This leads to the fact that the finer matter also pushes the coarser matter inwards. Huygens found out that the centrifugal force that acts on a body is equal to its gravity ( centripetal force ). He also postulated that normal matter must for the most part consist of empty space so that etheric matter can penetrate it evenly. He concluded that the fine ethereal matter moves much faster than the earth rotates. At this time Newton developed his theory of gravity based on attraction, which Huygens found insufficient due to the lack of a mechanical justification. Newton's finding that gravity is subject to the law of distance surprised Huygens, and he tried to take this into account by assuming that the speed of the ether would decrease with increasing distance, i.e., basically obey Kepler's 3rd law .

criticism

Newton objected to the theory that the flow resistance in the direction of motion must lead to noticeable deviations from the orbits , which is not observed. Moons also often move in different directions, which speaks against the vortex view. According to Horst Zehe, Huygens' theory also destroys itself, because a gravitation theory should explain Kepler's laws from the gravitational mechanism and should not assume it.

Currents

Isaac Newton assumed around 1675 that gravitation arises from the fact that the gravitational ether is comparable to a liquid that condenses on the surface of normal matter. This creates a current that carries along the surrounding masses proportionally to 1 / r².

Similar to Newton, but worked out in more mathematical detail, Bernhard Riemann assumed around 1853 that the gravitational ether represents an incompressible fluid and normal matter should be understood as "sinks" in this ether. That means, etheric matter is destroyed or absorbed by the bodies proportionally to their masses and thus transferred to another world or dimension, so that a current arises around each body, which pulls all surrounding bodies with it towards the center of mass.

Iwan Ossipowitsch Jarkowski assumed in 1888 that the absorbed ether is neither destroyed nor liquefied, but is converted into new chemical elements , which is supposed to cause the earth to expand .

criticism

As with Le Sage gravitation, the traceless disappearance of energy in the body violates the law of conservation of energy , and there must also be a flow resistance in the direction of movement. A creation of new matter is also not observed.

Static ether

In contrast to his first explanation, Newton later (1717) proposed a static ether that becomes thinner and thinner in the vicinity of the celestial bodies, and analogous to the static buoyancy in liquids, there is a force acting in the direction of the earth. (However, Newton gave no reason why the density should decrease with the law of distance ). He minimized the usual resistance in the direction of movement for moving bodies in liquids by assuming an extremely low density of the gravitational ether. Newton then distanced himself from all mechanical explanations by pointing out that he did not want to set up any hypotheses and, according to the testimony of some of his friends such as Nicolas Fatio de Duillier or David Gregory, assumed that gravitation is based directly on God's will .

Like Newton, Leonhard Euler assumed around 1760 that the gravitational ether would lose density in accordance with the law of distance in the vicinity of the bodies, but even he gave no reason for this decrease in density. Like Huygens, Fatio and Le Sage, he assumed that matter had very fine pores, which the ether could easily penetrate in order to maintain the mass proportionality.

criticism

As Newton and Euler themselves admitted, there is no justification here why the density of the aether should change at all. James Clerk Maxwell also pointed out that in this “ hydrostatic ” model there is an enormous load on the aether, which can be defined as rigid, which is around 3000 times stronger than the most resistant steel known at the time.

waves

Robert Hooke speculated in 1671 that gravitation may arise from bodies generating waves that rush through the aether in all directions. Other bodies that interact with these waves then move towards the source of the wave. Hookes saw this as an analogy to the fact that small objects on a disturbed water surface move towards the center of the disturbance.

A similar theory was mathematically worked out by James Challis from 1859 to 1876. He calculated that the case of attraction occurs when the wavelength is large compared to the distance between the gravitating bodies. If the wavelength is small, the bodies repel each other. Through a combination of these effects, he tried to explain all the other powers.

criticism

Maxwell objected that this constant regeneration of waves must be accompanied by an infinite consumption of energy, which is not compatible with the law of conservation of energy. Challis himself admitted that due to the complexity of the processes he had not reached a final result.

Pulsation

Here z. B. by Kelvin (1871) and Carl Anton Bjerknes (1871–1880) the ether is understood as a liquid, whereby normal matter should pulsate within this liquid. As has been found in experiments in liquids, 2 bodies attract each other when their pulsations are in phase , and a repulsive force results when their pulsations are out of phase. This hypothesis is among others of Sir George Stokes and Woldemar Voigt been studied.

criticism

In order to explain the universal gravity it has to be assumed that all pulsations of the universe are in phase, which seems very artificial. The aether would also have to be practically incompressible in order to guarantee the attraction over a greater distance. And, as Maxwell said, the constant regeneration and destruction of the ether must be explained.

Other historical speculation

Pierre Varignon explained in 1690 that all bodies are exposed to shocks from an etheric matter from all directions. At a certain distance from the earth there should now be a limit beyond which the ether particles cannot get. According to him, bodies fall to earth when the distance between the surface of the earth and the body is less than the distance between the body and the boundary. In his opinion, this implies that more and stronger impacts take place on the upper side of the body than on the underside.

The physicist Mikhail Wassiljewitsch Lomonossow , who was very influential within Russia , assumed in 1748 that the effect of an etheric matter is proportional to the total surface area of ​​the elementary components of which the matter is composed (as did Huygens and Fatio before him). Like this he also assumes an enormous permeability of matter. However, no further information was given by him, how exactly the ether particles affect the matter, so that the law of gravitation results from it.

In 1821 John Herapath tried to apply the kinetic gas theory model he helped to develop to gravity. He assumed that the ether is comparable to a gas , which is heated by heat radiation from the matter and loses density, so that the other bodies are pushed into these regions of lower density. As Taylor showed, this assumption was wrong: The density of this ether gas decreases at higher temperatures due to the thermal expansion, but the speed of the ether particles increases in the same proportion, so there is no attraction.

See also

literature

  • EJ Aiton: Newton's Aether-Stream Hypothesis and the Inverse Square Law of Gravitation . In: Annals of Science . 25, 1969, pp. 255-260.
  • Hereward Carrington: Earlier Theories of Gravity . In: The Monist . 23, 1913, pp. 445-458.
  • Paul Drude: About remote effects . In: Annals of Physics . 298, No. 12, 1897, pp. I-XLIX. doi : 10.1002 / andp.18972981220 .
  • Thomas Proctor Hall: Physical Theories of Gravitation . In: Proceedings of the Iowa Academy of Science . 3, 1895, pp. 47-52.
  • Georg Helm: About the mediation of remote effects through the ether . In: Annals of Physics . 250, No. 9, 1881, pp. 149-176. doi : 10.1002 / andp.18812500912 .
  • Caspar Isenkrahe: On the reduction of gravity to absorption and the laws derived from it . In: Treatises on the history of mathematics. tape 6 . Leipzig 1892, p. 161-204 ( quod.lib.umich.edu ).
  • James Clerk Maxwell: Atom . In: Encyclopædia Britannica Ninth Edition . 3, 1875, pp. 36-49.
  • James Clerk Maxwell: Attraction . In: Encyclopædia Britannica Ninth Edition . 3, 1875, pp. 63-65.
  • JW Peck: The Corpuscular Theories of Gravitation . In: Proceedings of the Royal Philosophical Society of Glasgow . 34, 1903, pp. 17-44.
  • Poincaré, Henri: Lesage's theory . In: Science and Method . Nelson & Sons, London / New York 1908, p. 246-253 ( Wikisource ).
  • Samuel Tolver Preston: Comparative Review of some Dynamical Theories of Gravitation . In: Philosophical Magazine . 39, No. 237, 1895, pp. 145-159.
  • Taylor, William Bower: Kinetic Theories of Gravitation . In: Smithsonian report . 1876, pp. 205-282.
  • F. Van Lunteren: Nicolas Fatio de Duillier on the mechanical cause of gravitation . In: MR Edwards (Ed.): Pushing Gravity: New Perspectives on Le Sage's Theory of Gravitation . C. Roy Keys Inc., Montreal 2002, p. 41-59 .
  • Horst Zehe: Nicolas Fatio de Duillier's theory of gravity . Gerstenberg, Hildesheim 1980, ISBN 3-8067-0862-2 .
  • Jonathan Zenneck : Gravitation . In: Encyclopedia of Mathematical Sciences, including its applications . 5, No. 1, 1903, pp. 25-67.

supporting documents

  1. ↑ Taylor (1876), Literature
  2. ↑ Drude (1897), literature
  3. ↑ Maxwell (1875, Atom), literature
  4. ^ Poincaré (1908), literature
  5. ↑ a b René Descartes: Principles of Philosophy: From the Visible World , Figure 8 to Section 23, to which Descarte refers again and again in many other sections of his treatise "From the Visible World".
  6. ↑ See also: Shmuel Sambursky: The Path of Physics: 2500 Years of Physical Thought Texts from Anaximander to Pauli - Artemis Zurich / Munich 1975. - p. 324 in the part of the texts by Descartes p. 311 ff.
  7. ↑ Descartes: See section 24, where Descartes assumes that not only the matter of the sun and the fixed stars, but of the whole sky is liquid.
  8. ↑ Descartes: See Section 30., where Descartes mentions a blade of grass floating in a vortex as an example
  9. ↑ a b c toe (1980), literature
  10. ↑ Ch. Huygens, Traité de la lumière ..., Leyden 1690; (English translation SP Thomson, Dover Edition, New York 1962)
  11. ^ C. Huygens: Discours de la Cause de la Pesanteur (1690) . In: Société Hollandaise des Sciences (ed.): Oeuvres complètes de Christiaan Huygens . tape 21 . The Hague 1944, p. 443-488 ( gallica.bnf.fr ).
  12. ↑ Van Lunteren (2002), Literature
  13. ↑ Isaac Newton: About gravitation….  : Texts on the philosophical foundations of classical mechanics; Latin-German text, transl. and ext. by Gernot Böhme. - Klostermann, cop., Frankfurt / M. 1988. (Klostermann texts. Philosophy) - A fragment in which Newton deals with Descartes' vortex physics.
  14. ^ I. Newton: Newton's Principia, The mathematical Principles of Natural Philosophy (1687) . Daniel Adee, New York 1846 ( archive.org ). ; New translation by Bedrnard Cohen and Ann Whitman, University of California Press, Berkley 1999, The circular motion of fluids. Pp. 779–790, “End of Book 2”
  15. ↑ Aiton (1969), literature
  16. ↑ B. Riemann: New mathematical principles of natural philosophy . In: R. Dedekind, W. Weber (Ed.): Bernhard Riemann's works and collected estate . Leipzig 1876, S. 528-538 ( quod.lib.umich.edu ).
  17. ↑ IO Yarkovsky: hypothesis cinetique de la gravitation universal connexion et avec la formation of elements chimiques . Moscow 1888.
  18. ^ I. Newton: Opticks . 4th edition. William Innys, St. Pauls 1730 ( books.google.de ).
  19. ^ L. Euler: Fiftieth Letter (August 30, 1760) . In: Letters to a German Princess . tape 1 . Leipzig 1776, p. 173-176 ( books.google.at ).
  20. ↑ a b c Maxwell (1875), literature
  21. ↑ a b c Taylor (1876), see literature
  22. ^ J. Challis: Notes of the Principles of Pure and Applied Calculation . Cambridge 1869 ( archive.org ).
  23. ↑ a b Zenneck (1903), literature
  24. ^ P. Varignon: Nouvelles conjectures sur la Pesanteur . Paris 1690 ( gallica.bnf.fr ).
  25. ↑ M. Lomonossow: On the Relation of the Amount of Material and Weight (1758) . In: Henry M. Leicester (ed.): Mikhail Vasil'evich Lomonossov on the Corpuscular Theory . Harvard University Press, Cambridge 1970, pp. 224-233 ( archive.org ).
  26. ↑ J. Herapath: On the Causes, Laws and Phenomena of Heat, Gases, Gravitation . In: Annals of Philosophy . tape 9 . Paris 1821, p. 273-293 ( books.google.at ).