The Complex Itinerary of Leibniz’s Planetary Theory by Paolo Bussotti

Author:Paolo Bussotti
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.7 Final Remarks on Leibniz’s Gravity Theory

In this chapter I have tried to show how strong the connection between planetary theory, cosmology and theory of gravity is, and how these three sections of Leibniz’s physics were developed jointly, although it is possible to identify the two Tentamina and the Illustratio as specific works concerning planetary theory, while Leibniz’s ideas on cosmology and gravity are disseminated in numerous works. However, it is impossible to get a complete idea of planetary theory without specific references to gravity theory and cosmology. We have also seen the evolution of Leibniz gravitational theory and of his ideas on the origin of gravity. In this context, until the publication of Newton’s Principia, the main concern of Leibniz was to provide a plausible mechanism for gravity, in the tradition of Descartes, Hobbes and Torricelli, although with personal ideas which, often, did not coincide with those of these other scientists. After publication of the Principia and with the full comprehension that a theory of gravity based on mechanical models had to fulfil two requests: 1) to explain from the phenomenological point of view how gravity is produced; 2) to explain from a physical-mathematical standpoint the inverse square law, Leibniz’s researches became wider and more profound. In this context Huygens’ works were fundamental for Leibniz, but the critical comparison with authors such as Papin, Catalan, De Volder, Fatio de Duillier, Johann and Jakob Bernoulli was important, as well.61 In particular, in the years following the publication of the Principia, the problem of guaranteeing the validity of the inverse square law inside the vortex theory took a prominent role in Leibniz’s thought. Progressively, Leibniz also adhered to the idea that gravity on the earth was an action quite similar to the force responsible for planetary movements, even though it is difficult to establish whether he had fully realized it was exactly the same force. There are numerous passages in Leibniz’s works and letters which testify to this situation. Among them, I have chosen these three which seem to me particularly significant. Many others could be selected.

One of the most interesting documents is the letter to Des Billettes in December 1696, where Leibniz dealt with many questions concerning gravity. In particular he wrote:As to gravity (pesanteur) Mr. Newton taught us a proportion, of which I knew already something; namely that the planets are such that the gravities or attractions are in inverse proportion as the squares of the distances. […] Now, I have found this agrees with the action of light rays. For, a subtle, but dense (solide) fluid will recede from the centre and form something as emission rays. This is also in agreement with the receding from the centre along the tangent, property which Kepler first applied to gravity. In this Mr. Descartes followed him. But the sole instantaneous conatus of the centrifugal force is not enough to originate either light or gravity, in the manner imagined by Mr. Descartes. It is necessary that a very movement of emission is produced, as a wind which blows and which requires time.

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