Nebular Hypothesis Of Laplace
French mathematician Laplace propounded his 'nebular hypothesis' in the year 1796. He elaborted his concepts about the origin of the solar system and the earth in his book entitled 'Exposition of the World System', Laplace's nebular hypothesis was in some way similar to the gaseous hypothesis of Kant. It appears that Laplace's hypothesis is just the modified version of Kant's hypothesis. It may be pointed out that Laplace propounded his hypothesis without math ematical formulation. In fact, Laplace postulated his hypothesis after removing the inherent weak points and erroneous concepts of Kant's hypothesis which suffered from three basic defects e.g.
(1) Large amount of heat cannot be generated due to the collision of cold particles of primordial matter.
(2) Mutual collision of particles cannot generate motion in the primordial matter and the random motion of the particles cannot generate circular motion (rotatory motion) in the primordial matter.
(3) The angular velocity of rota tory speed of the nebula cannot increase due to increase in the size of the nebula.
In order to remove the aforesaid defects
Laplace assumed certain axioms for the postulation of his nebular hypothesis to solve the riddle of the origin of the earth.
(1) He assumed that there was a huge and hot gaseous nebula in the space. Thus, he solved the problem of heat of the nebula through this assump tion.
(2) From the very beginning huge and hot nebula was rotating (spinning) on its axis.
(3) The nebula was continuously cooling due to loss of heat from its outer surface through the process of radia tion and thus it was contiunuously reducing in size due to contraction on cooling.
Evolution
The nebular hypothesis of Laplace commanded continued great respect for more than 150 years due to its simplicity but as the knowledge about the universe and its related phenomena as well as the knowledge of science and technology of the modernworld increased, the nebular hypothesis began to fade away due to severe criticisms of the hypothesis by the scientists on various grounds.
It is significant to point out that the nebular hypothesis not only tried to explain and slove the problems of the origin of the earth but it also described the constitution of the earth.
According to this hypothesis the planets and also the earth were initially in gaseous state. The earth was changed into liquid state on cooling and ultimately the solid crust was formed. But some scientists claimed that the earth retained a solid condition throughout its period of growth. How ever, the nebular hypothesis is renderd untenable on the basis of the following demerits.
Laplace's nebula was also an isolated system. therefore, it should have conserved its total angular momentum even after the separation of irregular ring but this is not the case as the sun contains only one or less than one per cent of total angular momen tum. Thus, this hypothesis is against established mathematical formulation.
In fact, the sun must have acquired most of the angular momentum of our solar system but this did not happen. Laplace did not explain the causes for this anomaly. It may be concluded that the present day total angular momentum of our solar system, originally concen trated in the nebula, is not quite sufficient for the separation of ring from the nebula by excess of rotatory motion.
Based on above arguments the nebular hy pothesis of Laplace is rendered untenable.
