Astronomy and Cosmology

Author(s): L C Jain
Publisher: Rajasthan Prakrit Bharti Sansthan Jaipur

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first prove that it is not at the center of the circle. The next thing is for us to see whether the earth is or is not at the center around which I say it turns, and in which you say it is situated. And prior to this, it is necessary that we declare ourselves as to whether or not you and I have the same concept of this center. Therefore, tell me what and where this center is that
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you mean.
It was left for the genius of Isaac Newton to formulate the laws of celestial mechanics in the Principia on the basis of the works of Galileo, Kepler, and Tycho Brahe, consistent with the Copernican system. Discussing about the absolute and relative, true and apparent, mathematical and common, Newton writes, "Absolute time, in astrnomy, is distinguished from relative by the equation or correction of the apparent time. For the natural days are truly, unequal, though they are commonly considered as equal and used for a measure of time; astronomers correct this inequality that they may measure the celestial motions by a more accurate time. It may be that there is no such thing as an equable motion whereby time may be accurately measured. All motions may be accelerated and retarded, but the flowing of absolute time is not liable to any change." 13
Albert Einstein contributed to relativistic mechanics replacing Newtonian space-time by a super-edifice of four-dimensional continuum on the basis of Riemannian geometry and tensor calculus of Levi Civita. Writing on the fundaments of theoretical physics, he explains, "The general theory of relativity owes its origin to the attempt to explain a fact known since Galileo's and Newton's time but hitherto eluding all theoretical interpretations the inertia and the weight of a body, in themselves two entirely distinct things, are measured by one and the same constant, the mass. From this correspondence follows that it is impossible to discover by experiment whether a given system of coordinates is accelerated, or whether its motion is straight and uniform and the observed effects are due to a gravitational field (this is the equivalence principle of general relativity theory). It shatters the concepts of the inertial system, as soon as gravitation enters in, It may be remarked here that the inertial system is a weak point of the Galilean-Newtonian mechanics. For there is presupposed a mysterious property of physical space, conditioning the kind of coordinate-systems for which the law of inertia and the Newtonian law of motion hold good." 14
Einstein's theory of gravitation on introduction of a cosmological term results in two types of universes; Einstein's closed spherical universe
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op. cit.
13. Cf. Principia Mathematica, op. cit.
14. Einstein, Ideas and opinions, London, 1956, p. 330.
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