Gravity: The Force that Governs the Universe

Abstract

In this chapter, we describe the phenomenon of gravitation. It is the weakest of the four known fundamental forces, and yet it governs the nature of astrophysical objects like stars and galaxies, and the structure and evolution of the whole Universe. Gravity was first recognised as a universal force between any two particles of matter by Isaac Newton in the seventeenth century. Using his famous law of gravitation and the calculus that he independently developed, Newton was able to describe the motion of objects near the surface of the Earth, the orbit of the Moon around the Earth and the orbits of planets around the Sun. In spite of all its successes, Newton’s theory is not consistent with the special theory of relativity. That led Albert Einstein to develop an astonishing new theory, known as the general theory of relativity, which describes gravity as the expression of curvature in space-time, produced by matter and energy. Einstein’s theory is compatible with special relativity and it agrees with Newton’s theory when gravity is weak as in the Solar system. It is able to account for a small discrepancy in the observed orbit of the planet Mercury compared to the orbit predicted by Newton’s theory, predict the correct value for the bending of light due to gravity and predict the existence of gravitational waves. The gravitational bending of light is at the basis of the gravitational lensing, which has proved to be an important tool for astronomers.