4 SOCIAL SCIENTIST It was shown by Galileo that the acceleration of a body under gravity does not depend on its mass or composition or structure. This is a very important result and the whole of the General Theory is based on this proposition of Galileo. Einstein's principle of equivalence goes beyond the above principle of equal fall. Galileo's principle tells us that by using any mechanical process it is impossible to distinguish between a gravitational field and a field due to acceleration such as within an accelerating lift. This is because all bodies will have the same acceleration in such fields. Einstein generalised this and said that it is impossible by any physical process whatever to distinguish between such fields. Tt is shown in the General Theory that a gravitational field is associated with the nature of the geometry in the region of the field. Einstein identified the gravitational field with the non-Euclidean nature of the geometry. The scheme of the General Theory is the following: from the way the gravitating bodies are distributed, we calculate the geometrical structure of the space by using Einstein's field equations. Then we find the geodesies (shortest paths) in this space. Free test bodies travel along geodesies. Thus we can calculate the paths of bodies in gravitational fields. Einstein published his theory of gravitation in 1916, first applying it to the solar system. Here the major mass is the sun, and the geometry is decided mainly by this. The field equations of Einstein are very complicated and cannot be solved in their generality. However, by assuming that the geometry is symmetric around the sun the equations can be solved. Einstein showed that the theory led to the following three main consequences: (a) The path of a ray of light near the sun is bent to 1.7 seconds of arc (one/two thousands of a degree), thereby correcting his earlier estimate by 100 per cent. (b) The paths of planets are not closed ellipses but elliptical curves turning slowly round the focus. In the case of Mercury this accounted for the discrepancy of 43 seconds of arc per century. This was very close to the observed results. (c) Light emitted from the sun and received on the earth will be redder in comparison with the light emitted by identical atoms on earth. This is called the gravitational red shift. This has been verified. All the three predictions of Einstein's theory have been verified to a very high degree of precision. Apart from these three crucial consequences, Einstein's theory predicted a new phenomenonc ailed gravitational waves, A number of scientists making astronomical observations have come to the conclusion that "the only straightforward interpretation of the data is that gravitational waves exist and carry energy away from an orbiting system at the rate predicted by general relativity".2