Chapter 2. Gravity – the “blue” field
Newton’s insight that the moon is a falling object led to a theory of gravity that explained the elliptical orbits of the moon and planets. Although Newton did not treat gravity as a field, he strongly rejected the concept of action-at-a-distance. The idea that gravity is a field was introduced in 1915 by Albert Einstein. After many false starts, Einstein succeeded in deriving a gravitational field equation from his Principle of Equivalence, which states that gravitational forces are indistinguishable from inertial forces. This theory is known as the General Theory of Relativity. The success of general relativity in explaining the discrepancy in the orbit of Mercury and in predicting the deflection of starlight (confirmed a year later) caused Einstein to achieve a world-wide fame that surpassed even that of Newton. One consequence of Einstein’s theory is the existence of gravity waves, which were detected for the first time in 2015.