The mystery of action at a distance 1011 The gravitational force

We begin with a topic which we have already dealt with in Chapter 5, apparently far removed from electricity, the relevance of which will become clear shortly. Objects fall because a mysterious force called gravity attracts them to the earth. As a child learns to walk it learns the art of balancing to combat this force. Familiarity may have taken away the sense of mystery, but the puzzle remains: How is it possible for things to be attracted to the earth without a visible link or connection? It is the first example we meet of a number of curious phenomena in nature of action at a distance.

Isaac Newton recognised gravity as a fundamental law of nature. A logical argument further led him to assume that as the separation of the masses increased, their mutual attraction would decrease in proportion to the square of the distance. This led him to the formula, to be confirmed by experiment, for the force of attraction between two masses m1 and m2 separated by a distance d:

Gm1m2 ~d2

where G is a constant which has the same value everywhere in the universe.

Hypotheses non fingo' — Newton did not wish to speculate on any philosophical implications of such 'action at a distance', In Book III of his Principia he acknowledges:

'But hitherto I have not been able to discover the cause of these properties of gravity...I frame no hypotheses...it is enough that gravity really exists and acts according to the laws we have described.'

As might be expected, Newton had no shortage of critics. In a letter to Gottfried von Leibnitz (1646-1716), Christiaan Huygens wrote: 'I am far from satisfied nor do I feel happy about theories built on his (Newton's) principle of attraction, which to me seems absurd.' The French philosopher René Descartes (1596-1650) expressed his reservations poetically: 'We must assume that...these souls of material particles are endowed with knowledge of a truly divine sort, so that they may know without any medium what takes place at very great distances, and act accordingly.'

Leaving aside philosophical reservations, the fact remains that every piece of matter, merely by its existence, exerts an influence which permeates the space around it and attracts other matter. This influence decreases gradually with distance, but never completely dies away. We call it gravitation.

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