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Gravitation is the force which occurs between every body of mass no matter how small, within the universe.
The first physicist to mathematically formalise gravitation was Sir Isaac Newton (1687). Newton assumed that Space and Time existed as Absolute things, and that matter existed as discrete 'particles' that moved in Space and Time. Gravity was assumed to be an attractive force proportional to the sum of the masses and inversely proportional to the square of the distance between them. The force of Gravity was also assumed to act instantly at-a-distance, though Newton never claimed to describe Gravity, only to calculate its force / effect on other matter. Newton writes;
 (Sir Isaac Newton) It is inconceivable that inanimate brute matter should, without mediation of something else which is not matter, operate on and affect other matter without mutual contact. ... That gravity should be innate, inherent and essential to matter, so that one body may act upon another at-a-distance, through a vacuum, without the mediation of anything else by and through which their action may be conveyed from one to another, is to me so great an absurdity that I believe no man, who has in philosophical matters a competent faculty of thinking, can ever fall into it. ... So far I have explained the phenomena ... by the force of gravity, but I have not yet ascertained the cause of gravity itself ... and I do not arbitrarily invent hypotheses. (Newton. Letter to Richard Bentley 25 Feb. 1693)
In 1915 Enstein's general theory of relativity replaced Newton's mechanics as the favoured theory for calculating the path of objects in gravitational fields. However, Einstein's General Relativity is not a theory of forces, but rather, assumes that matter (energy density of space) causes a curvature of the Space-Time continuum, and thus bodies in a gravitational field do not experience forces, but instead travel along a curved path in space-time. Einstein writes;
 (Albert Einstein) The first attempt to lay a uniform theoretical foundation was the work of Newton. In his system everything is reduced to the following concepts:
1) Mass points with invariable mass;
2) Action at a distance between any pair of mass points;
3) Law of motion for the mass point.
In Newtonian physics the elementary theoretical concept on which the theoretical description of material bodies is based is the material point, or particle. Thus matter is considered a priori to be discontinuous. This makes it necessary to consider the action of material points on one another as action-at-a-distance. Since the latter concept seems quite contrary to everyday experience, it is only natural that the contemporaries of Newton - and indeed Newton himself - found it difficult to accept. Owing to the almost miraculous success of the Newtonian system, however, the succeeding generations of physicists became used to the idea of action-at-a-distance. Any doubt was buried for a long time to come. (Albert Einstein, 1950)
Forces acting directly and instantaneously at a distance, as introduced (by Newton) to represent the effects of gravity, are not in character with most of the processes familiar to us in everyday life.
Newton's theory of motion, considered as a program for the whole of theoretical physics, received its first blow from Maxwell's theory of electricity. It became clear that the electric and magnetic interaction between bodies were effected, not by forces operating instantaneously at a distance, but by processes which are propagated through space at a finite speed. In addition to the mass point and its motion there arose according to Faraday's concept a new kind of physical reality, namely, the field.
The greatest change in the axiomatic basis of physics - in other words, of our conception of the structure of reality - since Newton laid the foundation of theoretical physics was brought about by Faraday's and Maxwell's work on electromagnetic phenomena.
According to general relativity, the concept of space detached from any physical content (matter, objects) does not exist. The physical reality of space is represented by a field whose components are continuous functions of four independent variables - the coordinates of space and time.
Since the theory of general relatively implies the representation of physical reality by a continuous field, the concept of particles or material points cannot play a fundament part, nor can the concept of motion. The particle can only appear as a limited region in space in which the field strength or the energy density are particularly high.
The non-mathematician is seized by a mysterious shuddering when he hears of "four-dimensional" things, by a feeling not unlike that awakened by thoughts of the occult. And yet there is no more common-place statement than that the world in which we live is a four-dimensional space-time continuum. Space is a three-dimensional continuum. ... Similarly, the world of physical phenomena which was briefly called 'world' by Minkowski is naturally four dimensional in the space-time sense. For it is composed of individual events, each of which is described by four numbers, namely, three space co-ordinates x, y, z, and the time co-ordinate t.
Editor: Haselhurst
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(Albert Einstein) Gravitation had indeed been deduced from the structure of space, but besides the gravitational field there is also the electromagnetic field. This had, to begin with, to be introduced into the theory as an entity entirely independent from gravitation. Terms which took account of the electromagnetic field had to be added to the fundamental field equations. But the idea that there exist two structures of space independent of each other, the metric-gravitational and the electromagnetic, was intolerable to the theoretical spirit. We are prompted to the belief that both sorts of field must correspond to a unified structure of space.