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2.4 Newtonian Gravity

Newton observed that large scale gravitational systems in the nearby universe, including the Earth itself, were spherical in shape and centered around a ball of matter consisting of gas and/or rock. He noted that, 'gravitation' seemed to be a property of all matter, and that even the smallest possible objects emit 'gravitation' proportional to their mass. He further assumed that matter could be sub-divided indefinitely, and invented the concept of the 'material point', whereby infinitesimally small particles of matter (geometrical points) emit gravity and are influenced by it. Hence, using his newly invented calculus, he showed that a large volume of matter could be treated as if its mass was concentrated at a single point which he called its 'center of gravity'. With this simplification, he showed that the motions of planets around the Sun, could be accounted for as the trajectories of such points.

Newton also used calculus to show that the gravitational field within a spherical shell of matter is zero, and that the field outside the shell is equivalent to the same quantity of mass as if it were located at the center of the shell. Using these concepts, it is possible to calculate the gravitational field within the Earth's surface. This shows that the field reaches its maximum value at the surface, and that at the center, the field is zero.

Newton's observation that gravity is a spherical force system is a strong indicator of its origin. In modern terms, it means that gravitational forces converge towards the center of the system. That is, adjacent 'lines' of force point towards the same central point. This is in contradiction to Einstein's Principle of Equivalence, in which gravitational acceleration is said to be equivalent to inertial acceleration within a box perpetually accelerating through free space.

For most calculations involving gravity - even in astrophysics, Newton's theory is still used in preference to General Relativity. However, Newton's theory is by no means perfect, and is invalid where masses become very small, as in particle physics. The reason for this is that his material point concept is incompatible with atomic theory. Matter can be subdivided, but only down to the atomic level. The primary particles, protons, electrons and neutrons do not divide into parts which have gravitational properties. The reasons for this will be explained in the next chapter on the structure of particles. Newton's gravitational theory also breaks down at long distances where gravitational curvature interacts with the structural curvature of the hyper-spherical continuum. This is why all the matter in the universe has not gathered itself into one huge galaxy. The physical reasons for this will be given in a later section of this chapter.

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