The same gravitation we humans experience as a simple everyday force, is in fact a very remarkable phenomenon, which even today is not well understood (see Verlinde, chapter 6.2).

In the first two parts of the basic work of Western physics, the Philosophiae Naturalis Principia Mathematica (Lat.: "The mathematical principles of natural philosophy, in short the Principia) by sir Isaac Newton, published on July 5, 1687, the concept of gravity is remarkably avoided: These parts form a long list of (historical) measurement data of all kinds of natural phenomena, but gravity is not mentioned yet by name. Only in Part 3 titled: De mundi systemate (Lat .: "the organization of the world"), Newton describes his universal gravitational theory. Part 3 starts with 9 hypotheses, followed by a long list of theorems and concludes with:  

"Hitherto, I have explained the phenomena of celestial bodies and our sea by gravitational force, but I did not provide the cause for it. In any case, this force emanates from some cause, that penetrates into the centers of the sun and the planets without diminishing strength; it is not working proportional to the surface of the parts it acts upon, but to the overall amount of matter, and its effects spread in every direction, diminishing force in proportion to the square of the distance....."

"However, I have not been able to deduce the cause of the properties of gravitation, and I don't make up hypotheses. What is not deduced from the phenomena, must be called hypothesis. Hypotheses, however, whether physical or metaphysical, whether concerning hidden properties or mechanical ones, do not have a place in experimantal philosophy. In this philosophy, theorems are deduced from phenomena, and generalized through induction. This way, the impenetrability, mobility and collision of bodies, and the laws of motion have been learned. It is sufficient that gravity exists, operates according to the laws postulated by us, and is an adequate explanation for all movements of celestial bodies and our seas."

This makes it clear, as Newton explicitly states, that this theory has been obtained inductively; iTherefore, it is in fact only a gravitational law. The Principia does explicitly not give an explanation for the existence of gravity or of a force at a distance.

Newton's law of universal gravity states that two masses (m1 and m2) exert an attractive force F that is proportional to both masses (m1 and m2) and inversely proportional to the square of the distance r (see the figure, where G is a constant).

                                            
                                               Gravitational law of Newton (Dennis Nilsson, CC BY 3.0)

Newton was very clear about it, the 'law of gravitation' applies to every body, large or small; it is universal. So the planets orbit around the Sun by virtue of a mutually attractive force between the Sun and its planets: Planets, therefore, orbit the Sun in the same way as navigational and other man-made satellites orbit the Earth. At first glance this seems to be strange, because this 'Gravity' seems far from strong enough!

Gravity, as described with the above formula, decreases very quickly as the distance between the bodies increases; the constant part (GxM1xM2) is divided by the square of the distance between the two bodies; with each doubling of the distance to the center of the bodies, the gravity will weaken to about a quarter. The sun's radius being about 700,000 km, the force of gravitation at 700,000 km above its surface (and therefore 2 * 700,000 km from its centre), is just one quarter of what it is on the sun's surface.

At first glance, the gravitational force between Sun and Earth, which are located at an average distance of about 150,000,000 km from each other, seems to be completely inadequate to be able to attract each other. But when you properly calculate, there still is a considerable forcer at that distance, so in practice Newton's law of universal gravity seems to be correct. Not quite so: For example, the perihelion shift of Mercury, which was discovered in 1855 by the French Urbain Le Verrier. Although the deviations are very small, this actually means that our solar system is unstable based on Newton's law of universal gravity. That is why the search for solutions, such as mutual influence by the planets themselves or by unknown celestial bodies, is going on; think e.g. of the hunt for planet Vulcanus. However, a satisfactory solution has not been found sofar, which implies that gravity as an attracting energy or force, as Newton supposed with his law, is not the correct explanation. So gravity is not an attracting force or energy.

What then can gravity be?

Gravity is one of the most important 'forces' in our daily world; therefore it is very annoying that its source can not be found. Hopefully the reader can imagine that gravity must have been a physical trial throughout the ages. And the more so because the term 'gravity' has acquired a different meaning in Western physics, through Einstein of all people, - I will come back to it later - I suppose, that this trial must have been a great challenge for a puzzler like Einstein: I think he solved the problem.

For a puzzler like Einstein this must have been a great challenge.

In order to solve the problem, Einstein's research has almost certainly been based on an already known relation between gravity and light, which had been predicted in the years 1783 and 1796 by John Michell and Pierre-Simon Laplace respectively. In 1801, this relationship was further investigated by Johann Georg von Soldner, who clearly showed that light was influenced by gravity: light is deflected by gravity.

As a possible cause of this phenomenon, which is also called gravity lens, only gravity can be supplied. But because we have already determined that gravity is not an attracting force or energy, it must be excluded as a cause, especially because radiation has no mass. So, light can not be attracted anyway. With the alleged attraction by celestial bodies of radiation being out of the question, a piece of highly unconventional western physics emanates.


Continue to: 5.2.1. Einstein's solution

­