In 1687, English physicist Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. While Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of “action at a distance” which his equations implied. He never, in his words, “assigned the cause of this power”. In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity.
Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science. (Christianson 1984) He lamented the fact that “philosophers have hitherto attempted the search of nature in vain” for the source of the gravitational force, as he was convinced “by many reasons” that there were “causes hitherto unknown” that were fundamental to all the “phenomena of nature”.
These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer is yet to be found. In Newton’s 1713 General Scholium in the second edition of Principia: «I have not yet been able to discover the cause of these properties of gravity from phenomena and I feign no hypotheses… It is enough that gravity does really exist and acts according to the laws I have explained, and that it abundantly serves to account for all the motions of celestial bodies.
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 and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it. » (Christianson 1984) Although Newton’s description of gravity is sufficiently accurate for many practical purposes and is therefore widely used, it is limited to domains where gravitational potential is a small fraction of the speed of light squared.
The more accurate general relativity theory of gravity must be used in general cases. General relativity results in Newtonian gravity in the limit of small potential, so Newton’s law of gravitation is often said to be the low-gravity limit of general relativity. (Christianson 1984) These objections were mooted by Einstein’s general relativity theory in which gravitation is an attribute of curved space-time instead of being due to a force propagated between bodies.
However, there is now the question of why mass and energy curve space-time. Modern physicists concern that there is no prospect of identifying the mediator of gravity. Newton himself felt the inexplicable action at a distance to be unsatisfactory. (Christianson 1984) Newton’s theory requires that gravitational force is transmitted instantaneously. Given classical assumptions of the nature of space and time, this is necessary to preserve the conservation of angular momentum observed by Johannes Kepler.
However, it is in direct conflict with Einstein’s theory of special relativity which places an upper limit—the speed of light in a vacuum—on the velocity at which signals can be transmitted. It is said that Newton’s theory does not fully explain the precession of the perihelion of the orbit of the planets, especially of planet Mercury. There is a 43 arc second per century discrepancy between the Newtonian prediction (resulting from the gravitational tugs of the other planets) and the observed precession. (Christianson 1984)
The predicted deflection of light by gravity using Newton’s theory is only half the deflection actually observed. General relativity is in closer agreement with the observations. The observed fact that gravitational and inertial masses are the same for all bodies is unexplained within Newton’s system. General relativity takes this as a postulate. Although Newton’s theory has been superseded, most modern non-relativistic gravitational calculations are based on Newton’s work because it is a much easier theory to work with and sufficient for most applications. (Christianson 1984)