Newton's law

Newton's law says that one may imagine interaction between two masses, let's call them $M$ and $m$, as if between them acted an attractive force (callled also "gravitational force" or more properly a "Newtonian force" $F_{N}$) such that $F_{N}=GMm/r^{2}$, where $G$ is Newtonian gravitational constant, $M$ and $m$ are the interacting masses, and $r$ is a distance between centers of gravity of these masses.

Newton didn't believe that it is a real force acting between masses $M$ and $m$ since he didn't believe in possibility of action at a distance through vacuum, and mainteined that it is just a mathematical coincidence that accidentally produces such an effect but its physical reasons are still not known. For him such attractive force looked extremly mysterious and improbable and he refused to believe in possibility of existence of "attractive gravitational force".

The reason for masses approaching each other got clarified in an experiment by MIT with extremely precise clocks which proved that time slows down in vicinity of a mass. It showed that each mass is rather pushed in the direction of the other with an inertial force resulting from the particle choosing its most proable position in spacetime but being prevented from occupying that position by some obstacle presses against this obstacle with inertial force considered "gravitational". The force disappears after removing the obstacle and the particle finds itself in a "free fall" in which no forces act on it and it is free to choose for itself the most probable position of the lowest internal energy $E=mc^{2}$, where $m$ is rest mass of the particle, and $c$ is speed of light.