Newton's Law of Universal Gravitation says that every mass (<math>M_1</math>) in the universe attracts every other mass (<math>M_2</math>) in the universe with an attractive force <math>F_N</math>, also called "gravitational force" or more properly "Newtonian force", inversely proportional to the square of the distance between them and directly proportional to the product of their masses, such that:
- <math>F_N=GM_1 M_2/r^2</math>,
- where <math>G</math> is the Newtonian gravitational constant (9.8 <math>m/s^2</math>), <math>M_1</math> and <math>M_2</math> are the interacting masses, and <math>r</math> is a distance between their centers of gravity.
Newton didn't believe that the gravitational force is a real force acting between masses <math>M_1</math> and <math>M_2</math>. He maintained that it was just a mathematical coincidence that his Law of Universal Gravitation correctly described the effect of gravitation but that the physical reasons for the appearance of an attractive force was unknown. To Newton, the attractive force looked extremely mysterious and improbable and he refused to believe in the possibility of the existence of and "attractive gravitational force" acting at a distance.
The reason for masses apparently being attracted to each other was clarified by an experiment conducted at MIT using extremely precise clocks. The experiment proved that time slows down in the vicinity of a large mass such as the Earth. The experiment showed that each mass is 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; it therefore presses against this obstacle with inertial force called "gravitational force". The force disappears after removing the obstacle and the particle is in "free fall" in which no forces act on it and it is free to occupy the most probable position of the lowest internal energy <math>E=mc^2</math>, where <math>m</math> is the rest mass of the particle, and <math>c</math> is the speed of light in vacuum.