Newton's Law of Universal Gravitation has been described as follows: It is a force between any two bodies and is "directly proportional to the product of their masses and inversely proportional to the square of the distance between them. ... The measure of the force of gravitation on a given body on earth is the weight of that body." While certain of Newton's theories have not stood the tests applied in the 20th century, his law of universal gravitation has stood: "In the general theory of relativity, gravitation is explained geometrically: matter in its immediate neighborhood causes the curvature of the four-dimensional space-time continuum."