"My colleagues and I have successfully experimentally demonstrated that the force of gravitation between two test bodies varies with their orientation in space, relative to a system of distant stars," Mikhail Gershteyn, a visiting scientist at the MIT Plasma Science and Fusion Centre, told United Press International from Cambridge, Mass..

Isaac Newton first described G in 1687 as a fundamental component of his universal law of gravity. Two masses, Newton wrote, attract each other with a force proportional to their mass that falls off rapidly as the bodies move farther and farther apart. Albert Einstein later used G in his own field equations that fine-tuned Newton's original laws. In Einstein's universe, gravity is the effect on bodies moving through space that is curved or warped by the presence of matter.

The constant G describes gravity's attractive force precisely and appears in equations for any gravitational field, whether the field is between planets, stars, galaxies, microscopic particles or rays of light. Centuries of measurement have firmly fixed the value of G as the complex formula 6.673 times 10 to the minus 11th power, times meters travelled per second times the number of kilograms, squared.

Gravity is a relatively very weak force, yet it is strong enough to hold planets in orbit and to mash great gobs of matter into incredibly dense, infinitesimally small black holes.

If G varies under any circumstances, scientists would have to rewrite virtually every physical law, including a long-accepted feature of the universe -- isotropy, or the condition that a body's physical properties are independent of its orientation in space...