If observed, the Nordtvedt effect would violate the strong equivalence principle, which indicates that an object's movement in a gravitational field does not depend on its mass or composition.
The effect is named after Kenneth L. Nordtvedt, who first demonstrated that some theories of gravity suggest that massive bodies should fall at different rates, depending upon their gravitational self-energy.
Nordtvedt then observed that if gravity did in fact violate the strong equivalence principle, then the more-massive Earth should fall towards the Sun at a slightly different rate than the Moon, resulting in a polarization of the lunar orbit.
To test for the existence (or absence) of the Nordtvedt effect, scientists have used the Lunar Laser Ranging experiment, which is capable of measuring the distance between the Earth and the Moon with near-millimetre accuracy.
[2][3] Measurements of Mercury's orbit by the MESSENGER Spacecraft have further refined the Nordvedt effect to be below an even smaller scale.