In theoretical astrophysics, the
Nordtvedt effect refers to the relative motion between the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
which would be observed if the gravitational
self-energyIn theoretical physics and quantum field theory a particle's self-energy represents the contribution to the particle's energy, or effective mass, due to interactions between the particle and the system it is part of...
of a body contributed to its gravitational mass but not its inertial mass. If observed, the Nordtvedt effect would violate the strong equivalence principle, which shows that an object's movement in a gravitational field does not depend on its mass or composition.
The effect is named after Dr.
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In theoretical astrophysics, the
Nordtvedt effect refers to the relative motion between the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
which would be observed if the gravitational
self-energyIn theoretical physics and quantum field theory a particle's self-energy represents the contribution to the particle's energy, or effective mass, due to interactions between the particle and the system it is part of...
of a body contributed to its gravitational mass but not its inertial mass. If observed, the Nordtvedt effect would violate the strong equivalence principle, which shows that an object's movement in a gravitational field does not depend on its mass or composition.
The effect is named after Dr. Kenneth L. Nordtvedt, from Montana State University, who first demonstrated that some theories of gravity suggest that massive bodies should fall at different rates, depending upon their gravitational self-energy.
If gravity does 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. To test for the existence (or absence) of the Nordtvedt effect, scientists have used the
Lunar Laser Ranging ExperimentThe ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors previously planted on the Moon and the time delay for the reflected light to return is determined. Since the speed of light is known with...
, which is capable of measuring the distance between the Earth and the Moon with near-millimetre accuracy. Thus far, the results have failed to find any evidence of the Nordtvedt effect, demonstrating that if it exists, the effect is exceedingly weak.