Earth-moon model backs general relativity.

Einstein's general theory of relativity predicts that both mass and energy respond to gravity, so even gravity's energy would feel the tug of gravity. Yet, some theories exempt gravitational energy from the force's attraction.

Curious to know which theory is right, physicists in Seattle have conducted new laboratory tests and combined them with findings from previous direct measurements of lunar and Earth motion.

In the Nov. 1 PHYSICAL REVIEW LETTERS, the team at the University of Washington reports that the acceleration of gravitational energy doesn't differ from that of other forms of mass and energy--a finding that supports general relativity. If there's a difference, it must be less than one-tenth of a percent, the researchers say.

"It's a very nice result," comments Clifford M. Will of Washington University in St. Louis. However, gravity specialists weren't "terribly worried" that the accelerations would be different, he notes.

Massive objects convert some mass to energy when forces--electromagnetic, gravitational, and others--draw their parts together. For the rock and dust that coalesced long ago to form Earth, about 460 trillionths of the mass became gravitational energy, scientists have calculated. Only 20 trillionths or so of the matter that formed the moon made the transition.

The Seattle scientists, led by Eric G. Adelberger and Blayne R. Heckel, suspended objects with compositions similar to that of the Earth and of the moon in a rotating torsion balance. That extremely sensitive device detects twisting forces. Adelberger describes the experiment as "dropping a little, tiny Earth and a little, tiny moon toward the sun." The test compares the acceleration of dissimilar materials due to solar gravity.

Einstein's theory holds that gravity accelerates objects equally, regardless of mass, energy, or composition. This notion, called the equivalence principle, has roots reaching back to Sir Isaac Newton, Galileo, and beyond (SN: 9/22/90, p. 183).

General relativity clashes with another major physics theory known as quantum mechanics. Many scientists think that finding a flaw in general relativity, perhaps in the equivalence principle, may lead to a way to reconcile the two theories.

For decades, scientists have been bouncing laser beams from Earth off reflectors on the moon to compare the accelerations of these bodies toward the sun. So far, those investigations have demonstrated equal acceleration of the bodies to a precision of 13 decimal places.

However, the experiments don't distinguish between acceleration due to gravitational energy and that due to other forms of mass and energy, Adelberger notes. A slim possibility exists that two different types of equivalence-principle violation cancel each other

In that scenario, the bodies' different compositions might tend to make Earth accelerate slightly more rapidly than the moon, while their unequal gravitational energies would lead the moon to accelerate slightly more rapidly than Earth. The result: an illusion that the equivalence principle reigns.

Masses of objects in a laboratory are so small that their gravitational energy is negligible. So the Seattle team set out to cleanly determine whether the composition disparity between Earth and the moon alone affects acceleration. The group used a pair of stainless steel cylinders to mimic Earth with its heavy iron core and a pair of quartz-magnesium cylinders to act as the relatively lightweight moon. Each cylinder weighed 10 grams.

The team found that in their precise measurements, composition caused no difference in acceleration. Consequently, contributions of gravitational energies to accelerations of Earth and the moon, while not directly tested, must also be equal, albeit to lesser precision.