A Delicate Balance: High-Precision Gravity Measurements

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On January 8, 1986, an article in the New York Times dramatically announced, "Hints of Fifth Force in Nature Challenge Galileo's Findings." footnote 1

Discovery of a fifth force in nature would be news indeed. There are only four recognized forms of interactions in nature. Two of them are perceptible only on the atomic scale of things: the strong force and the weak force. The other two, gravity and electromagnetism, operate over larger distances and are familiar to us in everyday life.

Sir Isaac Newton is credited with first deriving our modern understanding of the force of gravity. When two bodies are separated by a distance, they each "feel" the other's presence in the form of an attraction due to the force of gravity. The strength of the force depends on the masses of the bodies divided by the square of the distance between them. So, the larger the masses, the larger the force; and the greater the distance between them, the smaller the force. Gravity has not been thought, however, to depend on the composition of the masses. That's why, according to Einstein's gravitational equivalence principle, a kilogram of lead and a kilogram of feathers ought to fall to earth at exactly the same rate (in the absence of air drag).

The 1986 article claimed that there was another, fifth force that acts between two masses but, unlike gravity, depends on their composition. This paper touched off a cascade of experimental and theoretical work. The fifth force as originally proposed and its natural generalization was soon ruled out in experiments performed by a group headed by Eric Adelberger and Blayne Heckel at the UW.footnote 1

Although the original fifth force may be down, experimental work to look for new forces and violations of the equivalence principle is not out. Scientists now realize that there may be new forces that are so weak that existing instruments could not detect them. This has spawned some exquisitely elegant work here in the Pacific Northwest that continues to advance the frontiers of precision gravity measurements. Two independent groups of UW physicists have carried out some of the most delicate measurements of the force of gravity ever made.

The Eot-Wash Groupfootnote 2 of Eric Adelberger, Blayne Heckel, and Jens Gundlach uses a highly sensitive torsion balance. The balance, a tray suspended by a thin wire, contains test bodies made from two different materials. A force that acts differently on the two materials would cause a tiny twist of the wire; this could be the proof of a new force or a violation of the equivalence principle. The group was able to confirm that the equivalence principle holds to better than two parts in a millionth of a millionth. With this apparatus it was also possible, for the first time, to verify that gravity is the only force exerted by dark matter, the mysterious stuff that constitutes more than 90% of the mass of the Universe, but has never been seen directly.

UW physicist Paul Boynton and colleagues use a different kind of torsion pendulum. It consists of a carefully machined ring of half aluminum and half beryllium about the size of a small spool of ribbon. The pendulum is suspended by a thin wire which allows it to rotate. The researchers make use of a 130-meter tall granite cliff near the town of Index in the Cascade Mountains of Washington. If there exists a force that pulls the aluminum towards the granite cliff more than it pulls the beryllium, then there should be a fractional change in the period of the torsion pendulum when the entire pendulum is rotated. Another of Boynton's set-ups makes use of Rattlesnake Mountain at the Hanford site in southeastern Washington State.

Both groups are performing "null" experiments that are carefully designed to yield very precise values of "zero" out to many decimal places unless a previously undetected phenomenon is at work. Future tests by Boynton will rigorously test the inverse-square relation in the law of gravity. Adelberger's and Boynton's experiments have "greatly improved our experimental understanding of the gravitational interaction," says current physics department chairman Stephen D. Ellis.

  1. Recounted in The Rise and Fall of the Fifth Force: Discovery, Pursuit, and Justification in Modern Physics, Allan Franklin, American Institute of Physics, New York, 1993.
  2. "We named our research group this way in the honor of Baron Eotvos who did equivalence principle tests around the turn of the [19th] century. Eotvos pronounced properly almost sounds like Eot-Wash, hence the concoction," says Jens Gundlach, a member of Adelberger's research group.

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