Focus: Newton’s Still Correct
You might think that Newton’s law of gravity is about as solid as any principle in physics. But if some of the latest grand unification theories are correct, gravity may not operate exactly the way we expect. The 19 February PRL reports experiments that test Newton’s inverse-square law down to 200 µm, with at least ten times the sensitivity of previous tests. Newtonian gravity remains in tact so far, which eliminates some theories proposing extra spatial dimensions, beyond the three we know. But extra dimensions may still exist, “curled up” to smaller sizes.
While string theories promise to unify the four known types of forces in one framework, they also require at least six extra spatial dimensions. These dimensions are said to be curled up in a way that makes them normally invisible–like the width of a distant telephone pole on the horizon, you can’t see them until you come in close. Extra dimensions might solve other problems as well, says Jens Gundlach of the University of Washington in Seattle, such as the surprising feebleness of gravity compared with electromagnetism and the nuclear forces. According to this view, gravity’s effects spread into other dimensions, whereas the other forces are limited to our three-dimensional world. “Gravity is so weak because it’s diluted,” he says.
Theorists predicted that some of the extra dimensions might extend as far as a millimeter or so, making them accessible to tabletop experiments: With two “large” extra dimensions, the 1/ law of gravity would look more like 1/ at close range. Now a University of Washington team led by Eric Adelberger and Blayne Heckel, and including Gundlach, reports the first and most sensitive of a new generation of precision, short range gravity tests searching for extra dimensions.
Inside a high vacuum chamber, the team suspended a small metal ring from a torsion (twisting) pendulum and placed a slowly rotating disk below it. The ring and disk were perforated by ten holes each, and gravity tended to align the holes ten times per revolution. A second disk rotating just below the first one also had ten holes, but at positions designed to partially cancel the attraction of the upper disk to the ring. A mirror affixed to the pendulum reflected a laser beam and allowed Adelberger and his colleagues to detect slight rotations of the ring. Gundlach explains that the experiment was difficult partly because electrostatic fields–static electricity–could easily overwhelm the tenuous gravitational force. The team carefully stretched a 20-micrometer-thick sheet of metal between the ring and disks to shield from such effects.
The researchers recorded the rotation of the ring suspended at heights between 200 µm and 5 mm. At 2 mm–the height where cancellation was maximized–they saw the complete cessation of motion expected from Newtonian gravity, and their data at other heights were also completely consistent with the textbooks.
The team came up with a “brilliant design” and carried out a “beautiful experiment,” says Aharon Kapitulnik of Stanford University in Palo Alto, CA. One important aspect, he explains, is that they measured a complete cancellation of the gravitational force at one height but could then move the ring above and below that height to check that their apparatus detected gravity cleanly, free of background interference. Kapitulnik is working on a much shorter range measurement of gravity and says he’s never discouraged by so-called null results that do not disprove Newton. “It’s important to do it, and somebody has to do it,” he says.