COLLEGE PARK -- Sir Isaac Newton, who wrote the book on physics, can rest easy.
Using the most sensitive gravity-measuring device ever built, physicists at the University of Maryland have tested Newton's three-century old theory of gravity and found it remains an uncannily accurate description of the force, even on very small scales.
Dr. Ho Jung Paik and seven other researchers conducted their experiment in the physics building's sub-basement over a span of 33 straight nights, measuring changes in the basement's gravitational field generated by a swinging, 3,296-pound ball of lead.
They tested Newton's inverse square law, which holds that the attraction between any two objects is proportional to the square of the distance between them. That means that when you double the distance between two objects, the gravitational attraction becomes four times weaker.
In testing this law, the researchers found that it is at least 99.98 percent accurate at a distance of 1 meter -- just over a yard.
Gravity isn't just something that holds houses, trees and soda cans on Earth. Every object generates gravity. But the force is the weakest in nature. Things much less massive than moons or planets generally make too little of it to be felt.
Newton formulated his law simply by studying the motion of planets. But increasingly sensitive instruments can measure even the tiny amounts of gravity created by small objects.
Over the past 13 years, Dr. Paik said, he and his colleagues and students have built the most sensitive instrument yet, called a superconducting gravity gradiometer. The device, Dr. Paik said, is capable of detecting the gravity created by a person's fist.
Since 1979, Dr. Paik has been developing the gravity gradiometer for NASA's Superconducting Gravity Gradiometer Mission, a satellite intended to map the subtle differences in the gravity across the surface of the Earth. (These variations are due to the uneven distribution of mass in the planet.)
The mission, originally scheduled for 1998, has been postponed until at least 2005 because of NASA's budget cutbacks.
The present advanced instrument, which has the diameter of a beach ball, resembles three brass tubes that are fused in their centers. The device uses superconducting magnets and extremely delicate springs to measure tiny shifts, in three dimensions, in the gravitational field surrounding it.
In their experiment, Dr. Paik and his team swung their lead ball back and forth from the ceiling of the campus physics building's sub- basement, a few feet away from the gradiometer. (They built a concrete-block wall between the ball and the detector to keep the breeze kicked up by the ball from causing the detector to vibrate.)
Researchers calibrated the gradiometer and positioned it in relation to the swinging ball in such a way that, if the gravitational changes obeyed the inverse square law, the measurements along the gradiometer's three axes would always add up to zero.
In the late 1980s, some scientists thought they had detected variation in gravity's grip over short distances. But later tests showed those measurements were flawed.
Dr. Paik said his team's zero-sum or "null" technique was superior to previous experiments, which attempted to observe changes in the gravitational attraction between two objects as they were drawn apart. But this method, he said, requires extremely precise measurement of the mass of the two objects, which is very difficult.
Dr. Paik's team had problems of its own.
Vibrations from people walking upstairs in the physics building interfered. So the experiment was planned at night.
The huge lead ball set the building swaying slightly. This problem, Dr. Paik said, took two years to solve: The team finally was able to measure and filter out that movement using a special gyroscope.
After the monthlong experiment, the instrument came up with one consistent reading -- zero. "Getting a null result isn't quite so exciting," admitted Vol Moody, a research scientist at College Park. But the team was still gratified.
The previous most rigorous test of the inverse square law, Dr. Paik said, used submersible craft operating in the ocean, measuring the difference in gravity at various depths. The College Park experiment, he said, proved that Newton's law holds true on a scale 10 times smaller.
Dr. Paik said he thinks he can refine these measurements even further, proving Newton right at finer and finer scales.