Scientists Still Seeking a Precise Measurement of Gravity : Science: The known figure is accurate enough for now, physicists say, but far less precise than the established speed of light. The problem? Gravity is so weak that it is difficult to quantify.
At the doorstep of the 21st Century, as physicists probe deeply into the atom and out to the farthest reaches of space, an experiment planned on a lonely New Mexico mesa sounds downright quaint.
Its job: to measure how strong gravity is.
Yep, our old friend gravity.
Despite nearly 200 years of careful experiments, this familiar and fundamental force has eluded the kind of precision measurement that physicists are accustomed to obtaining.
The mass of the neutron, the electrical charge of a single electron and the speed of light in a vacuum, for example, all are known to an accuracy of 1 part per million or better. But the strength of gravity is known only to 128 parts per million.
That still can be equated with knowing the weight of a 790-pound grizzly bear to within the weight of a candy bar, which sounds pretty good to a layman. But for a handful of physicists, it’s a call to action.
“We aren’t to first base on measuring the strength of gravity,” said Alvin Sanders of the University of Tennessee. “I think that’s important, to know how big the darn thing is.”
The goal is not measuring Earth’s gravity, which can be determined very precisely. Rather, it’s the strength of gravity in general, as it applies to any two objects in the universe. This is the Newtonian constant of gravitation, called “G” or “big G.”
Sanders concedes that the current estimate for big G is good enough for now and that most physicists don’t care about pinning it down further.
But he maintains that greater precision will be needed in the future to help choose between competing fundamental theories of physics and to aid in understanding the interior of stars.
Some other physicists aren’t so sure.
“Do we really need it better?” asks Jim Faller of the joint Institute for Laboratory Astrophysics at the University in Colorado at Boulder, who has tried to improve the measurement.
“I’m not sure we do,” he said, “other than it’s sort of like climbing one of the great mountains and if you’re so clever as to figure out a way to do it, great.
“The measure of G is somehow embedded in the several-hundred-year-old culture of physics. You’re competing with some great minds of 100 years ago or 200 years ago. They weren’t dumb.”
So why is G so elusive? One reason is that gravity is extremely weak. When a magnet picks up a paper clip, it defeats the gravitational pull of the entire Earth. And scientists can’t even use Earth’s gravity in their experiments, in part because its density is too uneven.
So researchers are stuck with measuring the vanishingly small gravitational tug between objects small enough to fit into a laboratory. Measurements of something that small easily can be thrown off by such things as tiny, undetected variations in the density of the objects.
What’s more, since everything exerts gravity and there is no way to block it, scientists even have to worry about interference from gravitational tugs by other things.
“You and me and a car in the parking lot,” Sanders said, “a deer that walks by at 2 in the morning if you go to a rural area to get away from the cars--these things mess up the gravitational field in ways you just can’t predict.”
Gravity measurements have been affected by the gravitational tugs of passing field mice and the water deposited by sprinklers outside a lab building, Faller said.
His measurements of Earth’s gravity in the basement of one campus building gave different results during weekdays than during weekends. The apparent reason: gravitational attraction from students on the floors above during school days.
So scientists have to get away from it all, which is why Gabriel Luther of the Los Alamos National Laboratory in New Mexico plans to set up his experiment on the mesa overlooking the Rio Grande.
Luther plans to move his apparatus to the mesa soon and have it working a year after that.
The measurement device contains a small dumbbell hanging horizontally from a slender fiber. Two tungsten balls, each weighing 23 pounds, will rotate around the dumbbell. The gravitational tug of the balls on the dumbbell’s weights, which would tend to make the dumbbell rotate, then can be calculated precisely. Since the distances involved and the masses of all the objects are known, the results should reveal big G.
Luther, co-owner of the current precision record, is aiming to refine the measurement to 10 parts per million, a tenfold improvement.
Two proposed space-based experiments are aiming for 1 part per million, though some physicists say it will be hard to reach such accuracy without any way to fiddle with the instruments once they’re in orbit.
Ho Jung Paik of the University of Maryland in College Park designed one experiment as part of a larger investigation of gravity, which may soon get approval to fly in the year 2001.
His design calls for two pairs of cylinders, each with a smaller cylinder inside a bigger one. For each pair, a very dense mass would be moved back and forth through the inner cylinder. Its gravity would make the cylinders move and the motion could be precisely measured to yield a value for big G, Paik said.
Sanders is seeking funding for his experiment, which would involve a sphere a little bigger than a basketball and several the size of Ping-Pong balls. By observing how the larger ball affects the motion of the smaller ones as they float in weightless orbit around Earth, scientists should be able to calculate big G, Sanders said.
Clearly, after 200 years, the lure of trying to outfox the elusive big G lives on.
“Everybody solves the wrong problem,” Luther said. “You read all the previous measurements and say, ‘Aha, I know how to do it better,’ and then you try and you don’t do it better.”
