###### 1 May 2000

# Earth Loses Weight - Physicists Hone Gravity Number

##### by Kate Melville

Their new calculations for the Earth's mass came from work that could establish the most precise measurement ever achieved of Isaac Newton's gravitational constant.

According to the figures by Jens Gundlach and Stephen Merkowitz, Earth weighs in at 5.972 sextillion (5,972 followed by 18 zeroes) metric tons. Recent textbooks list the weight as 5.98 sextillion metric tons. Either way, that's about 1 trillion metric tons for each person on Earth. Put another way, 1 trillion metric tons is thought to be the total weight of all plant and animal life on the Earth's surface.

Gundlach, a UW research associate physics professor, and Merkowitz, a postdoctoral researcher, report their findings May 1 at a meeting of the American Physical Society in Long Beach, Calif.

"Gravity is the most important large-scale interaction in the universe, there's no doubt about it," Gundlach said. "It is largely responsible for the fate of the universe. Yet it is relatively little understood."

Gundlach uses the relationship between the Earth and sun to illustrate the mighty role of gravity. If the gravitational force that holds the Earth in its orbit around the sun were to be replaced by a steel cable (assuming it could be made with no mass), the cable would have to be two-thirds the diameter of the Earth to do the same job as gravity.

Newton's gravitational constant tells how much gravitational force there is between two masses - the Earth and sun, for instance - separated by a known distance.

The gravitational constant along with the speed of light and Planck's constant (a key value in quantum mechanics) are considered the three most fundamental and universal constants in nature. But while measurements of the other two constants have grown continually more precise through the years, the reverse has happened for the gravitational constant, called "Big G" in physics parlance.

In fact, new attempts to measure Big G in the 1990s brought results widely different from the previously accepted figure. That prompted the National Institute of Standards committee that establishes the accepted value to determine that there actually was 12 times more uncertainty about the figure last year than in 1987.

"That is a huge embarrassment for modern physics, where we think we know everything so well and other constants are defined to many, many digits," Gundlach said.

If accepted, the measurement by Gundlach and Merkowitz would reduce the uncertainty by nearly a factor of 100 from the currently accepted figure, making it far more precise than even the 1987 figure. Gundlach notes his numbers could change as additional data are analyzed in preparation for submitting the work for peer review.

To make their measurements, the researchers are using a device called a torsion balance that records nearly imperceptible accelerations from the gravitational effects of four 8.14-kilogram stainless steel balls on a 3- by 1.5-inch gold-coated Pyrex plate just 1.5 millimeters thick. The device, operating inside an old cyclotron hall in the UW nuclear physics laboratory, is similar in nature to one used 200 years ago to make the first Big G measurement. But it is computer controlled and contains numerous mechanical refinements that make the more precise measurement possible.

Gundlach acknowledged that the more precise calculation probably won't mean much to the average person.

"Just because we know the value of G won't make better cell phones," he said. "But it's something mankind should know because it's such a fundamental constant."