It might be time to revise those high school textbooks -- the Earth is apparently a little bit lighter than we thought. A new calculation for the mass of the Earth will be reported this weekend at the American Physical Society Meeting in Long Beach by Jens H. Gundlach of the University of Washington. Gundlach will be announcing a higher precision measurement of the gravitational constant (which describes the strength of gravity), usually denoted by the big letter G. This constant helps determine the weight of the Earth and everything else in the universe.

G has been of fundamental importance to physics and astronomy ever since it was introduced by Isaac Newton in the seventeenth century, but it has been relatively hard to measure. Gravity is actually weak compared to the other forces of nature, such as electricity and magnetism. However, it is still strong enough to keep us attached to the Earth and to keep the Earth in orbit around the sun.

In fact, G was not measured experimentally until 1797, when the British scientist Henry Cavendish set up a special balance for detecting the mutual influence of two hanging weights. (For a figure showing Cavendish's setup, go to the "Gravitational Constant" pictures on the Physics News Graphics website at www.aip.org/physnews/graphics.)

Through the years scientists have gotten better at probing the strength of gravity, but for such an important number, the precision in G was never as good as scientists wanted it to be. While some constants were known to parts per million or better, the uncertainty in G was only as good as 0.013% in the 1980s. Years later the situation actually got worse, mainly because of a highly respected but utterly perplexing experiment in Germany during the 1990s which yielded a G measurement that was radically different from the previously accepted value. As a result, the net uncertainty had worsened by 1998 to a value of 0.15%

The Washington experiment, which uses a number of metal spheres rotating around a pendulum, has changed all that (see second picture on Graphics website). Gundlach and his colleagues are able to measure G with an uncertainty of only 0.0015%, nearly a tenfold improvement over the 1980s uncertainty, and a hundredfold improvement over the 1998 uncertainty.

Using the new value of G and measurements and data from an Earth-orbiting satellite, the Washington researchers were able to provide a brand new calculation on the actual mass for the Earth. The estimate is M=5.972 x 10^21 metric tons. Recent textbooks quote the number as 5.98 x 10^21, so it would seem the Earth has lost a bit of weight, at least on paper.

PRESS CONFERENCE WILL BE HELD IN ROOM 201A, LONG BEACH CONVENTION CENTER AT 12:00PM PACIFIC DAYLIGHT TIME ON SATURDAY, APRIL 29TH, 2000.

CONTACT:

On April 28
Ben Stein
American Institute of Physics
301-209-3091
bstein@aip.org

April 29 - May 2
Phillip Schewe or Randy Atkins
Pressroom, American Physical Society meeting
Long Beach, California
562-499-7780

Jens Gundlach In Seattle 4/28 206-543-4080
In Long Beach - Mariot Hotel 562-435-8511

A lay-language summary of the results can be found at http://www.aps.org/meet/APR00/baps/vpr/layp11-03.html