G Whizzes Measure Gravity

Physicists are finally getting a grip on gravity. Gravity's strength, a number called Big G, is perhaps the most elusive of all the fundamental quantities. Over the past few decades a handful of physicists have dedicated themselves to measuring G more accurately, but to their dismay they've come up with wildly different values. Now, six groups using a variety of techniques have weighed in with new values of G, one of which is reported in today's Science, and they are all in rough agreement.

Measuring gravity is harder than it might seem because it is so weak compared to other forces such as electromagnetism. To accurately gauge its tiny tug, most experiments have to be carefully isolated from electrical and seismic disturbances and performed in a vacuum to minimize the push of atoms in the air. Even the gravitational attraction of objects near the apparatus can throw off the measurement.

The official value for G--chosen by an international panel in 1986--comes from a 1982 measurement by Gabe Luther, now at Los Alamos National Laboratory in New Mexico, and William Towler of the University of Virginia in Charlottesville. In their setup, a tiny barbell was hung from a long fiber made of quartz or tungsten; when disturbed, the barbell rotated lazily back and forth. When two huge tungsten balls were brought near, their gravitational tug on the barbell slowed its swing time by a split second. By measuring that difference, Luther and Towler pegged Big G with an estimated accuracy of better than a hundredth of a percent. Two later experiments came up with wildly different values, however, leaving researchers perplexed.

Now a whole gymnasium of new experimental approaches have started to deliver consistent results. In the study reported in Science, a group at the National Institute of Standards and Technology in Boulder and the University of Colorado dropped a weight through the hole of a large tungsten donut, then raised the donut above the release point and dropped the weight again. With the donut below, its gravitational tug made the weight fall faster. When the donut was overhead, it slowed the object's descent by a hair. From the tiny difference between the two drop times, the team teased out a value for G.

Others used variants of the Luther-Towler setup or measured the deflection of pendulums when large masses were wheeled close to them. The new results all point to a value of G hovering just above the 1986 value. One of the old outlying measurements was also brought into the fold when an error was corrected. Finally "a consensus is emerging," says Tim Armstrong, a physicist at New Zealand's standards laboratory. The payoff will be pure intellectual satisfaction: "You have to be an oddball to do this," admits Clive Speake, a physicist at the University of Birmingham, U.K.