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Weighty Matters: Is Newton's 300- year old law of g
Weighty Matters
Could Newton's 300-year-old law of gravity finally be succumbing to
age? Several recent findings seem to deviate from the theory, and now
the most meticulous test yet--a measurement of the gravitational field
in a mile-deep borehole in the Greenland ice sheet--has turned up
further evidence of a discrepancy.
The implications could be profound. Such small adjustments to gravity
are in fact predicted by all the most promising attempts to forge a
unified theory of the fundamental forces--the ultimate goal of
physics. These new effects, which some people call a fifth force and
even a sixth force, are expected to compare to gravity in strength,
but they act over perhaps a few hundres or thousands of meters,
whereas gravity has an infinite range.
One possibile consequence of such new effects is that within the range
of the new forces, Newton's inverse-square law (the strength of
gravity falls as the square of the distance between two masses) may
not be true. Another is that unlike standard gravity, which acts only
on mass, the new effects may depend on some aspect of an object's
compositin, such as the total number of baryons (protons and
nuetrons). Nearly a dozen experiments have sought--inconclusively--to
detect one of the effects (see "Force of a Different Color" in
"Science and the Citizen," December, 1987). The Greenland project is
the latest in a series of attempts to detect a violation of the
inverse-square law by measuring local gravitational fields and
comparing them to calculations based on the density of the surrounding
terrain.
An earlier experiment done inside an Australian mine found a repulsive
effect of roughly 1 percent of the strength of ordinary gravity,
acting over a range of a few hundred meters. A second experiment,
carried out on a 600-meter television tower in North Carolina, found
an attractive effect of about 2 percent of the strength of gravity
acting over a distance of 300 meters. The calculations worked out
even better when both an attractive and a repulsive effect were
presumed.
Skeptics argue that these apparent effects could result from anomalies
in local mass density, such as a hidden lode of metal ore. The
Greenland group, led by Mark E. Ander of the Los Alamos National
Laboratory and Mark A. Zumberge of the University of California at San
Diego, therefore chose a highly homogeneous site: a borehole
surrounded by a two-kilometer-thick expanse of ice. The team took
elaborate precautions: the bedrock was mapped by 42,000 high-frequency
radar scans, and careful surveys determined the height of the ice
surface to within a centimeter. A gravimeter took more than 100
readings at half a dozen locations, at depths of between 200 and 1,600
meters.
The researchers assumed the density of the bedrock might range between
2.7 and three grams per cubic centimeter; densities outside this range
are geologically improbable. Finally, different members analyzed the
data at least three times. Their preliminary conclusion: there
appears to be a single, attractive effect whose strength is between
1.7 and 3.9 percent that of gravity. It is thought to act over a
distance of somewhere between 10 meters and slightly more than one
kilometer.
The new findings agree with the results from North Carolina but seem
to contradict those from Australia. It may be possible to reconcile
all three results by including both an attractive and a repulsive
effect, but then the theoretical model "gets rather contrived,"
according to Ander's colleague Richard Hughes. To help determine
whether these effects are real or are instead caused by hidden
anomalies in the environment, the group is already planning future
experiments in the Pacific Ocean and in Antarctica, where the ice is
twice as thick as the ice in Greenland.
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