- A torsion pendulum with 10 holes is suspended by a tungsten
fiber so thin it is barely visible in the photo. The pendulum hangs less
than a millimeter above a rotating plate that also has 10 holes. The plate
exerts gravitational twist on the pendulum that is measured by bouncing
a laser beam off a mirror at the top of the pendulum. (UW Photo by Mary
Levin)Seattle - Feb. 19, 2001 University of Washington scientists using
gravity measurements to hunt for evidence of dimensions in addition to
those already known have found that those dimensions would have to occupy
a space smaller than 0.2 millimeter.
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- In making that finding, the team headed by UW physics
professors Eric Adelberger and Blayne Heckel gained new insights into gravity.
One of the biggest mysteries in physics is why gravity is so weak compared
with all other natural forces. A small magnet suspended above a table,
for instance, can easily overcome the downward gravitational pull of the
entire Earth and pick up a nail.
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- One idea, string theory, involves "extra" dimensions.
It requires that there be 10 space dimensions, and it usually assumes that
seven of those dimensions are curled up in regions so tiny they cannot
be detected by current technology. A new development in that theory suggests
that gravity's apparent weakness could be caused by a unique property allowing
gravity to leak off into "extra" dimensions, while everything
else is confined to the normal dimensions of length, width and height.
Extra dimensions would be a millimeter in size or smaller and, until now,
no experiment could have detected them. The Adelberger-Heckel team looked
for anomalies at small distances that might signal the presence of extra
dimensions.
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- "Gravity is the only way to see these extra dimensions
but the very weakness of gravity has meant that there was no way to test
the theory," said Adelberger. "No one had even been able to detect
the gravitational attraction between two millimeter-sized objects, much
less see if the force could be stronger than expected."
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- The UW team recently measured the strength of gravity
at a distance of just 0.2 millimeter and found no deviation from the gravitational
pull predicted by the inverse-square law devised by Isaac Newton. "No
one had ever detected that gravity even existed at distances less than
a millimeter," said Heckel.
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- "We find that if the extra dimensions exist, they
have to be smaller than two-tenths of a millimeter," Adelberger said.
"This doesn't say the extra dimensions idea is crazy. It's just not
as easy as the simplest picture."
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- The research team already is fabricating equipment to
test to 0.1 millimeter or less, and has begun planning an experiment to
test at even smaller distances.
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- The findings, to be published in the Feb. 19 edition
of Physical Review Letters, are based on results using a ring suspended
just above a rotating disk. The ring contains 10 small holes and hangs
by a tungsten fiber just 20 microns (less than one-thousandth of an inch)
thick. The rotating disk, with 10 similar holes, exerts gravitational pull
on the pendulum, twisting it back and forth 10 times for every revolution
of the plate. The amount of twist is measured by shining a laser beam off
a mirror mounted on the ring.
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- The device, housed in the UW's Nuclear Physics Laboratory,
has its main parts coated with gold, and a gold-coated copper foil less
than one-thousandth of an inch is stretched between the ring and the disk.
Those precautions are designed to prevent electrical forces from interfering
with the gravitational tug being measured.
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- The Adelberger-Heckel team will continue trying to measure
at smaller distances to explore whether some gravitational force might
be spilling over from other very tiny dimensions.
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- "We don't know if these ideas are right, but they
are revolutionary," Adelberger said. "There would be profound
consequences if the ideas are correct."
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