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Unformatted text preview: New Measurements of G Deepen
Uncertainties About its Value ew measurements of the Newtonian gravitational constant — the number
that describes the strength of gavity —
depart signiﬁmtly from the accepted value
established in the 1980s, deepening
uncertainties as to its actual value.
Affectionately dubbed “Big G it is the least
accurately known of all the fundamental
constants; the accepted value of 6.6726 x
10“ m3/kg—sZ is known with a relatively
high uncertainty of 0.01 percent. Big G is arguably the most difficult con—
stant to measure because, among other
reasons, gravity is the weakest of all
forces and it is impossible to shield deli—
cate measurements from the gravity
inﬂuences of buildings and other nearby
objects. Underscoring this difficulty,
three scientists from international labs
— the German Bureau of Standards, the
Measurement Standards Laboratory of
New Zealand, and the University of
Wuppertal in Germany « reported new
measurements of G which disagreed
widely with one another and with the
standard value in an invited session of
the Precision Measurement and Funda
mental Constants Topical Group at the
April meeting. The Wuppertal value was 0.07 percent
below the currently accepted value (cor
responding to 7 standard deviations), the New Zealand measurements were
0.07—0.08 percent below (748 standard
deviations) and the German Bureau of
Standards value was a whopping 06
percent above (60 standard deviations).
“There hasn’t been this big a difference
in the value of G for many years,” said
Eric Adelberger (University of Washing—
ton) of the results. Although the techniques differed, the
groups all essentially determined G by
measuring the gravimtional effects of cy—
lindrical masses acting on objects
suspended above the ground. For ex—
ample, the New Zealand group used a
compensated torsion balance in which
the gravitational torque was balanced
by an electrostatic torque from an elec
trometer, which was calibrated by
accelerating the entire apparatus. Researchers at Los Alamos, which
helped set the 19803 standard, are un
dertaking a new measurement of G
which may be five times as precise as
current measurements, and may shed
light on these puzzling results, A firmly
established value of G is important to
delineate between grand unified theo—
ries that attempt to relate G to
fundamental constants associated with
the other three physical forces (see fig—
ure on page 7). THREE NEW MEASUREMENTS or G, the Newtonian gravitational constant, disagree signiﬁcantly
with the accepted value and with each other. The
least well known of all the fundamental constants,
the accepted value of G was established in the
19803 to be 6.6726 :r 0.0009 in units of
10’11 m3 kg] 5’2. G is arguably the most difficult
constant to measure because, among other rea sons, gravity is the weakest of the four known fun
damental forces and it is impossible to shield deli—
cate measurements from the gravitational inﬂu
ences of buildings and other nearby objects.
Underscoring this diﬁiculty, scientists from three
labs (the University of Wuppertal in Germany, the
Measurement Standards Laboratory of New Zea
land and the German Bureau of Standards—PTB
Braunschweig) reported new and differing measure
ments of G at the April APS meeting. In the
same units as above, the Wuppertal value was
6.6685 i 0.0007 ; the New Zealand measurements
gave 6.6656 i 0.0006 ; and the German Bureau of
Standards value was a whopping 6.7154 i 0.0006.
Although the techniques differed, the groups all es
sentially determined G by measuring the gravita
tional effects of cylindrical masses acting on ob
jects suspended above the ground in static
equilibrium. According to Hinrich Meyer (Wupper
tal), we are “forced to conclude that G is known
with an uncertainty of 6 x 10‘4 and not 1.28 X 10—4” as previously thought. Meyer’s group
hopes for a more precise determination of G by the
end of 1995, while researchers at Los Alamos, the
lab that helped set the 1980s standard by measur
ing oscillations, are also undertaking a new meas
urement of G. JUNE 1995 PHYSICS TODAY 9 ...
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 Pottebaum

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