BigG - New Measurements of G Deepen Uncertainties About its...

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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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BigG - New Measurements of G Deepen Uncertainties About its...

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