THE FREE-FALL DETERMINATION OF THE UNIVERSAL CONSTANT OF GRAVITY by JOSHUA

Date The nal copy of this thesis has been examined by the signatories, and we nd that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. The Newtonian constant of gravity, G, is the constant of proportionality that scales the magnitude of the gravitational force between masses. G is the least precisely known of all fundamental constants. Recent high-precision experiments to measure G h a ve produced highly discordant results, with values spread over a range of 0.7. The situation is almost incredible; the second digit of the value is in contention and experiments disagree by a s m uch as 50 of their individual error estimates. We have developed a new determination that uses an absolute free-fall gravimeter to sense the gravitational attraction between a known source mass and a falling test mass. We use a one-half metric ton source mass whose position is alternated from above to below the dropping region, either decreasing or increasing the observed acceleration of the test mass. The source mass generates a diierential signal of 8 parts in 10 7 of the local acceleration due to the Earth. This method is independent of many of the systematic errors associated with the traditional torsion balance experiments upon which the accepted value of G is based. Using this free-fall" method in two data runs of approximately two w eeks, we h a ve found a value of G at the level of 1100 ppm. Our value is: G = 6.6873 0.0076 X10 ,11 m 3 kg-s 2. This value lies in the approximate center of the range of recent results ACKNOWLEDGEMENTS Many people have ooered assistance with my experiment. Tim Niebauer originally suggested the method of using the gravimeter to measure the acceleration signal of a source mass to determine G. But there is no doubt in my mind that the person most responsible for the free fall experiment w as Jim Faller. Jim provided the energy and drive to get the experiment ying, and in many r e s p e c t s drew me along for the ride. His was the insight that lead to our source mass design, and also provided an example of the type of thought that I most enjoy attempting. Doug Robertson was like a second advisor to me. While Jim discussed and argued diierent points of apparatus with …