Determination Of The Newtonian Gravitational Constant G With Time-of-swing Method Using High-Q Silica Fibers

The gravitational constant G,which describes the strength of the gravitational interac-tion between objects,is the first introduced fundamental physical constant in history,while its measurement accuracy is the worst due to the weakness and unshielablity of gravity.What especially confused the experimental physicists is the large discrepancy between the G values obtained by different groups in the world.The largest difference between the four-teen G values adopted by the Committee on Data for Science and Technology(CODATA-2014)reaches up to 550 ppm.The most likely explanation for this situation is there may exist some undiscovered or not correctly recognized systematic errors in these experiments.Our laboratory proposed to perform the G measurement using two independent methods,the time-of-swing method and the angular acceleration feedback method,with an expected precision of better than 25 ppm.By comparing the two different methods,we try to find the possible error sources and improve the confidence level of G value.This thesis is about the G measurement with the time-of-swing method.During my Ph.D.I mainly participated in and finished the following work:Research on coated silica fibers with quality factors of 5 × 104.G measurements were performed using these coated silica fibers and effectively reduced the anelasticity,one of the main system errors in this method,from about 212 ppm to about 6 ppm.A series of experimental studies have been carried out to improve the accuracy of the distance between the source masses:The support of the source mass is improved to a kind of three-point mount,the stability and anti-vibration ability of which are tested and better than 0.1 ?m,meeting the experimental requirements.It is pointed out that the difference of the distance between two sphere before and after vacuum pumping is an error term that should be considered.An air bearing method is used to test the deviation of the mass center and centroid of spheres.Finally the distance between the two spheres is measured with an uncertainty of 0.37 ?m,which contributes an uncertainty of less than 9.53 ppm to the value of G.The influence of the electric field and the magnetic field were analyzed and relevant modulation experiments were carried out respectively to evaluate their contribution to the G value.In order to check whether there is systematic error related to the experimental device,another set of independent apparatus was built.On this apparatus G measurement was performed using different silica fiber,torsion pendulum,source masses and obtained a result with an uncertainty of 15.59 ppm.Finally,the experiments for determing G with time-of-swing method using high-Q silica fibers were completed,and the systematic errors in the experiments were compre-hensively analyzed.7 results were obtained using 4 different silica fibers.The combined G value is(6.674184 ± 0.000078)× 10-11 m3 kg-1s-2,and the relative uncertainty is 11.64 ppm,which reaches the best level of current G measurement.This result is agreed with the result of the angular acceleration feedback method performed by other members in our group,(6.674484 ± 0.000078)× 10-11 m3 kg-1s-2,within 3? error range.The results obtained by two completely independent methods will make a contribution to nail down the true value of the gravitational constant G.