The Gravitational Constant: Theory and Experiment 200 Years After Cavendish

Conference on The Gravitational Constant: Theory and Experiment 200 Years After Cavendish (23-24 November 1998, Institute of Physics, Portland Place, London, UK) This conference was held to mark the bicentenary of the publication of Cavendish's determination of Newton's constant of gravitation. The aims of the conference were to: (i) assemble the key workers in the field, and give them an opportunity to present their latest results for G and to discuss planned experiments (ii) discuss the current status of G in physical theory (iii) celebrate the torsion balance as a detector in gravitational experiments and to discuss recent innovations in weak-force detection (iv) celebrate the achievement of the Right Honourable Henry Cavendish, who founded this field of research 200 years ago. The conference was attended by some forty-five people, three-quarters of whom were from outside the UK. The participants came from eleven countries around the globe, representing practically the whole of the international community now engaged upon measurements of G. As far as we know, this was the first international conference that has ever been dedicated to the measurement of the gravitational constant. We, as organizers, were delighted that all the key researchers in the field supported the event and in most cases made a substantial effort to attend. We believe that, thanks to their efforts, these proceedings represent an accurate and complete picture of current work in this field of research. We must add, however, that the experiments reported here are nearly all still in progress; the results given are preliminary and likely to be much improved over the next year or two. Indeed, at the end of the conference it was agreed that another similar meeting would be useful in two or three years' time. The programme comprised 14 invited papers and seven contributed papers. The invited papers appear first, followed by the contributed papers, in a specially dedicated section in this issue of Measurement Science and Technology. The modern interpretation of Cavendish's determination of the mean density of the Earth is that it was the first precise measurement of the constant of gravitation, G. The uncertainty achieved by Cavendish in his torsion balance experiment was about 5%. Today, some two hundred years after his pioneering work, the CODATA Task Group on Fundamental Constants is envisaging an uncertainty of 0.15% for its 1999 evaluation of G. The accuracy of our knowledge of G has thus advanced at a rate of less than one order of magnitude per century, which is painfully slow in comparison with progress in determining other physical constants. Table 1. Values for G. The first three entries in the table are included to illustrate the recent disparity in results. All other entries are results reported at the conference and described in these proceedings. Figure 1. Values of G given in table 1. At the conference, six groups announced new preliminary results. These new values are given in table 1 and illustrated in figure 1, together with the 1986 CODATA value. The disparity in recent values that provoked the current flurry of activity in measurements of G is illustrated by the inclusion of the result from Physikalische-Technische Bundesanstalt (PTB), Germany, (Michaelis et al 1995), and the original result from the Measurement Standards Laboratory (MSL), New Zealand, (Fitzgerald and Armstrong 1995). Gillies presents an excellent review in this issue of these measurements and other results reported prior to the conference. The unweighted mean of the results presented at the conference is 6.6772 × 10-11 m3 kg-1 s-2, with an uncertainty of 840 parts per million (ppm). The ensemble of new results therefore agrees with the CODATA value within this uncertainty, and given the diversity of the methods employed, provides some reassurance that it remains a robust estimate of Newton's constant. The PTB value remains an intriguing mystery. It is worth noting that the current progress on new determinations reported by Gundlach, Newman and Ni, leads us to expect values for G with uncertainties at the level of a few ppm within the next few years. A highlight of the conference was Damour's presentation of the theorists' view of the role of G. This was a fascinating exposition of the current attempts to construct theories that welcome gravitation into the fold of quantum theories. One was left with the tantalising hope that the rest of the physics community will finally appreciate the relevance of G in the not-too-distant future. The torsion balance, as developed by Cavendish, remains the most popular choice for weak-force physicists. However, it was interesting to note that, even after 200 years, promising innovations are still being made. For example, Luther described his development of a bifilar suspension for his next determination of G, and Richman reported a preliminary value for G made, for the first time, using a torsion-strip balance. These innovations are aimed at eliminating anelastic effects in the traditional torsion fibre. Our understanding of this phenomenon has progressed significantly over the past decade or so. Speake gave an overview of this topic, and also presented Kuroda's talk in his absence. Isobel Falconer offered us a spellbinding insight into both the psyche and formidable scientific achievements of Henry Cavendish. As organisers, we believe that the conference's aims were achieved and that the efforts of all concerned culminated in an occasion that was a worthy commemoration of this remarkable man and his measurement. References [1] Michaelis W, Haars H and Augustin R 1996 Metrologia 32 267-76 [2] Fitzgerald and Armstrong 1995 Bull. Am. Phys. Soc. 40 975