Future generations of gravitational wave detectors plan to use cryogenics in order to further reduce thermal noise associated with the mirror test masses and their suspensions. It is important that the thermal conductivity of candidate materials for these mirror suspension systems, and any additional thermal resistance associated with the required bonding/jointing, is characterised. Results are presented here for composite single-crystal silicon substrates, with multiple hydroxide catalysis bonds present, in order to assess the thermal conductivity of the bond layers. An average bond thickness of 460 nm is observed within the oxide-bond-oxide interfaces, with a calculated thermal conductivity rising from 0.013 -> 0.087 Wm-1K-1 across a temperature range of 9 -> 300 K. This confirms that the thermal conductance through hydroxide catalysis bonds, with geometries being considered for third generation gravitational wave detectors operating at 20 K or 125 K, would have a negligible impact on the required heat extraction for cryogenic operation. Therefore, the use of hydroxide catalysis bonding, as successfully demonstrated within room temperature gravitational wave detectors, remains an attractive solution for building future cryogenic instruments with reduced thermal noise and enhanced astrophysical reach.

I will talk about the family of experiments known as precision measurement and null experiments. These experiments, often associated with the determination of a fundamental constant or the testing of one of the accepted laws of Nature at some extreme of magnitude, have always provided a challenge to both a scientist's cunning and his or her experimental skills. In my talk I will use as one example, C.V. Boys' 1894 measurement of big G. [C.V. Boys, one of my physics heroes, wrote papers that evidenced his humanity and sense of humor and are, accordingly, very enjoyable to read. Unfortunately, journal editors go to great lengths to remove from today's publications any hint of humanity, humor, or enjoyment derived from doing science.]

We have experimentally demonstrated that the effective thermal expansion coefficient of a fused silica fibre can be nulled by placing the fibre under a particular level of stress. Our technique involves heating the fibre and measuring how the fibre length changes with temperature as the stress on the fibre was systematically varied. This nulling of the effective thermal expansion coefficient should allow for the complete elimination of thermoelastic noise and is essential for allowing second generation gravitational wave detectors to reach their target sensitivity. To our knowledge this is the first time that the cancelation of the thermal expansion coefficient with stress has been experimentally observed.

Retroreflector packages have been carried to the moon by the Apollo 11, Apollo 14, and Apollo 15 missions, as well as by Luna 17. Laser ranging from the earth onto these packages should eventually yield information on polar motions and crustal movements accurate to a few centimeters, and on UT1 to 100 microsec. Present (1971) error of the range measurements is 30 cm, but accuracy to 3 cm should be obtainable with improvements in methods and equipment.

I discuss here our new determination of the Newtonian constant of gravitation as well as some general comments. Great care was exercised in carrying out the experiment and in our detailed analysis. On completing our measurement, Harold and I were reminded of Cavendish's description of his 1798 experiment, “The apparatus is very simple.” (That statement also applies to the experiment that I report on here.) We would add, “The measurement is very hard.”

The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sèvres, France in September 2005. The organization of ICAG2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented. The g-values for the sites A and B of the BIPM gravity network at a height of 0.90 m are (980,925,702.2 ± 0.7) μGal and (980,928,018.5 ± 0.7) μGal, respectively. This result is in a good agreement with that obtained in ICAG-2001.

The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sèvres, France in September 2005. The organization of ICAG-2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented.

Many applications using bonded optical components have stringent requirements on the strength, rigidity, stability and alignment of the bonds. Hydroxy-catalysis bonding fulfills these requirements. Here we investigate methods by which the bonding time may be extended to better aid the precise prealignment of optical components through controlling the temperature and concentration of the bonding solution.

In a ballistic gravimeter, when a test body rotates during free fall an optical path error results, unless the centre of mass (COM) and the optical centre of the corner cube are exactly co-located. The error associated with the rotation needs to be held to 1 muGal (1 muGal == 10(-8) m s(-2)) or less, so as to not compromise the accuracy of today's absolute gravimeters. This error is systematic, difficult to monitor and can be decreased in only two ways: (1) by reducing the rotation rate of the failing object and/or (2) by minimizing the distance between the COM of the dropping object and the optical centre of the corner cube embedded in it. We report here on a new direct and extremely sensitive method for minimizing the distance between the COM and the optical centre.

This paper describes investigations into the mechanical losses of bonds created by hydroxy-catalysis bonding. Evaluation of the magnitude of such losses is important for determining thermal noise levels in bonded suspensions for gravitational wave detectors. Three samples were investigated with bonds of varying geometries and surface areas. In two cases, the bonds were between two pieces of fused silica, whilst in the third a fused silica piece was attached to a sapphire substrate. In each case sodium silicate solution was used as the bonding agent. The thickness and Young's modulus of the bond material were evaluated enabling values for the intrinsic mechanical loss factor of the bonding material to be obtained.

The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sèvres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively, calculated using a combined adjustment of the absolute and relative data.

In this paper, we describe the conceptual design for the suspension system for the test masses for Advanced LIGO, the planned upgrade to LIGO, the US laser interferometric gravitational-wave observatory. The design is based on the triple pendulum design developed for GEO 600—the German/UK interferometric gravitational wave detector. The GEO design incorporates fused silica fibres of circular cross-section attached to the fused silica mirror (test mass) in the lowest pendulum stage, in order to minimize the thermal noise from the pendulum modes. The damping of the low-frequency modes of the triple pendulum is achieved by using co-located sensors and actuators at the highest mass of the triple pendulum. Another feature of the design is that global control forces acting on the mirrors, used to maintain the output of the interferometer on a dark fringe, are applied via a triple reaction pendulum, so that these forces can be implemented via a seismically isolated platform. These techniques have been extended to meet the more stringent noise levels planned for in Advanced LIGO. In particular, the Advanced LIGO baseline design requires a quadruple pendulum with a final stage consisting of a 40 kg sapphire mirror, suspended on fused silica ribbons or fibres. The design is chosen to aim to reach a target noise contribution from the suspension corresponding to a displacement sensitivity of 10−19 m Hz−1/2 at 10 Hz at each of the test masses.

Summary form only given. The author describes some of the work directed towards the measurement of G, g, and g/spl mu//spl nu/. A number of recently completed measurements of G have had, and a number of ongoing measurements have, the aim of improving the accuracy of the first of these "gs" from today's 1.5 parts in 10/sup 2/ to better than one part in 10/sup 4/ (to somewhat better than the accuracy with which it was thought to be known 10 years ago). By contrast, our ability to accurately measure the second "g" has, during the past forty years, enjoyed a remarkable 3 orders-of-magnitude improvement. Lastly, sometime in the next decade or so, the final "g" can be expected to provide us with new insights into things such as the formation of massive black holes and galaxy evolution as well as a variety of other objects of astrophysical interest. The role of precision measurement science in carrying out these measurements, and, indeed, in underpinning all of scientific progress, is highlighted. Finally, the commonality of these three different measurements is pointed out.

The application of technological advances has dramatically improved our ability to measure the absolute value of g, the free-fall acceleration due to gravity. Over the past thirty years this improvement has been nearly three orders of magnitude! Today, the value of gcan be determined with an accuracy approaching 1 µGal (10-8 m/s2) and measured with a precision that is at least ten times better. This paper reviews the history of and the reasons for this progress as well as taking a look to the future.

We describe the design of and measurement results from a new cam-driven compact and portable absolute gravimeter. The resulting instrument will serve a multitude of field (because of its compactness) and laboratory (including standards) applications

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented.