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Theo Murphy international scientific meeting organised by Dr Terry Quinn CBE FRS, Professor Clive Speake and Professor Jun Luo
The Newtonian constant of gravitation, G, is the only fundamental constant of physics for which the uncertainty given in successive CODATA evaluations has increased. Measurements made since 2000, using a variety of methods, now show a spread of values more than ten times their estimated uncertainties. We shall explore possible reasons for this and hope to come to some proposals for new measurements that might resolve the present impasse.
Biographies of the key contributors are available below and you can also download a programme (PDF). Recorded audio of the presentations will be available on this page shortly after the event.
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This is a residential conference, which allows for increased discussion and networking. It is free to attend, however participants need to cover their accommodation and catering costs if required.
Places are limited, therefore pre-registration is essential. Please either:
Dr Terry Quinn CBE FRS, BIPM
Terry Quinn obtained a B. Sc in physics at the University of Southampton in 1956, then a D. Phil. at the University of Oxford in 1963. From 1962 to 1977 he was at the National Physical Laboratory working on temperature and later mass measurement. He moved to the International Bureau of Weights and Measures (BIPM) at Sèvres, France, in 1977 as Deputy Director becoming Director in 1988 until his retirement in 2003. While at the BIPM, in addition to being much involved with the organization of international metrology and the development of the proposal to redefine the units of the SI in terms of fundamental constants, he participated in work related to balances, fine suspensions and mass standards. Latterly he led work on the determination of G using a torsion strip balance, the final result being published in 2013. He was elected a Fellow of the Royal Society in 2001.
Professor Clive Speake, University of Birmingham, UK
Clive Speake received his undergraduate degree at University of Birmingham in 1979. His PhD work was awarded at University of Cambridge in 1984. He became a researcher at Bureau International des Poids et Measures in 1984 where he worked on mass metrology with Terry Quinn. After a year sabbatical at JILA, University of Colorado, where he worked on an experimental test of the inverse square law of gravity with James Faller, in 1989, he became a lecturer at University of Birmingham. He is now Professor of Experimental Physics and his current research interests include laboratory tests of gravity and development of high precision optical and cryogenic instrumentation with over 80 publications.
Dr Jun Luo, HUST Wuhan, China
Jun Luo received the B.Sc and M.Sc in theoretical physics from Huazhong University of Science and Technolog (HUST) in 1982 and 1985, respectively. Then received the PhD degree in solid geophysics from Institute of Geodesy & Geophysics, Chinese Academy of Sciences, Wuhan, P.R.China, in 1999. He is presently the vice president of HUST, and also academician of the Chinese Academy of Sciences. His research fields is precision measurement physics, major in gravitational experiments, including measurement of Newtonian gravitational constant G, experimental test of Newton inverse square law, experimental test of weak equivalence principle, experimental detection of the upper limit on the photon mass, space inertial sensors and laser ranging, atomic interference gravimeter.
Dr George Gillies University of Virginia USAThe large masses in measurements of G: some examples and considerations
George T Gillies was born in Rugby, North Dakota. He attended North Dakota State University, receiving the B.Sc. in Physics in 1974. He then attended the University of Virginia, receiving the M.Sc. and Ph.D. in Engineering Physics in 1976 and 1980, respectively. He was a postdoctoral fellow at the University of Virginia and the U.S. National Bureau of Standards, and thereafter accepted a position as a physicist with the International Bureau of Weights and Measures in Sèvres, France. In 1983 he returned to the U.S. and worked for the U.S. Department of Energy’s Oak Ridge Complex until 1985 when he became a faculty member of the University of Virginia’s School of Engineering and Applied Science. He is presently a Research Professor of Mechanical Engineering and Biomedical Engineering. His research interests include medical device development, precision measurement technology, and gravitational physics. He is also the co-founder of Stereotaxis, Inc., NexGen Medical Systems, Inc., and EpiEP, Inc. He is a Fellow of the American Physical Society, the Institute of Physics, and the American Institute of Medical and Biological Engineering, and serves as Deputy Editor of the Institute of Physics journal, Reports on Progress in Physics.
Simple spheres, cylinders, and sometimes complex configurations of them, have been the geometries employed most frequently for the attracting masses used historically in measurements of the Newtonian gravitational constant G. We present a brief overview of the range of sizes, materials, and configurations of the attracting masses found in several representative experimental arrangements. As one particular case in point, we present details of the large tungsten spheres designed originally by J. W. Beams, which have been incorporated into several different apparatuses for measuring G over the past 50 years. We also consider the question of possible systematic dependence of the results and their precision on the size of the large masses/mass systems that have been employed to date. We close with some considerations for possible future work.
Professor Gary Gibbons, University of Cambridge, UKThe role of G in fundamental physics
Biography not yet available
Dr Jens Gundlach, University of Washington, USAG measured with a rotating torsion balance
Dr Stephan Schlamminger, NIST, USAA measurement of G using a beam balance and 13 tons of mercury
Stephan Schlamminger graduated from the University of Regensburg, Germany, in 1998 with a diploma in physics (Dipl. Phys.). He then went to the University of Zurich, Switzerland, to pursue a doctorate in experimental physics working on an experiment to determine the gravitational constant, G, using an experiment that had been built by Professor W Kuendig, Dr J Schurr, and Dr F Nolting. He graduated with a PhD (Dr sc nat) in 2002. In the next year he went to the University of Washington in Seattle, WA, to work as a post-doctoral fellow with Professor J H Gundlach on various torsion balance experiments, most notably a precise test of the principle of equivalence (does gravitational mass equal inertial mass?). During the first years in Seattle he continued to assist Dr R E Pixley from the University of Zurich in a final analysis of the big G data. After four years at the University of Washington he was promoted to the rank of Research Assistant Professor. Since 2010 he has been working on an experiment, called watt balance, to determine the Planck constant, h, at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. Currently he is working on building a next generation watt balance that will be used to realize the unit of mass in the United States after the system of units has been redefined in the near future.
The Zurich G experiment is unique among modern G experiments in at least five aspects: (1) A modified commercial balance was used to measure the gravitational force allowing for easy calibration using weights. (2) The measured gravitational signal was large, approximately 8 μN. (3) Liquid mercury was used as field mass avoiding density gradients. (4) Only moderate precision (0.1 mm) was required for the relative distances between test and field masses as each test mass was located at a double extremum of the force field acting on it. (5) The measurement direction was along the direction of local gravity making the measurement independent of varying horizontal gravity gradients.
We believe that these five key differences allow for a reliable and independent determination of the gravitational constant G. Our final result was G=6.674 252(124) x 10-11 m3kg-1s-2. It has a relative uncertainty of 18.6 ppm.
Dr Jun Luo, HUST Wuhan, ChinaG using a torsion pendulum time of swing
A new determination of the Newtonian gravitational constant G is presented by using a torsion pendulum with the time-of-swing method. Compared with our previous measurement with the same method, several improvements greatly reduced the uncertainties as follows: (1) measuring the anelasticity of the fiber directly;(2) using spherical source masses minimizes the effects of density inhomogeneity and eccentricities;(3) using a quartz block pendulum simplifies its vibration modes and minimizes the uncertainty of inertial moment; (4) setting the pendulum and source masses both in a vacuum chamber reduces the error of measuring the relative positions. We have performed two independent G measurements, and the two G values differ by only 9 ppm. The combined value of G is 6.67349(18) 10-11m3kg-1s-2 with a relative uncertainty of 26 ppm.
Professor Riley Newman, University of California at Irvine, USAMeasurement of G using a cryogenic torsion pendulum
Riley Newman’s training and early work is in the field of experimental high energy particle physics. He received an undergraduate degree and PhD from Reed College and the University of California, Berkeley respectively, and then pursued experimental particle physics as an Assistant Professor at Columbia University in New York. While at Columbia Riley conducted an experiment to search for violation of rotational invariance of the weak interaction manifested as anisotropy in the beta decay of unpolarized nuclei in a reference frame with fixed orientation with respect to distant stars – an experiment which ignited in him a strong interest in “table top” tests of fundamental physics. In 1973 he took a tenured position at the University of California, Irvine where in subsequent years he has conducted a series of experiments to test the gravitational inverse square law and to search for possible new forces in nature.
Results of G measurements from years 2000 to 2006 are reported. The G determination is based on measurement of a roughly 1 millisecond change in the 130 second torsional period of a pendulum suspended between a pair of ring-shaped copper masses when the ring pair is rotated 90 degrees. Measurements were made using three different torsion fiber materials (as-drawn CuBe, heat treated CuBe, and as-drawn Al5056), operating at multiple torsional amplitudes. Features of the measurements include: 1) operation at cryogenic temperature near 3K, for reduced anelastic fiber effects, reduced thermal noise, and increased frequency stability, 2) operation at large pendulum oscillation amplitudes corresponding to extrema of the torsional period shift, to reduce effects of torsional amplitude error, 3) the pair of source mass rings, which produced an extremely uniform field gradient, and 4) a thin plate pendulum, minimizing sensitivity to pendulum imperfections. The large distance (about 33 cm) between pendulum and source masses facilitated multipole analysis of their coupling and reduced sensitivity to source mass and pendulum imperfections. A price paid for this was the extremely small period change signal, ranging from 1.7 to 0.2 milliseconds depending on fiber and torsional amplitude
Dr Harold Parks, Sandia National Laboratories, USAG with a suspended laser interferometer
Harold V Parks received the PhD degree in physics from the University of Colorado, Boulder, in 1998. From 1999 to 2001, he was a National Research Council Postdoctoral Fellow with the National Institute of Standards and Technology and, at the University of Colorado and JILA, he performed an experiment to measure the constant of gravitation with a suspended laser interferometer. From 2001 to 2003, he was a Research Fellow with Bureau International des Poids et Mesures (BIPM) and worked on the BIPM torsion balance measurement of the constant of gravitation. Since 2004, he has been a Member of the Technical Staff with Sandia National Laboratories, Albuquerque, NM, where he leads the DC electrical standards project at the Sandia Primary Standards Laboratory.
We will discuss our 2004 measurement of the Newtonian constant of gravitation G using a suspended Fabry-Perot interferometer. The apparatus consists of two simple pendulums hanging from a common support. Each pendulum has a length of 72 cm and their separation is 34 cm. A mirror is embedded in each pendulum bob forming the Fabry-Perot cavity. A laser locked to the cavity measures the change in pendulum separation as the gravitation field is modulated due to the displacement of four 120 kg tungsten masses. We obtain a value of (6.67234 ± 0.00014) × 10-11 m3 kg-1 s-2 for G which is well below the current CODATA value.
Dr Terry Quinn CBE FRS, BIPMG using a torsion-strip balance
Two measurements of G have been made at the BIPM, the first published in 2001 and the second in 2013, which agree to within the standard uncertainty of each. Both used the same principles of operation, although for the second measurement the apparatus was almost totally rebuilt, namely, a torsion balance with four masses suspended from a thin wide torsion strip that could be used in two different methods of measurement. One was a simple deflection method, which we refer to as the Cavendish method, and the other using electrostatic servo-control. The advantage of a wide heavily loaded torsion strip is that the restoring torque can be made almost entirely gravitational and thus lossless. The Cavendish and servo methods are largely independent so that if the results agree, as they did, the range of possible systematic errors is constrained to those common to both. Included in these are dimensional metrology and density uniformity in the masses which were carefully studied.
Professor Jim Faller, JILA Colorado, USAPrecision measurement: The Sine Qua Non for big G determinations
Born in Mishawaka, Indiana, Jim is an experimental physicist. He attended Indiana University (AB) and Princeton University (PhD). His research interests include physics, geophysics, astrophysics, gravitation, experimental relativity, determination of the fundamental constants, and precision measurement including null experiments that are designed to look for possible invalidations of accepted physical laws at some extreme of magnitude. Known as the father of modern absolute gravimetry, he also proposed the lunar laser ranging experiment. Most recently, together with Harold Parks, he made a new determination of big G, the Newtonian Constant of Gravitation. He is presently a Fellow Adjoint of JILA, an Adjunct Professor of Physics at the University of Colorado at Boulder, and a Visiting Professor at the University of Glasgow (Scotland). Though “retired” he continues to work daily as a scientist. He also serves regularly as a consultant to MicrogLacoste, a company that designs and manufactures precision absolute and relative gravimeters.
Determinations of the Newtonian Constant of Gravitation fit into the often times unappreciated area of physics called precision measurement — an area that also includes null experiments and determinations of the fundamental constants. The determination of big G — a measurement that on the surface seems deceptively simple — continues to be one of nature’s greatest challenges to the skills and cunning of experimental physicists. In spite of the fact that on the scale of the universe its effects are big enough to almost single handedly hold it together, on the scale of a laboratory, its effects are so small that they are hardly noticeable buried — as they are — in a sea of recognized as well as unrecognized noise sources. It is all of this that makes the precise value of this fundamental, but seemingly unrelated-to-the-rest-of-physics, constant so exceedingly difficult to determine. Nevertheless, it is connected, if by nothing else, via the error budgets of all precision measurements. Because of this connection, there is much to be learned from the 300-year history of determinations of this fundamental constant. Furthermore, it will continue to be a measurement on which metrologists will want to hone their laboratory skills for generations to come.
Professor David Bartlett, University of Colorado, USAA phenomenologist looks at discordant measurements of G
In 1989 there were two positive measurements of the Fifth force, V=-(GM/r)(1+alpha Exp[-r/lambda]). Experiments up a TV tower in North Carolina and down a mine shaft in Australia agreed that alpha was not zero and lambda was a few hundred meters. They disagreed, however, on the sign of alpha. Wes Tew and I helped both experimental groups to recognize that they had not treated the local topography correctly. Since then John Cumalat and I have been finding evidence for a different modification of Newtonian gravity, V=-(GM/r) Cos[k_o r], 2 pi/k_o= 400 parsecs.
Somewhere between an experimentalist and a theorist is one who scans the works of others to see what may have been missed. My working hypothesis is that human activity during the measurement of G significantly affects the measurement itself. Noise caused by the gravity gradient of humans may have been a reason why one experiment elevated its apparatus several meters above the floor. Perhaps the experimenters themselves can dispose of this issue for their own measurement. I conclude with a speculation on the possibility of a future “atomic” value of G.
Professor Clive Speake, University of Birmingham, UKSystematic effects in G experiments
The possible sources of systematic uncertainty in determinations of G are legion. I will review some of the known systematic effects that can bias the measured value of Newton’s constant. These will include anelasticity and the calibration of electrostatic torque transducers. I will refer to the determination undertaken at the BIPM and University of Birmingham to establish the value of G (Phys Rev Letts 2013) which is described by Terry Quinn in another talk at this conference. I will describe how these sources of uncertainty can be thought, reasonably, to have been made insignificant in this work.
Dr Barry Wood, CODATA task Group on Fundamental Constants, NRC Ottawa, CanadaRecommending a value for the Newtonian Gravitational Constant
Barry Wood received his PhD from the University of Toronto in 1981 and then joined the Electrical Standards group of the National Research Council in Ottawa, Canada. Since that time he has been involved in various electrical measurements including the Josephson effect, the quantum Hall effect, cryogenic current comparators, impedance standards, the calculable capacitor and most recently the watt balance. He is chairman of the Consultative Committee of Electricity and Magnetism's working group on proposed changes to the SI, Chairman of the US National Academies’ 2006 review panel for the Electronic and Electrical Engineering Laboratory of the National Institute for Standards and Technology (NIST), the past Chairman of the CODATA Task Group on Fundamental Constants and a Fellow of IEEE.
The primary objective of the CODATA Task Group on Fundamental Constants is ‘to periodically provide the scientific and technological communities with a self-consistent set of internationally recommended values of the basic constants and conversion factors of physics and chemistry based on all of the relevant data available at a given point in time’. I will discuss why the availability of these recommended values is important and how it simplifies and improves science. I will outline the process of determining the recommended values and introduce the principles that are used to deal with discrepant results. In particular, I will discuss the specific challenges posed by the present situation of gravitational constant experimental results and how these principles were applied to the most recent 2010 recommended value. Finally, I will speculate about what may be expected about the next recommended value of the gravitational constant scheduled for evaluation in 2014.
Dr Jun Luo, HUST Wuhan, ChinaFuture G work at HUST
Jun Luo received the B.Sc and M.Sc in theoretical physics from Huazhong University of Science and Technology(HUST) in 1982 and 1985, respectively. Then received the PhD degree in solid geophysics from Institute of Geodesy & Geophysics, Chinese Academy of Sciences, Wuhan, P.R.China, in 1999. He is presently the vice president of HUST, and also academician of the Chinese Academy of Sciences. His research fields is precision measurement physics, major in gravitational experiments, including measurement of Newtonian gravitational constant G, experimental test of Newton inverse square law, experimental test of weak equivalence principle, experimental detection of the upper limit on the photon mass, space inertial sensors and laser ranging, atomic interference gravimeter.
The Newtonian gravitational constant G holds an important place in physics. Though there have been about 300 measurements of G since the first laboratory measurement by Cavendish over 200 years ago, its measurement precision is the worst among all the fundamental physics constants. Up to now, even for the seven most precise values of G with their assigning uncertainties within 50 ppm, they are only consistent with each other in the range of about 500 ppm. It seems clear that to improve the accuracy of the G measurement, further investigating and depressing more possibly systematic errors is needed extremely. In order to find the potential errors in different method, two different methods, the time-of-swing method and the angular acceleration feedback one, were all used to determine the G value in our cave laboratory. In this talk, we will present some new progress about the G measurement with the two different methods.
Dr Guglielmo Tino, University of Florence, UKG measured using atom interferometry
Guglielmo M. Tino attended the University of Napoli, receiving the Degree in Physics in 1987. He then got the Ph.D. in Physics from Scuola Normale Superiore of Pisa in 1992. He was a faculty member at University of Napoli until 2001 when he moved to Firenze where he was appointed Full Professor of Physics of Matter. He spent research periods in Paris, at the Laboratoire Kastler-Brossel de l'Ecole Normale Supérieure, and in Boulder at JILA and NIST. His research interests include Precision spectroscopy and stable lasers, Atomic clocks, Atom interferometry, Precision measurements of fundamental constants, Laser cooling of atoms, Bose-Einstein condensation, Tests of fundamental physics laws, Experiments in space. He is the author of more than 100 scientific papers and editor of two books. Tino is a member of the Board of the Atomic Molecular and Optical Physics Division (AMOPD) of EPS and Chair of the Board of the European Group for Atomic Systems (EGAS) of EPS.
I report on the first precise determination of G using laser-cooled atoms and quantum interferometry to probe gravity. We reached a precision of about 100 ppm. Such a conceptually different experiment is important to identify the so far elusive systematic errors thus improving the confidence in the value of G.
Dr Terry Quinn CBE FRS and Professor Clive Speake How to resolve the present impasse?
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