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The Newtonian constant of gravitation, a constant too difficult to measure?

Event

Starts:

February
272014

09:00

Ends:

February
282014

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Theo Murphy international scientific meeting organised by Dr Terry Quinn CBE FRS, Professor Clive Speake and Professor Jun Luo

Events details

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. This meeting explored possible reasons for this and generated 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 can be accessed below, by clicking the '+' symbol next to each speaker.

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Schedule of talks

Session 1: Introduction and background

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A measurement of G using a beam balance and 13 tons of mercury

Dr Stephan Schlamminger, NIST, USA

Abstract

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.

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G measured with a rotating torsion balance

Dr Jens Gundlach, University of Washington, USA

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The large masses in measurements of G: some examples and considerations

Dr George Gillies University of Virginia USA

Abstract

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.

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The role of G in fundamental physics

Professor Gary Gibbons, University of Cambridge, UK

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Session 2: Results from recent determinations of G

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G using a torsion pendulum time of swing

Dr Jun Luo, HUST Wuhan, China

Abstract

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.

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G using a torsion-strip balance

Dr Terry Quinn CBE FRS, BIPM

Abstract

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.

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G with a suspended laser interferometer

Dr Harold Parks, Sandia National Laboratories, USA

Abstract

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.

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Measurement of G using a cryogenic torsion pendulum

Professor Riley Newman, University of California at Irvine, USA

Abstract

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

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Session 3: How do we understand and deal with widely dispersed values of G

4 talks Show detail Hide detail

A phenomenologist looks at discordant measurements of G

Professor David Bartlett, University of Colorado, USA

Abstract

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.

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Precision measurement: The Sine Qua Non for big G determinations

Professor Jim Faller, JILA Colorado, USA

Abstract

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.

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Recommending a value for the Newtonian Gravitational Constant

Dr Barry Wood, CODATA task Group on Fundamental Constants, NRC Ottawa, Canada

Abstract

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.

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Systematic effects in G experiments

Professor Clive Speake, University of Birmingham, UK

Abstract

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.

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Session 4: Proposals for future work and open discussion

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Future G work at HUST

Dr Jun Luo, HUST Wuhan, China

Abstract

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.

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G measured using atom interferometry

Dr Guglielmo Tino, University of Florence, UK

Abstract

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.

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How to resolve the present impasse?

Dr Terry Quinn CBE FRS, BIPM

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The Newtonian constant of gravitation, a constant too difficult to measure? Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ