Support 100 years of independent journalism.

  1. Science & Tech
13 March 2014updated 01 Jul 2021 11:42am

In search of the notorious Big G: why we still know so little about gravity

Gravity is pathetic and so is our understanding of it.

By Michael Brooks

Gravity is pathetic. The Oscar statuette, for instance, has a mass of 3.85 kilograms but it is pulled down to earth by a force so weak that you can buy a £2.99 fridge magnet that can beat it. It’s shameful that the gravitational pull of the entire earth can be overcome by a cheap piece of magnetised steel.

Gravity is by far the weakest of the fundamental forces of nature (the fridge magnet puts the far stronger electromagnetic force to work). It is so weak that its strength is proving difficult to measure accurately. In late February, while Alfonso Cuarón, the director of the sci-fi film Gravity, was on tenterhooks waiting for the Oscars result, the world’s experts on gravity assembled just outside Milton Keynes in an attempt to sort out this most embarrassing problem.

Numbers such as the strength of gravity, the speed of light and the charge on an electron are known to physicists as the “fundamental constants”. They are in some ways the sticking plaster of physics. We can explain the origin of most things but we know the values of the fundamental constants only by measuring them – there is no way to work them out from a theory.

These days, most are very well defined – but not gravity. It is the only fundamental constant for which our uncertainty over its value has got worse over the years.

The gravitational constant is sometimes known as “Big G”. This differentiates it from “little g”, which describes how fast things accelerate towards Planet Earth when free to fall. The first accurate measurement of Big G was made in 1798. Henry Cavendish used a torsion balance, a device in which two lead weights are attached to the ends of a metal bar. The bar hangs horizontally by a metal wire attached to its midpoint. Cavendish then brought other weights close to one of the lead weights and measured how much the gravitational attraction between the weights twisted the wire. From that measurement, he calculated the strength of gravity.

Select and enter your email address Quick and essential guide to domestic and global politics from the New Statesman's politics team. A weekly newsletter helping you fit together the pieces of the global economic slowdown. The New Statesman’s global affairs newsletter, every Monday and Friday. The New Statesman’s weekly environment email on the politics, business and culture of the climate and nature crises - in your inbox every Thursday. Our weekly culture newsletter – from books and art to pop culture and memes – sent every Friday. A weekly round-up of some of the best articles featured in the most recent issue of the New Statesman, sent each Saturday. A newsletter showcasing the finest writing from the ideas section and the NS archive, covering political ideas, philosophy, criticism and intellectual history - sent every Wednesday. Sign up to receive information regarding NS events, subscription offers & product updates.
  • Administration / Office
  • Arts and Culture
  • Board Member
  • Business / Corporate Services
  • Client / Customer Services
  • Communications
  • Construction, Works, Engineering
  • Education, Curriculum and Teaching
  • Environment, Conservation and NRM
  • Facility / Grounds Management and Maintenance
  • Finance Management
  • Health - Medical and Nursing Management
  • HR, Training and Organisational Development
  • Information and Communications Technology
  • Information Services, Statistics, Records, Archives
  • Infrastructure Management - Transport, Utilities
  • Legal Officers and Practitioners
  • Librarians and Library Management
  • Management
  • Marketing
  • OH&S, Risk Management
  • Operations Management
  • Planning, Policy, Strategy
  • Printing, Design, Publishing, Web
  • Projects, Programs and Advisors
  • Property, Assets and Fleet Management
  • Public Relations and Media
  • Purchasing and Procurement
  • Quality Management
  • Science and Technical Research and Development
  • Security and Law Enforcement
  • Service Delivery
  • Sport and Recreation
  • Travel, Accommodation, Tourism
  • Wellbeing, Community / Social Services
Visit our privacy Policy for more information about our services, how New Statesman Media Group may use, process and share your personal data, including information on your rights in respect of your personal data and how you can unsubscribe from future marketing communications.

Cavendish’s accuracy was five parts in 1,000. Over 200 years later, our accuracy stands at roughly one part in 10,000. Given that modern measurements use lasers and electronic devices and Cavendish used a mirror and a candle, it hardly counts as a great improvement.

What’s worse is that our measurements of Big G are getting less accurate. The latest measurement, reported at the end of last year, reduced the overall value by 66 parts per million but the uncertainty
of the value increased from 100 parts per million to 120 parts per million.

The measurement was taken by Terry Quinn, emeritus director of the International Bureau of Weights and Measures in Paris. At its meeting in February, he argued that it was time researchers admitted that everyone must be making some basic errors in their method and that they should give up on making any more unilateral measurements.

The experts now agree that future experiments seeking the value of Big G will be done in big collaborations, with the proposals for equipment and methodology being scrutinised by everyone in advance to minimise the chance of further embarrassment.

It will, they say, mimic the way that researchers worked together to find the Higgs boson. That gave us the secret of mass: the hope is that if the physicists all pull together, they can finally work out exactly what size of force brings that mass down to earth.

Topics in this article: