Science, Maths & Technology

The Gravitational Constant

Updated Tuesday 1st June 2004

Dr Alan Cooper explains how the gravitational constant can be demonstrated through table-top experiments

Big G and Maskelyne

Each of the really fundamental force laws of nature is very general, and needs to be linked to our own particular universe by measuring the value of a universal constant embedded in the law.

In the case of Newton it is G, the gravitational constant (as distinct from g which is merely the acceleration at the surface of the Earth and so of local interest only).

Once you know G here, you know it everywhere in the Universe.

Maskelyne measured it in Scotland, in fact, in 1776. A mountain pulls a pendulum sideways and the Earth pulls it downwards. Measuring the (very small) deflection allows the pull of the mountain alone to be deduced, and that gives G.
graphical representation of the equation described Copyrighted image Icon Copyright: Used with permission

Twenty years later, the Duke of Devonshire, Henry Cavendish, attempted the measurement of the attraction between metal spheres suspended in his laboratory. (Cavendish was a brilliant physicist little known because of his extremely retiring nature.)

Whereas the mass of the mountain Schiehallion had to be estimated, the masses of the spheres could be accurately measured. This was a bold and pioneering experiment, especially for a man nearing 70.

It was remarkably successful (only 2% above the true value) but it was a century before the same method could start to be refined, thanks to the invention of quartz fibres by C.V. Boys.

Even so, G has remained the most difficult of the basic physical constants to measure accurately. It is for this reason that astronomers adopted the practice of relating their measurements to the AU, rather than directly to the metre (Why this is important is discussed in What does the AU mean?) and that discussion of the AU is inevitably paralled by discussion of G.

The measurements are very elegant, but they create an odd situation. The value of G is used mainly to understand astrophysics on the grand scale, from stellar structure to galaxy formation to cosmology. Yet its measurement comes from almost literally table top experiments; an extrapolation by at least a million million million times.

The danger is clear; why should gravity be exactly the same on the different scales? For instance, classical physics did not survive the relatively modest extrapolation in the opposite direction, down to atomic scales; it was replaced by quantum physics. In the last ten years or so there have been attempts to look for a departure from the inverse square law at distances of tens of metres.

Methods similar to those of Cavendish had to be further developed to test the equally important question of whether gravitational pull depends on the composition of a body ("Would a polystyrene Earth have the same AU?") or only on mass. It is essential to the truth of General Relativity that it should not be related to composition.

 

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?

Other content you may like

Crater calculator Copyright free image Icon Copyright free: Fredrik / NASA activity icon

Science, Maths & Technology 

Crater calculator

How big a crater would be made by an impact from a body hurtling from space? It depends... but our calculator will give you an idea. You can choose either to compute crater size from the projectile size, or to compute the projectile size from the crater size.

Activity
How was the moon made, and when did it happen? Copyright free image Icon Copyright free: Hans video icon

Science, Maths & Technology 

How was the moon made, and when did it happen?

Dave Rothery shares some new thinking about our nearest neighbour.

Video
5 mins
New Horizons finally gets up close with Pluto – for 15 minutes Copyrighted image Icon Copyright: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute article icon

Science, Maths & Technology 

New Horizons finally gets up close with Pluto – for 15 minutes

After a journey of over four and a half billion miles, New Horizons gets some face time with Pluto. But not too much.

Article
Night sky puts on a meteor shower to celebrate Rosetta’s closest approach to the sun Creative commons image Icon mLu.fotos under CC-BY under Creative-Commons license article icon

Science, Maths & Technology 

Night sky puts on a meteor shower to celebrate Rosetta’s closest approach to the sun

The Perseids coincide with Rosetta making its closest approach to the Sun, explains Monica Grady.

Article

Science, Maths & Technology 

Moon Rocks

Researcher Mahesh Anand of The Open University's Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR) is undertaking analysis of lunar basalt collected during the moon landings of the late 1960s and 1970s. The results may provide clues into the origins of the earth. To find out more, follow the research links.

Video
5 mins
Sky notes: July Copyrighted image Icon Copyright: OU image library article icon

Science, Maths & Technology 

Sky notes: July

A guide to the what's happening in the night sky in July.

Article
Is There Anybody Out There? Copyrighted image Icon Copyright: NASA article icon

Science, Maths & Technology 

Is There Anybody Out There?

ET, Close Encounters, Mars Attacks… - the movies are full of extra-terrestrial landings and contact with aliens. In fact, all cultures have represented alien beings in their art, stories and writing at some time or other. But what does science say?

Article
Explore the Solar System Copyrighted image Icon Copyright: photos.com activity icon

Science, Maths & Technology 

Explore the Solar System

Explore the planets in our Solar System with our interactive guide.

Activity
Registering and Tracking Asteroids Copyrighted image Icon Copyright: NASA article icon

Science, Maths & Technology 

Registering and Tracking Asteroids

Professor David W Hughes explains how asteroids are registered and tracked to keep watch for any potential 'Near Earth Asteroids'

Article