The Weight of the World

Published by Oliver & Boyd Ltd 1968. Cover illustration shows the torsion balance used by Charles-Augustin de Coulomb to measure electrostatic force, and by Henry Cavendish to measure gravitational force, leading to a value for the gravitational constant.

Measurement lies at the heart of modern science and technology, and throughout history getting it right has represented some of the most significant examples of turning imagination into reality.  It’s a subject which we examine in the latest edition of Odyssey, which has just been issued.

Whether it is measurement of distance, weight, size, force or any other aspect of physics as we know it, scientists in the past have had to be remarkably inventive – and imaginative – in their approaches to such challenges, and never more so than when attempting to establish the weight of the planet Earth itself.  Yet that is exactly what the great physicist and chemist Henry Cavendish, probably best known for the discovery of hydrogen, achieved over two centuries ago back in 1797.

In his 1968 book The Great Experiments in Physics, Henry S Lipson, then Professor of Physics at the University of Manchester Institute of Science and Technology, describes how Cavendish built a device to measure the gravitational pull of two large lead spheres on two smaller ones.  Since he knew the gravitational force of the Earth on the smaller spheres, by a simple ratio he could work out the mass of the planet Earth.  His result was not far different to the best estimates today.

Professor Lipson explains how it was “an excellent example of the design of a scientific experiment” and “of prime importance in physics because it provided a measure of the masses of the planets and of the Sun – a first step in the understanding of the Solar System” through leading to a measurement of the gravitational constant G.  But all this was achieved not through creating some vast, expensive piece of equipment requiring the involvement of many technicians – Cavendish did it effectively in a workshop in his garden as what we might call a “tabletop” experiment.  And he basically got it right.

We assume that the days of such breakthroughs, with the inspired genius working alone to produce something which changes the shape of science and technology, have passed.  Terry Pratchett and Stephen Baxter give a tongue-in-cheek gesture in this direction in their 2012 science fiction novel The Long Earth with the idea of a “stepper”, which requires only a few electronic components and a potato to permit some people to travel to parallel worlds.  No, tabletop breakthroughs are no more.

But that might not be the case.  In Quantum Gravity in the Lab (Scientific American, April 2019), Tim Folger describes the inspiration of Cavendish’s work, and how Markus Aspelmeyer at the University of Vienna has plans for a small-scale experiment which could lead to evidence that gravity might be a quantum phenomenon.  It has long been thought that more and more powerful machines – Large Hadron Colliders on the grand scale – are the only way to investigate what happens at the quantum level, but some physicists are looking to high precision, rather than high energy, as the way forward.

He draws an analogy with Einstein’s observations in 1905 on Brownian motion – seemingly random movements of pollen grains in water could be explained by collisions with water molecules, even though you can’t see the molecules themselves.  Similarly, it should be possible that events on the incredibly small scale could influence phenomena that are revealed by tabletop experiments.

So, by measuring the gravitational attraction between small spherical masses, vastly smaller than Cavendish’s lead spheres, it may be feasible to test the interaction of gravity with their quantum properties.  The technology to do this isn’t there yet, but it should be in the foreseeable future.  And if gravity could yet be explained through the laws of quantum mechanics, physics will have taken a major step forwards.  There may still be scope for imaginative research by individuals in the lab to reveal the secrets of the universe, and perhaps to change the shape of the future for us all.


Richard Hayes, Assistant Editor (Odyssey)

With a lifetime’s interest in science, history and human behaviour, Richard Hayes focuses his writing on how the imagination has created the world in which we live, and where it may lead us in the future.  Odyssey, the e-magazine of the British Interplanetary Society, draws on the rich treasury of science fiction to explore many fields of speculation, enabling us to glimpse what might yet be, both here on Earth and out amongst the stars.

For related Odyssey posts, please click here: Neither Here Nor There – The Uncertainties of the Quantum Universe | Quantum Imagination

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