Hypothetically speaking.....No.2

In the second of our series of articles that pays tribute to the great theories, and the great minds behind them, Laboratory News explores Isaac Newton and his theory of universal gravitation

In science, discovery of evidence is vital. It is the currency on which knowledge develops and advances. But without some way to frame it - to draw it together - evidence can become abstract and dislocated. This is the role of the theory.

Theories can be specific in what they set out to do, or can describe grand laws that extend to the very limits of human knowledge, but all give scientists and researchers a direction in which to advance science.

However, it is a testament to human ingenuity that advancing scientific discovery can quickly render a theory outdated, leaving it discarded on the scientific hard shoulder.
Indeed, theories that truly describe all that they set out to, and stand up to the onslaught of evidence that follows are rare events. But those that do give science a vital skeleton upon which future discovery can be hung, and can cast even the most humble of facts in a starring role that can change the way we view the world forever.

This month Laboratory News looks at a theory that did exactly that - Isaac Newton’s theory of universal gravity.

Newton: Standing on the shoulders of giants

It keeps our feet firmly on the ground, dictates the orbit of planets, controls the tide and, famously, causes apples to fall from trees. But we haven’t always known the fundamental importance of the force known as gravity.

Newton was not the first to recognise gravity as a force of attraction. His great contribution was to understand that the same force that causes a thrown rock to fall back to Earth is also responsible for keeping the planets in orbit around the sun and the moon in orbit around the Earth.

It was this conceptual leap that threw the thinking of the time into a complete spin. It was a revelation, and one that was not easily accepted. With his theory, Newton had made a fundamental break with the past in stating that there was no difference between earthly and celestial phenomena.

Newton was continuing a revolution – started when Copernicus firmly moved the Earth away from the centre of the universe - that changed forever the way science was done. His statements were not based on assumptions or suppositions, but on mathematical proof set out in detail. It was this obsession with mathematical proof that set Newton’s theory of universal gravitation above that of one his great heroes.

Descartes had thought that all matter in space was contained in a thin fluid – the aether – which held all celestial bodies in place. At the time, his theory was just as plausible as Newton’s, the only difference was that Descartes had absolutely no proof to back up his – Newton did.

On July 5, 1687 Newton put forward this evidence and his theories in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy).

The book is essentially three works in one. The first two describe the motion of bodies, but it is in the third book – The System of the World – that Newton applies his idea of universal gravitation to the motion of the planets, moons and comets within the solar system. Principia was the first and, some say, the greatest ever work written on theoretical physics.

Professor William Whewell of Cambridge University, author of History of the Inductive Sciences (1837) said of Newton’s theory: “The law of gravitation is indisputably and incomparably the greatest scientific discovery ever made, whether we look at the advance which it involved, the extent of the truth disclosed, or the fundamental and satisfactory nature of this truth.”

But what went into Newton’s radical idea? Was it a sudden moment of inspirational insight, or was it honed from years of thought and mathematical toil?

History would have us believe that a falling apple landing on Newton’s head suddenly provided the conceptual leap that lead to his understanding of universal gravitation.                                                     

In reality, the idea came to Newton on the back of years of his own work, and that of others. A glimpse at the story behind Newton’s ideas of gravity reveals a long line of scientific icons. Before Newton, Galileo Galilei corrected a common misconception - started by Aristotle - that objects with different mass fall at different rates. To Aristotle, it simply made sense that objects of different mass would fall at different rates, relying more on philosophic thought experiments than experimentation. Galileo, however, used experiments that actually observed falling objects of different mass to prove his ideas.

It was Galileo’s ideas along with Johannes Kepler’s work on how planets circle the sun that Newton took inspiration from. He wondered if he could connect the two ideas of gravity on Earth and the motion of celestial bodies. Could it be the same force that was responsible for both?

In the summer of 1665 the plague epidemic reached Cambridge, where Newton was based, and forced the university to close. Newton decided to return to Woolsthorpe – a village in Lincolnshire where he grew up – and it was here that the legend of the falling apple is based. Falling apple or not, Newton made the link and struck upon the theory of universal gravitation.

However, the story does not stop here. Earlier on in his career Newton had worked on theories of light, observing that white light was made up of a spectrum of coloured light. It was this finding that started a bitter rivalry with Robert Hooke, a rivalry that spilled over to Newton’s ideas of gravity.

Hooke had claimed that he could prove an inverse square relation to Kepler’s law of elliptical orbits – something that was vital to universal gravitation, and had been proved years before by Newton. Even so, Newton had yet to publish his ideas meaning that astronomer Edmund Halley (of comet fame) and architect Sir Christopher Wren - who had suspected this relation and were hunting for proof- did not know of his findings.

Hooke could not provide a proof, and so Halley paid a visit to Newton, who reacted with disinterest having already solved the problem. However, the thought that Hooke was thinking about gravity caused Newton to quickly send Halley a nine page treatise proving Kepler’s law. Halley was impressed and after persuasion Newton agreed that he should publish his findings along with his other theories on motion and, of course, universal gravitation. 

Newton began writing, working for 18 months revising and rewriting the short paper he had sent to Halley until it grew into the Philosophiae Naturalis Principia Mathematica.

History’s cruel sense of humour is exposed in a final twist to the story. The Royal Society, having exhausted their funds on an extravagant edition of The History of Fishes, could not pay for the publication and so it was at Edmund Halley’s expense that the Principia was finally published.

The theories in Principia brought Newton fame, publicity and financial security. In his laws of motion he laid the groundwork for classical meachanics, and in his law of universal gravitation he was the first to show that the motion of bodies on Earth and those of the planets are governed by the same set of natural laws. His conceptual leap had come on the back of the contributions of many great minds. Newton himself famously said in a letter to his rival Hooke: “If I have seen further it is by standing on the shoulders of giants.”

Scientists now know the Earths garvitational
field is not regular, as depicted in this geoid diagram

This sums up most of the great scientific theories of our time. It is by building up idea upon idea that incisive theories emerge. Newton’s story shows that even the smallest insight into the humblest of facts – an apple falling towards to centre of the Earth - can prove to be the keystone in a ground breaking theory.

By Phil Prime, assistant editor, Laboratory News