Flamsteed Astronomy Society

‘Do we need a new theory of gravity?’ by Dr Stuart Clark — January 5, 2009

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The Flamsteed audience was delighted to welcome back Stuart Clark after an absence of only one year.  Stuart’s writing career has been unfolding (as has his music career we understand!) and most recently he was pleased to be appointed Senior Editor for Space Science at ESA, the European Space Agency.

Stuart returned to talk about gravity — Since 1687 Sir Isaac has served us well, but there continue to be significant areas where our use of Newton’s laws is failing to explain what we observe.  We may well be on the verge of ‘new physics’ — entirely new aspects of the Universe may open up to us, or...   maybe Newton’s ‘laws’ aren’t universal.  Maybe there are circumstances where Sir Isaac doesn’t hold sway.

It wouldn’t be the first time.  Stuart reviewed the development of theories about gravity through history — Kepler was the first to give mathematical form to the action of gravity, in elliptical planetary orbits, with his laws published about 400 years ago.  It was Sir Isaac Newton around 80 years later in 1687, who provided the rigorous maths that linked an inverse square law of attraction to Kepler’s elliptical orbits.  In addition to the inspired maths, Newton’s great insight was the universal nature of gravitational attraction — acting both on the Moon and a falling apple alike — and showed how gravitational attraction worked together with uniform linear motion to produce orbits.  Newton and others successfully used his laws to explain how the tides worked and describe the orbits of comets.  Even in today’s space age, Sir Isaac’s maths is good enough to plan a shot to the Moon or Mars.  Sir Isaac never speculated on what gravity was, just how it acted.

But all was not well almost from the start.   In 1781 William Herschel discovered the first planet found in modern times — named Uranus.  As Uranus’ orbit was carefully measured astronomers realised it wasn’t behaving according to Sir Isaac.  Why not?  By the 1840s the favourite idea postulated an undiscovered planet beyond Uranus whose gravity was pulling on Uranus and perturbing its orbit.  Working independently, two mathematicians, Urbain Le Verrier in Paris, and John Couch Adams in Cambridge, calculated where the proposed new planet might be found — these were very demanding, pioneering calculations.  How they worked to get their predictions checked and how the race was won by Le Verrier is a great story — using Le Verrier’s calculations the new planet was discovered in 1846 by the Berlin Observatory and named Neptune.  In this test, Sir Isaac prevailed — careful measurements since have shown that Newton’s laws fully describe the orbits of Uranus and Neptune, indeed all of the solar system planets... except Mercury.

The planet closest to the Sun, Mercury, doesn’t behave itself either.  Mercury’s orbit exhibits ‘precession’ and according to Sir Isaac it shouldn’t (well, not so much, anyway).  All other things being equal, we would expect the point of Mercury’s closest approach to the Sun, its perihelion, to fall in the same place on every orbit, but it doesn’t — Mercury’s point of perihelion moves slowly round the Sun, changing from orbit to orbit.  It ‘precesses’.   The undiscovered planet hypothesis had worked well for Uranus/Neptune and so the same idea was proposed to explain Mercury’s antics — there must be an undiscovered planet between Mercury and the Sun, small and hidden in the blinding glare of the Sun.  It was even given the name Vulcan...  but it was never found.  There were claims of discovery, but they have never been substantiated.  Vulcan doesn’t exist, at least outside of Star Trek.  Astronomers scratched their heads until 1915 or so, then, ta-ra!  New physics!

Albert Einstein presented the world with an entirely new view of how gravity worked. His theory of General Relativity described gravity not as a force of attraction but as a consequence of warping of the space-time continuum.  The equations of general relativity precisely described Mercury’s orbit — a compelling argument in favour of the theory.  The theory also predicted a different value for the bending of light near a massive body like the Sun.  This prediction was confirmed by Arthur Eddington’s measurements made during the 1919 solar eclipse.  Einstein had shown that Newton’s ’laws’ don’t work in all circumstances, but they do in most.  Physicists and astronomers are very reluctant to throw Sir Isaac out with the bathwater.

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[Pics Mike Dear]

Dr Stuart Clark by Mike DearDr Stuart Clark by Mike Dear