March 1, 2010

Do we need a new theory of gravity?

Dr Stuart Clark

Report by: Mike Dryland

Dr Stuart Clark

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.

So, with the help of Albert, is all well with Sir Isaac now? Sadly, no. There are at least two areas of observation where Newton and Einstein fail to account for what we can see.

The first is on the scale of the very large. Both in the rotation of individual galaxies, and in the movement of galaxies within clusters, Newton doesn’t come up with the right answer — stars within galaxies, and galaxies within clusters, are moving much faster than they should according to Newton. Much more mass than we can see, is needed, and distributed differently than that observed, for Newton’s laws to work here. Astronomers have come up with a ‘Vulcan’-type idea to try and account for this ‘missing mass’ — it’s called dark matter. It’s mass that must be there according to Sir Isaac, but can’t be seen. Dark Matter turns out also to be just what the particle physicists ordered — the physics of the very large and the very small suddenly converge and all sorts of research funding comes tumbling out. The latest hypotheses of sub-atomic structure propose a family of so-far-undetected particles some of which could be the astronomers’ dark matter — they don’t interact with light (can’t be seen or detected via electromagnetic radiation effects), but they exercise gravitational attraction. Oodles of research time and dollars are being poured into the search for these particles and also into calculating and looking for large-scale effects from gravitational lensing, to clumping and clustering during star and galaxy development. So far the search for direct detection of these particles has come up empty. A US experiment recently logged two events which may be dark matter effects, but the odds are no better than 1 in 5. As for the large-scale effects, maybe there are other explanations — maybe Newton’s laws don’t work in regions of very low acceleration.

Some physicists have proposed small modifications to how Newton’s laws might operate in regions of very low acceleration — these theories are called MOND (modified Newtonian dynamics), TeVeS (tensor vector scalar theory), and MOG or STVG (modified gravity). These approaches have enjoyed some success in explaining the large-scale anomalies, but they are criticised for being empirical. That is, the equations have been tweaked ‘after the event’ to fit the observations — there is no underlying hypothesis from which the equations can be derived. It has to be said that the same criticism can also be directed at the dark matter hypothesis. In the absence of any direct detection of dark matter particles, it is just as likely that Newton was a bit wrong. Maybe we should just go on what we can see. Dark Matter (never mind Dark Energy) seems to share some characteristics with the infamous and unlamented luminiferous aether. Physics seems still to need a defining test to decide between dark matter and MOND, much as the cosmic microwave background emerged finally to kill off Steady State Theory in favour of the Big Bang.

Are these large-scale effects the only rain on Sir Isaac’s parade? Not quite. There is the Pioneer Anomaly. The Pioneer probes launched in the early 1970s, are now on the very edge of the Solar System almost in deep space, and they’re not where Sir Isaac says they should be. Pioneer 10 is 400,000 km off course according to Newton. Why so? In truth there could be many mundane explanations — a leak of fuel perhaps, but the possibility of finding anomalies to Newton right here in our own backyard are sufficiently intriguing to warrant a very careful re-analysis of the Pioneer telemetry — all 95,000 measures of it. The jury is still out. Another line of enquiry involves efforts to design experiments that can take place in regions and times of very low accelerations right here on Earth.

So do we need a new theory of gravity? One way or another, the answer seems to be ‘yes!’. Either Newton needs to be modified, or the existence of dark matter will be proven, in any case opening up all sorts of new possibilities. Stuart thinks that within 5 to 10 years we will be staring at an entirely new picture.