Scotland is the centre of a gravity revolution
IAN JOHNSTON
A SHOCKWAVE tore through the space-time continuum that is the global astronomical community this week with the news that researchers at St Andrews University have apparently rewritten the laws of physics.
For one of the basic tenets of astronomy - the universal force of gravity - is now under serious challenge from a radical, competing theory which in the words of one observer threatens to "open Pandora's Box".
Dr HongSheng Zhao, of St Andrews, and his Belgian collaborator Dr Benoit Famaey believe that gravity actually changes depending on where it is in the universe, and have thrown down a challenge for their doubters to prove them wrong.
Rather than exerting a uniform pull that gradually reduces as an object gets further away, Dr Zhao and supporters of a controversial theory developed in the 1980s by the physicist Moti Milgrom believe gravity is "boosted" in the outer reaches of galaxy.
This can be used to explain why stars at the ends of the spiral arms of galaxies do not break free and fly off into space.
But the idea of "flexi-gravity" runs contrary to the established theory that mysterious "dark matter" - a mass of objects that cannot be seen by astronomers - holds the stars together by providing the mass to create the requisite amount of gravity. According to critics, the new theory should not even be given house room by the great minds of astronomy as it runs so contrary to established thinking.
The long-lived and well- respected theories of Isaac Newton and Albert Einstein would have to be thrown out the window, they protest.
Despite these claims Dr Zhao and Dr Famaey worked on the Milgrom theory - which was largely dismissed in the 1980s but recently refined in a manner that brought it back to international attention - and have now defined how gravity changes in a new formula.
This makes the extra mass provided by dark matter, which Dr Zhao describes as "an assumption", unnecessary for there to be the kind of gravity that keeps galaxies together. "It is a modification to the law of gravity as prescribed by Newton and Einstein," he said.
Newtonian physics can be used to predict the movement of the planets successfully, but the problem comes when it is used to calculate the movement of stars. Newton's formula was that the gravitational force of an object was equal to the gravitational constant multiplied by its mass and then divided by distance squared. The further away something is from the source of the gravity, the less powerful it is. At the edges of the universe, the force is so weak it is known as "nano-gravity".
Dr Zhao said: "If you use Newton's theory, you will find something very odd. The theory predicts the stars should be flying out of the galaxy, but in reality they are staying inside. Because people liked Newton so much, they put in dark matter in galaxies to keep the stars inside."
Where others add in extra mass to galaxies to hold them together, he and Dr Famaey were gripped by the idea that the gravitational constant could be "boosted".
"But how much do you want to boost it by? How do you put in a boost factor in a way that keeps the theory beautiful. It's a tricky thing," Dr Zhao said.
"Our aim was to put in the right amount of boost and still keep its intrinsic beauty... beauty meaning it doesn't violate fundamental principles.
"You need a large boost at the outskirts of the galaxy but you don't need that near the sun."
This required a gravitational constant that was anything but, and actually changed depending on where it was in the universe. This presented a fundamental problem. "There cannot be many laws of gravity. There has to be only one function that works," he added.
"By trial and error, only one set of functions comes out to be both simple and explain the data. This formula explains it very well and does it sort of effortlessly.
Everything is fixed and it fits.
"We have made the theory complete, connected the loose pieces of theory by one nice formula, and we think this formula has deep physics behind it. This theory is now fully specified so we can check it now."
This is the crucial part of any scientific theory: if it is capable of being tested and therefore falsified, it can be taken seriously. If scientists fail to knock it down, the theory gains credibility.
The new formula will be debated at a meeting at Edinburgh's Royal Observatory in April, when Dr Zhao and Dr Famaey, of the Free University of Brussels, will demonstrate their new formula on dark matter and gravity to an audience of experts from ten countries. They can expect a stormy ride.
"When people say 96 per cent of the universe is dark, that's an assumption," Dr Zhao said. "We don't need to introduce huge amounts of dark matter to explain the astronomical phenomena.
"The theory doesn't exclude dark matter. It just turns out if you do this, there is no real need for dark matter." He added that dark matter was still present but was simply not dense enough to affect a galaxy.
"It has an effect on the universe as a whole, but not on galaxies," he said.
However, Professor George Efstathiou, director of the Institute of Astronomy at Cambridge University, was unimpressed. "The physics of general relativity is a consequence of deep-rooted principles in physics. It's not something you can negotiate," he said.
"If you tinker with Newton's laws you are giving up these deep-rooted principles. You are also giving up general relativity. I wouldn't call it a theory. You cannot consider it on anything like an equal footing with general relativity."
Dr David Bacon, an advanced fellow at Edinburgh Royal Observatory, said he would have agreed with Prof Efstathiou until two years ago, when the physicist Jacob Bekenstein refined the Milgrom theory. "I think there has been a sea change with Bekenstein," Dr Bacon said.
"What HongSheng Zhao has done is refine Bekenstein's theory and it is now a good fit to some of the evidence - the evidence from the motion of stars. It is certainly very intriguing work."
http://news.scotsman.com/scitech.cfm?id=253972006
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