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Sunday, 18 November 2012

The Astrophysics of Gravity Modification

The concept of “gravity modification” is commonly associated with cranks – whether it’s backyard inventors with their antigravity machines or, as in last week’s post (Space, Gravity and the Flying Saucer) ufologists and their space drives. But the subject has another, much less well-known, side to it.

To most people, gravity is simply the force that pulls things down towards the surface of the Earth. That’s the kind of gravity the cranks are talking about – the force their antigravity devices and space drives are meant to counter. But there is a lot more to gravity than that. Not only does it pull objects towards the Earth (and other planets), but it keeps the planets in their orbits around the Sun. It’s also—according to textbook astrophysics—the force that holds stars together in a galaxy, and galaxies together in clusters of galaxies. But that’s where the problems start.

Observations have consistently shown that the stars and gas at the outer edges of galaxies are moving too fast. If the only force acting on them was the gravity produced by visible matter, the force wouldn’t be strong enough to hold them in their orbits: they would be flung out into intergalactic space. The fact that they aren’t is awkward to say the least. The textbook response is to say that a galaxy contains large quantities of invisible “dark matter”—much more than can be seen with optical, radio and infrared telescopes. No-one knows what the dark matter consists of, or where it came from, but it’s the easiest way to make the observed facts fit the theory.

There is another alternative, of course. Instead of trying to fiddle the observations to fit the theory, you could change the theory to fit the observations. Obvious as this may seem to many people, it’s something scientists are extremely reluctant to do – although a few of them have been brave enough to try.

Way back in the early 1980s, I spent a couple of years working at the Kapteyn Astronomical Institute in the Netherlands. One of the staff members who was there at the time has since become something of a celebrity (see Seth Shostak on SETI, 1983). Another was a theoretician named Robert H. Sanders, who was one of the first people to speculate that the high speeds observed in the outer parts of galaxies might be explained by a modification to the standard theory of gravity.

In 1984 Bob Sanders published a paper with the intriguing title of “Anti-Gravity and Galaxy Rotation Curves” (Astronomy and Astrophysics, volume 136, page L21). In it, he suggested that the anomalous observations could be explained if the formula for the gravitational field was modified by the addition of what is known as a “Yukawa potential” (more on which later). I remember discussing the paper with Bob at the time, and it was clear that he viewed it more as an interesting intellectual exercise rather than a serious scientific proposal. However, there were other professional astrophysicists at the time who took gravity modification more seriously. Specifically, a group led by Mordehai Milgrom at the prestigious Weizmann Institute in Israel came up with a theory of Modified Newtonian Dynamics (MOND) – which is respectable enough to have its own Wikipedia page.

When you look at the universe on scales larger than galaxies—in other words clusters and superclusters of galaxies—things get even worse for the standard theory of gravity. Not only is dark matter needed in even greater quantities, but also a completely new and equally ad hoc concept: dark energy. Gothically sexy as this might sound, it’s really just another contrived way of making the facts fit the theory. The alternative approach—adjusting the theory to fit the facts—is once again a minority activity. But some people are bold enough to try – Baojui Li of the University of Durham is one of them. There was an article by him in the August issue of the Royal Astronomical Society’s Astronomy and Geophysics magazine (pictured above), and I was interested to see that his theory once again involves the introduction of a Yukawa-like term. This creates a situation where the force measured in regions of relatively high density (such as the solar system) is a close match to Newtonian gravity, while the force in low density extragalactic regions is completely different—and able to match the observations without the need for all that sci-fi stuff like dark matter and dark energy.

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