Modified Newtonian Dynamics

I mentioned Mordehai Milgrom’s theory of Modified Newtonian Dynamics (MOND) in my post about The Astrophysics of Gravity Modification last year. I have a nostalgic interest in this subject because at the time it originated, in 1983, I was doing research on the dynamics of galaxies – and that’s what MOND is all about. In those days the subject of galaxy dynamics suffered from a massive “elephant in the room” problem... and the elephant is still there now. The basic observations – rotation curves and velocity dispersion profiles – don’t match the predictions of Newtonian dynamics. The standard solution, in 1983 and today, is to postulate some form of invisible “dark matter” to explain the discrepancy. MOND is an alternative, and less popular, theory that proposes a modified version of Newtonian dynamics that does away with the need for dark matter.

For the last 30 years, there have been two competing theories – MOND and dark matter – that explain the observations equally well. Most physicists have sided with dark matter simply because it’s a less radical departure from the standard paradigm than MOND would be. But just a few days ago I came across a new paper by Milgrom’s team which appears to scores a major point in favour of MOND. The paper shows that there is good agreement between velocity dispersion measurements of recently discovered satellites of the Andromeda galaxy (pictured here, from the Sloan Digital Sky Survey) and MOND predictions that were made before the measurements were carried out. In other words, the MOND theory was able to predict the outcome of the observations before they were made – something any self-respecting scientific theory ought to be able to do... but the dark matter theory can’t. All you can do with dark matter is to infer, after a set of observations have been made, the distribution of dark matter that would be needed in order to produce the observed results.

There are two aspects to theoretical physics: the equations, and the interpretation of those equations. Often the validity of the equations is demonstrated long before there is any kind of consensus as to their interpretation (quantum mechanics is a case in point). So even if the MOND equations do predict the observations, they can still be interpreted in different ways. It may be that the Newtonian form of gravity has to be modified for very weak fields (possibly as a consequence of quantum effects)... but on the other hand it may be that MOND is just telling us something about the density distribution of dark matter.

I’m not one of those people who desperately wants mainstream science to be proved wrong and fringe theories to turn out to be correct. In fact in most cases, my money is on mainstream science. But if there’s one area where I think it might end up with egg on its face, it’s in the MOND versus dark matter arena. One of the frequent criticisms scientists have of pseudoscience is that, unlike “real” science, it has no predictive power. But here is one case where the fringe theory appears to have more predictive power than the mainstream one!