One of the stranger concepts in modern astronomy is Dark Matter. Developed to explain certain discrepancies in the motions of stars within galaxies, it says that there must be a significant mass of material distributed widely through and around galaxies which we cannot detect from here on Earth – it’s “dark”. There is another theory, though, called Modified Newtonian Dynamics (MOND). The idea is that Newton’s laws of gravity, accurate and reliable as they are, are only useable within a range of conditions that we consider normal. Albert Einstein had already offered an alternative physics which continued to work at extremely high speeds and masses, and MOND suggests a similar change for extremely long distances.
As we all know, under Newtonian physics, gravity follows an inverse square law, meaning that if you double the distance between the centres of gravity of two objects, the gravitational attraction becomes only a quarter as strong. This makes sense because it’s based on simple geometry and applies to anything dispersing through 3D space. The light from a star, for example, also appears a quarter as bright if you double your viewing distance. But MOND suggests that the weakening might happen slightly more slowly than this so that as the distances get really high (hundreds of thousands of light years, for example) then the gravitational attraction will be a lot stronger than you would otherwise have predicted.
The problem with MOND is not that it doesn’t make sense, or that the maths don’t work out – they do. Rather, when the predictions of theory are compared with what we actually see through our telescopes, Dark Matter consistently matches up better than MOND. As the evidence accumulates, Dark Matter becomes more and more entrenched as the accepted explanation, and MOND finds itself with fewer and fewer supporters.
Still, all is not lost for MOND because the underdog theory has now gotten a boost: using MOND-based models, and assuming no dark matter whatever, astronomers have successfully predicted the orbital speeds of stars in 15 faint dwarf galaxies that hover around the nearby Andromeda spiral galaxy. MOND can already explain galaxies that spin like the Milky Way — not surprisingly, since the theory was invented to do just that. But this is its first test in galaxies that aren’t spinning as a whole, but whose individual stars are instead following their own random orbits. MOND predicted how fast those stars should be moving, and, says Stacy McGaugh of Case Western Reserve University, lead author of a paper on the predictions, “It’s spot-on.”
So while the evidence is increasingly stacked against MOND, it’s still in for a fighting chance!