MOND--A New Idea in Physics

I’m not an astrophysicist. I don’t even play one on TV. But I’m fascinated by books and articles about physics and cosmology even when my feeble mind understands only a small fraction of what they have to say.

I’ve just read an article in the August issue of Discover magazine about a relatively new idea in physics that may revolutionize our understanding of gravity. I wish I could provide a clear explanation, but I can’t. All I can do is say that it may supplant the dominant notion that invisible “dark matter” is largely responsible for the gravitational pull affecting such celestial processes as the orbital speeds of stars around their galaxies and the bending of light from distant stars and galaxies.

You see, scientists have long observed that certain celestial phenomena don’t precisely follow Newton’s laws of motion. For instance, Newton’s laws decree that stars at the outer edge of a spiral galaxy should orbit more slowly around the core than do stars closer to the core which is the galaxy’s center of gravity. That is, the farther away an object is from another object’s center of gravity, the less affected it is by the other object’s gravitational pull, and the orbital velocity of an object should therefore be slower the farther away it is from the object it’s orbiting. Yet, astronomers have observed that stars at the edge of spiral galaxies orbit at the same velocity as do stars close to the center.

Now there are essentially three ways that scientists can deal with results that contradict accepted theory. They can question the results, they can question the theory, or they can invent some new phenomenon or force to rereconcile accepted theory with observed fact. Astrophysicists chose the third way by coming up with the idea that approximately 80% of the mass in the universe consists of matter—so-called “dark matter--that can’t be seen by existing methods, and that this matter, concentrated outside a galaxy, exerts a gravitational pull on the stars at the rim of a spiral galaxy causing them to orbit as swiftly as do stars near the center of the galaxy.

Yet, an Israeli physicist named Mordehai Milgrom felt uncomfortable with this and hit upon a different approach to resolving the contradiction. He questioned existing theory, and when he modified Newton’s classic equation F = ma (Force = mass x acceleration) to F = ma2 / a0, he could account for the orbital velocities of stars without invoking a mysterious kind of matter that no one has ever seen. His idea has come to be called MOND-- “modified Newtonian dynamics.” A few years later, Milgrom’s collaborator, Jacob Bekenstein, published a masterful paper that elaborated on and strengthened Milgrom’s fundamental idea.

MOND is still a decided underdog to dark matter and dark energy as the predominant explanation for certain puzzling celestial phenomena involving gravity. But if scientists continue to be unable to detect dark matter and energy, and MOND is found to predict and describe observed phenomena just as well if not better than dark matter and energy do, it may grow in popularity and come to revolutionize astrophysics and cosmology.

For some reason, I love to read about and ponder, in my own rudimentary way, these grand, sweeping ideas about the physical universe, and I get a little excited when some new idea comes along that may overturn old ones.