The prevailing phenomenological theory of modified gravity is MOND, which stands for Modified Newtonian Dynamics. What baffles me is how MOND managed to acquire this status in the first place.

MOND is based on the ad-hoc postulate that the gravitational acceleration of a body in the presence of a weak gravitational field differs from that predicted by Newton. If we denote the Newtonian acceleration with a, MOND basically says that the real acceleration will be a‘ = μ(a/a0)a, where μ(x) is a function such that μ(x) = 1 if x >> 1, and μ(x) = x if μ(x) << 1. Perhaps the simplest example is μ(x) = x/(1+x).

OK, so here is the question that I’d like to ask: Exactly how is this different from the kinds of crank explanations I receive occasionally from strangers writing about the Pioneer anomaly? MOND is no more than an ad-hoc empirical formula that works for galaxies (duh, it was designed to do that) but doesn’t work anywhere else, and all the while it violates such basic principles as energy or momentum conservation. How could the physics community ever take something like MOND seriously?