The second law states that a force is proportional to an object's mass and its acceleration. But since the 1980s, some physicists have eyed the law with suspicion, arguing that subtle changes to it at extremely small accelerations could explain the observed motion of stars in galaxies.
Stars move at speeds that suggest that galaxies have far more mass than is visible, which astronomers attribute to dark matter. But if Newton's second law could be modified ever so slightly, it would obviate the need for dark matter. The hypothesis, known as modified Newtonian dynamics (MOND), was proposed in 1981 by Mordehai Milgrom, then at Princeton University.
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