Modified Newtonian dynamics close to home.

IN HIS PERSPECTIVE “SEEING THROUGH DARK Matter” (3 August, p. 607), Stacy McGaugh describes the success of a modified Newtonian dynamics (MOND) in explaining the flat rotation curves of galaxies without invoking dark matter. In addition to noting that MOND is in accord with available data and observations, it is possible to directly test Newtonian dynamics in the laboratory, even at low accelerations. This has recently been done in a test of Newton’s second law (F = ma), and perfect agreement with Newton was found down to accelerations of 10 m/s, three orders of magnitude below the scale at which MOND should set in (1). Similarly, Newton’s gravitational law has been tested to very small accelerations (i.e., with very small masses at small distances), and no deviations from the law were needed to describe the solar system (2). Thus, while no observational data disagree with MOND, recent laboratory tests indicate that Newtonian dynamics also explain galaxy rotation curves and apply to accelerations in the galaxy tails.