Newton’s second law of motion

Newton’s second law of motion passes another test. Physicists at the University of Washington used a swiveling torsion pendulum (shown here), in which the restoring force is provided by a very thin twisting tungsten wire. The pendulum, which had a natural period of 795 seconds and an amplitude decay time of 15 days, is inside a vacuum chamber that was installed on an air-bearing turntable. Newton’s law implies that for small oscillations, the pendulum’s frequency should be independent of the amplitude. Looking for slight departures from that independence, the Washington researchers operated the pendulum for 20 days at various amplitudes, some as small as 13 nanoradians; at such tiny twists, Brownian excitation of the pendulum was a considerable factor in interpreting the results. The researchers found the second law to be valid down to accelerations as small as 5 × 10⁻¹⁴ m/s². That is a 1000-fold improvement in sensitivity over the best previous test, carried out back in 1986. The new result tightens the constraints on speculative modifications of Newtonian dynamics that, for example, try to account for the rotation curves of galaxies or for the ongoing mystery surrounding an apparently unaccounted-for acceleration in the trajectory of the two very distant Pioneer spacecraft. (J. H. Gundlach et al., Phys. Rev. Lett. 98 , 150801, 2007 .)