A SPINNING disc may be all that is needed to overturn Newton’s second law of motion – and potentially remove the need for dark matter.
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.
Ground-based tests of MOND had been thought impossible because of the confounding motions of the Earth. But now, Vitorio De Lorenci of the Federal University of Itajubá, Brazil, and colleagues have devised an experiment to do just that (arxiv.org/abs/1002.2766).
The key is to cancel out the acceleration of Earth’s rotation, its orbit round the sun, and the orbit of the sun round the galactic centre. The basic idea was first proposed in 2007, when Alex Ignatiev calculated that the accelerations all cancel out for a millisecond at two particular points on Earth’s surface, twice a year. That makes the experiment possible in theory, but not feasible.
Now, De Lorenci’s team has figured out that a spinning disc can reproduce the effect any time and anywhere on Earth. Their calculations show that if the disc is positioned accurately and its speed precisely controlled, the acceleration at specific points on the disc’s rim would cancel out the accelerations produced by the motion of the Earth and the sun.
If the second law is correct at all accelerations, a measuring device mounted on the rim should register no anomalous force at these points. However, if MOND is correct, the device should feel an aberrant kick. “We are able to control the conditions to produce the MOND regime in any place at any time,” says De Lorenci.
However, the experiment can only test a version of MOND that says that all forces act differently at tiny accelerations. Another version postulates that just gravity would be affected, and this can only be tested in space. …