This experiment could affect Newton’s Second Law of Motion.
Push something with twice as much force, and it accelerates twice as quickly, right? This fact of life is at the heart of physics in the form of Newton’s Second Law of Motion, F = ma (force equals mass times acceleration). But what if it isn’t true for small accelerations? What if force at small accelerations is actually proportional not to acceleration but to acceleration squared?
A team of scientists from Towson University’s Jess & Mildred Fisher College of Science and Mathematics has proposed a bold new test of this idea using Modified Newtonian Dynamics (MOND), which successfully explains a number of puzzling observations on galactic and cosmological scales that can otherwise be understood only by postulating that the universe is dynamically dominated by huge amounts of unseen dark matter.
MOND is usually taken to apply only to gravitational forces. In a more audacious approach, some physicists suggest Newton’s Second Law might break down at small accelerations for any force F. This is known as an inertial, rather than gravitational formulation of MOND. If proven, this idea could reshape all of physics, with enormous possible implications.
But how to test a theory that departs from standard physics only when accelerations are more than 10 orders of magnitude smaller than 1 g (the acceleration due to gravity at the surface of the Earth)?
The Satellite Test of the Equivalence Principle (STEP) is an existing experimental plan to test whether all objects fall with the same acceleration in the same gravitational field. The TU team’s idea uses STEP in a different but complementary way. Rather than focusing on differences in acceleration between the test masses, they propose to use STEP’s measurements of absolute acceleration (which have a sensitivity approaching 10-18 g), together with the fact that STEP’s accelerometers exert small but known spring-like restoring forces on the test masses.
As stressed by team member and associate professor of physics James Overduin, it is the serendipitous combination of these two features that uniquely enables STEP to detect violations of Newton’s Second Law, even at accelerations eight orders of magnitude below the MOND scale. Team leader and visiting scientist Jonas Pereira said the implications of the tests would not be apparent on Earth since the accelerations associated with the planet's rotation and orbit around the sun would block them. However, they could completely alter our understanding of the laws governing the motion of the vast majority of matter elsewhere in the universe.
TU student Alexander Poyneer, who presented the team’s results at the 21st International Conference on General Relativity and Gravitation in New York City last month, adds that while the idea may be revolutionary, its implementation is remarkably simple.
“We are basically just checking whether a mass on the end of a spring obeys Newton’s laws when accelerations are small,” he noted. The team’s paper was published in the prestigious journal Physical Review Letters on Aug. 12.