Physical Review Letters this week features a paper on a topic that might not seem to be in dispute: Newton’s Second Law of Motion:
We have tested the proportionality of force and acceleration in Newton’s second law, F=ma, in the limit of small forces and accelerations. Our tests reach well below the acceleration scales relevant to understanding several current astrophysical puzzles such as the flatness of galactic rotation curves, the Pioneer anomaly, and the Hubble acceleration. We find good agreement with Newton’s second law at accelerations as small as 5×10-14 m/s2.
I’m writing this on the road, so I don’t have full-text access to the article, and I don’t see it on the arXiv, so I can’t really discuss the experimental details. Physics New Update says that it’s another of those torsion pendulum experiments I’m so fond of, but the details are a little sparse.
I really enjoy seeing this sort of work published. Not only is it just really cool to see anything measured at the part in 1014 level, but it makes an important point about science: even the oldest and most basic laws of physics are still open to question. If there’s any principle in physics that you might expect to have been completely settled by now, Newton’s Second Law would be it, but it’s still something worth experimental investigation.