Newton: Still Right

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.


  1. #1 Uncle Al
    April 14, 2007

    Riley Newman at UC/Irvine noted the 20-micron diameter tungsten torsion fiber was not proven to obey Hook’s law. Given the oscillations’ small amplitudes and their decay over time one would need a very fancy dance to obtain data deviations plus Hook’s law violation exactly compensating over the course of the experiments.

    U/Wash has the most extraordinary vertical torsion balances on the planet. They are committed to repeating their failed experiments rather than trying something new and allowed within existing theory. An Eötvös experiment contrasting solid single crystal spheres of space group P3(1)21 (right-handed screw axes) and P3(2)21 (left-handed screw axes) alpha-quartz test masses would differentiate between metric (General Relativity) and otherwise indistinguishable by prediction non-metric (affine, teleparallel, nocommutative) theories of gravitation. Cultured quartz of astounding perfection and purity is a commercial product.

  2. #2 Uncle Al
    April 14, 2007

    A parity Eötvös experiment is a universal vacuum background detector. As the Earth spins and orbits a torque must be impressed if the vacuum is not empty.

    HK Moffat, “Six lectures on general fluid dynamics and two on hydromagnetic dynamo theory,” pp. 149-234 in R Balian & J-L Peube (eds), Fluid Dynamics (Gordon and Breach, 1977)
    3.5 megabytes
    p. 175-176 (pdf p. 25-27) calculation of the chiral case.

  3. #3 agm
    April 15, 2007

    I saw that blurb. It sounded like it was pretty much just a write-up announcing the publication of the results you’ve already discussed.

  4. #4 Jonathan Vos Post
    April 15, 2007

    Uncle Al: (1) Spelled: “Hooke’s Law.”
    amount by which a material body is deformed (the strain) is linearly related …'s_law

    (2) This test might also be sensitive to the Axis of Evil. The rotational axes of galaxies seem not to be random, but too-otften parallel, and the circular polarization of quasars also seem to be polarized. One hypothesis is that inflation was not spherically symmetric. Another hypothesis is a pervasive magnetic field soon after the Big Bang.

  5. #5 Uncle Al
    April 15, 2007

    Thank you! Science is empirical. Somebody should do the experiment. Then, we will know. Theory can thereafter catch up as necessary.

  6. #6 Perry
    April 15, 2007

    well this is cute. But one can do experiments that show quantum/relativistic effects well before 1 part in 10^14! But its still a cool little experiment.