Modified Newtonian dynamics, or MOND, attempts to explain the properties of galaxies by suggesting a different gravitational model at small centripetal accelerations. Despite skepticism from the scientific community, MOND has survived by correctly predicting known observations.
Brown and Mathur had previously attempted to disprove MOND but found it held solid. So they decided to investigate its predictions in a more local environment — our solar system. Once far enough away from the sun, MOND begins predicting distinct effects.
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“The fact that MOND was going to tinker with the solar system was surprising to us. That’s why we thought it would be ruled out. It does a good job of explaining galaxies, but it can’t also explain the solar system. And that’s where we were wrong,” Brown says.
Brown realized the area where MOND starts applying is the same space where Planet Nine may be. The disturbed orbits of the Kuiper Belt objects could be clustering because of the pull of another body we cannot see. This same idea led to the discovery of Neptune in 1846. MOND’s predictions, Brown and Mathur thought, would not align with the observed Planet Nine data — a surefire blow against the hypothesis.
“Instead, we found that MOND exactly predicts the data that had been observed,” Brown says.
It remains to be seen if this clustering results from the pull of Planet Nine or the pull of the Milky Way galaxy, as suggested by MOND. It could even be the result of observational bias. Brown and Mathur hope more observations of distant Kuiper Belt objects will help them determine if the alignment, currently based on a small dataset, has statistical significance.
Brown says, “If the alignment goes away and it’s no longer statistically significant, in my mind that indicates MOND made a prediction that is not observed. That’s a blow against MOND.”
But if MOND continues to persist, its implications would shake the very foundations of physics. Our current understanding of cosmology rests on the existence of dark matter, another hypothesis that explains galactic properties not predicted by traditional Newtonian dynamics. Dark matter and MOND are at odds with each other — the proof of one would negate the other.
“If MOND was shown to be correct, that would keep people up at night,” Brown says. “But for me, I’m wondering what’s the true law of gravity.”
Brown and Mathur’s findings will help keep MOND alive for now — another correct prediction in a sea of them. But it will take far more to either prove or disprove MOND. The investigation continues, leaving us to wonder still what is out there in the dark.
