Newton's law of gravity works beautifully until you look at systems where gravitational accelerations drop below about \(10^{-10}\ \mathrm{m\,s^{-2}}\). At that threshold, galaxies stop behaving the way Newton predicts, and the usual fix is to invoke dark matter. But there's another possibility: maybe gravity itself changes at very low accelerations. That idea is called Modified Newtonian Dynamics, or MOND.
The trouble with testing MOND has always been scale. Galaxy rotation curves are suggestive but messy - full of baryonic uncertainties and modeling assumptions. What you really want is a clean, local laboratory. Wide binary stars turn out to be exactly that.
A wide binary is two stars gravitationally bound to each other but separated by thousands of astronomical units, far enough apart that the gravitational acceleration between them drops into the MOND regime. If MOND is right, their relative orbital velocities should be systematically higher than what Newton predicts. If Newton (plus dark matter) is right, they shouldn't.
At Ohio State, I'm building the most precise observational test of this prediction. I combine Gaia astrometry, which gives exquisite proper motions and parallaxes, with high-resolution radial velocities from facilities like Keck, VLT, and Magellan. The radial velocity component is critical: Gaia gives you the sky-plane motion, but you need the line-of-sight velocity to reconstruct the full 3D relative velocity of each pair.
In Saad & Ting (2025), I introduced a technique to measure differential radial velocities of wide binaries from high-resolution, high signal-to-noise spectra, and applied it to 85 pairs from the C3PO survey. The method reaches a precision of 8 to 15 \(\mathrm{m\,s^{-1}}\) per pair, a median improvement of roughly \(24\times\) over Gaia DR3, finally bringing line-of-sight velocities to the accuracy this test demands. Folding 57 gravitationally bound systems into a hierarchical Bayesian model of their 3D orbits, I inferred the global MOND acceleration scale \(a_0\) directly from the data and found tension with MOND: the canonical value is excluded at \(3.1\sigma\) for the simple interpolating function and \(1.9\sigma\) for the standard form. The data and code are on GitHub.
In a companion paper, Saad & Ting (2026), I showed just how sensitive these measurements are to the orbital modeling itself. Chae et al. (2026) had reported a gravitational anomaly, a gravity boost of \(\gamma \approx 1.6\), in a high-quality sample of 36 wide binaries. Reanalyzing the same data with a hierarchical Bayesian model that forward-models the full 3D orbits, I recovered \(\gamma = 1.00 \pm 0.24\), fully consistent with Newtonian gravity. The anomaly reappears only when the semi-major axis is replaced by a geometric de-projection of the observed separation, which pins the discrepancy on the orbit modeling rather than on new physics. The code is on GitHub.