A new line on the wide binary test of gravity
  The relative velocity distribution of wide binary (WB) stars is sensitive to
the law of gravity at the low accelerations typical of galactic outskirts. I
consider the feasibility of this wide binary test using the `line velocity'
method. This involves considering only the velocity components along the
direction within the sky plane orthogonal to the systemic proper motion of each
WB.
  I apply this technique to the WB sample of Hernandez et. al. (2019),
carefully accounting for large-angle effects at one order beyond leading. Based
on Monte Carlo trials, the uncertainty in the one-dimensional velocity
dispersion is $\approx 100$ m/s when using sky-projected relative velocities.
Using line velocities reduces this to $\approx 30$ m/s because these are much
less affected by distance uncertainties.
  My analysis does not support the Hernandez et. al. (2019) claim of a clear
departure from Newtonian dynamics beyond a radius of $\approx 10$ kAU, partly
because I use $2\sigma$ outlier rejection to clean their sample first.
Nonetheless, the uncertainties are small enough that existing WB data are
nearly sufficient to distinguish Newtonian dynamics from Modified Newtonian
Dynamics. I estimate that $\approx 1000$ WB systems will be required for this
purpose if using only line velocities.
  In addition to a larger sample, it will also be important to control for
systematics like undetected companions and moving groups. This could be done
statistically. The contamination can be minimized by considering a narrow
theoretically motivated range of parameters and focusing on how different
theories predict different proportions of WBs in this region.
