Adaptivity is exponentially powerful for testing monotonicity of halfspaces

We give a $\mathrm{poly}(\log n, 1/\epsilon)$-query adaptive algorithm for testing whether an unknown Boolean function $f: {-1,1}ⁿ \to {-1,1}$, which is promised to be a halfspace, is monotone versus $\epsilon$-far from monotone. Since non-adaptive algorithms are known to require almost $\Omega(n^{1/2})$ queries to test whether an unknown halfspace is monotone versus far from monotone, this shows that adaptivity enables an exponential improvement in the query complexity of monotonicity testing for halfspaces.