[en] Conservation of angular momentum and the isotropy of space prohibit orthopositronium (o-Ps) from annihilating into two photons. Nevertheless, an observed o-Ps decay rate in excess of theory tempts one to speculate on the existence of a ubiquitous field that would couple to Ps, allowing a small partial rate for the long-lived o-Ps state to annihilate into two photons. We apply a 3.4-kG magnetic field to a sample of Ps to quench the m=0 states in the laboratory frame of reference, and search for a 12-h periodicity in the 3γ yield. The observed effect is consistent with the assumption that space is isotropic and, at the 80% confidence level, is too small to explain the observed o-Ps decay-rate discrepancy with theory