Extra dimensions and invisible decay of orthopositronium

We point out that some models with infinite additional dimension(s) of Randall-Sundrum type predict the disappearance of orthopositronium $(o-\mathrm{Ps})$ into additional dimension(s). The experimental signature of this effect is the $o-\mathrm{Ps}\to \mathrm{invisible}$ decay of orthopositronium which may occur at a rate within three orders of magnitude of the present experimental upper limit. This result enhances existing motivations for a more sensitive search for this decay mode and suggests additional directions for testing extra dimensions in nonaccelerator experiments.