  We point out that some models with infinite additional dimension(s) of
Randall-Sundrum type predict the disappearance of orthopositronium (o-Ps) into
additional dimension(s). The experimental signature of this effect is the o-Ps
-> 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 non
accelerator experiments.
