  The de Broglie-Bohm theory is a hidden variable interpretation of quantum
mechanics which involves particles moving through space with definite
trajectories. This theory singles out position as the primary ontological
variable. Mathematically, it is possible to construct a similar theory where
particles are moving through momentum space, and momentum is singled out as the
primary ontological variable. In this paper we experimentally show how the two
theories lead to different ontological descriptions. We construct the putative
particle trajectories for a two-slit experiment in both the position and
momentum space theories by simulating particle dynamics with coherent light.
Using a method for constructing trajectories through the primary and derived
(i.e. non-primary) spaces, we compare the ontological pictures offered by the
two theories and show that they do not agree. This contradictory behaviour
brings into question which ontology for Bohmian mechanics is to be preferred.
