  Bohmian mechanics is a theory that provides a consistent explanation of
quantum phenomena in terms of point particles whose motion is guided by the
wave function. In this theory, the state of a system of particles is defined by
the actual positions of the particles and the wave function of the system; and
the state of the system evolves deterministically. Thus, the Bohmian state can
be compared with the state in classical mechanics, which is given by the
positions and momenta of all the particles, and which also evolves
deterministically. However, while in classical mechanics it is usually taken
for granted and considered unproblematic that the state is, at least in
principle, measurable, this is not the case in Bohmian mechanics. Due to the
linearity of the quantum dynamical laws, one essential component of the Bohmian
state, the wave function, is not directly measurable. Moreover, it turns out
that the measurement of the other component of the state -the positions of the
particles- must be mediated by the wave function; a fact that in turn implies
that the positions of the particles, though measurable, are constrained by
absolute uncertainty. This is the key to understanding how Bohmian mechanics,
despite being deterministic, can account for all quantum predictions, including
quantum randomness and uncertainty.
