  The actual value of the quantum vacuum energy density is generally regarded
as irrelevant in non-gravitational physics. However, this paper gives a
non-gravitational system where this value does have physical significance. The
system is a mirror with an internal degree of freedom which interacts with a
scalar field. We find that the force exerted on the mirror by the field vacuum
undergoes wild fluctuations with a magnitude proportional to the value of the
vacuum energy density, which is mathematically infinite. This infinite
fluctuating force gives infinite instantaneous acceleration of the mirror. We
show that this infinite fluctuating force and infinite instantaneous
acceleration make sense because they will not result in infinite fluctuation of
the mirror's position. On the contrary, the mirror's fluctuating motion will be
confined in a small region due to two special properties of the quantum vacuum:
(1) the vacuum friction which resists the mirror's motion and (2) the strong
anti-correlation of vacuum fluctuations which constantly changes the direction
of the mirror's infinite instantaneous acceleration and thus cancels the effect
of infinities to make the fluctuation of the mirror's position finite.
