  Zitterbewegung, a force-free trembling motion first predicted for
relativistic fermions like electrons, was an unexpected consequence of the
Dirac equation's unification of quantum mechanics and special relativity.
Though the oscillatory motion's large frequency and small amplitude have
precluded its measurement with electrons, zitterbewegung is observable via
quantum simulation. We engineered an environment for 87Rb Bose-Einstein
condensates where the constituent atoms behaved like relativistic particles
subject to the one-dimensional Dirac equation. With direct imaging, we observed
the sub-micrometer trembling motion of these clouds, demonstrating the utility
of neutral ultracold quantum gases for simulating Dirac particles.
