 The study reports experimental investigation of a harmonic oscillator made of both condensed lithium atoms in an optical lattice, revealing crossover from sinusoidal to nearly photon-like propagation at high velocities and observing intrinsic relativistic defacing and monopole oscillations. All observed dynamics are in agreement with long-standing but untested relativistic predictions. This article was authored by Kurt M. Fujiwara, Zachary A. Geiger, Kevin Singh, and others.