  Time crystals are many-body states that spontaneously break translation
symmetry in time the way that ordinary crystals do in space. While experimental
observations have confirmed the existence of discrete or continuous time
crystals, these realizations have relied on the utilization of periodic forces
or effective modulation through cavity feedback. The original proposal for time
crystals is that they would represent self-sustained motions without any
external periodicity, but realizing such purely self-generated behavior has not
yet been achieved. Here, we provide theoretical and experimental evidence that
many-body interactions can give rise to an inherent time crystalline phase.
Following a calculation that shows an ensemble of pumped four-level atoms can
spontaneously break continuous time translation symmetry, we observe periodic
motions in an erbium-doped solid. The inherent time crystal produced by our
experiment is self-protected by many-body interactions and has a measured
coherence time beyond that of individual erbium ions.
