  Gauge theories form the foundation of modern physics, with applications
ranging from elementary particle physics and early-universe cosmology to
condensed matter systems. We perform quantum simulations of the unitary
dynamics of a U(1) symmetric gauge field theory and demonstrate emergent
irreversible behavior. The highly constrained gauge theory dynamics is encoded
in a one-dimensional Bose--Hubbard simulator, which couples fermionic matter
fields through dynamical gauge fields. We investigate global quantum quenches
and the equilibration to a steady state well approximated by a thermal
ensemble. Our work may enable the investigation of elusive phenomena, such as
Schwinger pair production and string-breaking, and paves the way for simulating
more complex higher-dimensional gauge theories on quantum synthetic matter
devices.
