  Hydrodynamics accurately describes relativistic heavy-ion collision
experiments well before local thermal equilibrium is established. This
unexpectedly rapid onset of hydrodynamics -- which takes place on the fastest
available timescale -- is called hydrodynamization. It occurs when an
interacting quantum system is quenched with an energy density that is much
greater than its initial energy density. During hydrodynamization, energy gets
redistributed across very different energy scales. Hydrodynamization precedes
local equilibration among momentum modes, which is local prethermalization to a
generalized Gibbs ensemble in nearly integrable systems or local thermalization
in non-integrable systems. Many theories of quantum dynamics postulate local
(pre)thermalization, but the associated timescale has not been quantitatively
studied. Here we use an array of 1D Bose gases to directly observe both
hydrodynamization and local prethermalization. After we apply a Bragg
scattering pulse, hydrodynamization is evident in the fast redistribution of
energy among distant momentum modes, which occurs on timescales associated with
the Bragg peak energies. Local prethermalization can be seen in the slower
redistribution of occupation among nearby momentum modes. We find that the time
scale for local prethermalization in our system is inversely proportional to
the momenta involved. During hydrodynamization and local prethermalization,
existing theories cannot quantitatively model our experiment. Exact theoretical
calculations in the Tonks-Girardeau limit show qualitatively similar features.
