Fluid dynamics accurately describes relativistic heavy-ion collision experiments long before local thermal equilibrium is established. This unexpectedly rapid onset of hydrodynamics (occurring on the fastest available timescale) is called hydrodynamics. This occurs when an interacting quantum system is quenched with an energy density much greater than the initial energy density. During hydrodynamization, energy is redistributed on very different energy scales. Fluid mechanics precedes local equilibria between momentum modes. This is either local preheating to generalized Gibbs ensembles in nearly integrable systems or local thermalization in non-integrable systems. Many theories of quantum dynamics postulate local (pre-)thermalization, but the relevant timescales have not been quantitatively studied. Here we use a series of 1D Bose gases to directly observe both hydrodynamics and local preheating. After applying a Bragg scattering pulse, hydrodynamics are revealed in the fast redistribution of energy between distant momentum modes occurring on timescales associated with Bragg peak energies. Local preheating is seen in the slow redistribution of occupancy between nearby momentum modes. We find that the time scale of local preheating in our system is inversely proportional to the momentum involved. During hydrodynamics and local preheating, existing theories cannot quantitatively model the experiment. Exact theoretical calculations at the Tonks-Girardeau limit show qualitatively similar features.

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