  One of the central problems in many-body physics, both classical and quantum,
is the relations between different notions of chaos. Ergodicity, mixing,
operator growth, the eigenstate thermalization hypothesis, and spectral chaos
are defined in terms of completely different objects in different contexts,
don't necessarily co-occur, but still seem to be manifestations of closely
related phenomena. In this dissertation, we study the relation between two
notions of chaos: thermalization and spectral chaos. We define a quantity
called the Total Return Probability (TRP) which measures how a system forgets
its initial state after time $T$, and show that it is closely connected to the
Spectral Form Factor (SFF), a measure of chaos deriving from the energy level
spectrum of a quantum system. The main thrust of this work concerns
hydrodynamic systems -- systems where locality prevents charge or energy from
spreading quickly, this putting a throttle on thermalization. We show that the
detailed spacings of energy levels closely capture the dynamics of these
locally conserved charges. We also study spin glasses, a phase of matter where
the obstacle to thermalization comes not from locality but from the presence of
too many neighbors. Changing one region requires changing nearby regions which
requires changing nearby-to-nearby regions, until only catastrophic
realignments of the whole system can fully explore phase space. In spin glasses
we find our clearest analytic link between thermalization and spectral
statistics. We analytically calculate the spectral form factor in the limit of
large system size and show it is equal to the TRP. Finally, in the conclusion,
we discuss some ideas for the future of both the SFF and the TRP.
