  The eigenstate thermalization hypothesis (ETH) insists that for nonintegrable
systems each energy eigenstate accurately gives microcanonical expectation
values for a class of observables. As a mechanism for ETH to hold, we show that
the energy eigenstates are superposition of uncountably many quasi eigenstates
of operationally defined "time operator", which are thermal for thermodynamic
isolated quantum many-body systems and approximately orthogonal in terms of
extremely short relaxation time of the fidelity. In this way, our scenario
provides a theoretical explanation of ETH.
