  The Eigenstate Thermalization Hypothesis (ETH) posits that the reduced
density matrix for a subsystem corresponding to an excited eigenstate is
"thermal." Here we expound on this hypothesis by asking: for which class of
operators, local or non-local, is ETH satisfied? We show that this question is
directly related to a seemingly unrelated question: is the Hamiltonian of a
system encoded within a single eigenstate? We formulate a strong form of ETH
where in the thermodynamic limit, the reduced density matrix of a subsystem
corresponding to a pure, finite energy density eigenstate asymptotically
becomes equal to the thermal reduced density matrix, as long as the subsystem
size is much less than the total system size, irrespective of how large the
subsystem is compared to any intrinsic length scale of the system. This allows
one to access the properties of the underlying Hamiltonian at arbitrary energy
densities/temperatures using just a $\textit{single}$ eigenstate. We provide
support for our conjecture by performing an exact diagonalization study of a
non-integrable 1D lattice quantum model with only energy conservation. In
addition, we examine the case in which the subsystem size is a finite fraction
of the total system size, and find that even in this case, a large class of
operators continue to match their canonical expectation values. Specifically,
the von Neumann entanglement entropy equals the thermal entropy as long as the
subsystem is less than half the total system. We also study, both analytically
and numerically, a particle number conserving model at infinite temperature
which substantiates our conjectures.
