  Despite the importance of non-equilibrium statistical mechanics in modern
physics and related fields, the topic is often omitted from undergraduate and
core-graduate curricula. Key aspects of non-equilibrium physics, however, can
be understood with a minimum of formalism based on a rigorous trajectory
picture. The fundamental object is the ensemble of trajectories, a set of
independent time-evolving systems that easily can be visualized or simulated
(for protein folding, e.g.), and which can be analyzed rigorously in analogy to
an ensemble of static system configurations. The trajectory picture provides a
straightforward basis for understanding first-passage times, "mechanisms" in
complex systems, and fundamental constraints the apparent reversibility of
complex processes. Trajectories make concrete the physics underlying the
diffusion and Fokker-Planck partial differential equations. Last but not least,
trajectory ensembles underpin some of the most important algorithms which have
provided significant advances in biomolecular studies of protein conformational
and binding processes.
