We start from the Planck thermodynamic description of light that leads to black-body radiation. We discuss Einstein’s introduction to spontaneous emission and the first evidences leading to the realization of the laser. We detail the nonequilibrium descriptions of lasers as heat engines. We define random lasers and detail their introduction from Letokhov. We show their link with modern optical computing and the Ising and Hopfield models. This chapter illustrates the idea of complexity, the relation with replica symmetry breaking, and the role in optical computing and Ising machines.

In this chapter, twenty-five carefully selected flow-related phenomena are analyzed, with the purpose of consolidating the understanding of the subject and strengthening the ability to link theory with practice. These flow phenomena include some interesting examples such as the principle of lift, the thrust of a water rocket, the mechanism of a faucet et al. Another type of the examples includes those with controversial explains in some popular science books or websites, such as the pressure of jet flow, the pressure change by a passing train, the reason why the water does not spill when a cup is upside-down et al.