phenomena in lasers. The results obtained enabled the signal, which contains the relevant information, to be separated from the unwanted (uncontrollable) random noise and gave rise to the Lax-Onsager regression theorem. This work now underlies many aspects of the design of optical communications devices and was part of Mel’s lifelong, deep interest in random processes. Most recently he worked in the area of inverse scattering techniques needed to extract information from noisy measurements, such as using light scattering to study clouds with lidar (light detection and ranging) techniques; searching for oil-bearing layers using acoustic backscattering; and detecting possible tumor nodules in the human breast using pulsed, noninvasive infrared light. These major directions of his work do not exhaust all of his significant contributions in physics, as we shall describe below. In his own resume Mel listed eight areas of physics where he had made significant scientific contributions: (1) multiple scattering of waves; (2) multiphonon processes in solids; (3) application of group theory to solids; (4) coherence and fluctuations in classical and quantum systems; (5) nonlinear interaction of light with sound and other excitations in solids; (6) high-power lasers; (7) phonon production and phonon optics; and (8) hot phonon interaction with electrons in semiconductor quantum wells and heterostructures. Mel also did early work on quantum transport theory.
Mel was an indefatigable worker who maintained and regularly used three offices: at City College, at Bell Labs, and at home, all crammed with piles of documents. Somehow he always seemed to know in which office he had placed any specific article. He had an inexhaustible curiosity and interest in nearly every branch of physics. More broadly he was deeply interested in any field of human endeavor that could be made quantitative. This included fields as diverse and as far from theoretical physics as finance (long before