Parity-time symmetric acoustics

Symmetries are one of the most fundamental properties of nature, and are the basis of many physical phenomena. For example, in quantum mechanics the Hermitian Hamiltonian is employed to describe the microscopic processes that obey certain symmetries, where the Hermitian ensures not only purely real spectra of operators but also conservation of probability. In 1998,Bender and Boettcher found that the non-Hermitian Hamiltonians that respect parity (P) and time(T) symmetry might have the same properties. Since the time-dependent Schrödinger equation and paraxial wave equation are similar in form, it is possible to extend the PT symmetry into classical wave open systems. In this article, we first review the discovery of PT symmetry breaking in quantum systems. Then we introduce the theory of PT-symmetric acoustics, describe certain recent experimental observations that include some very intriguing effects, and discuss the future prospects of this rising field.