In his paper, "Quantum mechanics, matter waves and moving clocks", Müller has suggested that "The description of matter waves as matter-wave clocks... has recently been applied to tests of general relativity, matter-wave experiments, the foundations of quantum mechanics, quantum space-time decoherence, the matter wave clock/mass standard, and led to a discussion on the role of the proper time in quantum mechanics. It is generally covariant and thus well-suited for use in curved space-time, e.g., gravitational waves."