Matter waves were first proposed by Louis de Broglie and are sometimes called de Broglie waves. They form a key aspect of wave–particle duality and experiments have since supported the idea. The wave associated with a particle of a given mass, such as an atom, has a defined frequency, and a change in the duration of one cycle from peak to peak that is sometimes called its Compton periodicity. Such a matter wave has the characteristics of a simple clock, in that it marks out fixed and equal intervals of time. The twins paradox arising from Albert Einstein 's theory of relativity means that a moving particle will have a slightly different period from a stationary particle. Comparing two such particles allows the construction of a practical "Compton clock".