The frequency of the wave associated with a massive particle, such as an atom, is too high to be used directly in a practical clock and its period and wavelength are too short. A practical device makes use of the twin paradox arising from the theory of relativity, where a moving particle ages more slowly than a stationary one. The moving particle-wave therefore has a slightly lower frequency. Using interferometry, the difference or "beat frequency" between the two frequencies can be accurately measured and this beat frequency can be used as a basis for keeping time.