WikiMechanics

Bidang, Iban people. Sarawak, Upper Rajah River, early 20th century, 39 x 79 cm. The unusually small size and simple rusa and tangkong motifs suggest this was a girl's bidang. From the Teo Family collection, Kuching. Photograph by D Dunlop.

Let P be a material particle, and consider that it may be a clock in motion. The orbital period of P is

For material particles, the mechanical energy is

where γ is the Lorentz factor. Then

If P is isolated and the frame is rigid, then the elapsed time between some inital and final events is given by

If P is a clock, this is the elapsed time that it would indicate between events. For comparison, set γ=1 to define

This is the elapsed time that would be recorded if the clock was at rest. These numbers are related as

Since the Lorentz factor for a particle in motion is always greater than one, the moving clock always reports less elapsed time than the stationary clock. This effect is called time dilation.