If a particle moving near the speed of light carried a clock with it, the clock would run slow compared to a stationary clock on Earth. In the terminology of Einstein's theory of special relativity, the particle experiences time dilation. The following equation is the factor by which time slows:
Gamma (), called the "Lorentz factor," grows very large as the particle velocity (v) approaches the speed of light (c). This means that the faster a decaying particle travels, the longer it appears to live to a stationary observer. This counterintuitive and confusing aspect of the theory of special relativity is demonstrated every day by cosmic rays. If time weren't dilated for the fast-moving cosmic ray particles, they would decay long before reaching the Earth's surface.