The **speed of light** in a vacuum (denoted **c )** is a universal physical constant

important in the laws of physics. Its exact value is 299,792,458 meters per second or about 300,000 kilometers per second. It is also the speed at which all massless particles (photons, gluons and gravitons) and field perturbations travel in vacuum, including electromagnetic radiation and gravitational waves. Particles with nonzero rest mass can approach *c*, but can never actually reach it. In the special and general theories of relativity, *c* appears in the famous equation of mass–energy equivalence, *E* = *mc*^{2}.

The **light-year** or **lightyear** is a unit of length used to express vast astronomical distances and is about 9.46 trillion kilometers (9.46×10^{12} km) , or the distance that light travels in vacuum in one year (365.25 days). The light-year is most often used when expressing distances to stars and other distances on a galactic scale. A *parsec* (denoted pc) is about 3.26 light-years.

Although the speed of light is always a constant in the laws of physics, its limits in interstellar travel are circumvented by various faster-than-light techniques and devices, and an understanding of the new laws of physics and dimensions >= 7d.