Light travels at 186,282 miles per second, or approximately 670,616,629 miles per hour, in a vacuum. As of 2014, it is assumed that no object that has mass can travel at or above this speed.
The speed of light in a vacuum is constant; however, light speed can be altered significantly when passing through objects. Light filtered by the relatively unobstructed atmosphere of Earth, for example, is only slowed slightly, while the speed of light passing through a diamond is halved. The light year, or the distance light travels in a year, is frequently used to calculate distances in space and is a measurement of both time and distance.