What is the speed of light?
Light travels at exactly 299,792,458 meters per second in a vacuum, about 300,000 kilometers or 186,000 miles every second. Nothing travels faster.
The speed of light in empty space is a fixed number. Since 1983 the meter itself has been defined using it: one meter is the distance light travels in 1/299,792,458 of a second. That makes the speed exact by definition rather than a measurement with an error margin.
At this speed, light could circle the Earth about 7.5 times in one second. It takes about 1.3 seconds to reach the Moon and about 8 minutes and 20 seconds to reach us from the Sun. Light slows down slightly in air, and more in water and glass, which is why lenses can bend it.
Why nothing goes faster
Albert Einstein's theory of relativity shows that the speed of light is a cosmic speed limit. As an object with mass speeds up, it needs more and more energy, and reaching light speed would take an infinite amount. Physicists write the speed of light as c, the same c in the famous equation E = mc².
Did You Know?
The first good estimate of the speed of light came in 1676, when Danish astronomer Ole Rømer noticed that eclipses of Jupiter's moon Io happened earlier or later depending on Earth's distance from Jupiter. Calculations based on his work gave a value about 25 percent too low, but they proved that light does not travel instantly.