Speed of Light
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Definition
The exact speed at which light travels in vacuum: 299,792,458 meters per second, which since 1983 has been used to define the meter.
Explanation
Because the speed of light is fixed by definition, the meter is now defined as the distance light travels in 1/299,792,458 of a second. At this speed, light circles the Earth 7.5 times per second and travels from the Moon to Earth in about 1.28 seconds. Nothing with mass can reach this speed: at 99% of c, time dilation already causes a factor of 7 difference between moving and stationary clocks.
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