光速
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定义
光在真空中的精确速度：299,792,458米每秒，自1983年起用于定义米。
解释
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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