光速
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定義
真空中を光が進む正確な速度：毎秒299,792,458 m。1983年以降、メートルの定義に使用されている。
解説
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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