群速度的定义是包络波上任一恒定相位点的推进速度。
该流星群速度可达每秒66公里。
The shooting stars are falling at a speed of 66 km per second.
当介质的密度增加时（包含更多的原子），群速度就降低。
When the medium density increases (with more atoms), the slow group velocity decreases.
尽管先前的研究已经证实，光的“群速度”可以超越光速c，而“信号传播速度”——光携带的信息的传播速度——不能超越光速。
Although previous studies have found that the "group velocity" of light can travel faster than c, the "signal velocity" - the speed at which information travels - cannot.
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