我不知道是你沒看到,還是你英語有困難。。
下麵是我給的連接裏的。。
Closing speeds
The rate at which two objects in motion in a single frame of reference get closer together is called the mutual or closing speed. This may approach twice the speed of light, as in the case of two particles travelling at close to the speed of light in opposite directions with respect to the reference frame.
想聽聽你的看法。。