我不知道是你没看到,还是你英语有困难。。
下面是我给的连接里的。。
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.
想听听你的看法。。