我觉得你说的不对所谓光速不变的推论光速是速度的极限，乃是指绝对速度而言，也就是说任何物体对于一个特定的参照系的运行速度都不超过光速，而不是说任何两个物体的相对速度都不超过光速。
举个例子：如果两个物体从两个相反方向以相对于某个惯性系M 0.9c的速度迎面接近，那么在一个物体上的观测者将观测到另外一个物体的运动速度是0.95c迎面而来；但是在处于M惯性系中的观察者，两个物体的相对速度却是1.8c。
about relativistic addition of velocities
first, want to state that, no object can travel faster than the speed of light in ANY reference frame.
second,I think in your case, A is travelling in the speed of 0.9C while B is at 0.9C but opposite direction. Well, A in the reference frame of B, its velocity is NOT 0.95C. You can disprove this from
u=(v+u')/(1+vu'/c^2).
My result from calculation is 0.994C.
third, the relative velocity is the velocity of A with respect to B. There is no point to say, the relative velocity of A and B in the frame M, is 1.8C.
Waiting for your enlightenment.
second,I think in your case, A is travelling in the speed of 0.9C while B is at 0.9C but opposite direction. Well, A in the reference frame of B, its velocity is NOT 0.95C. You can disprove this from
u=(v+u')/(1+vu'/c^2).
My result from calculation is 0.994C.
third, the relative velocity is the velocity of A with respect to B. There is no point to say, the relative velocity of A and B in the frame M, is 1.8C.
Waiting for your enlightenment.
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