因为我们必须重新定义动量，牛顿力学的动量是会随坐标系而改变的，
两个完全一样的球反向做直线运动且速度相等，
从质心座标看动量为 0，但以球的观点来看不为零，
所以 in general p = mv is not general。
we redefine momentum as p = mv[1-(v/c)^2]^-0.5
and by the def. of force: F = dp/dt
we can know if we want to accelerate a body(m =/= 0) is impossible,
because we need infinite force,
so we can't accelerate a body(m =/= 0) to 299792458 m/s .
someone will say if we reach the speed of light,the mass will be infinite,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
we should be care that we redefine momentum, not mass,
so that statement is not good, I don't like it.