going to a rotated frame will continue to say

即便坐标系旋转了
that the rotated vector is equal to the rotated vector.

旋转后的矢量仍旧相等
That's because equal vectors upon rotation will remain equal vectors.

因为相等的矢量经过旋转仍然相等
That's the whole point.

这就是重点
The total incoming momentum four-vector, call it P incoming,

总的入四动量我们把它叫做 P入
and this is the outgoing momentum, call it the P outgoing.

这是总的出四动量叫做 P出
If P incoming is equal to P outgoing for me,

如果对我来说 P入等于 P出
so two vectors that equal in space-time,

这两个矢量在时空里就是相等的
for the rotated observer they will be equal.

对旋转了的观察者也是一样
Do you understand why?

能理解吗
Because my total incoming vector will undergo Lorentz transformation

因为我的总入矢量经过洛伦兹变换后
to become your incoming vector.

会变成你的总入矢量