有一个人用这个例子说明相对论是错的,怎么反驳他?

有一个人用这个例子说明相对论是错的,怎么反驳他?
If an observer with velocity v heads towards a beam of light one would have expected that the measurable velocity of the light beam would have been c + v.However according to the Special Theory of Relativity because time slows down and length decreases with velocity,the measured velocity of the beam would still be c.In other words a change in space and time for the observer slowed the new velocity of c + v back down to c again.However if the observer now heads in the opposite direction with the same velocity one would have expected that the measurable velocity of the beam without any relativistic effects,would now be c – v.But on this occasion a change in space and time for the observer would have to increase the measured velocity of light,the exact opposite of the case with c + v.But how could this be if time slows and length decreases with velocity,for the opposite to occur one would have expected that time would have needed to have speeded up and length increased?However both cannot be the case so therefore the speed of light could not remain constant when an observer’s velocity changed with respect to either magnitude or direction.

呵呵,我知道他哪里出了问题了,楼上们回答的不是他的疑问所在.
他是觉得,在顺光和逆光跑的人测量光速,同样这个人都是钟慢（相对于地面）,不是钟快,而一个应该是c+v一个应该是c-v,为什么测的光速都是c.他认为如果一个效应是钟慢,一个效应是钟快,还能理解.这个不对称的结论让他迷惑.
举个例子,你的速度是100,你测向着你开的火车v和背着你开火车v,速度应该是100-v和100+v.可是你自己的判断,仍然是“火车的速度是v”,即使你的测量给出的分别是100-v和100+v.因为你用了不同的计算方法来获得这个火车速度,考虑到顺着和逆着的情况.
你的具体问题,无论当你顺光和逆光,都是钟慢,但是你在这两种情况下计算光速的方法是不同的.（当然你实际看到的都是c,我是说这个中间的计算过程,如果你非要把地面上的人引入做参考来计算这个问题的话）你根据你的观测值（可能不同）,但是考虑到顺光和逆光后,计算之后得到的都是c.
这个问题和相对论没有关系.只是一个小的逻辑问题.