Now let's suppose that both trains are at the railroad station loading and unloading passengers and baggage. A half-hour passes. As we look at the other train through our window, we see that our train seems to start moving, smoothly and slowly. For a minute or so, our train seems to travel at a uniform speed. Our special speedometer shows that the relative speed between the two trains is 20 mph. But as we look out our window, we suddenly see the last coach of the other train disappear from sight and notice the motionless station behind it. So we are not moving after all. The other train has been moving!

　　现在我们假设两列火车都在火车站上在上下乘客或装卸行李。过了半小时以后，当我们透过车窗看另一列火车时，我们看到我们所乘的火车好像是在缓慢地平稳地开始开动了。约有一分钟左右，我们所乘的火车似乎是在以均匀的速度向前运行。我们特制的测速仪显示这两列火车的相对速度是每小时20英里。但当我们往窗外一看，这才突然看见另一列火车的最后一节车厢从我们的视野中正在消失，于是我们看到那一列火车后面留下的是不动的火车站。所以，我们所乘的火车根本就没有开动。原来是另外一辆火车在开动!

　　This peculiar and often frustrating experience is an effect of relative motion. At the train station we cannot tell whether it was our train that changed its speed from 0 mph to 20 mph or whether it was the other train that changed its speed from 0 mph to 20 mph. Only after the other train pulled out of the station could we see that it, and not our train, was moving.

　　这种奇特的而又经常使人沮丧的经历就是相对运动所产生的效应。在火车站上我们辨别不出窨是我们所乘坐的那一列火车把车速从每小时0英里改变成每小时20英里。只有当另一列火车驶出火车站之后，这我们才能明白原来是它在开动，而不是我们所乘坐的这列火车在开动。

　　Now let us again raise the question that was raised at the beginning of this article: can we be absolutely certain that the other train did indeed increase its speed, and in this case pull out of the station?

　　现在让我们再次提出本文开头所提出过的那个问题：我们能否绝对肯定确实是另一列火车加快了车速，在此情此景驶出了火车站呢?

　　If your answer is yes, then you are wrong again. All we can be certain of is that the relative speed between the two trains changed.

　　如果你回答说是的，那么你就又错了。我们所能够完全肯定的一点只是这两列火车之间的相对速度变了。

　　These examples illustrate an important principle in the special theory of relativity. If A appears to be moving at a steady speed relative to B, we cannot know for sure if it is A that is really moving. Perhaps A is standing still, and B is moving. Or perhaps both are moving. According to relativity, there is no experiment that can be devised to solve the problem. As there is no way of deciding which of the two objects is moving, we can choose either one as the moving object. The reason is that their motion is relative, and relativity, as we have seen, means that we have a choice.

　　这些例子阐明了特殊的相对论中的一个重要的原理。假如我们觉得A似乎是在用相对于B的一种很均匀的速度在移动，我们就不能肯定地确切知道是否就是A在真的移动。也许A仍在原地不动而是B在移动。或者，AB两者都在移动。根据相对论的说法，根本就不能构想出任何一个实验去解答这个问题。因为根本没有办法来确定这两个物体中哪一个物体在移动，所以我们可以把两个物体中的任何一个物体认定是在移动着的物体。理由就是他们的运动是相对的。正如我们所理解的那样，相对论就意味着我们可以选用一种解释了。