特别广播英语第 78 课：You Have a Choice 你可以选定一种解释-4

These examples illustrate an important principle in the special theory of relativity. 这些例子阐明了特殊的相对论中的一个重要的原理。

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. 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。

Perhaps A is standing still, and B is moving. Or perhaps both are moving. 也许A仍在原地不动而是B在移动。或者，AB两者都在移动。

According to relativity, there is no experiment that can be devised to solve the problem. 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。

As there is no way of deciding which of the two objects is moving, we can choose either one as the moving object. 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。

The reason is that their motion is relative, and relativity, as we have seen, means that we have a choice. 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。

This principle - that if two objects are in uniform motion relative to each other, it is impossible to decide which one is moving and which one is at rest - applies to all objects moving uniformly in a straight line through the universe. 如果两个物体在以相互间相对的同等的运动速度在运动，若想判定哪一个物体在动，哪一个物体原地不动，这是不可能的。这一原理适用于整个宇宙中凡是作直线等速运动的一切物体。

In relativity you'll find that whenever you have a choice among things that are equally possible, you are dealing with relative things. 按着相对论的说法你就会发现，每当你在众多同样可能的事物中需要选定一种解释时，你就是在跟相对的事物打交道。

For example, time, which is measured with clocks and watches, is relative because it can be shown that there is more than one system of time. 例如，时间，虽然能用钟表加以测量，也是相对的，因为有不止一种时间体系可以来表示时间。

All systems of time are equally possible and you can choose any system you with. 所有的时间体系都同样有可能表示时间，所以你可以选用你所喜欢的时间体系来表示时间。