Two trains are traveling side by side and at the same speed along parallel tracks. We are seated in one of the trains, and with us we have a special speedometer that measures their relative speed. Since the trains are traveling at the same speed, their relative speed is zero; the speedometer therefore reads "0."

兩列火車以相同的車速并排地沿著兩條平行的軌道往前行馳。我們坐在其中的一列火車上帶著特制的速度測量儀，可以測出兩列火車之間的相對速度。因為這兩列火車是以相同的車速前進的，所以二者的相對速度是零;因此，測速儀上的讀數為“0”。

Suddenly the other train seems to start pulling ahead of ours. The speedometer shows a reading of 10 miles per hour. The other train has apparently increased its speed. But can we be absolutely certain of this increase?

突然間另一列火車使我們覺得好像是開始趕到我們所乘坐的火車前頭去了。這時測速儀上顯示出來的讀數是每小時10英里。很顯然，另一列火車已經加快了速度。但是，我們能不能肯定是另外那輛火車加快了速度呢?

If your answer is yes, you are wrong. You are wrong because all that we know is that the relative speed between the two trains changed from 0 mph to 10 mph. Nothing more. This change could have been brought about in one of two ways:

如果你回答說是的，那你就錯了。你之所以是錯了，我們所知道的僅僅是這兩輛火車之間的相對速度由每小時0英里增加到了每小時10英里。僅此而已，這車速的變化很可能是由于下列兩種原因中的任何一個原因所造成的：

The other train increased its speed.

另一列火車增加了車速。

Our train decreased its speed.

我們的火車降低了車速。

There are thus two possible explanations to account for the change in speed, but we don’t know which one is right. Furthermore, regardless of which explanation we choose, the end result will be the same: the other train will arrive at the station first. So it makes no difference whether we say that the other train increased its speed or that our train decreased its speed.

因而對于車速的改變就可能有兩種不同的解釋，但是我們不知道哪一種解釋是對的。進一步來說，不管我們選定了上述兩種解釋中的哪一種，反正最終的結果是同樣的：另一列火車將首先抵達終點站。所以，不論我們說另一列火車加快了車速，還是說我們所乘的火車減慢了車速，實際上這兩種說法沒有什么區別。

Since both explanations lead to the same result, you can choose either one. Whenever two things are relative, you can choose either one of them. The converse is also true: whenever you have a choice between two things that are equally possible, then the things are relative.

既然上述兩種解釋會導致同樣的結果，那么你選定任何一種解釋都可以。不論任何時候，只要兩件事物兩種情況是相對的，那你就可以選定其中一種解釋。逆真理亦真，每當你從有同樣可能的兩件事物中選定其中之一時，那么這兩種事物之間就是相對的。

There is no reason, except convenience, for choosing one explanation over the other. The relative speed between the trains remains the same, 10 mph; and the end result will be the same.

除為了方便這個理由而外，我們選定了一種解釋而不選另一種解釋根本就沒有其他別的原因。如果兩列火車之間的相對速度依然相同，都是每小時10英里，那么最終的結果也會是相同的。

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!

現在我們假設兩列火車都在火車站上在上下乘客或裝卸行李。過了半小時以后，當我們透過車窗看另一列火車時，我們看到我們所乘的火車好像是在緩慢地平穩地開始開動了。約有一分鐘左右，我們所乘的火車似乎是在以均勻的速度向前運行。我們特制的測速儀顯示這兩列火車的相對速度是每小時20英里。但當我們往窗外一看，這才突然看見另一列火車的最后一節車廂從我們的視野中正在消失，于是我們看到那一列火車后面留下的是不動的火車站。所以，我們所乘的火車根本就沒有開動。原來是另外一輛火車在開動!

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.

這種奇特的而又經常使人沮喪的經歷就是相對運動所產生的效應。在火車站上我們辨別不出是不是我們所乘坐的那一列火車把車速從每小時0英里改變成每小時20英里。只有當另一列火車駛出火車站之后，這我們才能明白原來是它在開動，而不是我們所乘坐的這列火車在開動。

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?

現在讓我們再次提出本文開頭所提出過的那個問題：我們能否絕對肯定確實是另一列火車加快了車速，在此情此景駛出了火車站呢?

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.

如果你回答說是的，那么你就又錯了。我們所能夠完全肯定的一點只是這兩列火車之間的相對速度變了。

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.

這些例子闡明了特殊的相對論中的一個重要的原理。假如我們覺得A似乎是在用相對于B的一種很均勻的速度在移動，我們就不能肯定地確切知道是否就是A在真的移動。也許A仍在原地不動而是B在移動。或者，AB兩者都在移動。根據相對論的說法，根本就不能構想出任何一個實驗去解答這個問題。因為根本沒有辦法來確定這兩個物體中哪一個物體在移動，所以我們可以把兩個物體中的任何一個物體認定是在移動著的物體。理由就是他們的運動是相對的。正如我們所理解的那樣，相對論就意味著我們可以選用一種解釋了。

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.

如果兩個物體在以相互間相對的同等的運動速度在運動，若想判定哪一個物體在動，哪一個物體原地不動，這是不可能的。這一原理適用于整個宇宙中凡是作直線等速運動的一切物體。

In relativity you’ll find that whenever you have a choice among things that are equally possible, you are dealing with relative things. 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. All systems of time are equally possible and you can choose any system you with.

按著相對論的說法你就會發現，每當你在眾多同樣可能的事物中需要選定一種解釋時，你就是在跟相對的事物打交道。例如，時間，雖然能用鐘表加以測量，也是相對的，因為有不止一種時間體系可以來表示時間。所有的時間體系都同樣有可能表示時間，所以你可以選用你所喜歡的時間體系來表示時間。