You Have a Choice 你可以选定一种解释
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."
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?
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:
The other train increased its speed.
Our train decreased its speed.
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.
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.
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.
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!
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.
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?
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.
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.
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.
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。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。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。