Now we must remember that one assumption about the motion of the universe as a whole is exactly as good—or bad—as another. No possible experiment can distinguish between them. Hence on the Principle of Relativity, we have left no absolute measurement of time or space. Whether two distances in different directions are to be called equal or not—whether two events in different places are to be called simultaneous or not—depends on our arbitrary choice of such an assumption, or “frame of reference.” All the various schemes of measurement corresponding to these assumptions will, when applied to any imaginable experiment, predict exactly the same phenomena. But, in certain important cases, these predictions differ from those of the old familiar theory, and, every time that such experiments have been tried, the result has agreed with the new theory, and not with the old.
We are therefore driven to accept the theory of relativity, strange as it is, as being more nearly “true to nature” than our older ideas. Fortunately, the difference between the results of the two become important only when we assume that the whole visible universe is moving together much faster than any of its parts are moving relatively to one another. Unless we make such an unwarranted assumption, the differences are so small that it takes the most ingenious and precise experiments to reveal them.
The Generalization
Not content with all this, Einstein proceeded, a few years ago, to develop a “general” theory of relativity, which includes the effects of gravitation.
To make this idea clear, let us imagine two observers, each, with his measuring instruments, in a large and perfectly impervious box, which forms his “closed system.”
The first observer, with his box and its contents, alone in space, is entirely at rest.
The second observer, with his box and its contents, is, it may be imagined, near the earth or the sun or some star, and falling freely under the influence of its gravitation.
This second box and its contents, including the observer, will then fall under the gravitational force, that is, get up an ever-increasing speed, but at exactly the same rate, so that there will be no tendency for their relative positions to be altered.
According to Newton’s principles, this will make not the slightest difference in the motions of the physical objects comprising the system or their attractions on one another, so that no dynamical experiment can distinguish between the condition of the freely falling observer in the second box and the observer at rest in the first.