In the present case the state of the twin remaining on earth is truly Galilean (if we neglect the earth’s rotation and the curvature of its orbit); but this Galilean condition no longer applies to the twin who reverses his course when he reaches the star. In this latter case there is acceleration, and the well-known symptoms of accelerated motion, namely, forces of inertia, would make their appearance and would bring about well-defined physical disturbances. As a matter of fact, we can amend the problem of the two twins so as to eliminate acceleration by assuming that the so-called travelling twin passes both earth and star without ever stopping or reversing his course. In this case the motions of both twins are Galilean; and, true enough, the special principle of relativity allows us to consider either one of the twins as at rest and the other as in motion. But, again, we should see that exactly the same relative agings followed, whether we considered one twin or the other as being at rest; hence the critic’s statement that motion produces a slowing down of time is a [Pg 237] loose one unless explained more fully. We may make these points clearer by examining the slightly modified example we have suggested. First, suppose that the earth twin, the earth and the star are all at rest; and that the so-called travelling twin travels without a stop past earth and star. We will further suppose that a rod extends from earth to star. To return to the numerical values selected at the beginning of the chapter, calculation would show that the duration of the passage of the travelling twin from earth to star would be twenty years in the opinion of the earth observer and only ten in the opinion of the wanderer. Why this difference? Because, as we have already mentioned, since a relative motion exists between the so-called wanderer and the rod, the distance from earth to star, or the length of the rod, is contracted to half its length when computed by him. And here we must note that this contraction is not due to the so-called wanderer’s motion (for motion, unless accelerated, has no absolute significance). It is due to the relative motion between the wanderer and the rod, or the earth-star distance. If, therefore, the wanderer were to consider himself at rest and the earth-twin, together with the earth and star, travelling past him, exactly the same conclusions would endure; since once again the rod, or earth-star distance, would be animated with exactly the same relative velocity in respect to the so-called wanderer, and it is relative velocity, and not absolute velocity, that counts. We see, therefore, that no inconsistencies can arise whether we consider one twin or the other as at rest. Let us now revert to the complete problem where the wanderer returns to earth. In this case, the wanderer has led an entirely different life history from his brother, since he, and he alone, has suffered acceleration.