The word "paradox" was deliberately introduced to attract the attention of the public. The twin paradox is not a paradox. The correct choice of words is the "thought experiment". The time dilation in relativity is relative. Every observer sees that the clock of another moving observer slows down. However, the twin paradox shows that time can indeed slow down. The twin paradox is not mutual... But I think I have discovered the trick. Everything depends on who determines the distance of the trip. Does the twin who remained on Earth do that, then this twin ages faster. And vice versa: Does the travelling twin do that in his frame during the outward journey, then this twin ages faster. The difference is the length contraction. The twin who sees the route from another reference system sees this route shortened. Therefore, he needs less time to travel this route. In usual representation this makes the twin on earth. He chooses a star that rests in his frame. In his view, the traveling twin must fly the full length of the route. That's why the journey takes the longest. The traveling twin, however, sees the route shortened due to the length contraction. As a result, his journey from his perspective takes shorter. Now we let the traveling twin determine the destination of the journey. He finds an object, e.g. a meteroid, in the direction from the other side of the earth (see picture below). This meteroid rests in the frame of the traveling twin. Together they fly relative to the earth (sun). From the perspective of the traveling twin, the earth should pass the full distance to the meteroid. That's why the journey takes the longest in his frame. However, because of the length contraction, the twin on Earth sees the meteroid closer. That's why the journey will be shorter for him. In addition, the twin paradox is mutual. And it gets even worse. What is the difference between these two cases purely practical? In the first case the traveling twin flies fewer light-years forth and back. His brother is getting older than him. In the second case the traveling twin flies a bit further and back. His brother is getting younger than him. It does not look like a natural law. More: Einstein's Clock |

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