Twin Paradox
The twin paradox is a thought experiment that illustrates a consequence of Einstein's theory of special relativity. The theory of special relativity makes two fundamental claims: The speed of light, c, is a constant and never changes; and, the laws of physics must be identical for all observers who are either stationary or in constant velocity motion. If one sits inside a windowless van that rides along at a constant speed, it is impossible to tell if one is moving or standing still. Any physics experiments that are conducted in the van would have the same results whether the van speeds along or sits in a parking lot.
An effect of a fixed speed of light is that measurements of time and space vary depending on the motion of the observer. Since most observers generally move much slower than that of light, the effects of special relativity exist but are negligible. As an observer's speed approaches the speed of light, however, time and space change from the position of the observer.
One application of time dilation is the twin paradox thought experiment. Twins Larry and Arthur separate. Larry goes for a ride in his rocket ship, while Arthur remains behind on Earth. Larry's spaceship travels very quickly--close to the speed of light. Since Larry is moving at such a high speed, the passage of time on the watch he wears is slower than that of the watch worn by Arthur. When Larry returns to Earth, his brother Arthur has grown very old, while he has aged very little in comparison.
As mentioned above, a postulate of the special relativity is the relativity of uniform motion. In other words, time dilation should be symmetrical because neither Arthur nor Larry is in a privileged frame of reference. From Arthur's perspective, Larry travels at a high speed, so his clock must be the one that slows down. But because Larry travels at a constant speed, he can rightfully claim that Arthur is the one moving, so Arthur's clock must slow down. So why is Arthur much older than Larry when Larry returns to Earth?
The reason is that Arthur and Larry were not in constant motion relative to each other. Larry's rocket ship turns around midway through his journey. This introduces a third reference frame into the thought experiment which breaks the symmetry between the reference frames of Larry and Arthur. It is true, that from Larry's perspective, Arthur will age more slowly on the outward and return legs of his trip. But there is a gap in Arthur's timeline which Larry doesn't see as he changes direction, and thus changes reference frames, at the midway point. This gap accounts for Arthur's advanced age when Larry returns to Earth.
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