Why is it said that a person traveling in spaceships at near speed of light comes back younger than his counterpart on earth?
To start, look at the left-hand diagram. Say we have two people, Purple and Yellow. They are the same age. They are both at point A, and want to get to point B. Now, every time Purple wants to get to point B, he goes 100 miles to his North, then 100 miles to his West. Yellow also has been to point B, but she goes 120 miles to her North, and only 70 miles to her East to get there.
Since they are using different directions as "North," they both get to the same ending point at B.
Now, take a look at the diagram on the right. Yellow gets in her spaceship and flies away very fast. She follows her space, and her time. Purple also follows space and time, but they are his space and time. Since Yellow went longer in space but shorter in time than Purple did, she is younger. This is because age is a measure of time, and Yellow's time is short.
Submitted by Rajeev (Tokyo, Japan)
(April 8, 1998)
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