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Become a nitrogen atom in the nitrogen cycle in our Traveling Nitrogen Classroom Activity Kit/Game. See all our games, activity kits and classroom activities.

# 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)

#### Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information.