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Why a person travelling in spaceships at near speed of light comes back younger than his counterpart on Earth? - Windows to the Universe

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# Why is it said that a person traveling in spaceships at near speed of light comes back younger than his counterpart on earth? Does it mean that the body metabolism is affected by the speed?

#### This question involves both the concept of space-time and the theory of relativity. To begin with, take a look at the left-hand diagram. Say we have two people, Purple and Yellow. They are the same age. Say 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 North, then 100 miles West. Yellow also has been to point B, but she insists that you have to go 120 miles North, and only 70 miles East to get there. Yellow and Purple argue, discuss, and debate for a while, and finally figure out that Purple uses his compass and the magnetic North Pole to navigate, while Yellow has always found her way using the North Star. They have been using different "reference frames." So, both Yellow and Purple were correct in their directions, from their own point of view. Thus, Purple can go 100 miles (magnetic) North and 100 miles West and end up at the same place as Yellow who is going 120 miles (polar) North and only 70 miles West. Now, take a look at the diagram on the right. Say Yellow gets in her spaceship and flies off at near the speed of light for a while. When she comes back, she will be younger than Purple. Because she has gone so far, so fast, she has gone a farther distance in the 'space' direction than the 'time' direction, as compared to Purple. Since her 'time' in her reference frame is shorter than that of Purple, she is younger, even though they have both arrived at the same place in space-time.

Submitted by Rajeev (Tokyo, Japan)
(April 8, 1998)

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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.