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Chapter 25: Relativity

Difficulty Level: At Grade Created by: CK-12

Cartoon of twin astronauts that are of different age illustrating time dilation

According to the special theory of relativity, time on an object traveling near the speed of light passes more slowly than time on a non-moving reference object.  This phenomenon is called time dilation.  There is an interesting consequence of time dilation.  Imagine two twins, one who stays on earth (non-moving reference object) while the other twin travels into space on a space ship traveling at 90% of the speed of light.  After reaching some distant point, the traveling twin immediately turns around and returns to earth.  If the trip covered 4.3 light years (distance), the stay-at-home twin sees the trip take 9.6 years on his normal clock.  The traveling twin finds that the trip according to his watch (having undergone acceleration and high speed velocity) takes only 4.0 years.  The closer the traveler gets to the speed of light, the slower time moves and the less the traveler ages.

Therefore, when the traveling twin returns home, he will find that his twin brother has aged 5.6 years more than he has.

In this unit, you will come face to face with Einstein’s special theory of relativity and some of the very strange conclusions resulting from it.

Chapter Outline

Chapter Summary

The Theory of Special Relativity is very interesting and can be used to address some of the inconsistencies with Newton’s Laws that crop up when dealing with objects in different frames of reference.  It has been often stated, however, that the Theory of Special Relativity was ‘an idea whose time had come’ and that if Einstein had not proposed it when he did, that someone else would have before long.  The Theory of General Relativity, however, is considered to be one of the most fantastic achievements of the human mind in all of history.  General Relativity explains how gravity affects both space and time, and the consequences can be truly fascinating. 

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Date Created:

Oct 11, 2013

Last Modified:

Jan 08, 2015
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