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# Chapter 25: Relativity

Difficulty Level: At Grade Created by: CK-12

According to the Theory of Special 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 (the non-moving reference object in this case) while the other twin travels into space on a rocket traveling at 90% of the speed of light. After reaching some distant point, the space-traveling twin immediately turns around and returns to Earth. If the trip covered a distance of 4.3 light-years, the stay-at-home twin would perceive the trip as having taken 9.6 years on his normal clock. The traveling twin would find, however, that the trip according to his watch (having undergone acceleration and high velocity) has taken only 4.0 years. The closer one gets to the speed of light, the slower time moves and the less one 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 chapter, you will come face-to-face with Einstein’s theories 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 that the Theory of Special Relativity was “an idea whose time had come” and that if Einstein had not proposed it when he did, someone else would have introduced the same concept 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 its implications can be truly fascinating.

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Date Created:
Aug 02, 2016