http://www.ck12.org Chapter 22. The Special Theory of Relativity
22.2 Time Dilation and Length Contraction
Objectives
The student will:
- Understand the two postulates of the special theory of relativity.
- Understand what is meant by time dilation and length contraction.
- Understand the twin paradox.
Vocabulary
- length contraction:Shortening of distance.
- time dilation:Clocks moving relative to an observer run more slowly compared to the clocks that are at rest
relative to the observer. - twin paradox:Illustrates questions of the relativity of time.
Introduction
The surprising results for the speed of light were counter to all logic. Einstein’s special theory of relativity was a
resolution to this, which started from two postulates:
Postulate 1: The laws of physics must have the same form in all inertial frames of reference.
Postulate 2: The speed of light through a vacuum is constant, regardless of the motion of the source or
observer.The first postulate means that we cannot solve the paradox by saying that the ground observer correctly sees how
the light moves, and the observer in the car or plane measure the speed wrong for some reason. It has to be that the
observer in the plane is correct when they see light move away at the same speed.
Check Your Understanding
A space ship traveling with velocityv= 0. 4 capproaches a space station housing a light beacon. When the ship is
still some distance away, the station turns on the light beacon, as shown in the diagram below.