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Chapter 7Relativity
Relativity
Special Theory of Relativity – Einstein 1905– space contraction and time dilation
General Theory of Relativity – Einstein 1916– theory of gravitation
7.1 Special Relativity
Problems with Electromagnetic Radiation– Sound requires a medium?– Shouldn’t electromagnetic waves also need a
medium?– Physicist postulated a medium called ether.
Thought Experiments
What is the relative speed between you and the other car?
Answer: 35m/s - 25 m/s = 10 m/s
Car
You
35m/s
25m/s
Thought Experiments
What is the relative speed between you and the beam of light?
Answer: 300,000,000m/s - 25 m/s = 299,999,975 m/s ? No, no, no! Real Answer: 300,000,000 m/s
Flashlightat rest
You
300,000,000m/s
25m/s
Conclusion
Experiments show that “the speed of light in a vacuum is the same for all observers, independent of their motion”.
This suggests that something is wrong with the concept of ether.
Michelson and Moley in 1887 showed with an experiment that the speed of light did not depend of the motion of the observer.
Problems with SimultaneityExplosions in a Train Example
At Rest
There is no experiment we can perform to tell us which inertial from is moving and which is at rest.
There is no ‘preferred’ inertial frame.
7.2 Time Dilation
MIRROR
MIRROR
Light Clock at RestL
Tick-Tock Time for the Clock at Rest
c
L2to
Light Clock in Motion
MIRROR
MIRROR
Tick-Tock Time for the Clock in Motion
c
2/vtL2t
21
22
MIRROR
MIRROR
MIRROR
MIRRORv
v
End of Chapter 7