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