Relativity

Chapter 26

Background Physics is concerned with describing nature based on

observation and measurement. Principle of Classical Relativity: - The laws of

mechanics are the same in all inertial (non accelerating) reference frames.

Maxwell (late 1800’s) determined the laws governing electricity and magnetism. Also identified light as an electro-magnetic wave traveling at 3.0 X 108 m/s.

3.0 X 108 m/s with respect to what frame of

reference???

Background Since all waves were mechanical, scientists presumed

that light was also mechanical and traveled through an ever-present and far reaching ether.

The ether must be the reference frame with respect to which we measure the speed of light…. It must be the ‘preferred’ reference frame.

Michelson and Morley set out measure the ether. Famous experiment!!

No ether.

Einstein Postulate 1: All of the laws of physics

are the same in all inertial reference frames. Laws of mechanics as well as laws governing electricity and magnetism.

Postulate 2: The speed of light in a vacuum has the same value in all inertial frames of reference.

Speed of light is constant: Imagine observing a car traveling at 60 mi/hr on the

highway from three different inertial frames of reference:

From the side of the road From another car traveling alongside at 20 mi/hr From a car traveling in the opposite direction at 50

mi/hr

Light measured from different frames of reference would have exactly the same value… 3.0 X 108 m/s. HOW?

Our measurement of speed involves a measurement of distance and time. We assume these quantities are constant and measureable. Are they???

Homework

Read Sections 27.2 through 27.4 – Time Dilation and Length Contraction

Do # 12 – 15, 17, 20 – 23, 26, 28, 30