1. Relativity Chapter 26

2. 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 10 8 m/s.  3.0 X 10 8 m/s with respect to what frame of reference???

3. 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.

4. 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.

5. 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 10 8 m/s. HOW?  Our measurement of speed involves a measurement of distance and time. We assume these quantities are constant and measureable. Are they???

6. Homework  Read Sections 27.2 through 27.4 – Time Dilation and Length Contraction  Do # 12 – 15, 17, 20 – 23, 26, 28, 30