1. Special Relativity

2. Interferometer  A coherent light source can be split and recombined. Phase shifts for different paths  A Michelson interferometer observes the spacing between fringes. Time-of-flight differences mirror 2 mirror 1 semi- silvered mirror

3. Michelson-Morley  In 1887 Michelson and Morley used the interferometer to try to measure the relative velocity of the ether. One path into and against etherOne path into and against ether One path across etherOne path across ether  The time difference depends on the velocity of the ether.

4. No Ether  The earth changes velocity direction as it orbits the sun. Changing v relative to the etherChanging v relative to the ether Measure fringes in different orientationsMeasure fringes in different orientations Measure in different seasonsMeasure in different seasons  The experiment saw no shift in fringes. Theories to preserve ether failedTheories to preserve ether failed

5. Relativity Principle  FitzGerald (1887) and Lorentz (1892) developed transformations consistent with electromagnetism. FitzGerald-Lorentz contractionFitzGerald-Lorentz contraction Explains Michelson-MorleyExplains Michelson-Morley  Poincare (1898) proposed that the speed of light must be the same for all observers. Unprovable postulateUnprovable postulate Still accepted etherStill accepted ether

6. Einstein’s Theory  In 1905 Einstein independently derived Lorentz’s equations from two principles. No need for ether I. The laws of physics are the same in all inertial frames. II. The speed of light in a vacuum is the same for all inertial frames, from all sources.

7. Simultaneity  Einstein’s special theory of relativity required that neither distance nor time was absolute. Only the speed of lightOnly the speed of light  Simultaneity is relative to the observer. Simultaneous pulses for ASimultaneous pulses for A

8. Relative Time  Another moving observer sees events as separate in time. Time is relative Like coordinates and velocity  Neither observer is wrong. Each correct in their frame next