Simultaneity Time Dilation Length Contraction

1. SPECIAL RELATIVITY Space, time, motion, revisited • Simultaneity • Time Dilation • Length Contraction

2. Special Relativity Postulates • The laws of physics are same for all observers in inertial reference frames (IRF) • The speed of light in vacuum is c,regardless of relative speed between source and observer. Note: Luminiferous Ether is now superfluous • Rexamine basic concepts – space, time, speed,.. • Demand operational definitions • -> consensus based on observation

3. Is simultaneity absolute ? Operational defn Two clocks in different places strike Noon simultaneously if YOU, standing halfway between them, see them strike Noon at the same time. Simultaneity Moving Observer George is moving uniformly relative to YOU and the clocks In George’s IRF, the clocks do not strike Noon simultaneously Both YOU and George are correct Simultaneity is a relative concept

4. Relativity of Time Two light clocks, synchronized at relative rest Tick-tock time is T for both YOU and George Now light clocks inrelative motion George is moving relative toYour IRF…

5. You say light travels further in George’s clock, so his tick-tock lasts TG by Your clock, longer than T (2nd Postulate). • Conclude moving clock runs SLOWER TIME DILATION • NOTES: • Dilation occurs for ALL clocks, • not just light clocks • Effect is tiny unless clocks move • at nearly the speed of light • George sees no change in his clock, • tick-tock T (1st Postulate)

6. Relativity of Length Put clocks at each end and You (halfway) on a train moving relative to George sitting on a tunnel. 1. Front clock strikes Noon as it exits tunnel 2. Back clock strikes Noon at it enters tunnel Suppose You see them strike Noon simultaneously you conclude train fits exactly in tunnel George sees back clock strike Noon first, front clock later concludes train is shorter than tunnel Moving objects are… LENGTH CONTRACTED