1. Simultaneity2. Time Dilation3. Length Contraction

SPECIAL RELATIVITYSpace, time, motion, revisited

Special Relativity Postulates1. The laws of physics are same for all observers in

inertial reference frames (IRF)2. 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

Simultaneity

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.

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

Relativity of Time

Two light clocks, synchronized at relative rest

Tick-tock time is T for both YOU and George

Now light clocks in relative motionGeorge is moving relative to

Your IRF…

• 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 DILATIONNOTES:• 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)

Relativity of LengthPut 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 tunnel2. Back clock strikes Noon at it enters tunnel

Suppose You see them strike Noon simultaneouslyyou conclude train fits exactly in tunnel

George sees back clock strike Noon first, front clock laterconcludes train is shorter than tunnel

Moving objects are… LENGTH CONTRACTED