Special Relativity

AP Physics

Relative Velocity of Light

Either

1. Addition of Velocities are WRONG

2. Laws of E and M are not the same in all inertial frames of reference

Principle of Relativity

The Postulates1) Laws of Physics are same in all inertial frames of reference (at rest or constant v)

2) Speed of light in a vacuum has the same value (3 x 108 m/s) regardless of the motion of the observer or source.

Note: There is no preferred frame of reference

Consequences of Relativity

• No Absolute Length

• No Absolute Time

• Simultaneous or NOT

• Mass Increase

• Equivalence of Mass and Energy

• New Math - for velocity addition

Simultaneity2 events simultaneous in one reference frame are in general NOT simultaneous in a 2nd frame moving relative to the first.

There is no preferred inertial frame of reference!

Time Dilation - The Light Clock

Light bounces off the mirrors. Each up and down “bounce” is a tick on the clock.

Time Dilation

Time Dilation

Time Dilation

Moving clocks run slower than those at rest by a factor of -1

Experimental Proof - Muon Decay

What will the lifetime of a muon measured if it is moving at 0.60 c? The mean lifetime at rest is 2.2 x10-6 s.

Answer: 2.8 x 10-6 s

Another Problem

Mr. Smith travels at 0.99 c for 1 year (to him). How much time goes by for his physics class at relative rest?

Twin Paradox