1. Special Relativity Contents: The End of Physics Michelson Morley
Postulates of Special Relativity
Simultaneity
Time Dilation

2. The End Of Physics 1900 +/- 5 The End of physics
Black Body Radiation -> Quantum mechanics
Velocity of light With Respect to ether
Maxwell’s Equations…
But what does light move through?
Ether
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3. Michelson Morley So what does light travel through ?
Path parallel to ether should take a bit longer
Tried it in January
Tried it in June…
The Earth is at rest in ether
Or the path parallel to the ether shrinks
Or, there is no ether
So what does light travel through ?
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4. Einstein’s Gedanken If we caught up to an EM wave what would we see:
                               
B and E fields with nothing to create them
This is impossible
Therefore we cannot catch up to light waves…
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5. Postulates of Special Relativity
First Postulate:
The laws of physics are the same in all inertial frames of reference
There is no experiment you can do to ascertain whether you are moving.
(i.e. all motion is relative)
Second Postulate:
The speed of light is the same no matter what speed the source or observer is going
(i.e the speed of light is a law of physics)
(examples of this with cars/observers)
                               
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6. Simultaneity We can measure speed of light, and time of arrival
                               
We can measure speed of light, and time of arrival
Observer 2 sees pulses at same time
Observer 1 sees the light from B first, then A
Simultaneity is relative…
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7. Time Dilation
                               
How the clock works
What proper time is
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8. Time Dilation Earth observer sees light traveling at speed of light
                               
Earth observer sees light traveling at speed of light
Time is longer (i.e the clock runs slower)
Derive
Moving clocks run slower
Examples: (time elapsed, velocity, factor of c)
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9. factor
You can safely ignore relativistic effects to about .5 c

10. Whiteboards:
Time Dilation
1 | 2 | 3 | 4
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11. Moe and Joe have clocks that tick every 10. 00 seconds
Moe and Joe have clocks that tick every seconds. Joe is flying by at .85 c. What time does Moe see Joe’s clock take to tick? W
19 s

12. Two stopwatches, one sitting still, one moving in a very large circle
Two stopwatches, one sitting still, one moving in a very large circle. The clock at rest ticks every .800 s, the moving one ticks every s. What speed is the moving watch going?
answer W
.75 c or 2.24 x 108 m/s

13. Moe and Joe both have identical watches
Moe and Joe both have identical watches. Joe is moving with respect to Moe at .85c. If 15 s elapse on Moe’s clock, how much time has elapsed on Joe’s clock?
answer W
7.9 s

14. Muons have a mean life of 2. 2 x 10-6 s at rest
Muons have a mean life of 2.2 x 10-6 s at rest. What speed are they traveling if they they have an average lifetime of 2.6 x 10-6 s?
How far would they travel before decaying on the average?
answer W
.53c, 1.6 x 108 m/s, 415 m

15. Paradoxes Planet X Joe stays on earth
                               
Planet X
Joe stays on earth
Moe goes to planet x that is 30 Ly away at .99 c
Comes back
How old Moe is/Joe is
Does this violate Special relativity?
Inertial means no acceleration
What they see
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16. Paradoxes Planet X Planet X It’s all relative
                               
Planet X
Planet X
It’s all relative
Moe sees our clocks slow down too
Postulates only apply to inertial frames of reference
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