Postulates of Special Relativity

1
Postulates of Special Relativity
1. Physics is invariant among inertial
(Galilean) reference frames.
You cannot do an experiment that gives you the
absolute velocity of your reference frame.
2. The speed of light is the same in all inertial
reference frames.
Vector addition for velocities does not work
between reference frames
2
Consequences Time Dilation
Time to make round trip
L
3
Consequences Time Dilation (continued)
L
vx
vxt
Time to make round trip
4
Time Dilation
Conclusion
If I compare my clock with a moving clock, my
clock will tick faster by a factor of g.
5
Time Dilation - Example
Observation
We see neutrons with velocities of (1- ½ x
10-20)c coming from the center of the galaxy
hitting the earths atmosphere. How can they get
here?
Useful numbers
Distance to center of galaxy 26 x 103 light
years
Neutron lifetime 14.7 minutes
ct 2.8 x 10-5 light years
6
Interesting Note about Center of Galaxy
Rotational data from stars close to center of
galaxy
7
Consequences Length Contraction
Time to make round trip
L
8
Consequences Length Contraction (continued)
L
d2
vx
d1
9
Consequences Length Contraction (continued-II)
On the car
On the ground
Conclusion
A moving yard stick is shorter than a stationary
one.
10
Time to Get to the Stars
Tom
Dick
v0bc.9c
L
What does Dick measure?
Answer Dick better measure the same thing!
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Time to Get to the Stars
Tom
Dick
v0bc.9c
L
Toms Length
Dicks observation of Toms length
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Time to Get to the Stars
Tom
Dick
v0bc.9c
L
Harry
v0bc.9c
Can we get Harry to agree also?
No can do! Well get back to Harry when we
discuss Lorentz transforms.
13
Simultaneity
What the man on the train saw
Signals from the front of the train and back of
the train arrived at the same time.
14
Simultaneity (continued)
What the man on the ground saw
Second
First
In order for the signal from the back of the
train to arrive at the same time as the forward
signal, the back signal had to be emitted first!
15
Paradox Ladder and the Barn Door
15 m
10 m
Solution for farmer Move ladder by at .866 c
and pull in!
But what does the bug on the ladder see?
16
Conclusions

• Time moves slower in a moving frame.
• Lengths are shorter in moving frames.
• What is simultaneous in one frame is not
  simultaneous in another.

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Goal of the Course
Determine the method of predicting what you will
measure in your inertial reference frame given
what I measure in mine.
Like rotating coordinates
Inertial frame physics
Rotating frame physics (rotating frame changes
physics)