# CH39 ModernAndRelativity

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```					             Modern Physics
• Change of mode for class:
– Much faster pace.
– Easier, shorter homework problem
assignments.
• Purpose:
– Get an overview of Modern Physics in two
weeks.
– For more thorough coverage, take Physics
215 in the fall.
Modern Physics
• Theory of Relativity
• Quantum Mechanics, a.k.a. Wave
Mechanics.
• Nuclear Physics
Relativity
Physics in 1900
• A strong sense that we were at the end of
physics. Nothing but engineering left.
• Newton’s laws of motion & gravity
• Maxwell’s equations were awe-inspiring.
• But …
Two inconvenient experiments
• Michelson-Morley experiment to measure
motion of the earth through space.
• Black-body radiation.
1887: Michelson-Morley
Experiment

Images from Wikipedia.

• Measured zero
velocity change!
Explanations – or speculation
• Ether drag. Earth drags
ether locally along with it,
so no ether wind.
• Einstein: what if we just
assume the result as a law
of nature?
Einstein’s idea
•    Began with only two
postulates initially:
1. The speed of light is
independent of the speed of
the observer or the source.
2. The laws of physics are the
same in all inertial reference
frames.
– Basis of his Theory of
Special Relativity
Implications of postulates

• Consider two observers, one moving c/2
with respect to the other.
• Light pulse passes observer one at speed
c.
• How fast does observer two see it
moving?
Conclusion:
• Either:
– Clocks are “broken”, or
– Meter sticks are broken, or
– Both!
Time dilation

t p
t               .
2
v
1
c2
Time dilation
t p
t               .
2
v
1 2
c
Length contraction

2
v
L  Lp 1  2 .
c
Length contraction

2
v
L  Lp 1  2 .
c

V
Length contraction

2
v
L  Lp 1  2 .
c

V
Contradictions?
• How can both of us think the other’s meter
stick is shorter?
• How can both of us think the other is aging
more slowly?
Twin Paradox
• One travels to a distant star at close to the
speed of light, and returns.
• What is the trip like for each?
• What is the resolution of the paradox?
Mass difference
For momentum conservation to work in both
reference frames, mass cannot be the same in
both reference frames.

mp
m          .
v2
1 2
c
General theory of relativity
• Additional postulate: gravity, acceleration
indiscernible.
• Consequences:
– Gravitational time dilation.
– Bending of light
– Black holes

```
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