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1
Lecture 2:
The Special Theory of Relativity
I
•
MichelsonMorley Experiment
• Einstein’s postulates of special relativity
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Classical Relativity: Example
v
L
• A stick length of
L
is moving with constant speed
v
along
the positive
x axis
as observed
in reference frame
S.
• At
t
= 0, the midpoint of the stick is located at
x
= 0. At this time, two
ﬂashes of light are emitted, one from each end of the stick.
• An observer is riding along at the midpoint of the stick (reference
frame
S
’). At what time does she record the ﬂashes of light according
to classical relativity? Assume that the speed of light in the frame
S
is
known to be
c
.
• Another observer is at rest in frame
S
at
x
= 0. At what time does he
record the ﬂashes of light?
3
Classical Relativity: Example
v
L
• Observer riding along at the midpoint of the stick:
• Sees ﬂash from A arrive at:
• Sees
ﬂash from B arrive at:
• Observer at rest at
x
= 0:
• Sees ﬂashes arrive simultaneously at:
t
A
'
=
L
2
c
!
v
( )
t
B
'
=
L
2
c
+
v
( )
t
A
=
t
B
=
L
2
c
A
B
This is correct
only if light did
indeed travel in
“ether.”
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The MichelsonMorley Expt.
• Prevalent view in late 1800s: light propagates in the “ether.”
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This note was uploaded on 05/08/2008 for the course PHYS 237 taught by Professor Stephonalexander during the Spring '08 term at Pennsylvania State University, University Park.
 Spring '08
 STEPHONALEXANDER
 Theory Of Relativity, Special Relativity

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