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Recap from Fri
Consequences of Special Relativity:
1) objects shrink in direction of motion
2) time slows down in moving frame
3) mass increases in a moving frame
4) velocities don't add up in Newtonian fashion
1905 Special Relativity: only deals with motion at constant velocity;
gravity is not mentioned until general relativity (all frames, including
accelerating ones)
Example #1 of special relativity: spaceships
2 frames of references, one stationary and one moving with a constant
velocity (mirrors equidistant on opposite sides of each spaceship)
light arrives simulataneously at the same time and light hits
mirrors at the same time .
.. Stationary frame of reference: How does
light do this?
both agree light travels the same distance; it is simultaneous but
light did no hit mirrors at the same time.
Moral: defining what is simultaneous between moving frames is
tricky/not obvious
The speed of light is finite (has to travel, spaceship was moving)
Example #2: The train
2 observers at each' end of the train; a light source in the middle of the ./
train trying to identify length of train (How? when each observer sees
the light, they identify the pole on the ground closest to them)
An observer on the ground: sees it's a combo of light moving and the
person on back of the train moving forward to meet the light, and the
person on the front of the train moving away from the light ~ the observers on the train 'did see
light at the same time', so logically, the train must have shrunk.
~ difference of opinion in length in each frame of reference.
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 Fall '08
 Heil
 Mass, Special Relativity

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