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# Special Relativity II

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```									Special Relativity II:
by
Robert Nemiroff
Michigan Tech

• Pronounced "Fiziks Ecks"
• Reviews the coolest concepts in physics
• Being taught for credit at Michigan Tech
o   Michigan Tech course PH4999
o   Aimed at upper level physics majors
o   Light on math, heavy on concepts
o   Anyone anywhere is welcome
• No textbook required
o   Wikipedia, web links, and lectures only
Special Relativity:
Doppler effect
• Object moving toward you
o   appears more blue -- called blueshift
o   appears to speed up (clock looks faster)
o   appears elongated
• Object moving away from you
o   appears more red -- called redshift
o   appears to slow down (clock looks slower)
o   appears contracted
Special Relativity:
Doppler Related Effects

• Doppler effect: moving toward a uniform color background
o   A green background ahead of you appears blue
o   A green background behind you appears red

• Transverse Doppler Effect
o   The time dilation effect of special relativity is directly measured
for an object moving transverse (neither toward nor away) from
you
Special Relativity:
Doppler Effects

• Aberational effect: moving toward a uniform star field
o Stars in your direction of motion appear to bunch
Special Relativity: Doppler effects

Earth's motion with respect to the cosmic microwave background
Special Relativity:

Can a wavefront of blinking lights appear superluminal?

Example: A row of lights blinks in sequence. Each light seen to blink one
second after the one to its right. You now find out that the lights are far away
and separated by light years. Does the "wave" of blinking exceed c?
1. Yes.
2. No.
3. Lights can't blink that fast.

Yes: blinking light sequence may appear to have v > c.

• No information is transferred with v > c.
• Observers located elsewhere may see different blinking pattern.
• No observer can use the blinking lights to communicate with v > c.
Superluminal Flashlight Sweep

You shine a flashlight inside a distant spherical dome one light year
away. In one second, you swing the light from shining to your left to
shining to your right. How fast does the spot from the flashlight
move on the dome?

1. Slower than c.
2. c.
3. Faster than c.
4. 5 sqrt(alpha - 2 gamma).
Superluminal Flashlight Sweep

3. Faster than c.

Although you would have to wait for two years to see the spot move
at all, when you saw it, the spot would move many light years across
the sky in one second.

The spot is not a physical thing, however, and the spot can carry no
information.
Special Relativity: