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Black Holes and
General Relativity
Stuff that warps your mind
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Goals
• Why should light be affected by gravity?
• What proof is there that it is?
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Are Black Holes Black?
• Why should light be affected by gravity?
Newton - “Whenever you have mass you have gravity.”
• But light has no mass!
• Einstein’s General Theory of Relativity (1915):
– New way of looking at gravity.
– Allows gravitational effects on light.
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General Relativity
2 Main Postulates:
1. The speed of light is always c.
Thou shalt not add your speed to the speed of light!
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General Relativity
2. Accelerating reference
frames are
indistinguishable from
a gravitational force.
The Star Tours ride at Disneyland is awesome!
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Gravity Bends Light
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Is there Proof?
Scientific Method:
1. Hypothesis: predict gravity bends light.
2. Test: The Sun has a lot of gravity and should
deflect star light.
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Solar Eclipse of 1919
• Hard to see stars near the Sun.
• Can see stars near the Sun
during an eclipse.
• Can predict time and position
of Sun during eclipse years in
advance.
• Measure the positions of stars SOHO spacecraft
6 months before when they are
up at night.
• During eclipse, measure theirResult: Theory of Relativity confirmed!
positions again.
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Gravitational Lensing
• If geometry right: gravity cause multiple images.
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Einstein Lenses
Copyright – Charles R Evans (UNC)
Einstein Ring
Copyright – L. J. King (U. Manchester)
Copyright – J. Rhoads, WIYN
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Abell 2218
Copyright – A. Fruchter (STScI)
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Space is Curved
• Einstein said to picture gravity as a warp in space.
– Sagan : A “pucker” in fabric of space.
• Kepler’s Laws
explained by
movement around
“puckers.”
• Everything moving is
affected, regardless of
mass.
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distance
time 
c
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Gravity makes time slow
F
F
S
S
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Proof?
•
•
•
•
GPS satellites use atomic clocks.
Position determined by timing radio signals.
In orbit Fg = ¼ Fg at Earth surface.
Clock on satellite runs fast by 45 msec per day.
D=vt
D=ct
DD = cDt
DD = 3 x 105 km/s * 45 x 10-6 s/day
DD = 13.5 km/day
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Concept Test
•
We think the Theory of General Relativity is correct
because:
a.
b.
c.
d.
e.
We have performed thought experiments showing that
accelerating rockets bend light and slow time.
We know that the speed of light is constant and that what
happens in an accelerating frame is also what happens in a
gravitational field.
We observe phenomena that confirm the results of our thought
experiments.
Our thought experiments confirm the predictions made by
General Relativity.
Einstein is always right; that’s why he’s called Einstein.
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Homework #14
• For Monday: Read articles
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