Transcript Document

Black Holes and
General Relativity
Stuff that warps your mind
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Goals
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Use the Scientific Method:
What proof is there of black holes?
What proof is there that black holes are black?
Why should light be affected by gravity?
What proof is there that it is?
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Black Holes
• Mass so great that the
velocity needed to
escape is greater than c.
• Need lots of mass in a
small volume.
R EH
 M
 3km  
 M Sun



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Seeing Black Holes
• Rips apart and heats up
anything it swallows.
• Very hot  thermal
emission peak in X-rays.
• So what do you look for:
–
–
–
–
Massive
Small
Dark in visible
Bright in X-rays
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Cygnus X-1
• 1970s
• Intense source of X-rays.
• “Near” star HDE226868.
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HDE226868
• Doppler shifts of
HDE226868
• Like before, we get mass
of star and unseen
companion.
Separation( AU )
Period( yrs) 
T otalMass
3
2
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The Companion
• Result:
Period = 5.6 days
Total Mass ~ 28 x Msun
• From spectral type of HDE226868 we estimate its
mass ~18 Msun.
• Companion M = 10 Msun!
• Massive!
• But where is its light?
• Dark!
• Can’t be a normal star, or even neutron star.
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X-ray Source?
• Star brightness fluctuates every 5.6 days.
• X-rays drop off every 5.6 days!
• Companion must be source of X-rays!
• REH = 30 km!
R EH
 M
 3km  
 M Sun



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Supermassive Black Holes
Distance
• Photograph the center of a
galaxy.
Velocity
• Make spectrum of light
from center.
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Heart of Darkness
• From Doppler shift get a
velocity.
• From picture get a
separation.
• From Kepler’s Laws get
a Total Mass.
Separation( AU )
Period( yrs) 
T otalMass
3
2
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The Dark Truth
• Observe:
V = 400 km/s within 26 LY of center.
• So:
Separation( AU )3
2
Period( yrs) 
– Period = 121,600 yrs
– Separation = 26 LY = 1,600,000 AU
• Total Mass in central pixel:
300,000,000 x Mass of Sun!
• But where’s all the light?
• Small, massive, dark  black hole?
• REH = 6.5 AU!
T otalMass
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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 their Result: 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 can
all be 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
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