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Transcript
Black Holes and
General Relativity
Stuff that warps your mind
11
Goals
•
•
•
•
•
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?
11
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  
 MSun 
11
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
11
Cygnus X-1
• 1970s
• Intense source of X-rays.
• “Near” star HDE226868.
11
HDE226868
• Doppler shifts of
HDE226868
• Like before, we get mass
of star and unseen
companion.
Separation ( AU )
Period ( yrs ) 
Total Mass
3
2
11
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.
11
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  
 MSun 
11
11
Supermassive Black Holes
Distance
• Photograph the center of a
galaxy.
Velocity
• Make spectrum of light
from center.
11
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 ) 
Total Mass
3
2
11
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!
Total Mass
11
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.
11
General Relativity
2 Main Postulates:
1. The speed of light is always c.
Thou shalt not add your speed to the speed of light!
11
General Relativity
2. Accelerating reference
frames are
indistinguishable from
a gravitational force.
The Star Tours ride at Disneyland is awesome!
11
Gravity Bends Light
11
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.
11
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.
11
Gravitational Lensing
• If geometry right: gravity cause multiple images.
11
Einstein Lenses
Copyright – Charles R Evans (UNC)
Einstein Ring
Copyright – L. J. King (U. Manchester)
Copyright – J. Rhoads, WIYN
11
Abell 2218
Copyright – A. Fruchter (STScI)
11
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.
11
distance
time 
c
11
Gravity makes time slow
F
F
S
S
11