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Black Holes Black Holes and General and General

RelativityRelativity

Stuff that warps your mind

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GoalsGoals

• 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.

SunEH M

Mkm3R

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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 X-

rays.• “Near” star

HDE226868.

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HDE226868

• Doppler shifts of HDE226868

• Like before, we get mass of star and unseen companion.

Mass Total

)(Separation)(Period

32 AU

yrs

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The Companion• Result:

Period = 5.6 daysTotal 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!

SunEH M

Mkm3R

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Supermassive Black Holes

• Photograph the center of a galaxy.

• Make spectrum of light from center.

Velocity

Dis

tan

ce

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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.

Mass Total

)(Separation)(Period

32 AU

yrs

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The Dark Truth• Observe:

V = 400 km/s within 26 LY of center.• So:

– 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!

Mass Total

)(Separation)(Period

32 AU

yrs

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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 Relativity2 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 6 months before when they are up at night.

• During eclipse, measure their positions again.

SOHO spacecraft

Result: Theory of Relativity confirmed!

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Gravitational Lensing

• If geometry right: gravity cause multiple images.

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Einstein Lenses

Copyright – J. Rhoads, WIYN

Copyright – Charles R Evans (UNC)

Einstein Ring

Copyright – L. J. King (U. Manchester)

1212Copyright – A. Fruchter (STScI)

Abell 2218

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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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c

distancetime

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Gravity makes time slow

S

F

S

F