Theres something strange at the center of our galaxy

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Theres something strange at the center of our
galaxy

• Modern large telescopes can track individual
  stars at Galactic Center
• Need infra-red (to penetrate dust?)
• Need very good resolution.
• We have been observing for past 10 years

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• The central object is
• Very dark
• Very massive (3 million solar masses)
• Must be very compact (Star S2 gets within 125 AU
  of the center)
• Currently the best case for any supermassive
  black hole

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Why are Supermassive Black Holes always found in
Galactic Cores?

• Gravity separates matter by density
• The dense element iron sinks to Earths core
• Heavy elements sink to centers of stars
• Binary stars and massive stars sink to the center
  of star clusters
• Supermassive black holes sink to the centers of
  galaxies
• Massive galaxies sink to the center of galaxy
  clusters
• This is a very general property of nature!

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Is there a Supermassive Black Hole in Every
Galaxy?

• Supermassive Black holes exist in centers of some
  galaxies
• But how widespread are they? Does every galaxy
  have one?
• Several teams set out to answer that question
• Use best resources (HST, large telescopes on
  ground etc.) to gather lots of data on many
  nearby galaxies.
• Systematic search for black holes
• They found them everywhere they looked, and
  discovered interesting patterns
• Correlation between size of black hole and the
  brightness of the galaxys bulge (but not the
  galactic disk)

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Patterns Galaxies and their Supermassive Black
Holes

• Even better correlation with stellar velocity in
  bulge

Correlation with the mass of the galactic bulge.
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Supermassive Black Holes

• Correlations between Supermassive Black Holes and
  their host galaxies are crucially important!
• Argues for a connection between the formation of
  the galaxy and the supermassive black hole.
• The details are still not well understood - an
  active area of current research!

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Active Galactic Nuclei(When good black holes go
bad!)

• Active Galactic Nuclear - AGN
• Discovery of AGN (3C273)
• What are AGN?
• Radio-galaxies
• AGN and colliding galaxies

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I The discovery of AGN

• Early 1960s
• Radio astronomers started to survey the sky
• Found many mysterious radio sources
• Quasi-stellar radio sources (quasars for short)
• Very difficult to identify radio images were too
  fuzzy to allow quasars to be localized on sky
• Cyril Hazard
• Used Lunar occultation to localize 3C273
• I.e., measure the precise time when the radio
  signal from the quasar is blocked by the Moon
• then use knowledge of the Moons position to
  determine position of quasar.
• Hazard could localize 3C273 to about 1 arcsec.

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3C273
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Quasars are Bright and Far Away

• M.Schmidt
• Took Hazards position and observed it with
  normal (optical) telescope in particular, he
  measured its spectrum
• Spectrum showed huge velocity, 50,000km/s
• Hubbles law ? huge distance (700Mpc)
• Thus, 3C273 must be very powerful to still appear
  bright when its so far away!
• About 1000x luminosity of our whole galaxy
• Source was variable ? must be small, about size
  of our solar system (Why?)

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Rapid Time Variation means Small Source Size

• What if the Sun stopped shining right now
  (instantaneously) - what would we see?
• Nothing unusual for 8 minutes!
• Then the center of the Sun would go dark -
  theres a little dark spot on the Sun today.
• The dark circle would expand toward the bright
  edge of the Sun and, 2 seconds later, all would
  be dark
• The 2 second delay happens because the center of
  the Sun is closer to us than the edges by about 2
  light seconds.
• The fastest change that we could see happening
  to the entire Sun would take 2 seconds - this
  limit is set by the objects size

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There are now many many examples of AGN
MCG-6-30-15
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II The Theory of AGN

• What powers AGN?
• Define efficiency of an AGN
• Think about efficiency of different processes
• Chemical burning, ??10-9
• Nuclear fusion, ??0.007
• Accretion onto a black hole, ??0.1
• Matter/anti-matter annihilation, ?1

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How to Power AGN

• Suppose AGN has power of 1040W and lasts for 10
  million years
• Then, what masses are needed?
• Chemical burning 2?1012 Msun
• Nuclear burning 2?109 Msun
• Accretion 2?108 Msun
• Matter/anti-matter 2?107 Msun
• Masses tend to argue for accretion (there are not
  large amounts of anti-matter in space!). And we
  know that billion solar mass black holes exist.

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• Now believe that AGN are indeed accreting
  supermassive black holes
• Matter cant fall straight in forms an accretion
  disk. Accretion disks power jets.

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III Radio galaxies
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Jets from Black Holes form Radio Galaxies

• About 1-10 of AGN are radio-galaxies
• Radio galaxies
• Possess huge jets that transport energy away from
  center and entirely out of the galaxy
• Processes associated with jets then give powerful
  radio emission
• Material in the jets is moving very close to the
  speed of light how do we know this?

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Faster than a speeding light ray?

• Superluminal motion
• Jets have blobs
• Some blobs appear to move faster than light!
• Motion faster than light is not allowed by the
  Special Theory of Relativity
• Whats going on?
• Superluminal motion is an optical illusion
  blobs partially catch-up with the light they
  emit.
• But needs jet to be traveling close to the speed
  of light
• No Warp-Drive yet.

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Wormholes and Timewarps?

• Wormholes
• Wormholes may exist inside black holes
• Build a stable wormhole and bring wealth and
  prosperity to your planet!
• Timewarps
• The idea of cause and effect
• The assumption of causality and the speed of
  light
• Can time-machines exist?

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Astrophysicists view of black holes stops at the
event horizon
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But there is structure inside the event horizon!

• Nothing escapes from inside the Event Horizon

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Non-rotating Black Holes The Schwarzschild
Solution

• Schwarzschild solution
• Mathematical description of a non-rotating black
  hole
• Discovered by Karl Schwarschild in 1916
• Any normal particles/light entering event horizon
  are doomed to hit the singularity at the center
  of the black hole
• Once there they will be totally destroyed
• But, the Schwarzschild solution has an
  interesting mathematical structure
• Suggests that particles traveling faster than
  speed of light can avoid the center
• Then pass through a wormhole and emerge from a
  white hole into a new region of normal space

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(Black Hole) Stuff goes in here
Avoids the Singularity of death
and comes out here! (White Hole)
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Do Schwarzschild Black Holes Really Exist?

• Probably not
• Real stars do not form Schwarzschild black holes
  because they rotate!
• Full Schwarzschild solution needs existence of
  white hole these violate key laws of physics
  (2nd law of thermodynamics).
• Even if they did exist, they are unstable! Try
  to pass through it and it collapses!
• An unstable wormhole wont bring prosperity to
  your planet.

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Rotating Black Holes The Kerr solution

• Kerr solution
• Mathematical solution describing rotating black
  holes
• Applicable to most real black holes in nature
• Discovered by Roy Kerr in 1960s
• More interesting than Schwarschild solution
• This time, dont need to travel faster than light
  to avoid the singularity!
• Singularity has a ring structure enter the
  wormhole by flying through the ring
• Are then propelled from a white hole and emerge
  into another region of normal space.

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Can Wormholes form in Rotating Black Holes?

• Again, seems unlikely
• White holes still violate physical laws
• Wormhole are still unstable
• Some researchers think that there is not a hole
  in the ring through which to pass
• Never trust a stranger selling low-mileage
  wormholes, although

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There is one in New York (if you know where to
look)!
Scientific American, Jan 2000
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Scientific American, Jan 2000
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Alright, then lets build a Time Machine instead

• Idea of cause and effect
• Foundation of scientific reasoning
• If one event can have an effect on the other
  event, they are said to be causally-connected.
• To have any meaning, the cause must happen before
  the effect!
• This requirement is called causality.
• Einsteins theory of special relativity
• What if a signal could be transmitted faster than
  light speed?
• Then Einsteins Special Theory of Relativity
  shows that you can change the order of
  causally-connected events.
• Can make effect happen before cause!
• This seems to make no sense. We disallow this!
• Causality seems to imply that no signal can
  travel faster than light

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Is my Time Machine ready yet?

• The big question Does Einsteins theory
  necessarily demand causality? Must Cause always
  precede Effect?
• Surprisingly, and in contrast to everyday
  experience, this is not at all obvious!
• To determine if Time Machines are possible, we
  probably have to understand the nature of
  space-time a lot better than we currently do.
• Extra Credit Project Invent a Time Machine (0.5
  points - due tomorrow).