Transcript Lecture 25&26

There’s something strange at
the center of our galaxy…
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Modern large
telescopes can track
individual stars at
Galactic Center
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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
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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
Why are Supermassive Black Holes
always found in Galactic Cores?
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Gravity separates matter by density
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The dense element iron sinks to Earth’s 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!
Is there a Supermassive
Black Hole in Every Galaxy?
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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…
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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
galaxy’s bulge (but not the galactic disk)…
Patterns: Galaxies and their
Supermassive Black Holes
Even better correlation with
stellar velocity in bulge…
Correlation with the mass
of the galactic bulge.
Supermassive Black Holes
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Correlations between Supermassive
Black Holes and their host galaxies are
crucially important!
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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!
Active Galactic Nuclei
(When good black holes go bad!)
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Active Galactic Nuclear - AGN
Discovery of AGN (3C273)
What are AGN?
Radio-galaxies
AGN and colliding galaxies
I : The discovery of AGN
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Early 1960s…
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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…
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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 Moon’s position to
determine position of quasar.
Hazard could localize 3C273 to about 1 arcsec.
3C273
Quasars are Bright and Far Away
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M.Schmidt
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Took Hazard’s position and observed it with “normal”
(optical) telescope… in particular, he measured its
spectrum
Spectrum showed huge velocity, 50,000km/s
Hubble’s law  huge distance (700Mpc+)
Thus, 3C273 must be very powerful to still appear
bright when its so far away!
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About 1000x luminosity of our whole galaxy
Source was variable  must be “small”, about size of our
solar system (Why?)
Rapid Time Variation means
Small Source Size
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What if the Sun stopped shining right now
(instantaneously) - what would we see?
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Nothing unusual for 8 minutes!
Then the center of the Sun would go dark - “there’s
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 object’s size
There are now many many
examples of AGN…
MCG-6-30-15
II : The Theory of AGN
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What powers AGN?
Define efficiency of an AGN
E   Mc
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2
Think about efficiency of different
processes
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Chemical burning, 10-9
Nuclear fusion, 0.007
Accretion onto a black hole, 0.1
Matter/anti-matter annihilation, =1
How to Power AGN
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Suppose AGN has power of 1040W and lasts for 10
million years
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Then, what masses are needed?
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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 can’t fall straight in… forms an
accretion disk. Accretion disks power jets.
III : Radio galaxies
Jets from Black Holes
form Radio Galaxies
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About 1-10% of AGN are radio-galaxies
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Radio galaxies
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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?
Faster than a speeding light ray?
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Superluminal motion
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What’s going on?
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Jets have “blobs”
Some blobs appear to move faster
than light!
Motion faster than light is not
allowed by the Special Theory of
Relativity
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.
Wormholes and Timewarps?
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Wormholes
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Wormholes may exist inside black holes
Build a stable wormhole and bring wealth
and prosperity to your planet!
Timewarps
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The idea of cause and effect
The assumption of causality and the speed
of light
Can time-machines exist?
Astrophysicists view of black holes…
stops at the event horizon
But there is structure inside the
event horizon!
Nothing escapes from inside the Event Horizon
Non-rotating Black Holes: The
Schwarzschild Solution
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Schwarzschild solution
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Any normal particles/light entering event
horizon are doomed to hit the singularity at
the center of the black hole
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Mathematical description of a non-rotating black hole
Discovered by Karl Schwarschild in 1916
Once there they will be totally destroyed
But, the Schwarzschild solution has an
interesting mathematical structure…
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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
(Black Hole)
Stuff goes
in here …
Avoids the
Singularity
of death …
and comes
out here!
(White Hole)
Do Schwarzschild Black Holes
Really Exist?
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Probably not…
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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 won’t bring
prosperity to your planet.
Rotating Black Holes:
The Kerr solution…
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Kerr solution
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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…
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This time, don’t 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.
Can Wormholes form in
Rotating Black Holes?
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Again, seems unlikely…
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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 “lowmileage” wormholes, although …
There is one
in New York
(if you know
where to
look)!
Scientific American, Jan 2000
Alright, then let’s build a
Time Machine instead
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Idea of cause and effect…
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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.
Einstein’s theory of special relativity
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What if a signal could be transmitted faster than light speed?
Then Einstein’s 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
Is my Time Machine ready yet?
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The big question: Does Einstein’s 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).