1. There’s 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…

3. 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

4. Gravity separates matter by density 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! Why are Supermassive Black Holes always found in Galactic Cores?

5. 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 galaxy’s bulge (but not the galactic disk)…

6. Patterns: Galaxies and their Supermassive Black Holes Even better correlation with stellar velocity in bulge… Correlation with the mass of the galactic bulge.

7. 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!

8. 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

9. 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 Moon’s position to determine position of quasar. Hazard could localize 3C273 to about 1 arcsec.

11. 3C273

13. Quasars are Bright and Far Away M.Schmidt 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! About 1000x luminosity of our whole galaxy Source was variable  must be “small”, about size of our solar system (Why?)

14. 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 - “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

15. There are now many many examples of AGN… MCG-6-30-15

16. 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

17. How to Power AGN Suppose AGN has power of 10 40 W and lasts for 10 million years Then, what masses are needed? Chemical burning… 210 12 M sun Nuclear burning… 210 9 M sun Accretion… 210 8 M sun Matter/anti-matter… 210 7 M sun 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.

18. Now believe that AGN are indeed accreting supermassive black holes Matter can’t fall straight in… forms an accretion disk. Accretion disks power jets.

19. III : Radio galaxies

21. 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?

22. 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 What’s 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.

23. 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?

24. Astrophysicists view of black holes… stops at the event horizon

25. But there is structure inside the event horizon! Nothing escapes from inside the Event Horizon

27. 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

28. (Black Hole) Stuff goes in here … and comes out here! (White Hole) Avoids the Singularity of death …

29. 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 (2 nd 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.

30. 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, 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.

32. 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 …

33. Scientific American, Jan 2000 There is one in New York (if you know where to look)!

34. Scientific American, Jan 2000

35. Alright, then let’s 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. Einstein’s theory of special relativity 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

36. Is my Time Machine ready yet? 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).