Black Holes. Class 17 Prof J. Kenney June 19, 2018

Transcription

1 Black Holes Class 17 Prof J. Kenney June 19, 2018

2 basic structure of (non-rotating) black hole

3 basic structure of (non-rotating) black hole SINGULARITY: all the mass of the black hole is crushed to incredibly small volume, with extremely high (infinite?) density

4 basic structure of (non-rotating) black hole EVENT HORIZON: the horizon beyond which light (or anything else) cannot escape the black hole. we have no way to learn anything about events, physical processes, etc that occur beyond the event horizon, i.e. between the event horizon and the singularity

5 basic structure of (non-rotating) black hole EVENT HORIZON: the horizon beyond which light (or anything else) cannot escape the black hole. we have no way to learn anything about events, physical processes, etc that occur beyond the event horizon, i.e. between the event horizon and the singularity what is located at event horizon?

6 basic structure of (non-rotating) black hole EVENT HORIZON: the horizon beyond which light (or anything else) cannot escape the black hole. we have no way to learn anything about events, physical processes, etc that occur beyond the event horizon, i.e. between the event horizon and the singularity what is located at event horizon? NOTHING

7 black holes are simple! detailed properties of things that fall into black hole are lost black holes have only 3 properties: mass spin (angular momentum) electrical charge (probably ~zero) black holes have no hair

8 region just outside some black holes can be complex & interesting jet comes from accretion disk not black hole accretion disk: rotating disk of gas surrounding (& perhaps spiraling into) black hole event horizon of black hole

9 Falling into Black Holes How to avoid it What it feels like What it looks like

10 What is the best way to stop being pulled into the black hole? A. Fire rockets continuously. B. Use anti-gravity. C. Go into orbit. D. Reduce the mass of the ship by jettisoning excess cargo. E. Just go in, don t be such a baby!

11 You guide your spacecraft into an orbit a few AU from a black hole. You know its mass is ~4 or 5 solar masses, but you want to measure it more precisely. How would you do this? A. It is impossible to see beyond the event horizon, so your estimate of 4-5 solar masses is as good as you can do. B. You can measure the period and the radius of your orbit and then use Newton's form of Kepler's 3rd Law. C. You cannot use Newton's form of the Third Law. You must use GR. D. Move to event horizon of the black hole. From this distance the mass can be calculated using GR.

12 how curvature of space (=strength of gravity) changes when masses shrink line marking solar radius curvature of space doesn t change out here if mass stays the same, but only size shrinks

13 why wouldn t earth get pulled into BH if sun turned into BH? GR effects important only if walls are steep curvature of space = strength of gravity event horizon

14 why wouldn t earth get pulled into BH if sun turned into BH? GR effects important only if walls are steep curvature of space = strength of gravity event horizon far from event horizon, curvature of space (=strength of gravity) remains the same

15 If you fall into a solar-mass black hole A. Time will appear to you to slow to a halt as you enter the event horizon. B. You will die, torn apart by tidal forces (spaghettification) C. You will die, incinerated by intensely hot radiation. D. You will die, crushed at the central singularity. E. You will pass through a wormhole into another part of the Universe. (You will die.)

16 Gravitational forces on person in strong gravitational field different r or r for different parts of body > stronger F g at feet than head: stretching F g at sides in different directions: squeezing

17 Gravitational forces Tidal forces

18 Tidal forces Gravitational force acting on extended body If we subtract the force at the center of mass, we get the differential gravitational force = tidal force To observer at center, the near & far sides are experiencing accelerations which differ from its own

19 examine gravitational force exerted by companion galaxy M (~point source) on 3 stars within extended galaxy m R M a N a C a F companion galaxy M gravitational acceleration on 3 stars: NEAR a N = GM/(R-r) 2 CENTER a C = GM/R 2 FAR a F = GM/(R+r) 2 r r extended galaxy m

20 view of differential acceleration (tidal acceleration) across extended galaxy R M Δa N Δa F companion galaxy M tidal acceleration on 2 stars: NEAR Δa N = a N - a C + a C (2r/R) = +2GMr/R 3 FAR Δa F = a F - a C - a C (2r/R) = - 2GMr/R 3 r r extended galaxy m toward companion away from companion

21 tidal forces: water tides on the 2 sides of the earth

22 Spaghettification by strong tidal forces near black hole!

23 big fractional difference in distance between head & BH center and feet & BH center tidal forces strong! tidal forces near small & large BHs stellar mass black hole small fractional difference in distance between head & BH center and feet & BH center tidal forces weak! supermassive black hole

24 Galactic center stars orbiting central Black Hole Infrared images of stars near galaxy center from

25 Gas cloud G2 approaching black hole in center of Milky Way G2: What is it? T=600 K Warm gas cloud Might have star inside G2 in 2012 Brγ emission from ionized gas Tail of gas stripped from main body by tidal forces

26 Gas cloud (like G2?) approaching Black Hole in center of Milky Way Closest approach: March 2014 simulation

27 Prof K. is investigating a supermassive black hole from a safe distance. He persuades Andrea to jump into the black hole. Will Prof K. ever see Andrea pass the event horizon? A. Yes, he will see her fall faster and faster until she disappears as she falls through the event horizon. B. Yes, but light from her will be so blueshifted that he would need X-ray eyes to see her. C. No, she will be tidally compressed to zero thickness and disappear from sight before she reaches the event horizon. D. No, she will appear to stop and hover forever just above the event horizon.

28 Gravitational time dilation Time passes more slowly in stronger gravitational fields Happens in ANY gravitational field! Effect becomes infinite at event horizon Related effect: gravitational redshift

29 When you fall thru the event horizon of a Black Hole A. The view of the outside world disappears, and you find yourself in darkness. B. The view of the outside world changes only slightly, and there is no clue that you have actually fallen through the horizon. C. You achieve enlightenment D. You go through a wormhole of doom

30 How do we see Black Holes?

31 How do we see Black Holes? 1. Stuff falls into them & heats up & emits lots of radiation BEFORE it falls past event horizon

32 region just outside some black holes jet comes from accretion disk not black hole accretion disk: rotating disk of gas surrounding (& perhaps spiraling into) black hole event horizon of black hole

33 The easiest black holes to see are those in binary star systems, where mass from evolving star is pulled onto BH accretion disk

34 Black hole nova disk but no surface Neutron star nova disk AND surface

35 Galaxy interactions cause central black holes to be fed, making active galactic nuclei (AGN)

36 How do we see Black Holes? 1. Stuff falls into them & heats up & emits lots of radiation BEFORE it falls past event horizon 2. Gravitational effects on nearby objects a. Stars or gas orbiting something massive and dark

37 Galactic center stars orbiting something massive & dark Infrared images of stars near galaxy center from Stellar motions provide evidence of central black hole M = RV 2 /G (Newton s laws)

38 How do we see Black Holes? 1. Stuff falls into them & heats up & emits lots of radiation BEFORE it falls past event horizon 2. Gravitational effects on nearby objects a. Stars or gas orbiting something massive and dark b. Gravitational lensing of background objects by foreground BH

39 How black holes distort light (gravitational lensing with multiple images) 1000 R s from BH 10 R s from BH

40 foreground BH distorting light of background galaxy

41 event horizon and accretion disk with & without gravitational lensing effects

42 event horizon and accretion disk with gravitational lensing effects upper side of disk from movie Interstellar lower side of disk

43 How do we see Black Holes? 3. detection of gravitational waves from merging black holes! (2016)

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45 Different sizes of Black Holes Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars

46 Different sizes of Black Holes Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars Galactic (Supermassive) M~ M sun exist! Form from mergers of stellar mass BHs

47 Different sizes of Black Holes Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars Galactic (Supermassive) M~ M sun exist! Form from mergers of stellar mass BHs Medium M~ M sun probably exist not yet seen

48 Different sizes of Black Holes Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars Galactic (Supermassive) M~ M sun exist! Form from mergers of stellar mass BHs Medium M~ M sun probably exist not yet seen Mini-holes M<<1 M sun??? Probably don t exist, no good way to form them

49 The Large Hadron Collider March 29, 2008 Asking a Judge to Save the World, and Maybe a Whole Lot More By DENNIS OVERBYE More fighting in Iraq. Somalia in chaos. People in this country can t afford their mortgages and in some places now they can t even afford rice. None of this nor the rest of the grimness on the front page today will matter a bit, though, if two men pursuing a lawsuit in federal court in Hawaii turn out to be right. They think a giant particle accelerator that will begin smashing protons together outside Geneva this summer might produce a black hole or something else that will spell the end of the Earth and maybe the universe.

50 Different sizes of Black Holes Mini-holes M<<1 M sun??? Probably don t exist, no good way to form them Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars Medium M~ M sun probably exist but not yet seen Galactic (Supermassive) M~ M sun exist! Form from mergers of stellar mass BHs plus accretion of gas & stars into central object

51 Different sizes of Black Holes Stellar M~3-50 M sun exist! Form from collapsed cores of massive stars how many exist? millions in every (large) galaxy how many have we detected? ~20 what is the nearest one? V616 Monocerotis, located about 3,000 light years away, ~11 times the mass of the Sun.

52 Different sizes of Black Holes Galactic (Supermassive) M~ M sun exist! Form from mergers of stellar mass BHs how many exist? 1 or 2 in every (large) galaxy how many have we detected? ~100 what is the nearest one? Sgr A*, in center of Milky Way Galaxy, ~25,000 light-years away, ~4x10 6 M sun

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54 White hole Purely Speculative!!

55 Wormhole? Hypothetical feature of spacetime A shortcut through spacetime? No evidence! Not a prediction of GR! (BHs are!) A valid solution of GR but may not exist in nature (many valid solutions of GR are not realized in nature..) Spacetime in BH probably too chaotic for safe passage

56 Good books on black holes Kip Thorne Black Holes & Time Warps Mitch Begelman & Martin Rees Gravity s Fatal Attraction

57 Great NOVEL!! Alan Lightman Einstein s Dreams

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59 What is actually located at the event horizon of a black hole? 1. Nothing 2. an infinitely dense concentration of mass 3. The outer boundary of a wormhole 4. a sphere of photons 5. The remnant of the star that collapsed to the black hole.

60 What lies on the other side of the event horizon of a black hole? 1. A parallel universe 2. A wormhole 3. A white hole 4. Empty space surrounding a singularity 5. The remnant of the star that collapsed to the black hole.

61 Which statement describing wormholes is correct? 1. A wormhole connects one black hole to a second black hole when the two form a binary pair. 2. A wormhole connects our universe with another universe. 3. Because wormholes, once established, are permanent, a wormhole could be used as a time travel machine 4. A wormhole, once established, collapses almost immediately.

62 The tidal force is the difference in the gravitational force between two parts of an object (e.g. between your head and your toes). The tidal force of a black hole will tear you apart 1. at the event horizon 2. somewhere inside the event horizon 3. at the singularity 4. inside the event horizon if the black hole has a stellar-size mass, or outside the horizon if the black hole is supermassive 5. outside the event horizon if the black hole has a stellar-size mass, or inside the horizon if the black hole is supermassive.

63 If a probe falls through the event horizon thinking that time goes by normally, then you, watching from outside, see the probe clock: 1. tick at the same rate as your clock 2. slow to a halt at the horizon 3. speed up to an enormous rate at the horizon

64 If nothing can escape from a Black Hole, how can its gravity escape? 1. Gravity is a curvature of space, and does not need to escape. 2. A person outside the BH experiences the gravity of the matter that long ago collapsed to, or fell into, the BH. 3. Gravity travels faster than light. 4. Gravity always exists everywhere but just gets amplified by black holes 5. Gravity can t escape

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66 Curved space around stars & BH Normal star Neutron star Event horizon

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69 Galactic center stars orbiting central Black Hole Infrared images of stars near galaxy center from