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Black Holes: Knowns and Unknowns Edmund Bertschinger, MIT March 7, 2016 Summary Black holes exist in nature, consistent with Einstein’s General Theory of RelaAvity. Time and space are severely distorted in a black hole. Black holes pinch off spaceAme; the singularity cannot be seen from outside. MaJer falling toward a black hole swirls around it in an accreAon disk, which can be very bright in opAcal and X-­‐ray light. X-­‐rays and gravitaAonal waves are good ways to study black holes. Black holes might be connected by wormholes, passages between widely separated regions or even different universes. General relaAvity breaks down at the singularity, and we have no good theory for what happens there. The Problem of Time at Sea What Ame is it? The Problem of Time in Space What Ame is it for ScoJ Kelly? In orbit around the Earth… Time warp! GPS satellite: Time speeds up by 38 microseconds/day compared to Earth’s surface. During 340 days, this becomes 0.013 seconds What if the Earth were 1,000,000 Ames more massive? How much would you weigh? How much slower would you age, compared with a distant astronaut? What is the escape speed from Earth’s gravity? What if the Earth were 1,000,000 Ames more massive? How much would you weigh? 80 million kilograms How much slower would you age, compared with a distant astronaut? 38 seconds per day What is the escape speed from Earth’s gravity? 11,000 kilometers/second = 0.037 c 1,000,000 Earths = 3 Suns Without nuclear fuel, such a heavy star could not support itself against gravity… Escape speed vescape=√⁠​2𝐺𝑀/𝑟 M = 3 Msun , r = 9 km è vescape = c What happens when a star collapses so much that not even light can escape? •  …if the semi-diameter of a sphere of the
same density as the Sun were to exceed
that of the Sun in the proportion of 500 to
1 … all light emitted from such a body
would be made to return to it…
John Michell, 1784 “…if the semi-­‐diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the propor8on of 500 to 1 … all light emi>ed from such a body would be made to return to it…” Oppenheimer and Snyder, 1939 “The star thus tends to close itself off from any communica8on with a distant observer; only its gravita8onal field persists.” Einstein and others never believed this could happen! “Various accidents may intervene to save the star, but I want more protec8on than that. I think there should be a law of Nature to prevent a star from behaving in this absurd way!” Sir Arthur Eddington, 1935 What is a black hole?
A massive spacetime curvature singularity,
(a point or ring of infinite density and tidal acceleration)
Surrounded by an event horizon
(a spacetime boundary between causally disconnected regions of
the universe)
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But what is a black
hole REALLY?
i was considering how
within night's loose
sack a star's
nibbling infin
-itesi
-mally devours
darkness the
hungry star
which
will e
.
-ven
tual
-ly jiggle
the bait of
dawn and be jerked
into
eternity. when over my head a
shooting
star
Burs
(t
into a stale shriek
like an alarm-clock)
-- e.e. cummings
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A Brief History of Black Holes
1784: John Michell (independently, Pierre-­‐Simon Laplace in 1796) propose the existence of “dark stars” 1915: Albert Einstein’s theory of General RelaAvity 1916: Karl Schwarzschild finds black holes as a soluAon to Einstein’s equaAons 1931: Subrahmanyan Chandrasekhar finds the maximum mass for a stable white dwarf star; cold heavier bodies cannot support themselves against gravity 1939: J. Robert Oppenheimer and Hartland Snyder predict that massive stars can collapse into black holes 19
A Brief History of Black Holes, contd.
1963: Roy Kerr solves Einstein’s equaAons for rotaAng black holes 1967: John Archibald Wheeler popularizes “black holes” 1972: Cygnus X-­‐1 first strong candidate for a black hole 1974: Stephen Hawking shows that quantum effects lead black holes to weakly emit radiaAon 1980s-­‐2000s: astronomical evidence grows for black holes 2015: Binary black hole observed to merge by LIGO 20
Black holes in Nature
The afterlife of giant stars
Black hole masses 3-35* solar masses
Giant sinkholes in galaxy centers
Black hole masses million to billion solar
masses
* Prior to LIGO, no stellar black holes were thought to exceed 15 Msun
21
How we found black holes (and neutron
stars) before 2015: X-rays!
Sky map
made by
Rossi
X-Ray
Timing
Explorer
satellite,
launched
1995
22
Before LIGO, how did astronomers
“see” black holes? With X-Ray vision!
23
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Galaxy PGC 04323’s supermassive black hole
swallowed a star. Reported October 21, 2015
25
Image credit: Chandra X-ray Center
September 14, 2015: LIGO measured a black hole
merger consistent with Einstein’s General Relativity!
26
Now for fun: try to fall into a black hole
27
Opening Pandora’s Box: Traveling Into a Spinning Black Hole 28
What happens to objects close to a black hole?
Calculate orbits with GROrbits!
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Can black holes give shortcuts through space? Shortcuts through space: Wormholes Daily Mail, October 23, 2011 Can black holes be used as Ame machines? Conventional Wisdom: Yes/No/
Maybe
Ring singularity: cross it, go backward in time
Spacetime is unstable at the inner horizon
The Kerr solution cannot describe a real black hole
inside the event horizon
Einstein’s theory may be correct,
but we can’t calculate it!
No exact solutions known for gravitational collapse
of spinning matter
Numerical methods lack the resolution to follow
collapse accurately into the inner horizon
Maybe the Kerr solution is closer to reality than
conventional wisdom…
Charged or spinnning
black holes:
Multiple sheets in (r,t)
Could allow travel to
other universes, if not
backwards in time Image from Wikipedia Summary Black holes exist in nature, consistent with Einstein’s General Theory of RelaAvity. Time and space are severely distorted in a black hole. Black holes pinch off spaceAme; the singularity cannot be seen from outside. MaJer falling toward a black hole swirls around it in an accreAon disk, which can be very bright in opAcal and X-­‐ray light. X-­‐rays and gravitaAonal waves are good ways to study black holes. Black holes might be connected by wormholes, passages between widely separated regions or even different universes. General relaAvity breaks down at the singularity, and we have no good theory for what happens there. GR can answer some, but not all, of these quesAons! •  Do black holes suck in everything around them? •  Are we in danger of falling into a black hole? •  What happens if you cross the event horizon? •  Can black holes give shortcuts through space? •  Can black holes be used as Ame machines? What are your quesAons?