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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) 17 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 18 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 24 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! 29 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?