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UT AST 350L - Lecture 20: Black Holes

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The History and Philosophyof Astronomy (Lecture 20: Black Holes)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)“The Black Hole of Calcutta”• 1756: British prisoners perished in tiny dungeonBlack Holes: First ideas • 18thcent. physics:  Newton’s theory of gravity• for object to escapefrom gravitational pullof planet or star, need:velocity > escape velocityBlack Holes: First ideas • 18thcent. physics:  Newton’s theory of light• light rays behave asstream of particles(“light corpuscles”)Black Holes: First ideas • John Michell (1783): combines corpuscular theory oflight with Newtonian gravity • object massive enoughso that speed of light< escape velocity: “Dark Star”- invisible to far away observerEnlightenment Astronomy • Pierre-Simon de Laplace(1749-1827)• Exposition de System du Monde(1796)• Dark Stars (i.e., black holes)- removed from the 3rdedition (1808)- What happened in between?The Downfall of the Dark Star Idea Thomas Young (1773-1829)• interference of light  wave-nature of light• light-waves not subject to gravity  light cannot be trapped  “Dark stars” idea forgotten!Einstein: General Relativity (1915-16) • Principle 1: “Matter tells space how to curve”• matter creates `dimples’ in otherwise flat spacetime!Einstein: General Relativity (1915-16) • Principle 2: “Curved space tells matter how to move”• particles move through spacetime along paths ofleast resistance (technically: `geodesics’)!General Theory of Relativity (1915-16) • Einstein’s Field equations: • a `tensor equation’  very complicated(10 coupled non-linear differential equations)(curvature of space)(matter content)Karl Schwarzschild: First Solution of GR Equations • 1873 (Frankfurt) – 1916 (Potsdam)• pre-eminent astrophysicistat beginning of 20thcentury• found first solution to Einstein’sGR (1916):  Gravitational field aroundspherical mass Predicts black holesSchwarzschild Geometry (1916)• consider (curved) spacetime around a star: `Schwarzschild radius’“Schwarzschild metric”• Q: What happens when r -> rS?Schwarzschild Geometry (1916)• curvature increases when star becomes more compact • dense star “shuts itself off” from rest of the universe!MorecompactBlack Holes Rediscovered: • Nothing (not even light) can escape from black hole • at the time termed:`Schwarzschild Singularities’Einstein Rejects Black Holes (1939) • Einstein argues: Black holes could never form in reality!- Cluster of particles in equilibrium:centrifugal force = gravityradiusspeedSpeed of lightRSJ. Robert Oppenheimer: American Prometheus • 1904 – 1967 • “Father of the Atomic Bomb”(leader of Manhattan project)• Founder of American schoolof theoretical physics (Berkeleyand Caltech)• argued that massive stars haveto implode  black holes exist!Oppenheimer: Manhattan Project Hiroshima, Aug. 6, 1945`Little Boy’ (Uranium bomb)• Oppenheimer feelsintense anguish (“the physicistshave known sin”)Gravitational Collapse of Massive Star • Oppenheimer and Snyder (1939):  brute force calculation black hole formation is inevitable!Gravitational Collapse of Massive Star • surprising result:for far-away observer,star’s surface `freezes’at event horizon(i.e., never crosses over)• however: for observerriding on top of collapsing surface, thisonly takes a finite time!• Q: How can both perspectives be right?John Archibald Wheeler: Popularizing Black Holes • b. 1911 • “Father of the Hydrogen Bomb”• Coined term “Black Hole”• gifted teacher, keen sensefor aphorisms(“Time is what prevents everythingfrom happening at once”)How to find Black Holes? • Strategy: Look for X-ray emission from gas fallingonto black hole!How to find Black Holes? • Cygnus X-1: a double star with an unseen massivecompanion (> 3 Suns) to a normal starUhuru: first X-ray satellite,Launched 1970Alternative explanation? • Q: could unseen object in Cyg X-1 be somethingelse?• A: No! Objects more massive than 3 Suns must be black holes!Discovery of Radio Galaxies and Quasars ( (1950s and 60s) • Promising idea: Radio galaxies and quasars are powered bygas falling onto supermassive black holes (million to billion Suns)Cygnus A: Radio imagePossible explanation:~ million lightyearThe Galactic Center: A SupermassiveBlack Hole • Radio Map (VLA)Sgr A*The Galactic Center: A SupermassiveBlack Hole • Estimated mass: ~ 3 million solar masses!Stephen Hawking: New look at black holes • b. 1942 •Lucasian Professor in Cambridge• predicts: black holes arenot really black they can evaporate Hawking radiationQuantum Mechanics of Black Holes • Hawking radiation Black Holes slowly evaporate!Black Holes• Early speculations:- 1780s and 90s: “dark stars” (Michell and Laplace)- relies on particle nature of light (`light corpuscles’)- abandoned in 1800s when wave-nature of light was found• General Relativity predicts them again:- Karl Schwarzschild (1916)- strong initial opposition- Oppenheimer and Snyder (1939) do first relativistic collapse calculation • Empirical proof:- 1970s: X-ray binaries (Cygnus X-1)- Supermassive black holes in center of quasars- best current evidence: center of our Milky


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