The History and Philosophyof Astronomy (Lecture 19: Einstein II)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)Albert Einstein: Revolutionary of Physics • 1879 (Ulm) – 1955 (Princeton)• revolutionized concepts ofspace, time, and gravity- Special Relativity (1905): E=mc2- General Relativity (1915): new theory of gravity• co-founder of quantum theory photonsOverview: Einstein’s Life Nov. 2Nov. 7Einstein in Berlin: 1914-32• 1914: Max Planck secures Einstein’s appointmentas professor at the Prussian Academy of Sciences(founded 1700 by Leibniz, German equivalent of Royal Society)Berlin 1914: World Center of Science• Einstein becomes part of vibrant scientific community!Berlin 1914: Outbreak of WW I• All European nations excited to go to war!• Einstein takes lone pacifist stand!General Theory of Relativity (1915-16) • Big Q: What is gravity???Newton (1687): -- `action-at-a-distance’-- doesn’t really answer the question Einstein (1915): -- principle of equivalence’-- great idea: GRAVITY = ACCELERATION-- his “happiest thought”General Theory of Relativity (1915-16) • Principle of Equivalence (gravity = acceleration)General Theory of Relativity (1915-16) • Principle of Equivalence (gravity = acceleration)(1g upwards)General Theory of Relativity (1915-16) • Big Q: What is gravity???• principle of equivalence: GRAVITY = ACCELERATION• you don’t feel effect of gravity in freely falling spaceship(`weightlessness’)• it is possible to “transform away” gravity (locally)!• But: you can’t get rid of gravity completely! tidal forces betray gravity! globally, gravity is non-uniform!General Theory of Relativity (1915-16) • gravity betrayed:• gravity: neighboring particles in free-fall slowlyconverge (experience: `tidal forces’)General Theory of Relativity (1915-16) • tidal effect of gravity  similar to behavior of`straight’ lines on curved surfaces• Einstein’s great idea: gravity is related to curvature ofspace! (surface = 2 dimensional space)(ants are told to crawl straight ahead!)General Theory of Relativity (1915-16) • Principle 1: “Matter tells space how to curve”• matter creates `dimples’ in otherwise flat spacetime!General Theory of 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)Einstein vsNewton • compare views on planetary motion:Einstein vsNewton • Old view (Newton): Gravity acts `at-a-distance’(i.e., speed of gravitational disturbance = infinity) • New view (Einstein): Gravity acts via gravitational waves (`ripples in spacetime’)(i.e., speed of gravitational disturbance = speed of light) • try to detect them with LIGO and LISA!LIGO Experiment (Laser Interferometer Gravitational-wave Observatory)• tries to detect gravitational wavesGeneral Theory of Relativity (1915-16)- Predict `gravitational redshift’(light escaping from massive body becomes redder) - Predict gravitational bending (deflection) of light!(most famous of the 3 tests; `experimentum crucis’) - Successfully explain anomaly in Mercury’s orbit(`perihelion precession’) • Einstein proposes 3 classical tests of GR:General Theory of Relativity (1915-16)• Anomalous orbit of Mercury:perihelion• Newton’s theory could not explained the observedshift of Mercury’s perihelion • Einstein’s theory (GR) could!!!General Theory of Relativity (1915-16)• Gravitational redshift:• Photon climbing out of gravitational well loses energy less energy = lower frequence = redder color!•was successfully measured only in 1960 (Pound & Rebka)General Theory of Relativity (1915-16)• Einstein Tower: built to measure gravitational redshift• built by Erich Mendelsohn (1921; expressionism)General Theory of Relativity (1915-16)• Bending of light-rays:• Prediction: Sun’s gravity deflects light from distant stars!General Theory of Relativity (1915-16)• Observe bending of light during Solar eclipse!• Einstein’s prediction: Stars shifted by 1.75”Eddington’s Eclipse Expedition (1919)• Eddington’s observation confirms Einstein’s theory!After 1919: Einstein the Legend • Einstein becomes world-famous!• travels the globe• hobnobs with high-society• begins to exert significantinfluence in politics(New York Times)1919: Divorce and 2ndMarriage • marriage with his cousin Elsa Einstein (1919-36)Einstein visits America • Einstein at City Hall, New York (1921)Einstein visits America • Chaplin to Einstein: “People love me, because theyunderstand everything I say, and they love you, because they understand nothing of what you say…”Einstein’s Favorite Recreational Spots • his beloved summer house in Caputh (near Berlin)• many famous and powerful visitorsEinstein’s Favorite Recreational Spots • Island of Hiddensee (Germany’s `Martha’s Vineyard’)Solving Einstein’s Equations of GR• 1916: Karl Schwarzschild predicts black holes Karl Schwarzschild (1873-1916)`Schwarzschild radius’Solving Einstein’s Equations of GR• 1917: Einstein constructs model of the universethat is eternal and static • begin by postulating `cosmological principle’:“Universe looks everywhere the same!”(homogeneity and isotropy)• today, we knowthis is an excellentapproximation, butin 1917, it was merelyan inspired guess!Solving Einstein’s Equations of GR• 1917: Einstein constructs model of the universethat is eternal and static • finite but without boundarySolving Einstein’s Equations of GR• Big problem: model is not stable!• equations predict collapse  that seems