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Lecture 27: Black Holes Stellar Corpses: white dwarfs collapsed cores of low-mass stars supported by electron degeneracy white dwarf limit 1.4 Msun neutron stars collapsed cores of high-mass stars supported by neutron degeneracy neutron star limit about 3 Msun black holes collapse to a singularity General Relativity the equivalence principle: gravity and acceleration are equivalent – i.e., one cannot discriminate between being at rest in a gravitational field and being accelerated in the absence of gravity gravity = acceleration General Relativity mass causes space-time to curve: Imagine space-time as a fourdimensional rubber sheet. Any object with mass causes this sheet to become deformed. General Relativity the curvature of space-time tells matter how to move: What we perceive as gravity arises from the curvature of space-time. Masses follow the ‘straightest possible paths’ possible given the curvature. Strange consequences of the Equivalence Principle: gravitational time dilation: time runs slower near a massive object flashes take a longer time to reach flashes take a shorter time to reach Strange consequences of the Equivalence Principle: gravitational time dilation: time runs slower near a massive object gravitational redshifting: light escaping from a massive object is shifted towards lower frequencies/longer wavelengths Strange but true: observations confirming the predictions of general relativity gravitational lensing (bending of light by gravity) confirmed during a solar eclipse in 1919 Strange but true: observations confirming the predictions of general relativity gravitational lensing (bending of light by gravity) confirmed during a solar eclipse in 1919 precession of the perihelion of Mercury: general relativity predicts a correction to Newton’s Law, which fits the observations 574 arcsec per century Newtonian theory predicted 531 arcsec per century Strange but true: observations confirming the predictions of general relativity gravitational lensing (bending of light by gravity) confirmed during a solar eclipse in 1919 precession of the perihelion of Mercury: general relativity predicts a correction to Newton’s Law, which fits the observations gravitational redshifting: spectral lines from white dwarfs are shifted; direct confirmation in 1960 Black Holes general relativity predicts that there can be singularities in space-time, places where the density of matter becomes infinite ‘black holes’ are the name for one kind of ‘singular solution’ in the equations. Formation of a Black Hole The paths of photons in curved space-time Escape velocity from a black hole remember (from Chapter 5) the escape velocity is given by vesc = [2GM/R]1/2 what if the escape velocity was equal to the speed of light? this would set a maximum radius for which light could escape from an object with a given mass The Schwarzschild radius v2esc = [2GM/R] = c2 RS = 2GM/c2 or RS = [3.0 x M/Msun] km The Schwarzschild radius the larger the mass of a black hole, the larger the Schwarzschild radius once light or any object has crossed the Schwarzschild radius (or event horizon), it can never escape the force of gravity of the black hole. Black holes have no hair all information about the material that is inside the event horizon of a black hole is lost, except mass charge angular momentum Black hole Entropy Theorem The total amount of information (entropy) in the Universe cannot decrease (second law of thermodynamics) this is what lead Bekenstein and Hawking to the idea that Black holes must radiate Falling into a black hole stretched by tidal forces time slows down radiation is redshifted Observational Evidence there is evidence that black holes formed from collapsed stars exist in some X-ray binaries most promising candidate: Cygnus X-1: 18 Msun star orbiting an unseen companion with a mass of 10 Msun too massive to be a neutron star and too small to be an ordinary star Cygnus X-1 Supermassive Black Holes there is very good evidence from the motions of stars and gas near the centers of galaxies that most galaxies (including our own) contain ‘supermassive black holes’ – black holes weighing millions to billions of solar masses how these objects formed is still something of a mystery… M87 White holes, Wormholes, and tunnels through hyperspace black holes are only one of the several kinds of singularities in the equations of general relativity white holes are sort of like the opposite of black holes a wormhole is a black hole connecting to a white hole Einstein-Rosen bridge Wormhole the end