Black HolesGuiding QuestionsThe special theory of relativity and our conceptions of space and timeThe speed of light is the same to all observers, no matter how fast they are movingAn observer will note a slowing of clocks and a shortening of rulers that are moving with respect to the observerThe general theory of relativity as a description of gravitationThe general theory of relativity predicts black holesCertain binary star systems probably contain black holesSupermassive black holes exist at the centers of most galaxiesA non-rotating black hole has only a “center” and a “surface”Escape speed and the event horizon (Schwarzschild radius)Falling into a black hole is an infinite voyage as gravitational tidal forces pull spacetime in such a way that time becomes iJourneys to Black holesBlack holes evaporate JargonBlack HolesGuiding Questions1. What are the two central ideas behind Einstein’s special theory of relativity?2. How do astronomers search for black holes?3. What are super massive black holes, and where are they found?4. In what sense is a black hole “black”?5. In what way are black holes actually simpler than any other objects in astronomy?6. What happens to an object that falls into a black hole?7. Why do some pulsars emit fantastic amounts of X rays?8. Do black holes last forever?Recall Life Cycle of StarsThe special theory of relativity and ourconceptions of space and time• This theory, published by Einstein in 1905, is based on the notion that there is no such thing as absolute space or time• Space and time are not wholly independent of each other, but are aspects of a single entity called spacetimeThe speed of light is the same to all observers, no matter how fast they are movingAn observer will note a slowing of clocks and a shortening of rulers that are moving with respect to the observerThis effect becomes significant only if the clock or ruler is moving at a substantial fraction of the speed of lightThe general theory of relativity as a description of gravitation• Published by Einstein in 1915, this is a theory of gravity• A massive object causes space to curve and time to slow down• These effects manifest themselves as a gravitational force• These distortions of space and time are most noticeable in the vicinity of large masses or compact objectsThe theory of relativity predicts a number of phenomena, including the bending of light by gravity and the gravitational redshift, whose existence has been confirmed by observation and experiment• The general theory of relativity also predicts the existence of gravitational waves, which are ripples in the overall geometry of space and time produced by moving masses• Gravitational waves have been detected indirectly, and specialized antennas are under construction to make direct measurement of the gravitational waves from cosmic cataclysmsThe general theory of relativity predicts black holes• If a stellar corpse has a mass greater than about 3 M, gravitational compression will overwhelm any and all forms of internal pressure• The stellar corpse will collapse to such a high density that its escape speed exceeds the speed of light• Could a black hole somehow be connected to another part of spacetime, or even some other universe?• General relativity predicts that such connections, called wormholes, can exist for rotating black holesCertain binary star systems probably containblack holes• Black holes cannot been seen because they do not emit nor reflect light• Black holes that are in binary systems may be detected using indirect methods• In such a system, gases captured from the companion star by the black hole emit detectable X-rays• As material races toward a black hole, it heats and emits X-raysSupermassive black holes exist at the centersof most galaxiesThese are detected by observing the motions of material around the black holeA non-rotating black hole has only a “center” and a “surface”• A black hole is surrounded by an event horizon which is the spherical region at which the escape velocity is exactly the speed of light• The distance between the black hole and its event horizon is the Schwarzschild radius(RSch= 2GM/c2)• The center of the black hole is a point of infinite density and zero volume, called a singularityEscape speed and the event horizon(Schwarzschild radius)1. Gravitational escape speed for any object with mass M, radius R (G is grav. constant)2escGMVR=2. Suppose escape speed Vesc= c (speed of light)?3. Set Vesc= c , solve for R:223sunGM MRkmcM⎡⎤==⋅⎢⎥⎣⎦This is known as the Schwarzschild radius, or the event horizonStructure of a Kerr (Rotating) Black HoleIn the Ergoregion, nothing can remain at rest as spacetime here is being pulled around the black hole.• A black hole has only three physical properties: mass, electric charge, and angular momentum • A rotating black hole (one with angular momentum) has an ergoregion around the outside of the event horizon• In the ergoregion, space and time themselves are dragged along with the rotation of the black holeFalling into a black hole is an infinite voyage as gravitational tidal forces pull spacetime in such a way that time becomes infinitely long (as viewed by distant observer). The falling observer sees ordinary free fall in a finite time.Note: A black hole of Mass M has EXACTLY the same gravitational force outside the event horizon as an ordinary object of mass MFalling into a black hole – what would it be like?Journeys to Black holes1. A freely falling observer would pass right through the event horizon in a finite time, would be not ‘feel’ the event horizon’2. A distant observer watching the freely falling observer would never see her fall through the event horizon (takes an infinite time)3. Signals sent from the freely falling observer a regular intervals (‘clock ticks’) would be1. Dilated (take a longer time interval as measured by distant receiver2. Signals are redshifted4. Once inside the event horizon, no communication with ‘external’ world ever possible 1. But ‘incoming’ signals from external world can enter5. The tidal force near the event horizon 6. A black hole of mas M has exactly the same gravitational field as an ordinary mass M at large distancesBlack holes evaporateJargon• black hole• black hole evaporation• equivalence principle• ergoregion• event horizon• general theory of relativity• gravitational radiation•