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B LACK H OLE E VAPORATION by Robert Nemiroff Michigan Tech

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Physics X: About This Course Officially "Extraordinary Concepts in Physics" Being taught for credit at Michigan Tech o Light on math, heavy on concepts o Anyone anywhere is welcome No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at: "Starship Asterisk" then "Physics X" o

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B LACK H OLES AND E NTROPY You drop a hot ball into one black hole, and a cold ball into an identical black hole. The balls were otherwise identical. Are the black holes still identical? 1. Yes, black holes carry only mass, change, and spin. 2. No, now they are different. 3. Hey, you said you were only borrowing those balls!

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B LACK H OLE E VAPORATION B LACK H OLE E VAPORATION : B ACKGROUND Bekenstein & Hawking: Black holes are not black. BHs emit thermal radiation as if they have a temperature. Why? o Theory still somewhat controversial o Now less controversial than assuming truly black Virtual particles created everywhere Including just outside event horizon One of pair falls in, the other goes to infinity Falls in: dark energy; Bumped out: "normal" energy

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B LACK H OLE B LACK H OLE D ENSITY The higher the mass of the black hole (BH), the less the average density inside. Radius of BH: rs = (3 km ) (M / Msun) ~ M Called the "Schwarzschild radius" or the radius of the event horizon. Volume of BH: V ~ (4/3) π rs3 ~ M3 Average density of BH: Mass / Volume ~ M / M3 ~ 1/M2

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B LACK H OLE B LACK H OLE D ENSITY Average density of BH ~ 1014 (Msun / M)2 gm/cm3 Average density of 1 Msun BH: nuclear matter Average density of 107 Msun BH: water Average density of Universe mass BH: universe o coincidence?

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B LACK H OLE E VAPORATION B LACK H OLE E VAPORATION : P OWER Power emitted proportional to area of event horizon. Therefore inversely proportional to mass squared. P ~ (Msun / M)2 Watts A BH of mass Msun glows like a 100 Watt light bulb. That's the mass of an asteroid Accretion power of (present) microwave background radiation exceeds evaporation power for BHs with M > 10-8 Msun

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B LACK H OLE E VAPORATION B LACK H OLE E VAPORATION : T IME SCALE Evaporation time scale (in vacuum): tevap = 2 x 1067 (M / Msun)3 years

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B LACK H OLE E VAPORATION B LACK H OLE E VAPORATION : T IME SCALE Sun-sized black hole would take many times the current age of the universe to evaporate. A bus-sized BH would take about 1 second to evaporate at about 1022 Watts. Searches with GRB instruments so far have found nothing definitive.

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B LACK H OLE : I NFORMATION P ARADOXI NFORMATION P ARADOX Does the radiation that comes out of a black hole carry the information of the matter that fell in? 1. Yes, information is conserved. 2. No, black holes carry only mass, charge, and spin. 3. You woke me up to ask me what?

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B LACK H OLE : I NFORMATION P ARADOXI NFORMATION P ARADOX Controversial! Not experimentally verifiable. The information dropped into a black hole might be lost. radiated away gradually during evaporation. radiated at the end as the BH explodes. trapped in a stable Planck-mass remnant. trapped in a massive remnant. trapped in a new baby universe. otherwise determined by an as yet unknown quantum theory of gravity.

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