BLACK HOLE EVAPORATION

byRobert NemiroffMichigan Tech

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 requiredo Wikipedia, web links, and lectures onlyo Find all the lectures with Google at:

"Starship Asterisk" then "Physics X"  o http://bb.nightskylive.net/asterisk/viewforum.php?f=39

BLACK HOLES AND ENTROPY

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!

BLACK HOLE EVAPORATION: BACKGROUND

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

BLACK HOLE DENSITY

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

BLACK HOLE DENSITY

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?

BLACK HOLE EVAPORATION: POWER

Power emitted proportional to area of event horizon.Therefore inversely proportional to mass squared.

P ~ 10-28 (Msun / M)2 Watts

A BH of mass 10-15 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

BLACK HOLE EVAPORATION:TIME SCALE

Evaporation time scale (in vacuum):

tevap = 2 x 1067 (M / Msun)3 years

BLACK HOLE EVAPORATION:TIME 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.

BLACK HOLE: INFORMATION PARADOX

 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?

BLACK HOLE: INFORMATION PARADOX

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.