MadSci Network: Astronomy

Re: Radiation emition from black holes...

Date: Wed Jun 17 22:11:29 1998
Posted By: Bernard Gilroy, Secondary School Teacher, Science (Physics, Astrophysics), Holy Ghost Preparatory School
Area of science: Astronomy
ID: 898100957.As

Very insightful question.  The short answer is, it's not the black hole 
emitting radiation.  It's the _event horizon_!

    To grasp this, you have to understand a quirk of quantum mechanics; to 
wit, even the vacuum can produce pairs of particles spontaneously.  If the 
particles violate energy conservation, they're called "virtual particles" 
and they have to annihilate each other before a time delta_t = h / delta_E. 
 To do that, they have to be anti-particles.  This random creation and 
destruction is disturbing but it happens all the time ... including near 
black holes.  Let's zoom into near the horizon.

    If that happens, sometimes one of the particles pops into being headed 
towards the black hole and falls in.  In doing so, it gives up 
gravitational potential energy.  This allows the other particle to _gain_ 
the energy, giving it postive energy and become "real".  The first 
particle, of course, disappears from the Universe.  But the second, freed 
from the Uncertainty Principle restrictions, goes on forever.  Since it was 
created _outside_ the horizon, it's possible for it to escape, if its 
energy is big enough.  To an outside observer, the black hole seems to 
'radiate' this particle.

    Interesting sidebar: The particle has picked up energy, enough to make 
it real.  Where did that come from?  The gravity of the black hole, of 
course, and since the source of gravity is mass, the black hole loses mass. 
 It appears to evaporate as it radiates.

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