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Q & A: dispersing black holes?

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Most recent answer: 10/22/2007
Q:
Hi. I understand that a black hole forms because there is not enough energy to prevent it collapsing due to gravity. I’m not a physicist, so I apologise if these questions are a little naive: 1) Would it be possible to disperse the black hole by introducing enough energy to overcome the gravitational force? 2) According to relativity, mass and energy are sort of the same thing, aren’t they? If this is the case, what is the distinction between the two ideas - is energy in the form of mass ’dormant’ in some way? What is the relevance to the mechanics of black holes? Thanks for your time.
- Olly
Uni of Nottinham, UK
A:
I'm no expert, but i think these answers are ok.

1. Once the black hole has formed, anything you throw in to try to disperse it only makes it more massive. if it were not for quantum fluctuations, that would be a one-way street. Ultimately, black holes should evaporate via quantum effects on the event horizon, but the more stuff you throw in, the longer that takes.

2. Yes, in the notation that Einstein used, mass and energy are exactly the same thing, just measured in different units. However, the rest mass part of that is somehwat special in that it exists regardless of the momentum of the object, i.e. independent of whether the object is moving in your favorite reference frame. In modern terminology, sometimes that part is simply referred to as the 'mass'. In some processes (e.g. radioactive decay) particles change to types with different net rest mass, so there must be some other sort of energy released, i.e. some type associated with the momentum of the particles.
The important question is 'which quantity is conserved," i.e. which one doesn't change in time. That's the total mass-energy, counting all the terms, including a gravitational potential term. Usually we describe black holes in frames in which their velocity is small compared to the speed of light, and so are the initial velocities of the things that fall in, so you can get away with treating the black hole mass as the sum of the masses of the things that fell in.

Mike W.


The stuff that falls into black holes usually picks up a lot of speed as it drops in. Tidal forces tear the stuff apart, and it usually swirls around a bit before in an accretion disk before falling in. In the process, electrons can be stripped from atoms and accelerated along powerful magnetic fields. Black holes are often X-ray sources for this reason. The point here is that the gravitational potential energy of stuff before it fell in gets added to the mass of the black hole as well. It's probably a small fraction of the energy of the stuff falling in given its rest mass.

Tom

(published on 10/22/2007)

Follow-Up #1: Constituents of black holes

Q:
are black holes a kind of anti matter
- charlie
A:
Probably not, although in principle there could be black holes made from anti-matter.   The current theory is that they started out as monstrously big stars made of ordinary matter,  hundreds or thousands of times heavier than our sun,  that simply collapsed  by their own weight into a black hole.  Although there a few anti-protons in cosmic rays, a few parts in 10,000, the abundance is consistent with p p-bar production from high energy cosmic ray interactions.  There is not much anti-matter around at all, let alone enough to form a star or black hole.

LeeH

(published on 10/22/2007)

Follow-up on this answer.