Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: benm on 29/03/2019 09:55:31

Title: How is it possible that we have black holes of different sizes?
Post by: benm on 29/03/2019 09:55:31
Mwaba has a cosmic conundrum about black holes:

If I have understood correctly how is it possible that if at the center of a black hole there is a singularity (a point of mass with infinite density and the laws of physics break down) then how can we have black holes of differing "sizes"?

Can anybody shed any light?
Title: Re: How is it possible that we have black holes of different sizes?
Post by: Halc on 29/03/2019 11:45:55
The size of a black hole is a measure of the region beyond which nothing can escape.  This is the Schwarzschild radius, the point at which no amount of acceleration can keep you out.  The more massive the black hole, the larger this radius.

Current thinking trend is that no material actually passes through this event horizon due to the temporal singularity there, so there in fact is no dense center of a black hole.  All the mass is stuck to the surface.  I do not believe this is more than a trend, since there are very much those that say the material does indeed fall through.
Title: Re: How is it possible that we have black holes of different sizes?
Post by: evan_au on 29/03/2019 11:52:10
Black holes of different masses can form from the collapse of stars with different masses (say, 15x and 40x the mass of the Sun).
- These stellar-mass black holes can then merge to form larger black holes
- In the center of galaxies there is a black hole that may be millions or billions of times the mass of the Sun (but it is a bit of a mystery about how they got that big, early in the life of the universe).

The mathematical singularity predicted at the center of a black hole has zero size, even though it has a mass many times that of the Sun.
- But outside observers can't see the singularity, because it is surrounded by an Event Horizon
- Nothing that passes within the event horizon can escape, not even light (this is what makes them "black" holes)

If you had two black holes, say 10x and 30x the mass of the Sun:
- Both the singularities have the same size (zero)
- But the more massive one has an event horizon with a larger radius
- In comparison, the less massive one has a lower escape velocity (at a given distance), and light can escape from  points which are closer to the singularity.
 
See: https://en.wikipedia.org/wiki/Event_horizon

..crossover with Halc...