Why does a black hole grow when it consumes matter? Isn't this a contradiction?

Question

I've been thinking about this for a while but can't get my head around it.

A black hole's mass is such that its gravity can overcome light. Its event horizon, as I see it, is the boundary up to which light is "pulled back" into the black hole, from the black hole's point of view.

Surely then, if a black hole consumes more mass, therefore it's gravity would be greater and thus exert a greater force on the light, pulling it tighter, resulting in a smaller black hole...

Judging by the overwhelming amount of information out there suggesting a black hole increases in size, I've obviously got something wrong and would love to be enlightened..

Thx Mark.

Answer 1

Surely then, if a black hole consumes more mass, therefore it's gravity would be greater and thus exert a greater force on the light, pulling it tighter, resulting in a smaller black hole...

To use your logic, if the gravity is stronger, then things (and light) farther away that used to not be trapped can now be trapped by the stronger gravity, hence that boundary between where things are trapped and where things aren't trapped is now farther out, and that's the size of the black hole, so the size is larger.

There is a still a potential hole in your argument however. Which is that you just assumed the black hole could consume things. But if something happens it is reasonable to ask when and where it happened, which is a hard thing to answer for a black hole.

People on the outside never see a black hole form and they never see a black hole consume something. They see stuff that starts to look more and more like a black bole as time goes on. And they see new stuff join up and the new larger collection start to look more and more like a black hole.

This is basically because of extreme time dilation between the forming surface and the outside where we see the things near the forming surface going slowly.

And if it is an astrophysical black hole, there isn't even a surface we just see the original matter say the neutron star collapsing in slower and slower slow motion and getting redder and redder.

Answer 2

Surely then, if a black hole consumes more mass, therefore it's gravity would be greater and thus exert a greater force on the light, pulling it tighter, resulting in a smaller black hole...

Define size of a black hole:

The Schwarzschild radius :

The Schwarzschild radius (event horizon) just marks the radius of a sphere past which we can get no particles, no light, no information.

Note that the radius, which defines the volume from which there is no return, is proportional to the mass, so the larger the mass the larger the volume of the black hole trap.

Your misunderstanding comes from "pulling it tighter". The strength of the gravitational field of the larger in mass black hole extends further then when it was smaller in mass.

Answer 3

The size of the blackhole (i.e. its blackness) is not its actual size. It is the area where light simply cannot escape. The more mass the blackhole gets, the wider this area be due to increased gravity pull. It's actual size? We do not know but it is assumed as a singularity at its center.

Answer 4

The density of a black hole is concentrated at the singularity at its center, where density becomes infinite and the Theory of Relativity breaks down. Although the Theory of Relativity is insufficient to describe what happens at the singularity, it may be assumed that the density at that point can not increase. Therefore, accretion of matter does not increase the density at the singularity, but rather increases the Schwarzschild radius of the black hole, expanding the event horizon. Gravity increases, but so does the size of the black hole.

Answer 5

An important thing to realise is that the event horizon is not a boundary that light rays can rise up to before being drawn down again. Light rays inside a black hole never travel upwards towards the event horizon. When they enter the blackhole light rays travel inevitably to the central singularity. If an infalling observer falls through the event horizon and they have a lamp that shines in 'all directions' no light rays from that lamp will ever reach a higher altitude than the altitude they were emitted from. There is no residual ball of trapped light rays inside the event horizon that can be drawn into a tighter sphere.