Why do black holes grow?

Question

I'll use the term 'understand' here. By that, I mean that I understand in laymen's terms. From my questions, it should be obvious that I don't understand mathematically.

I understand how black holes form, and I understand how matter is accreted. I even understand how they will eventually evaporate. But, there's a lot that I don't grasp.

At the centre of a black hole is a singularity, and by definition that cannot grow any bigger (or smaller). It is infinitely dense and therefore can't gain any density. It doesn't have a definable position in space-time, so how can something 'hit' it? How does a singularity get bigger in some way -- how does it gain mass?

Then there's the idea that information cannot escape a black hole, but, surely, if adding matter increases its mass and that has an effect on the space-time surrounding the black hole, then some information is indeed escaping -- information about how much it has eaten.

And, how can something outside of space-time affect space-time?

Or, do I need to wait for quantum-gravity to be solved to get any answers?

Answer 1

It's best to distinguish black holes and the singularity at the center of a black hole. From the misnamed Quanta magazine article Black Hole Singularities Are as Inescapable as Expected,

A singularity is not a place where quantities really become infinite, but “a place where general relativity breaks down,” Chesler explained.

I wrote "misnamed" because the person of interest in that article, Paul Chesler, specifically told Quanta magazine that singularities are places where general relativity breaks down. The view that the predicted singularity at the center of a black hole indicates a place where general relativity breaks down is widespread amongst theoretical physicists. Nobody yet knows what will supersede general relativity to describe what happens will inside a black hole.

On the other hand, the view that black holes do exist is almost universal amongst physicists and astronomers. (The only holdouts are generally viewed as crackpots.) Black holes are about the only thing that can explain objects such as Cygnus X-1, the supermassive object at the center of our galaxy, (or for that matter, the supermassive objects that appear to reside at the center of almost every galaxy), quasars, the collisions detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), and many other observations that black holes are real. The singularity at the center of a black hole might or might not be real.

Why do black holes grow?

Except for Hawking radiation (which has not yet been observed, except in analogs using sound), black holes are inescapable. An object that crosses the event horizon makes a black hole gain mass, maybe change angular momentum, and maybe change charge. Per the no hair conjecture, mass, angular momentum, and charge are the only three characteristics of a black hole that can be observed from outside the black hole.

Answer 2

Don't think of the singularity as a "what", it is more like a "when". If you fall into a black hole you will end up at the singularity. Every direction in spacetime leads towards it. In this way it is like a "when" because just as we can't avoid the future, we can't avoid the singularity in a black hole.

Rather than think of a black hole as a singularity that is pulling things towards it, don't think of the singularity as a thing with density. Think of a black hole as a region of spacetime with very strong gravity. This region has three properties: mass, charge and angular momentum. You don't need to send information about the mass, charge and angular momentum, because those are properties of the spacetime, not properties of the singularity (remember the singularity isn't a "what").

The singularity doesn't gain mass, the black hole gains mass, when things fall into it.

The singularity does have a definable position in spacetime (that is about all it has). It is at r=0 in spherical coordinates. This justifies drawing it as a dot in the middle of the black hole. But that picture can be confusing, since it doesn't account for the bending of spacetime. You couldn't see the singularity because it is always in your future. That is why it is better to think of the singularity as a "when" not a "what".

Quantum gravity might show that singularities don't exist. But the General Relativity model of black holes is sufficient to describe how they have mass and how the mass can increase.