Black holes / density

Question

If a star explodes to form a black hole how does the gravitational field become infinite from one state to the next? ie: it seems additional mass has been added or is this simply a function of density?

Answer 1

It does seem odd that a star that isn't a black hole can explode, and therefore presumbly lose mass, and still form a black hole.

The explanation is that to form a black hole requires a high density not just a high mass. Even a small object such as, well, you or I could form a black hole if compressed enough, though obviously in practice that level of compression is impossible.

The explosion you're thinking of is a type II supernova. This happens because a large star is kept inflated by the heat it generates from fusion in its core. But large stars run out of fuel surprisingly suddenly, and when that happens the star starts collapsing under its own gravity. This collapse compresses the core enough to form a black hole then rebounds to blow the rest of the star to bits as a supernova.

So the black hole isn't formed as a result of the star getting more massive, but as a result of the star's core getting compressed enough to form a black hole.

Answer 2

This can be understood by showing in general where curvature comes from. Spacetime can curve naturally all on its own even in empty space.

Matter can connect together regions that have two different types of curvature.

One possible type of curvature is the funnel-shaped curvature outside a star or outside a black hole. As the star collapses inwards to for the black hole the location of the matter moves in. This means the location where the two types of curvature can meld together moves inwards.

But the curvature type outside was the funnel curvature, so when it fills inwards it fills inwards with more of the same type and so creates new, and stronger curvature in this region. (and it also actually creates space because this new region of the new type is actually larger than the old region of the bold type).

This is because the type of curvature in empty space is a type that is fully capable of creating more of itself, all the matter does is allow two different types (like the type inside and the type outside the star) to join up.

So as the star collapses the region that is now outside becomes more strongly curved and it persists and survives that way all by itself because it is perfectly natural.

Now eventually the star gets so compact we can't see what is going on inside the star anymore. This is what we call a black hole. Since we don't know what is going on we have to just look at the math because there could be new effects at higher densities we wouldn't know.

At this point, the black hole has formed (because the event horizon has formed) but the singularity inside has not yet formed. The event horizon is not caused by an infinite density or an infinite force or anything like that. The event horizon is what makes it black and we don't actually know for sure if a singularity forms.

The math indicates that we can't see the inside because the path of everything inside that event horizon (even light) is to head inwards and if that is the case it will compress inwards more and more no matter what tries to stop it, so it looks like they will collapse into a singularity. But again, we don't know for sure because we haven't seen it. Unlike the outside which we have seen.