How fast does a black hole collapse?

[Dinosaur]

What I am curious to know is how fast does a black hole collapse? Since gravity is distorting spacetime and it is spacetime that is collapsing and not mass moving through spacetime is there any limit to the "inward velocity" of the collapsing event horizon? I do have a good reason for wanting this information and since my math is not as hot as I would like it to be, maybe someone out there could enlighten me.

[rstevenson]

According to this very interesting page, the speed of the collapsing material equals c at the event horizon. The black hole continues to collapse beyond that point at an ever increasing rate, but the information that it is doing so can't reach us, so the black hole seems to stay as big as the event horizon -- from our point of view.

Rob

[Dinosaur]

Thanks’ Rob, but I guess what I really want to know is how rapidly the inner core collapses. I will go check the website you recommended, if that doesn’t give me the answer maybe we can discuss the matter further. Once again, thanks.
Greta website.

[Billy]

If a super nova is the outerlayers bouncing off the collapsed core of a massive star and igniting, then the collapsing of the core has to stop at one point for them to bounce back off. I read some where the iron core collapses from thousands of km in diameter to just a few km in 1/1000 of a second. this implies that it stoped collapsing. so what makes it start collapsing again and how fast does it collaps from the point of iron fusion (initial collaps) untill the surface of the collapsing core reaches the event horizon, and then how long after it hits the event horizon to reach plank length? im sure I wont get a responce befor my project is due but im still curious.

[Chris Peterson]

... and then how long after it hits the event horizon to reach plank length? im sure I wont get a responce befor my project is due but im still curious.

It doesn't collapse beyond the event horizon. That's the limiting size of the black hole. At this point, it is meaningless to think of anything being "inside" a black hole, which actually behaves like a fundamental particle with just a few parameters defining it.

[alter-ego]

It doesn't collapse beyond the event horizon. That's the limiting size of the black hole. At this point, it is meaningless to think of anything being "inside" a black hole, which actually behaves like a fundamental particle with just a few parameters defining it.

I agree that giving phyiscal meaning to the interior spacetime of a black hole is dubious at best, especially for an "outside" observer. However, GR does have the perplexing problem of predicting a gravitational singularity at the center of the black hole where spacetime has infinite curvature. The event horizon is a special boundary, but not one that precludes "predictable" phenomenon at smaller radii for a comoving observer (as in Rob's link). Gravitational callapse and the effects on spacetime at radii less than the event horizon are part of GR none the less. I don't know if GR can answer Billy's question, but collapse below the event horizon does occur in theory.

Personally, I've always believed if and when GR is revised, one of the results will be defining a finite collapse condition rather than a singularity. Don't get me wrong though, even though part of me wants to witness a momentous crack in GR, my money is always on Albert E.

[Chris Peterson]

I agree that giving phyiscal meaning to the interior spacetime of a black hole is dubious at best, especially for an "outside" observer. However, GR does have the perplexing problem of predicting a gravitational singularity at the center of the black hole where spacetime has infinite curvature.

Sort of. But this is why most physicists believe that GR fails under those conditions. The "inside" of an event horizon is probably outside the boundary conditions for GR, just as relativistic velocities are for Newtonian mechanics.

[alter-ego]

I agree that giving phyiscal meaning to the interior spacetime of a black hole is dubious at best, especially for an "outside" observer. However, GR does have the perplexing problem of predicting a gravitational singularity at the center of the black hole where spacetime has infinite curvature.

Sort of. But this is why most physicists believe that GR fails under those conditions. The "inside" of an event horizon is probably outside the boundary conditions for GR, just as relativistic velocities are for Newtonian mechanics.

Yes, your point is my belief also, even down to the relativistic velocity analogy for NM. I chose to defer judgement on the usefulness of GR for r < event horizon because I wanted to avoid speculative discussion. Specifically sticking to existing GR theory, I think mathematical statements can be made about the "inside". Whether they are correct or not is certainly open for discussion. We certainly seem to share the same view overall.

Given Billy's situation, he might appreciate knowing that the physics of BH collapse should likely be questioned, but can GR make any predictions about a collapse time to some other radius in any useful reference frame? That seemed to be what he's after.

Btw, I'm excited about the upcoming sub-millimeter, VLBA imaging of Sag A* BH silhouette. This will be the first GR test within the strong-field regime approaching the event horizon. Astronomers are optimistic about acquiring adaquate resolution (~20 microasec) for this test within the next few years. That amazes me we are that close to "seeing" a BH well enough to compare to a model.

[Billy]

that is exactly what I am after! The relative radius in question is plank length which if I understand is like 10^-35m. So how fast does the iron core of a supergiant collapse from the point of supernova down to plank length?

[Chris Peterson]

that is exactly what I am after! The relative radius in question is plank length which if I understand is like 10^-35m. So how fast does the iron core of a supergiant collapse from the point of supernova down to plank length?

As previously noted, the question cannot be answered using current physics. It isn't even certain that there is any collapse beyond the event horizon.

[Dinosaur]

Thanks to everyone who attempted to answer my question "How fast does a black hole collapse?” I have been otherwise occupied for the past couple of years, and would like now to re-open the debate. As I mentioned in my initial question, my maths are not up to scratch, so I have been taking some online maths course through Stanford University, still a ways to go though. So let me be a bit more explicit in my request, let’s go back to the Big Bang, within an impossible to imagine small unit of time all matter came into existence and inflated! Now what is to stop, in theory, the reverse happening inside a black hole, all the matter that makes up the original star disappears from the universe in an impossible small unit of time? (I know all of this happens inside the event horizon, so we can never see the result, but one can always do thought experiments.) Now the matter disappears but the gravitational distortion remains and Albert did say gravity and acceleration were equivalent so... the event horizon seems to persist for us on the outside because time ceases to exist on the inside, so I guess what I’m asking is can the collapse exceed the velocity of light?

[rstevenson]

... so I guess what I’m asking is can the collapse exceed the velocity of light?

As previously noted, the question cannot be answered using current physics.

But if it could be answered, the answer would involve the mathematics of relativity, so neither you (by the sounds of it) nor me (for sure) would be able to understand it.

Rob

[Chris Peterson]

(I know all of this happens inside the event horizon, so we can never see the result, but one can always do thought experiments.)

No, we can't always do thought experiments. The problem here isn't the math, it's the nearly complete lack of any physical theory to apply the math to. What you're suggesting isn't a thought experiment, but a guess. And honestly, you might as well say there's a massive leprechaun inside that eats the matter, for all we understand the inside of a black hole (again, assuming the "inside" even means anything in that case).