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When a black hole first forms (eg via gravitational collapse of a star), how quickly does the event horizon expand?

Quote from: evan_au on 17/12/2012 10:38:09When a black hole first forms (eg via gravitational collapse of a star), how quickly does the event horizon expand?Let's remember that the event horizon is a function of the gravitational mass of the body. And shortly before collapse, the body has already attained sufficient mass at it's surface to make escape only possible by reaching velocity very close to light speed anyway. In effect, when collapse occurs, the event horizon will propagate very close to the last spherical radius of the initial mass. In reality, the event horizon doesn't really expand. It just takes place very close to where the surface of the original mass last stood.

Now, if the black hole is formed during a supernova. Then could one have an expanding cloud of debris that would be outside of the event horizon, which could then start to rapidly contract?

Quote from: CliffordK on 21/12/2012 00:44:49Now, if the black hole is formed during a supernova. Then could one have an expanding cloud of debris that would be outside of the event horizon, which could then start to rapidly contract?Absolutely my friend........Your observations are correct and leads one to believe that it is more likely that; Either the event horizon stays fairly close to the original radius in the case of a moderate event or, in the case of a violent occurrence, the event horizon would most likely shrink due to loss of captured mass.

How I interpreted Evan's question was - how fast does this transition move

Does the event horizon just "appear", or does it expand out from the denser central core?

Quote from: evan_au on 22/12/2012 09:43:53Does the event horizon just "appear", or does it expand out from the denser central core? The radius at which escape velocity becomes the speed of light and determines the sphere of all events which can no longer be observed from outside. We call this the Event Horizon.

And this sphere is determined by the total mass of the body relative to it's volume. And this total mass/volume, when calculated, is called the packing fraction.

If the body is undergoing a supernova type explosion, then the total mass/volume will vary greatly over time until gravity is given opportunity to reassemble all local matter. In any case, the escape velocity of a body that reaches the speed of light is what we refer to as the event horizon. So where and how fast this radius occurs depends entirely upon the aggregate mass and energies of compression. I tend to think this final stage occurs at light speed because light speed is the threshold for these criterion. Black holes can form either thru gravitational attractions or thru external compressions. I think the easiest way to examine this question is to look at the least violent example of black hole formation. Let's start with a neutron star, or as some theories have suggested, a quark star. These bodies are rather stable and from this starting point we can add matter a little at a time until we reach a point where just one more small volume of matter initiates collapse. Interestingly enough, I feel that this point of mass/volume is a physical constant that we have yet to acknowledge. I'm familiar with the Schwarzchild solution; r=2Gm/c^2 but this is not the physical constant I'm referring to. This formula is only a relationship between constants. The constant I'm speaking about is: What number of neutrons, or total mass, when assembled together in empty space without perturbation or external energies of compression will initiate collapse?

When done thru this method, the event horizon will establish it's self where the surface of the former body existed. Now comes the 64 thousand dollar question. How fast will the former body shrink to the singularity leaving the event horizon where the former surface last stood? I have no proof but I believe that this event occurs at light speed.

Yes that's a nice way of looking at it. And the internal stuff - Agree no way but speculation.