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zonde
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I liked this graphical example of black hole formation posted by PAllen in another thread and I want to discuss it.
It is not unusual that arguments defending existence of black hole go like that:
1. Assume that BH exists.
2. Then observer falling into BH ...
I think that point 1. is begging the question fallacy. So any argument defending BH should be about formation of BH.
Therefore I think that this example of PAllen is excellent basis for discussions around black holes and so it is worth a closer look.
Now about example. Let's say that we are looking at star at the far edge of collapsing cluster. Let's assume further that we have idealized situation where light from that star is going exactly through the cluster's center of mass. That particular star will be gravitationally lensed (amplified) and it is a bit unclear if it will be redshifted or blushifted as it is moving toward us not away from us, right?
Another point is where exactly formation of event horizon starts? It does not appear at once but is expanding from some point by engulfing mass. So we have to have some seed black hole that is produced in collision of two stars near cluster's center of mass, right?
PAllen said:I think this point was made earlier in this thread, but I would like to pose it in a graphic form. This is the point that an eternal black hole as described by SC geometry almost certainly does not exist in our universe. Let's instead look at formation of black hole.
To be able to see the formation better from afar, let's have the far fetched scenario of a trillion stars of some super cluster collapsing with no net angular momentum, no accretion disk forming. I pick the far fetched number of a trillion stars because that allows the black hole to form while the stars are still well separated from each other, and individually resolvable (in principle) up until the last moments. Let's further assume there is a background of galaxies behind this collapsing cluster, but nothing in your line of sight in front of it.
What would you see? As the collapse occurred, you would see the cluster, as a whole, reddening, and more and more extreme Einstein rings from galaxies behind the cluster. Up until the last moments, you would see highly red shifted light from stars throughout the cluster - especially, you could see stars in the center of the cluster. Then, in a relatively brief period of time, the cluster would further redden/darken until it was blacker than even CMB radiation. Against the background galaxies, it would look, quite literally, like a black hole in the sky surrounded by an Einstein ring of light from galaxies behind it.
How would you want to interpret this? It is a mathematical fact that this is what you would see. Would you say that a trillion stars have actually vanished? Would you say that the stars in center magically are compressed invisibly against the not quite yet formed horizon (having jumped billions of miles from the center of the cluster to the edge of this black ball)? You could say there is an invisible ball of a trillion frozen stars, a millimeter larger than the theoretical event horizon. Then, if matter falls in, it soon vanishes and the black region grows slightly (after all settles down). Again, you could say the black ball is still just larger than the theoretical event horizon, with frozen stars throughout, and new matter somewhere at the outer edge.
If you prefer this interpretation, it is, indeed, not determinable from outside observations that further collapse has occurred inside the black region. However, I would than ask:
If look like a duck, ... . Isn't black hole a good description of the this scenario? Then if you ask, what would happen according to a ship orbiting one of those interior stars? GR has only one answer - further collapse (to a singularity), in very finite time for the ship.
It is not unusual that arguments defending existence of black hole go like that:
1. Assume that BH exists.
2. Then observer falling into BH ...
I think that point 1. is begging the question fallacy. So any argument defending BH should be about formation of BH.
Therefore I think that this example of PAllen is excellent basis for discussions around black holes and so it is worth a closer look.
Now about example. Let's say that we are looking at star at the far edge of collapsing cluster. Let's assume further that we have idealized situation where light from that star is going exactly through the cluster's center of mass. That particular star will be gravitationally lensed (amplified) and it is a bit unclear if it will be redshifted or blushifted as it is moving toward us not away from us, right?
Another point is where exactly formation of event horizon starts? It does not appear at once but is expanding from some point by engulfing mass. So we have to have some seed black hole that is produced in collision of two stars near cluster's center of mass, right?