How can a black hole form in finite "external" time?

The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...

"Too Many Kooks Spoil the Brothel" ha scritto
nel messaggio oups.com...
The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...

Recent theories suggest that the matter collaps stops at some time, so you
don't have to solve paradoxes...
See http://xxx.arxiv.org/ftp/astro-ph/pa...03/0503200.pdf

Luigi Caselli

Luigi Caselli wrote:
"Too Many Kooks Spoil the Brothel" ha
scritto
nel messaggio
oups.com...
The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer
and
nearer to forming a horizon. Given the finite age of the Universe,
I
cannot see how any could by now have formed. ...

Recent theories suggest that the matter collaps stops at some time,
so you
don't have to solve paradoxes...
See http://xxx.arxiv.org/ftp/astro-ph/pa...03/0503200.pdf

Luigi Caselli

Perhaps the black-hole problem applies to the above URL, too, cos I got
"Page cannot be found".

Apologies, I've now found the paper in question!

"Too Many Kooks Spoil the Brothel" wrote in
message oups.com...
The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...


It cannot be observed "from the outside"
That is why they are called black holes.

The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer
and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...

and that we know that you cant see, your question was...?

The whole thing(the original singularity) was a black hole.

Too Many Kooks Spoil the Brothel wrote:
The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...

The question (whether a black hole can form in finite external time)
does not have a well-defined answer. Even in special relativity,
simultaneity is relative, and your question is equivalent to: Is there
any time for an external observer that is simultaneous with the time
the black hole forms?

However some other related questions have absolute answers. Given two
events, A and B, we can answer the following questions:

1. Can a signal be transmitted from A to B? (Note that the signal
cannot travel faster than light or backwards in time.)

2. Can a signal be transmitted from B to A?

If #1 applies, we say that A is in the causal past of B. If #2
applies, A is in the causal future of B. It is also possible that
neither is true, i.e., that there is not enough time for a signal to
pass either way. If this is the case, whether A precedes B or vice
versa depends on the choice of reference frame.

Let's take the time a black hole forms to be the time the event horizon
comes into existence. Since not even light can escape from the event
horizon, the black hole's formation is not in the causal past of any
external observer. However, for any such observer, there comes a time
when the black hole's formation is no longer in their causal future.

So do black holes exist yet, or are there only frozen stars? Pick
whatever answer pleases you, and unless someone finds a way to travel
(or communicate) faster than light, no one will ever prove you wrong.

-- Jim Black

Too Many Kooks Spoil the Brothel wrote:

The collapse of matter towards the formation of a black hole should
slow down indefinitely, observed "from outside", as it comes nearer and
nearer to forming a horizon. Given the finite age of the Universe, I
cannot see how any could by now have formed. ...

You could have primordial black holes. Ones that were there at
the big bang.

John Anderson

"TMKStB" == Too Many Kooks Spoil the Brothel writes:

TMKStB The collapse of matter towards the formation of a black hole
TMKStB should slow down indefinitely, observed "from outside", as it
TMKStB comes nearer and nearer to forming a horizon. Given the
TMKStB finite age of the Universe, I cannot see how any could by now
TMKStB have formed. ...

This is discussed at length in Misner, Thorne, & Wheeler. Briefly,
you are correct that it requires an infinite amount of time for an
external observer to see the surface of the star cross the horizon.
However, the light emitted from the surface of the star is also being
gravitationally redshifted. For a star of mass M, it takes roughly
only 10 microseconds*(M/M_sun) for the gravitational redshift to reach
a large value. As a result, the star essentially instantaneously
fades from view.

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