Undergrad Time to Form Black Hole? | Observer Slows Time at Event Horizon

[Cobalt101]

Is it correct that, from the perspective of an observer, time slows down and ultimately stops at the event horizon of a black hole, implying that no black holes have had time to form in the universe ?

 

[PeterDonis]

Is it correct that, from the perspective of an observer, time slows down and ultimately stops at the event horizon of a black hole, implying that no black holes have had time to form in the universe ?

No. A massive object only takes a finite time to collapse to a black hole, according to an observer falling in with the collapse; and an observer falling into a black hole that already exists only takes a finite time by his own clock to reach the hole's horizon.

 

[Cobalt101]

No. A massive object only takes a finite time to collapse to a black hole, according to an observer falling in with the collapse; and an observer falling into a black hole that already exists only takes a finite time by his own clock to reach the hole's horizon.

Thanks - but the observer I was referring to was not falling in with the object, but rather was watching from a distance. So in particular if a black hole is formed by a collapsing star, from the perspective of the rest of the universe does the black hole ever form? (I understand that from the perspective of the collapsing star it only takes a finite time).

 

[rootone]

I think the observer would see what had been the star becoming increasingly red shifted until it was no longer visible.
However if it is gravitationally bound, say in a binary system, it could be detected indirectly that way, and possibly there might be a hot and visible accretion disk of remnant material surrounding the event horizon of what now is a black hole.

 

[PeterDonis]

if a black hole is formed by a collapsing star, from the perspective of the rest of the universe does the black hole ever form?

There is no such thing as "from the perspective of the rest of the universe". The black hole is a feature of the global spacetime geometry; either it is there or it isn't. If the collapsing object forms a horizon in a finite time, or an infalling observer reaches the horizon in a finite time, then the black hole is there. The fact that the distant observer can't see it doesn't change that.