Are black holes exists? (black hole formation)

We know that black hole (BH) cause the time flow around it to slow until zero in event horizon.
So it looks like that a moment before the BH created, the time around it is slow down, so actually it will take it inf. time to come to be a real BH.

That’s what it looks like from outside. But from the black hole’s point of view, time seems to be going ahead normally as the collapse continues.

SDMB alum ‘Bad Astronomer’ addressed this once:

Sounds like this is not a settled issue.

Classically, it takes an infinite amount of time, as measured by any external observer, for a black hole to form. But black holes, like everything else in the Universe, are not classical objects. And while we can say precious little about the quantum mechanics of black holes, one thing that we can say is that the infinite time for formation, even from the outside reference frame, is wrong (though unfortunately this doesn’t help us narrow down the right answer very much).

Has your sig ever been more appropriate?

Additionally, there could plausibly be primordial black holes that do not originate as a collapsing star, possibly avoiding the “should take infinite time to form” problem in another way.

They theoretically could have formed during the Big Bang, right?

How is babby black hole formed?

All your black hole are belong to us!

So, stupid question…if one were to “fall” into a black hole, with all the stretching of time and whatnot, how long would it take from the fallee’s perspective?

they need to do way instain massive star

It depends on the size of the black hole, but very short. Roughly the size of the black hole divided by the speed of light.

Glad I RTFT first, I was thinking the same thing–respectable merits of the question aside.

I burning your black hole!

Wait. It doesn’t stretch out to an endless point?

Or are you saying that - even though it took something less than infinite time - it wouldn’t appear that way to the subject?

The time flow of universe outside the falling astronout will accelerate. so the universe will come to its end before he will get to event horizon.

So this is still coherent to say that no black hole exists.
Or maybe we can say that the black hole exists only in the end of time.

In the reference frame of an outside observer, it takes a long time, though our theories are insufficiently developed to say just how long. In the reference frame of an infalling observer, it takes a short time.

The paper which the bad astronomer bases that on leaves me unsatisfied as it seems to be asking the wrong questions. A black hole is defined by its absolute horizon (i.e. its event horizon), but the question they’re asking is not whether an absolute horizon forms and the set-up they use doesn’t even seem to be capable of answering that question. In particular you need to know what is going on inside the collapsing star to answer that question, but the choice of modelling the star as a Nambu-Gotu domain wall is totally inadequate to tell you what happens on the inside. Of course Krauss knows more about physics than me.

Honestly it seems to me that the answer to this question depends on what simplifications are made.

The impulse…I…Could not frighth back! :mad: