# If event horizons (black holes) never form in a finite time, then how can they merge to create a common event horizon in a finite time? [duplicate]

There is another question about this topic, but that is asking about gravitational waves, and I am not. I am specifically asking about the timescale of the merger being finite (actually quick), whereas the black holes (and event horizons themselves) never really form.

There are perfectly good local coordinates in which there merger of two black holes (the formation of a common apparent horizon) happens at a finite time.

LIGO Observation Reconciled with Falling Object Never Seen to Cross Event Horizon

And this one:

This isn't some accounting trick, it means we will never see an event horizon form. At this point someone will usually pop up and say that means black holes don't really exist. In a sense that's true in our co-ordinate system, but all that means is that our co-ordinate system does not provide a complete description of the universe.

How can anything ever fall into a black hole as seen from an outside observer?

Now the first one says that the unformed (or not yet formed, because it takes infinite time for them to form) event horizons, merge in a finite time.

The second one says that from our far away view (our reference frame here on Earth), the black holes (and event horizons) never form.

As far as I understand, this means, that from our perspective here on Earth, we have two objects, with not yet formed event horizons, and these merge in a finite time to form a common event horizon. But doesn't this mean that the newly formed common event horizon cannot be formed (when viewed from our frame here on Earth) either?

Question:

1. If event horizons (black holes) never form in a finite time, then how can they merge to create a common event horizon in a finite time?
• Does this answer your question? So Black Holes Actually Merge! In 1/5th of a Second - How? Commented Jul 23, 2021 at 23:29
• The key to this is that usually, we assume that what we see is what happens. Roughly,within limits of experimental/measurement error and quantum uncertainty. A forming or existing black hole so distorts spacetime, that this is no longer even slightly true. So you have to ignore the notion that what you as an observer would see, even witn perfect vision, is at all remotely what happens. It may not be. You see an jnfalling object taking infinite time and becoming infinitely dim, never crossing the event horizon. Thats not at all what happens in the objects experience. ..... Commented Jul 25, 2021 at 22:50
• ...... It falls according to gravity, accelerating as General Relativity says, and completely unaffected by passing the event horizon. At that point you cant see it, but it doesn't care what you can see. So the solution is that sure they form, merge, infall, and so on, in accordance with General Relativity. But to an outside observer the conclusion of those processes is never quite seen. The passinf through the event horizon, the stable final product of the merger, and so on - youll see to an extremely close degree but never see all. Think 99.9999999999 and tack on as many 9s as you like. ... Commented Jul 25, 2021 at 22:52
• .... The same is true even for objects that pass a formed event horizon (which you'll never see completed either, from outside). At that point spacetime is so totally distorted that even with light speed drive, whatever direction you aimed it, even outward, youd find the centre of the black hole (the singularity or whatever it is) ahead of you. In absolutely every direction, its ahead of you. But you cant see it, because any signal from ahead of you that tried to reach you..... it too would find it was headed for the singularity as well, so you'd never see that photon or get that signal. Commented Jul 25, 2021 at 22:59
• Spacetime's a funny thing! Commented Jul 25, 2021 at 23:00