# Black hole formation as seen by a distant observer [duplicate]

Is black hole formation observable for a distant observer in finite amount of time? Specifically, let's take uniform non rotating distribution of 10 solar masses within solar radius, assume no outward pressure, and calculate its gravitational collapse. I assume distant observer would never witness complete formation of black hole with event horizon corresponding to entire 10 solar masses. What would be observable in finite amount of time, then? For example, would there be a black hole with smaller radius, while the rest of mass is still falling into its event horizon?

• Possible duplicates: physics.stackexchange.com/q/5031/2451 , physics.stackexchange.com/q/21319/2451 and links therein. Jan 1, 2013 at 15:33
• The link is more comprehensive, indeed. Although when trying to delete I was warned that 2 posts has been deleted as duplicate of this one, so I hesitated to confirm. Any expert in the matter is welcome to delete it. Jan 7, 2013 at 17:51

There is no unique way to answer this question. The problem is the following. You are asking if, at a specific proper time, a black hole of some radius exists. However, to do this you have to say something like "the black hole has a 10m radius at the same time that the observer's proper time is 5sec". Such simultaneous events are not well defined in GR unless you make an arbitrary choice of a coordinate system with a time coordinate. Nonetheless, if you make such a choice, it is true that the black hole will start with a radius of 0 inside of the infalling matter and then get larger. Eventually it will reach a maximum radius and then get smaller through Hawking radiation.

So what will an observer literally see if she observes the star collapsing? The answer is that the observer will see infalling matter slow down and thermalize. The matter will never appear to "fall in" as far as this observer is concerned. However, the particles/light emitted will be redshifted more and more (and before long the only outgoing particles of reasonably not-low energy will be from Hawking radiation).

This question is very unclear because you haven't said what you mean with "black hole formation being observable", neither did you specify by whom. I guess from the rest of your question that you mean an observer at timelike infinity? The observer can see mass collapse and "freeze" as the surface becomes very redshifted. He'd in principle also be able to see Hawking radiation come out how the black hole subsequently evaporates. Now it is not known whether black holes that have formed from collapse actually have a true event horizon or an apparent horizon (depends on the end stage of evaporation), so for me him seeing the matter freeze, settle in a "hairless" state, and witness Hawking radiation does account for "observing black hole formation."