Ok so if we have some matter(a star or whatever) and under classical laws of gravity(GR) this matter is going to form a black hole. If we're in the frame A of the collapsing matter then there will come a time(measured in that frame) that the matter crosses the event horizon after which no events that occur can be seen by an observer outside this horizon. But now if we are in the frame B of an observer outside the horizon how long will it take for the matter to cross the horizon? Surely we never see the matter cross the horizon because if we did we would be seeing an event which happened behind the horizon.(adsbygoogle = window.adsbygoogle || []).push({});

I concede that as the matter approaches the horizon it gets more and more red shifted but if we continue t use measuring equipment that can detected ever increasing wavelengths in theory we can still detect the matter.

In this sense we never see the full formation of a black hole before we in fact fall in black hole ourselves.

Now if we take into account hawking radiation on the horizon this surely implies that the horizon will shrink as observed from B. Eventually the black hole will get very small and thus even to the outside observer the matter(which is always present at the event horizon) will approach the origin.

If we go back to frame A all this evaporation has occurred while the matter is just about to cross the horizon but yet the horizon is shrinking so the matter is in a sense falling to the origin. At some point of coarse we need a theory of quantum gravity to know what happens next.

I'm just wondering what is wrong with my reasoning here?

I'm I correct in thinking that for an outside observer will never be a time at which the matter actually crosses the horizon?

I'm I also correct that according to Hawking's theory there will be a time at which the horizon approaches the Planck length?

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# Gravitational collapse to form a black hole

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