Archive of Astronomy Questions and Answers

If, from far away, nothing gets inside a black hole, how do they accrete and grow?

Matter does not have to fall THROUGH the event horizon to make a black hole for outside observers. All it has to do is get close enough to the horizon, by literally a few millimeters, so that it experiences a large relativistic dilation factor. The emitted light will be highly redshifted from gamma ray energies to radio wavelengths during this last few millimeters, and as seen by an outside observer, the matter will cease to emit light or energy. As seen by a distant observer, the object accretes mass which falls towards its horizon limit...and then seems to slow down and stop before 'winking out'. Only to an infalling observer will the matter actually be seen to pass across the horizon and fall into the singularity, but this is not ever observable to the outside party in a finite amount of time. Black holes are to general relativity what the Twin Paradox is to special relativity.