Question

Can a photon cross the event horizon from the perspective of an outside observer?

Answer 1

The only way to see the path of a light beam is to put some scattering dust in the path. In that way we observe the scattered radiation. I now consider the diagram of the Finkelstein coordinates below. Consider a pulse of photons traveling down the blue path or geodesic. We also think of there being some orbiting dust or scattering stuff that scatters some of these photons out along the red null geodesics. This is what we will see. The result is that we can never see the pulse of light actually reach or traverse the event horizon. These outwards null geodesics define the delay or tortoise coordinates for the observed entering photon. It will take an infinite amount of time for the exterior observer to witness this.

Answer 2

Explanation:

n practice, all event horizons appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view (as the horizon-crossing event's light cone never intersects the observer's world line).