There is an argument that matter can't ever cross the event horizon, that as it comes closer to the event horizon, outside observers see its time slow down and come to a crawl just outside of the event horizon. Some have suggested then that a black hole can never form, as any piece of matter that would cause the density of matter to go critical and form a true black hole, would always grind to a stop right before doing so. You can read more here. What about photons though? Can a photon cross the event horizon? If a photon can cross the event horizon, then the almost black hole might be able to turn into a true black hole. Is there any principle that would prevent an event horizon from forming? Might the matter inside the almost black hole expand to accommodate the energy of the photon?


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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.


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