Quite literally every science show talks about black holes with a similar phrase: so dense that nothing, not even light, can escape.
Let's say I am in space a safe distance from an object. I drop an orb of light into the object so that photons are exiting in all directions and watch it from above. Because the speed of light is a constant, I will measure photons traveling towards me at the speed of light.
Now let's dial up the gravity of this object. As we move past white dwarf, past neutron star, near the limit of a black hole the speed of the photons never changes, and instead they become redshifted more and more. Since the speed of light is a constant, they cannot slow down. They must always travel at
c. They have less and less energy though. So even at the threshold of collapse into a black hole, photons still all emerge at
c with infinitesimally small energy levels.
So why at this critical mass do they stop? It almost feels like at this point a zero-energy photon should emerge though completely unmeasurable and unable to interact with anything ever again still at
c. And if photons only travel towards the center at this gravity level regardless of how they are emitted, are they blue-shifted and actually gain energy on their way down? Perhaps I'm just picturing all of this wrong in the first place. Oh, and if this belongs in physics or another SE group I'm happy to delete/repost.