Black holes are very curious objects to scientists as these very real structures have profound effects on the Universe, yet we are limited to primarily theory and thought experiments to understand them. A classic and invaluable kind of thought experiment is a paradox; a system which implies a fundamental inconsistency yet cannot easily be shown to be itself, inconsistent. Now the firewall paradox for black holes has been solved by researchers at the University of York and, as intended, the solution has improved our understanding of all black holes.
The firewall paradox gets its name from the idea that just above the event horizon of a black hole, high energy particles would exist and burn away anything being drawn into the black hole. The problem with this idea is that the physics behind such a firewall cannot be localized to the area around a black hole, according to General Relativity, and this could then lead to an inconsistency with quantum mechanics. If one particle in an entangled pair struck the firewall, the information of that particle would be released as radiation, reinforcing the firewall, and the other particle would be able to observe this, and that is unallowable, creating the paradox. In other words, if Alice were falling into the black hole and shouted what it looks like to Bob, he may hear it, which is forbidden.
The researchers solved the problem though by showing that for all black holes the formation of the firewall depends on the entanglement of the particles. The stronger the entanglement the longer it takes before the radiation can be emitted and firewall formed, with maximally entangled particles never forming a firewall. Returning to the shouting Alice, the shout will either never come or come so late that Bob is no longer there to hear it.
Source: University of York