A **black hole firewall** is a theoretical attempt to resolve the paradox that arises when a photon falls into a black hole while entangled under quantum mechanics with another photon.

It has been known for some time that quantum mechanics and the theory of gravity under general relativity are inconsistent. The most famous consequence of this is the realization, by Stephen Hawking in 1974, that quantum particle pair production very close to the event horizon of a black hole could result in one of the particles appearing to be emitted out of the black hole, while the other falls in. This is called "Hawking radiation". This phenomenon creates problems for the information-theoretic aspects of black hole behavior, such as the "no-hair theorem", and these problems, and their proposed solutions, are active areas of theoretical research.

Various theories have been proposed for dealing with this. They generally posit that the event horizon of a black hole is not the smooth, featureless black surface that classical general relativity would suggest, but is somehow different.

It needs to be pointed out that whatever unusual features (such as the "firewall") exist, they exist only out to the "Planck length" above the event horizon. The Planck length is 1.6x10^{−35} meters, which is about 10^{−20} times the diameter of a proton. So ordinary quantum mechanics and general relativity work together smoothly down to distances much smaller than atomic or nuclear sizes.

While a theoretical firewall has been promoted as a way of salvaging the Theory of Relativity, modern refinements of the firewall require rejecting the Theory of Relativity.