I may have posted this before, but it’s a really fun read:
The firewall paradox involves a thought experiment where Alice—it’s always Alice—sits outside of a black hole waiting for it to mostly but not completely evaporate, and scooping up all the Hawking radiation it emits as it does so. For a black hole the mass of our sun, this would take about 1067 years (we’ll assume Alice has a really long grant). Then, Alice routes all the photons of Hawking radiation into her quantum computer, where she processes them in such a way as to prove that they did encode information about the infalling matter. Then, as the final step, Alice jumps into the black hole. The clincher is that, if you combine all the ideas about black holes that had previously been accepted, you can now make a firm prediction that Alice will encounter an end of spacetime right at the event horizon (in the physicists’ colorful language, she’ll “hit a firewall and burn up”). But this is totally contrary to the prediction of general relativity, which says that Alice shouldn’t notice anything very special at the event horizon, and should only encounter an end of spacetime at the singularity.
There are various ways out of this that aren’t very satisfying: you could deny that information escapes from black holes. You could say that general relativity is wrong, and what we’d previously called black holes are really just firewalls. You could argue that what happens inside a black hole isn’t even within the scope of science—since much like life after death, it’s not empirically testable by anyone who “remains on this side.” Or—and this seems like the “conservative” option!—you could admit that Alice can create a firewall by doing this crazy processing of the Hawking radiation, but insist that, if she doesn’t do the processing, then she’ll pass through the event horizon just like general relativity always said she would. But if you take this last option, then what Alice perceives as the structure of spacetime—whether she encounters an event horizon or a firewall—will depend on what she programmed her quantum computer to do.