Wigner’s Friend as an Oracle for Quantum Gravity

The firewall paradox is a thought experiment designed to test the consistency of quantum theory and general relativity.

It features two agents, Alice and Bob, who analyse the physics of a black hole emitting Hawking radiation from complementary perspectives. Alice, falling freely into the black hole, probes the smoothness of the event horizon — testing for the presence or absence of a "firewall". Meanwhile, Bob remains outside to collect the Hawking radiation and test for unitarity. The combination of Alice’s and Bob’s findings then yields a contradiction. In my talk, I will revisit this paradox, using insights learnt from Wigner’s friend thought experiments. The conclusion is that, in contrast to common belief, the contradiction in the firewall paradox does not need to originate from an incompatibility between quantum theory and gravity.

(This is joint work with Ladina Hausmann)