What’s Inside a Black Hole’s Horizon?

New theoretical work rekindles the question on whether black holes have an interior: Would a firewall destroy any observer crossing a black hole horizon?

New York City mayoral candidate Joe Lhota generated a public outcry when he indicated that he would not have shut down subway service for a pair of kittens lost in the tunnels. Even more outrage could have arisen from the discussions that took place at the Kavli Institute for Theoretical Physics during a talk on the recent work of Joseph Polchinski and Donald Marolf, both at the University of Santa Barbara, California. Physicists discussed a thought experiment that involved locking up a cat in a sealed box containing a vial of poisonous gas that could be released at any moment, triggered by a random radioactive decay. What would happen if one threw this whole “Schrödinger’s cat” contraption into a black hole horizon (see Fig. 1)? The cat’s ultimate fate is sealed the moment it passes the horizon: it will be crushed at the singularity at the center of the hole. But if an in-falling observer jumped into the hole together with the box, he or she could open the box to determine whether or not the cat has been killed by the poison at a certain point of its travel between the horizon and the singularity. Marolf and Polchinski presented arguments, now reported in Physical Review Letters [1], suggesting that there is no well-defined quantum mechanical calculation that could predict the outcome of the in-falling observer’s measurement. According to the authors, the solution to this seeming inconsistency is to postulate that box and cat go up in a burst of flames the moment they pass the horizon by hitting a “firewall.”