The Hawking radiation is the result of virtual particle pairs popping into existence near the event horizon, with one partner falling in and the other escaping. The black hole’s mass decreases as a result and is emitted as radiation.
Image: Courtesy of Joseph Polchinski
Susskind’s argument that information could be recovered without resorting to quantum Xeroxing proved convincing enough that Hawking conceded his bet with Preskill in 2004, presenting the latter with a baseball encyclopedia from which, he said, “information can be retrieved at will.” But perhaps Thorne, who refused to concede, was right to be stubborn.
Bousso thought complementarity would come to the rescue yet again to resolve the firewall paradox. He soon realized that it was insufficient. Complementarity is a theoretical concept developed to address a specific problem, namely, reconciling the two viewpoints of observers inside and outside the event horizon. But the firewall is just the tiniest bit outside the event horizon, giving Alice and Bob the same viewpoint, so complementarity won’t resolve the paradox.
Toward Quantum Gravity
If they wish to get rid of the firewall and preserve No Drama, physicists need to find a new theoretical insight tailored to this unique situation or concede that perhaps Hawking was right all along, and information is indeed lost, meaning Preskill might have to return his encyclopedia. So it was surprising to find Preskill suggesting that his colleagues at the Stanford workshop at least reconsider the possibility of information loss. Although we don’t know how to make sense of quantum mechanics without unitarity, “that doesn’t mean it can’t be done,” he said. “Look in the mirror and ask yourself: Would I bet my life on unitarity?”
Polchinski argues persuasively that you need Alice and Bob to be entangled to preserve No Drama, and you need the Hawking radiation to be entangled with the area outside the event horizon to conserve quantum information. But you can’t have both. If you sacrifice the entanglement of the Hawking radiation with the area outside the event horizon, you lose information. If you sacrifice the entanglement of Alice and Bob, you get a firewall.
“Quantum mechanics doesn’t allow both to be there,” Polchinski said. “If you lose the entanglement between the in-falling (Alice) and the outgoing (Bob) observers, it means you’ve put some kind of sharp kink into the quantum state right at the horizon. You’ve broken a bond, in some sense, and that broken bond requires energy. This tells us the firewall has to be there.”
That consequence arises from the fact that entanglement between the area outside the event horizon and the Hawking radiation must increase as the black hole evaporates. When roughly half the mass has radiated away, the black hole is maximally entangled and essentially experiences a mid-life crisis. Preskill explained: “It’s as if the singularity, which we expected to find deep inside the black hole, has crept right up to the event horizon when the black hole is old.” And the result of this collision between the singularity and the event horizon is the dreaded firewall.