WORMHOLES – tunnels through space-time that connect black holes – may be a consequence of the bizarre quantum property called entanglement. The redefinition would resolve a pressing paradox that you might be burned instead of crushed, should you fall into a black hole.

Knowing which hazard sign to erect outside a black hole isn’t exactly an everyday problem. For theoretical physicists, though, it reveals an inconsistency between quantum mechanics and general relativity. Solving this conundrum might lead to the sought-after quantum theory of gravity.

Solving the conundrum could lead to the much sought-after theory of quantum gravity

Relativity says if you fall into a black hole, you would die via “spaghettification” – a gradual stretching by ever-more intense gravitational forces. But last year, when Joseph Polchinski at the University of California in Santa Barbara and colleagues explored the quantum implications of black holes, they hit a problem. Black holes emit photons via something called Hawking radiation, and these are “entangled” with the interior of the black hole and also with each other. This breaks a quantum rule that particles can’t be entangled with two things at once.


To preserve quantum monogamy, Polchinski suggested last year that the black hole-photon entanglement breaks down. That causes a wall of energy at the black hole’s event horizon that wrecks relativity because anyone falling in would burn up rather turn to spaghetti. Welcome to the black hole firewall paradox.

Possible solutions abound but now two physics heavyweights, Juan Maldacena of the Institute for Advance Study in Princeton, and Leonard Susskind of Stanford University, California, have come up with the most audacious one yet: a new kind of wormhole that means the entanglement needn’t be broken in the first place.

First, the pair showed that these space-time tunnels, usually described by the maths of general relativity, also emerge from quantum theory, if two black holes are entangled. It’s as if the wormhole is the physical manifestation of entanglement.

The pair then extended this idea to a single black hole and its Hawking radiation, resulting in a new kind of wormhole (see diagram). Crucially, they suggest that this wormhole, which links a black hole and its Hawking radiation, may not be a problem for quantum monogamy in the way that normal entanglement is. As a result, the firewall needn’t appear, preserving relativity (arxiv.org/abs/1306.0533).

Patrick Hayden of McGill University in Montreal, Canada, finds the idea of wormholes from entangled black hole pairs convincing, but says more work is needed for the case of the black hole and a photon. Polchinski, meanwhile, is cautiously optimistic: “It certainly injects new ideas. But there is a lot that still needs to be filled in.”

There is still room for firewalls in the new wormhole definition. Maldacena and Susskind also outline how an observer outside the black hole could manipulate the Hawking radiation, creating a shock wave that travels down the wormhole and appears as a firewall. This may not screw up relativity because the firewall is optional, not intrinsic to the black hole. Maldacena hopes mulling these options will teach us about quantum gravity.