Every black hole has an event horizon. Nothing that moves inside a black holes event horizon will ever escape, not even light. Yet weve always understood event horizons to be less than dramaticif you were to cross one, you wouldnt notice anything immediately amiss.

Event horizons are important, however, for a number of reasons. Consider that according to the laws of quantum mechanics, a pair of virtual particles can jump into existence. Ordinarily, they quickly come back together and annihilate one another, but if the process happens near an event horizon, one particle can get sucked into the hole, leaving the other to drift into space. This implies that black holes radiate particles, a curious fact thatmany years ago. Eventually black holes lose so many particles that they shrink and die, having spewed their mass out into the cosmos in a stream of Hawking radiation.

Looking at the situation another way, black holes swallow mattera star here, a wayward astronaut therethen, over time, spit it back out into the cosmos as Hawking radiation. But because information can not be destroyedonly scrambledthe Hawking radiation must contain all the information about the stuff that fell in to the black hole. And the only way that this can happen is if all the Hawking radiation is entangledthat is, every particles quantum state co-depends on the quantum states of all the other particles in the Hawking radiation. (Entanglement is a weird and important quantum concept. If youd like to know more, I recommend .)

Remember, though, that Hawking radiation only exists because a pair of virtual particles popped into existence. One fell in, the other drifted out. These two particles must also be entangled. Unfortunately, the laws of quantum mechanics forbid promiscuous entanglementsa particle can be entangled with its twin, or the rest of the radiation coming out of the black hole, but not both.

And so we have a dilemma. In order for information to be conserved, particles in the Hawking radiation must be entangled each other. But in order to get the Hawking radiation in the first place, these particles must be entangled with the particles falling in to the black hole. Physicists used to think , since no single observer could detect both entanglements. But AMPS noticed that a particle coming out of the black hole could be turned around and sent in to the black hole, illuminating the double quantum correlations and causing no end of quantum mischief. To avoid this, they suggest that as the particle crosses the event horizon, the original quantum correlation breaks, producing a burst of energy. The net effect: a wall of fire.

(For more on the firewall paradox, Id recommend reading , , ,and , who first came up with the paradox along with his colleagues Ahmed Almheiri, Don Marolf and James Sullythe quartet now known as AMPS.)

The black hole firewall paradox has caused no small amount of wonder and confusion amongst particle physicists. It appears as though one of our core beliefs about the universe is wrong: Either particles can be promiscuously entangled, leading to quantum disaster (basically no one takes this option seriously; quantum theory and the no-promiscuous-entanglement rule are far too well supported by decades of experimental evidence), or information is not conserved (another non-starter), or black holes have firewalls (even Polchinski considers this a reductio ad absurdum), or we just dont fully understand whats really going on.

And so in an effort to sort the mess out, physicists gathered this week at the Kavli Institute for Theoretical Physics at UCSB to talk over the options. (Theyve been doing a great job uploading , so if youre interested in watching smart folks try to hash out knotty thought experiments in near-real time, you can follow along at home.) One of the most intriguing possibilities for a solution comes from , building on theand . Maldacena and Susskind posit that the solution to the firewall problem may come in the form of wormholes.

Wormholes! I feel like we havent talked about them since the 90s. Basically, wormholes are theoretical objects that connect two different points in space. Theyre allowed as possible solutions to Einsteins equations for general relativityindeed, Einstein and his colleague Nathan Rosen , which is why theyre also called Einstein-Rosen bridges. Unfortunately, wormholes arent perfectEinsteins equations also imply that nothing with nonnegative energy (that is to say: nothing that we know of) can traverse a wormhole, so theyre not going to make for useful intergalactic portals anytime soon.

Maldacena and , following Van Raamsdonk, posit that any time two quantum particles are entangled, theyre connected by a wormhole. They then go on to say that the wormhole connection between particles inside a black hole (the infalling virtual particles) and the particles outside of a black hole (the Hawking radiation) soothes out the entanglement problems enough so that we canat the event horizon.

Note that this requires a profound rethinking of the fundamental stuff of the universe. Entanglement, a deeply quantum phenomenon, is fundamentally wound into to the geometry of the universe. Or, to flip it around, quantum weirdness may be stuff that creates the substrate of spacetime.

Of course, nothing is settled yet. As Maldacena and Susskind write towards the end of :

At the moment we do not know enough about Einstein-Rosen bridges involving clouds of Hawking radiation to come to a definite conclusion. The AMPS paradox is an extremely subtle one whose resolution, we believe, will have much to teach us about the connection between geometry and entanglement. AMPS pointed out a deep and genuine paradox about the interior of black holes.

And if theres one great thing about paradox, its that their resolutions require radical breakthroughs. The equipment we build for the job may take us to places weve never dreamed.

from Wikimedia Commons courtesy of , Physics education group Kraus, Universit?t Hildesheim, , (background image of the milky way: )

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