Are black holes surrounded by walls of fire? Does this imply that one (or more) of our most cherished physical principles—and here I’m talking about biggies like quantum theory, the conservation of information or Einstein’s equivalence principle—is wrong? Any may our savior come in the form of wormholes? These are the questions consuming some of the world’s foremost theoretical particle physicists as they argue about potential solutions to what has become known as the “black hole firewall” problem—perhaps the most important paradox in physics since Stephen Hawking proposed his first black hole information paradox nearly four decades ago.
Every black hole has an event horizon. Nothing that moves inside a black hole’s event horizon will ever escape, not even light. Yet we’ve always understood event horizons to be less than dramatic—if you were to cross one, you wouldn’t 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 that Stephen Hawking pointed out many 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 matter—a star here, a wayward astronaut there—then, over time, spit it back out into the cosmos as Hawking radiation. But because information can not be destroyed—only scrambled—the 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 entangled—that is, every particle’s 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 you’d like to know more, I recommend this short video.)
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 entanglements—a 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 this might be OK, 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, I’d recommend reading Jennifer Ouellette at Cocktail Party Physics, Dennis Overbye in the New York Times, Zeeya Merali in Nature, Caltech’s John Preskill and UCSB's Joe Polchinski, who first came up with the paradox along with his colleagues Ahmed Almheiri, Don Marolf and James Sully—the 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 don’t fully understand what’s 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. (They’ve been doing a great job uploading videos of all the talks, so if you’re 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 Juan Maldacena and Leonard Susskind, building on the ideas of Mark Van Raamsdonk and Brian Swingle. Maldacena and Susskind posit that the solution to the firewall problem may come in the form of wormholes.
Wormholes! I feel like we haven’t talked about them since the ’90s. Basically, wormholes are theoretical objects that connect two different points in space. They’re allowed as possible solutions to Einstein’s equations for general relativity—indeed, Einstein and his colleague Nathan Rosen first discovered wormholes, which is why they’re also called Einstein-Rosen bridges. Unfortunately, wormholes aren’t perfect—Einstein’s equations also imply that nothing with nonnegative energy (that is to say: nothing that we know of) can traverse a wormhole, so they’re not going to make for useful intergalactic portals anytime soon.
Maldacena and Susskind, following Van Raamsdonk, posit that any time two quantum particles are entangled, they’re 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 can avoid the firewall at 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 their paper:
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 there’s one great thing about paradox, it’s that their resolutions require radical breakthroughs. The equipment we build for the job may take us to places we’ve never dreamed.