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.
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. 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 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.