Black holes are, needless to say, strange places, and over the years, I’ve written much about all the bizarre paradoxes and extreme questions they pose. All this weirdness is what makes them fun to study because solving some of these paradoxes and questions ultimately gets us closer and closer to figuring out how time and space works. Consider the science this way. When you build an airplane wing, you want to flex it as hard as you can until it snaps because that will tell you the limits of the materials you used and the soundness of your design. Much the same way as destructive testing helps engineers hone their craft, so does studying a place where physics seems, well, broken, helps scientists test the outer limits of their discipline. Of course when you have the broken fabric of space-time to piece together, some problems will be much harder to solve than others and one of the most persistent ones is whether black holes have firewalls.
What exactly is a black hole firewall? We don’t really know because it’s not supposed to be one of the defining features of its anatomy. Instead, it’s what happens when spontaneous quantum particles litter the cosmos in the wrong place. These particles constantly blink into existence as particle and anti-particle pairs which instantly annihilate each other, and where this won’t pose any problem anywhere else in the cosmos, when they appear too close to an event horizon of a black hole, one particle will get drawn in while the other is repelled into space, with a small flash of energy from the maw of the black hole which it must give up to keep with the laws of physics. It’s a fraction of a fraction of a nanowatt, but over the eons a black hole will exist, this add up as the black hole would eventually be unable to hold itself together and explode. At least this is the theory behind what we call Hawking radiation and it balances out the escaping particle’s energy and returns the swallowed matter back to the universe. So what’s the problem?
Well, the problem lays in a technicality that’s actually quite a big deal because it breaks a very fundamental principle of how quantum systems work. Entangled quantum particles are said to have a monogamy of entanglement, meaning that once you entangle one particle in a system, you can’t entangle it with another. To paraphrase a great explanation the source of which I just can’t recall, imagine quantum entanglement as rolling some dice and no matter what numbers come up for each individual dice, the sum of those numbers is the same with every roll. This is important because it lets us know that if we entangle a pair of dice and get 12 in our first throw, when we throw them again and one comes up as a 7, we understand that the other is 5 without even looking at it. Understanding how this works allows us to do some amazing experiments on the very nature of causality itself. But to make this balance out, we would have to know for sure that our 5 isn’t also being used to change the sum of another roll, elsewhere.
And this is exactly what a black hole’s event horizon allows. When one of our virtual particles is swallowed and the black hole gives off the teeny Hawking emission, the remaining particle and the emission are entangled. But the infalling anti-particle is still there, and they outgoing one is still entangled with it. Two independent quantum systems have created a mass-energy surplus, which is a very blatant violation of the laws of thermodynamics, and most solutions to this weird state of affairs involve even further violations of the laws of physics. Enter the firewall. Not really an ongoing phenomenon just beyond the event horizon, it’s instead a line crossing which would permanently sever the entanglement between the outgoing and infalling particles, leaving just a Hawking emission and the outgoing particle as a quantum system. This would release massive amounts of energy proportional to the event, and trap the particle in the black hole forever. It’s not a tidy solution, but it sort of works if you try not to think about it too hard.
Of course thinking too hard about things is what scientists do and they quickly pointed out that breaking quantum entanglement on a whim just doesn’t work, no matter how much energy you release to compensate for the inequality in the resulting equations. And that means, the firewall isn’t really the answer to what happens to the energy and information when black holes devour something. A new solution proposes that black holes actually spawn wormholes when they eat entangled particles. Those aren’t the conventional kind of wormholes we think of, they couldn’t be used to cross space and time on a whim, but they’re essentially a connection which keeps both the escaping and the infalling particle entangled. Recall our dice-based quantum system, and imagine that you roll a dice in NYC while someone else in Hong Kong rolls the other. Both will still amount to 12, and if your dice shows a 6, the one in Hong Kong will as well. But should you be unable to see what you rolled, you can count on a call telling you what the other dice is showing at the moment. That phone call? That’s more or less the wormhole in question.
Yes, this does basically jettison Hawking radiation and leads to its own weird conclusions about the fabric of the universe being composed of a constantly entangled quantum mesh, but that’s how science works. Slowly and carefully, we chip away at complex problems and flesh out all of the toy models until we can simulate real systems, and try and observe them and their behavior out in the wild. What happens to matter that falls into a black hole and if it’s still connected to a quantum system on the outside is still a wide open question. But the fact that it’s just so difficult to even try to answer what seems like a simple question at first glance, shows just how bizarre, complex, and self-contradictory the universe can be. Far from a steady, ordered system, it’s an incredibly wild mess that seems to barely be governed by its own rules should we look just a bit too close, and nowhere is this more evident than with black holes. They’re places where all we know about time and space is broken. But they’re also the places that could teach us the most about these laws, especially because that’s where they’re being tested at their extremes…