Alice and Bob are two of the most famous explorers you've probably never heard of. If there is a quantum experiment being discussed, Alice and Bob are usually involved, and they've been through a lot together. But in the last 50 years, classical physics and quantum mechanics have come into a direct conflict at the bleeding edge of the most extreme objects in the universe, black holes, and things have not turned out great for Alice.

See, Alice is a sub-atomic particle, and she's been everywhere from hanging out with Schrodinger's Cat to performing immensely complex computations in a quantum computer. But, if a recent theory about an especially thorny physics paradox is correct, Alice just might end her intrepid travels for good by falling past the event horizon of a black hole, only to be immediately incinerated by a massive wall of intense energy that runs all along the entire event horizon, forever beyond our ability to ever see it.

This black hole firewall, as it's become known, was immediately dismissed as ludicrous, and even insulting, when it was initially proposed in 2012, but nearly a decade later, scientists are still struggling to refute it, and the controversy could have profound implications for physics as we know it.

## A Brief History of Black Holes

Before we can wrangle with the mysterious interior of a black hole, we should start by describing what we know about black holes.

Black holes were first predicted by a humble English rector John Michell in 1783, who used Newtonian mechanics to posit the existence of "Dark Stars" whose gravity was stronger than a particle of light's capacity to escape it. However, the concept of black holes we are more familiar with arose from Albert Einstein and his theory of relativity in 1915.

Karl Schwarzschild, a German physicist and astronomer, read Einstein's 1905 paper on special relativity a few months and produced the first exact solution to Einstein's general gravitational equations, which impressed even Einstein himself. "I had not expected that one could formulate the exact solution of the problem in such a simple way," he wrote to Schwarzchild in 1916.

What Schwarzchild is perhaps best known for, however, is applying the math of Einstein's relativity and deriving the possible existence of black holes based on the escape velocity of light (much as Michell had done with Newtonian mechanics). Schwarzschild himself didn't believe that black holes actually existed, but his work provided the mathematical basis on which our modern understanding of black holes was built.

The key feature of the black holes he described was an event horizon, a boundary located a predictable distance from the center of the black hole's mass which represented the gravitational threshold where the escape velocity from the black hole exceeds the speed of light. On the outside of the event horizon, escape was possible, but once you passed that boundary, relativity meant you could never leave, since nothing can travel faster than light.

There have been some major developments in our understanding of black holes since Schwarzchild, but these basic features have stayed more or less the same since he first proposed them.

## Some Fundamental Features of Quantum Mechanics

Stepping away from the macroscale for a moment, we now need to dive below the level of the atom and discuss subatomic particles.

Subatomic matter does not behave in the same way as matter at the macroscale level. Instead, at the quantum level, the universe is governed by a strange world of probabilities and physics-defying features like quantum entanglement.

This feature of quantum entanglement, where two subatomic particles interact with one another and in the process become inextricably linked so that they behave as if they were a single object, seems to pay no mind to relativity, happily transmitting information between two entangled particles instantaneously over distances so vast that this information can be said to be traveling faster, sometimes exponentially faster, than light.

Einstein and other noted physicists in the first half of the 20th century were so bothered by some of the peculiarities of quantum mechanics, particularly quantum entanglement, that they went to great lengths to try to refute its results, but its math has held up sound and some of the fundamental laws have proved to be as unassailable as Relativity. Quantum entanglement isn't just predictable, it's become the bedrock of actual working technology like quantum computing.

Quantum mechanics isn't constructed using the same kind of math as classical physics, though. Classical physics relies on predictable mathematical techniques like calculus, while quantum mechanics is built largely on probabilities, the math of the card game, and the craps table.

The probabilities that form the basis of quantum mechanics, however, rely on an important principle that cannot be violated: the preservation of information.

If you roll a six-sided die, you have an equal one-in-six chance of rolling any of its values, but the probability that you will role * a* result is 1, which is the sum of adding up all the individual probabilities for all possible outcomes (in the case of the die, rolling a 1, 2, 3, 4, 5, or 6 all have one-sixth probability, so add all six one-sixths together and you get six-sixths, which is equal to 1). This summing up of probabilities in quantum mechanics is known as the principle of unitarity.

This predictive quality of probability relies on an even more fundamental rule, though, which is that knowing the current quantum state of a particle is predictive of its future state and* also* allows you to wind the particle back to its previous state.

Theoretically, if you had perfect knowledge of how a die was rolled, as well as the result, you could move back in time to identify which side was facing up when it was in your hand.

In order for this to work, though, that information about a previous quantum state must be preserved somehow in the universe. If it were to suddenly disappear, it would be like taking one of the die faces off the die and leaving nothing in its place.

When that die is rolled again, its five remaining sides still have a one in six probability, but now those sides add up to five-sixths rather than 1. So destroying information, like removing one of those die faces, breaks the quantum probabilities of that die roll.

This sort of transgression in quantum mechanics can't be allowed, since the information being destroyed directly leads to us not even being able to tell how many die faces we started out with originally and, thus, we couldn't actually know the true probabilities for anything.

Quantum mechanics as we know it would no longer work if quantum information is somehow destroyed.

What's more, there is also a principle in quantum mechanics known as monogamous quantum entanglement. Essentially, a particle can only be maximally entangled with one other particle, to the exclusion of all others, and this is key to how information in a quantum system is preserved.

There is a lot more to quantum mechanics than just these principles, but these are the essential ones to understanding how a black hole's event horizon could really be a gigantic, invisible shell of blazing hot energy.

## Hawking Radiation

When Steven Hawking did his most important work on black holes in the 1970s, he wasn't setting out to lay the foundation for a black hole firewall that annihilates anything unfortunate enough to fall into it, but may be what he did when he proposed the existence of Hawking radiation in 1974.

In even the emptiest of space, there is a roiling boil of quantum activity. It is thought that, spontaneously, virtual quantum particle and anti-particle pairs entangled together are constantly materializing and annihilating each other, drawing energy from the universe to create themselves and returning that same energy when they destroy each other.

Hawking realized, though, that if a pair of virtual particles materialize along the edge of a black hole's event horizon, though, one particle could fall into the black hole while its entangled partner on the outside is able to break free of the black hole and escape, producing what is now known as Hawking radiation.

The problem is that, according to the first law of thermodynamics, energy in a closed system must be conserved. If two virtual particles draw from the energy of the universe to materialize but don't immediately annihilate each other, then energy has been drawn from the universe without crediting it back. The only way something like this can happen is that the infalling particle must have negative energy in equal absolute value to the positive energy of the escaping particle.

But black holes, while immensely massive and energetic, aren't infinite — they have a defined mass, and any infalling, negative-energy particle subtracts an infinitesimally small amount of that black hole's mass when it enters. If the black hole doesn't accrete any additional material to add more mass, these tiny substractions due to Hawking radiation start to add up, and as more mass gets evaporated away, the evaporation of the black hole accelerates.

Eventually, enough Hawking radiation is emitted that the largest black holes shrink to nothing and simply wink out of existence.

## The Information Paradox

The challenge presented by Hawking radiation is that even if spacetime becomes infinitely warped at a black hole's singularity, it is held that whatever quantum information enters a black hole is still somehow preserved and therefore, theoretically, retrievable.

If nothing else, all that information hangs out at the black hole's infinite singularity and can at least still factor into any quantum probabilities so everything continues to add up to 1.

Critically, Hawking said that this radiation, even as it is still entangled with its infalling anti-particle, contains no encoded information about the black hole or its contents.

This means that all of the information that falls into a black hole never leaves it and would presumably evaporate into nothing, along with the black hole, due to Hawking radiation. This would take all of that information out of the overall quantum equation and the probabilities would suddenly stop adding up correctly.

Other physicists, like John Preskill of the California Institute of Technology, have argued that Hawking radiation actually becomes entangled with the area immediately outside the event horizon where the quantum information from infalling particles must be encoded. So long as the infalling particle and the outside particle do not share this information between them, quantum information need not be destroyed.

This was a tangled knot to begin with, but in 2012, a group of University of California, Santa Barbara, physicists proposed a solution to the information paradox that only seemed to make everything more contentious.

## The Great Black Hole Firewall Controversy

When attempting to wrestle with the information paradox in 2012, Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully — collectively known as AMPS — published a paper in the *Journal of High Energy Physics* arguing that along the edge of a black hole's event horizon was a swirling wall of energy so intense that it completely incinerated anything that touched it.

This was the result, AMPS argued, of the entanglement responsible for Hawking radiation being effectively severed by the event horizon, releasing an enormous amount of energy in the process. And since Hawking radiation is a constant process all along the edge of the event horizon, this energy is also being released constantly all across the event horizon.

What makes this theory so controversial is that this would violate another pillar of modern physics: the principle of equivalence. According to General Relativity, gravitational and inertial forces have a similar nature and are often indistinguishable. So, you would not be able to tell the difference between being in a stationary elevator in a gravitational field and an accelerating elevator in free space. This means that, if an observer were to pass the event horizon of a black hole, they should not notice anything amiss — at least not immediately — because it is still entangled to the observer outside the event horizon.

The tidal force of the singularity's incredible gravity would eventually tear the observer apart into a very long string of atoms, but depending on the size of the black hole, an observer could continue to float down toward the black hole's singularity for anywhere from a few microseconds to possibly a few decades before this spaghettification occurs.

If the black hole firewall theory is correct though, the infalling observer would not even make it past the event horizon, since the outside particle becomes Hawking radiation when its entangled counterpart falls into the black hole. In order for the quantum information inside the black hole to be preserved, the new Hawking radiation must become entangled with the area outside the event horizon.

Quantum mechanics forbids this kind of dual-entanglement. Either Hawking radiation does not entangle with the region along the event horizon, meaning that quantum information is lost for good, or its entanglement with the infalling particle must be severed at the event horizon, meaning equivalence breaks down, which inexorably gives rise to the black hole firewall.

This did not go over well with physicists, since undoing the equivalence principle would pull the entire foundation of spacetime out from under Einstein's relativity, which simply couldn't be possible given how regularly relativity has been validated through experimentation. If equivalence didn't hold, then all of those experiments had to have been a 90-plus-year series of flukes that happened to confirm a false idea by pure chance.

This wasn't lost on AMPS, who pointed out that if everyone wanted to keep equivalence, then they had no choice but to sacrifice the preservation of information or completely rewrite what we knew about quantum field theory.

## Attempts to Scale the Black Hole Firewall

Steve Giddings, a quantum physicist at the University of California, Santa Barbara, said the paper produced “a crisis in the foundations of physics that may need a revolution to resolve”.

When Raphael Bousso, a string theorist at the University of California, Berkeley, first read the AMPS paper, he thought the theory preposterous and believed it would be quickly shot down. "A firewall simply can’t appear in empty space, any more than a brick wall can suddenly appear in an empty field and smack you in the face," he said.

But as the years dragged on, no one has really been able to offer a satisfying rebuttal to put the controversy to rest. Bousso told a gathering of black hole experts who'd come to CERN in 2013 to discuss the black hole firewall that the theory, "shakes the foundations of what most of us believed about black holes... It essentially pits quantum mechanics against general relativity, without giving us any clues as to which direction to go next."

The controversy has produced some interesting counter theories though. Giddings proposed in 2013 that if Hawking radiation were to make it some short distance from the event horizon before its entanglement with the infalling particle is broken, the release of energy would be muted enough to preserve Einstein's equivalence principle. This has its own cost, though, as it would still require rewriting some of the rules of quantum mechanics.

Preskill, meanwhile, famously bet Hawking in 1997 that information was not lost in a black hole and soon after a theory was put forward by Havard University's Juan Maldacena argued that "holograms" could encode 3D information in a 2D space where gravity had no influence, allowing information to find its way out of the black hole after all.

This argument proved persuasive enough for Hawking, who conceded to Preskill that information could be saved after all. With this history, Preskill makes an odd champion for the idea that information loss is actually the least offensive solution to the black hole firewall, but that was the argument he put forward in the 2013 conference. Quantum mechanics might need a page-one rewrite if information is lost, he said, but it wasn't out of the question. "Look in the mirror and ask yourself: Would I bet my life on unitarity?" he asked attendees.

Another possible solution to the black hole firewall problem was proposed by Maldacena and Stanford University's Leonard Susskind in 2013: wormholes.

In Maldacena and Susskind's proposal, quantum entanglement and Einstein-Rosen bridges are both intimately connected and could be two ways of describing the same phenomenon. If wormholes from inside the black hole were able to connect the infalling particles to their outside partners, then a form of entanglement could be maintained that did not require breaking entanglement at the event horizon, thus sidestepping the need for a firewall.

For all their inventiveness though, no one seems to be totally satisfied with the answers, even if they are enjoying the excitement of the debate itself.

“This is probably the most exciting thing that’s happened to me since I entered physics,” Bousso said. “It’s certainly the nicest paradox that’s come my way, and I’m excited to be working on it.”