The Firewall Paradox

This article is (sort of) an expansion of a Twitter thread I wrote in January 2017 in an attempt to explain some of the work Emirati theoretical physicist Ahmed Al Mheiri was involved in. You can find the thread here: https://twitter.com/7anooch/status/826043463158337536

Any comments or questions are welcome.

Modern physics has made immense progress over the last century or so, with general relativity and quantum mechanics at it's foundation. We've made countless tests of Einstein's general relativity, which have all passed with flying colors. We've discovered a whole world of sub-atomic particles. Our smartphones wouldn't exist without quantum mechanics, GPS wouldn't exist without general relativity.

 General relativity (GR) and quantum mechanics (QM) are the two cornerstones of modern physics. The problem is that they don’t happen to be the best of friends. GR applies to large scale things (think: galaxies and stuff), quantum mechanics applies to small things (atoms, nuclear fusion and such). When we try to put GR and QM together, we get very confused. It just doesn’t work. They both work remarkably well, just not together. No one knows what a quantum theory of gravity looks like, and people have been trying hard to figure one out for decades. Even Einstein tried (and failed).

However, in certain situations we can try and put GR and QM together and try to figure out what happens: this is one of the reasons why black holes are cool (to the scientists studying them at least). Black holes are basically a theoretical physicist’s playground. Gravity is strongest around a black hole, so GR effects would be most pronounced. QM, as anywhere, would also apply if you look at the small scales. So around black holes is a great place to try and conduct what physicists call 'thought experiments': hypothetical experiments where we try to figure out what would happen in (often) extreme situations. Einstein's relativity was essentially a result of a thought experiment.

In the 1970s Stephen Hawking discovered that black holes aren’t actually black, they emit radiation (hence, light). We now call that Hawking radiation. There is a point  (actually, more like a spherical shell) around a black hole after which even light cannot escape. The pull of gravity dominates over other forces. We call that the event horizon.

Space is not empty. Quantum fluctuations sometimes create two particles (strictly speaking, a particle and it's anti-particle) out of nothing. So, occasionally two particles will pop up at an event horizon. When that happens, you can imagine one being right outside the event horizon, while the other one is inside. One gets sucked in, the other doesn’t: it gets released as Hawking radiation.

The event horizon is essentially a shell surrounding the black hole at a specific radius. Hawking radiation implies that there will always be radiation coming from the black hole when nothing really goes in, so in the end the black hole shrinks. From this we can also conclude that Hawking radiation doesn’t contain information. It's the result of a random particle that was created from nothing, and is just pure energy with no encoded information in it. So if the black hole eventually shrinks (until it is no more), and the Hawking radiation has no information in it, then is all the information that was once in the black hole lost?

This is the black hole information paradox. Information loss is forbidden by quantum mechanics, so the issue must be resolved in some way. Otherwise, there is something wrong with quantum mechanics. In quantum mechanics, particles can become entangled with each other. Einstein never believed in entanglement, he called it 'spooky action at a distance', which isn't  actually a bad explanation of what it is. Entangled particles share the same information, so if you manipulate a particle one way, the same will simultaneously happen to its entangled pair. It's one of the weirder things about QM.

Now we get to the fun part. In 2012 Ahmed with advisor and colleagues devised a thought experiment that turned theoretical physics on its head.

They discovered a new paradox, called the firewall paradox. The argument is very subtle, I try my best not to butcher it but the truth is I don't understand it very well either. Here's my attempt:

You have a Hawking pair: two particles that occur due to quantum fluctuations at the event horizon. These particles are also entangled: if something happens to one then it also happens to the other, no matter the distance that separates them. Only one of them crosses the event horizon and falls into the black hole. According to GR, anything that crosses the event horizon shouldn’t ‘feel’ a difference. Nothing special should happen to the particle, it wouldn't even notice that it crossed the event horizon. This is called the ‘no drama’ postulate and is one of the cornerstones of general relativity.

Also, one way to get around the information paradox issue is to say that outside the event horizon the information gets increasingly entangled with itself. So we can preserve information in this way and rest assured that none is lost. The problem here though, is that according to QM, the particle that stays outside can't both be entangled to the particle that falls in, and also to all the other particles outside.

And so, they discovered that something is wrong in this argument: Either Hawking radiation is not 'pure' (it does somehow have information! something is wrong with QM), the ‘no drama’ hypothesis is wrong (something is wrong with GR), or physics doesn’t behave ‘normally’ outside the horizon (again, something wrong with GR). The best way they can think of to resolve the issue (because both QM and GR are at stake here) is to give up the ‘no drama’ hypothesis. Both QM and GR and very well tested, so we really don't like to think that something is wrong with either one of them.

They hypothesize a ‘firewall’, as in: an actual wall of fire, that anything falling into the black hole will be incinerated by. This was the most 'conservative' compromise they could come up with. Even then, it's not conservative in the least.

When this was announced, everyone went crazy, most people thought someone would quickly figure out a solution. But of course, more than 4 years later, no one has figured out a convincing resolution for the firewall paradox. Physicists thrive on these sorts of paradoxes, because they illuminate the subtle things in our theories that don't quite add up. If you can't see the problem, you can't fix it.

You can google the ‘firewall paradox' find a lot more coherent explanations than mine, the firewall paradox created a large amount of buzz. Bear in mind, this is only about one paper Ahmed was part of. Maybe I'll try to make sense of his other work later.