Last August, Stephen Hawking tantalised the world by saying he'd worked out a solution to the “black hole paradox”.

He's now dropped the first detailed discussion of his hypothesis for the world to pore over, here at ArXiv in a paper entitled Soft hair on black holes.

The black hole paradox wouldn't have arisen if not for his own work, in a now 40-year-old paper that proposed “Hawking radiation”. That paper created a problem because it proposed a mechanism by which information is lost to the universe forever.

Physicists don't like information destruction any more than they like singularities. Physical laws let us use the present to predict the future, but black holes destroying information also destroys the determinism we rely on.

Hawking's 1970s paper, however, was written in the context of the mathematical understanding of the time, which assumed that black holes were featureless “bald” spheres, about which only mass, charge, and spin (angular momentum, to be more precise) could be known.

Since then, others have proposed one more thing that could be described about a black hole, if you had a way to observe it: “soft hairs” that preserve information about infalling quantum states.

What Hawking and his co-authors have done in this paper is to try and tackle a very thorny question: “if there is hair, what does it look like, and how does it preserve information?”

Here's where things get convoluted, so bear with us.

Hawking radiation

Throughout the universe, “virtual” particles and anti-particles are regularly coming into existence spontaneously, and annihilating each other with a tiny release of energy. The reason we know this happens is that we can see and manipulate vacuum fluctuations.

Hawking radiation is a special case: if a virtual particle comes into existence on one side of the event horizon, and its twin on the other side, one will fall inwards and the other one escape. The escaping particle steals a tiny bit of the black hole's mass with it; if you waited a sufficiently ridiculous number of billions of years, the black hole itself would boil away into space. But the information dropping in is destroyed.

Hair restorer

Hawking's proposal last August was that instead of disappearing into the interior of the black hole, information about what dropped in is stored at the event horizon. At the time, he didn't propose a mechanism for how it was stored.

That's what the ArXiv paper begins to address (with lots more to come). Hawking and co-authors Malcolm Perry (University of Cambridge) and and Andrew Strominger (Harvard University) flesh things out a bit.

The get-out-of-jail card is described as soft, “zero-energy” photons that retain the quantum state information of whatever fell into the black hole (he posits soft gravitons as well, but let's stick with photons).

Strominger gave this example to Smithsonian: since charge cannot be created or destroyed in the Universe, an infalling electron crossing the event horizon would leave a ghost of itself in the soft hair – a tiny variation in spacetime sufficient to describe the electron's charge.

What's missing?

Hawking isn't the first to discuss the idea of “hairy” black holes – in that sense, this is an incremental addition rather than an intellectual revolution.

The paper itself admits there's a lot of work to be done – phrases like “we take some steps” and “details will appear elsewhere” demonstrate that.

Sabine Hossenfelder of the Nordic Institute for Theoretical Physics writes, for example, that the “soft hair” theory focuses on one specific kind of information – information that describes charge particles.

To generalise from this paper, in other words, we need a theory that can describe other kinds of information.

Testable? Not yet

However, what the paper does propose is that the “hair” left behind by charged particles should be physically measurable, even if we don't yet know how:

“The soft photon modes on the horizon can be indeed excited in a physically realisable process, as long as their spatial extent is larger than the Planck length”

Since Hawking et al aren't proposing to wander over to the nearest black hole to check, The Register supposes the idea is that if the hypothesis is right, it should permit predictions to be made about black holes we've already observed.

Expect theoreticians to be hard at work figuring out how "soft hair" might show up in the radiation we see spilling from black holes as they swallow matter.

After the black hole

Another problem that Hossenfelder identifies is that Hawking hasn't yet provided a mechanism that preserves information after radiation has boiled away a black hole.

Over at Scientific American, there's a long interview with Andrew Strominger. ®