The 'Soft Hair' on Stephen Hawking's Black Holes


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The 'Soft Hair' on Stephen Hawking's Black Holes

A step toward solving the black hole information paradox.

On Tuesday, Stephen Hawking and friends posted a new paper to the arXiv server. It has a great, or at least peculiar, title: "Soft Hair on Black Holes."

What in the great wide universe could that possibly mean? Glad you asked.

The subject of the paper is a deeply vexing problem known as the black hole information paradox. This is the conundrum that arises when we ask what happens to information as it falls into a black hole. Does it persist in some form or is it lost? We hope that it persists in accordance with the rules of quantum physics, which demand that the probabilistic information governing a quantum state not vanish, but that sure doesn't seem to be the case.


Or at least it's really hard to imagine how the information inside of a black hole might stay intact, given that there is no conceivable way of accessing it. Is it to be found in the junk leftover after a dying black hole disappears in a final fit of radiation? Or can we access it through Hawking radiation, that slow dissipative fizz of energy that gives every black hole a finite (if very, very, very long) lifetime?

Before going on, let's restate the information paradox question in maybe more stark terms: Does there always exist a history? If there is a present, is there a past? If information can indeed be lost, well, then we can imagine something that exists but without a history.

Now, pause here for maybe five or 10 seconds to imagine a universe in which history itself is routinely gobbled up. Like in the Stephen King's The Langoliers.

Anyhow, Hawking doesn't have an answer to the information problem, but the new paper offers a tentative step toward an answer. This is where hair comes in.

Around 1973, the physicist John Wheeler declared that "black holes have no hair." The phrase is the origin of what came to be known as the no-hair theorem or no-hair conjecture. What it states is that black holes are essentially bald, or featureless. From the outside, they can be characterized by three parameters: mass, electric charge, and angular momentum. But nothing else.

If you were to have two black holes with the same mass, charge, and momentum, but one of them consists of antimatter and the other consists of regular matter, they would be completely identical. The same black hole, really.


Image: Ethan Siegel/Starts with a Bang

In the new paper, Hawking gives black holes hair. These are minute deformities in space-time that may exist around the event horizon of a black hole in the form of "super translations." The idea is that as a charged particle passes the threshold of a black hole, its information is stripped away and left just outside. The super-translation occurs as the incoming information jiggles the fabric of space-time a tiny bit, but enough to influence the radiation being emitted by the black hole.

Don't imagine this radiation as little blips of light. (That's how I usually think of radiation.) Imagine itself as individual rays of light blasting outwards at the speed of light. They're going at the speed of light, for sure, but right here at the boundary of a black hole they're sucked backwards with just enough force to make them seem to be just lingering there frozen at light-speed.

This is why you can't get out of a black hole. Nothing is faster than light!

Andrew Strominger, one of Hawking's co-authors on the new paper, gave an interview to Scientific American last week that explains it much better:

There's this symmetry of a black hole that we all knew about in which you move uniformly forward and backward in time along all of the light rays. But there's another symmetry, which is the new thing in this paper (though various forms of it have been discussed elsewhere). It's a symmetry in which the individual light rays are moved up and down. See, individual light rays can't talk to each other—if you're riding on a light ray, causality prevents you from talking to somebody riding on an adjacent light ray. So these light rays are not tethered together. You can slide them up and down relative to one another. That sliding is called a super-translation.

These super-translations can be viewed alternately as "soft" particles, which are just particles that have zero or basically zero energy. Hawking and Strominger are interested in "soft" gravitons (the theorized particle form of gravity) and photons (the particle form of light). These particles emerge as charged particles fall into the black hole, carrying information away but no energy.

The very-awesome Sabine Hossenfelder, the theoretical physicist and Backreaction blogger, is skeptical. The Hawking paper, for one thing, doesn't really explain how information actually winds up encoded in outgoing radiation, just that there's a mathematical solution allowing it. The paper also doesn't make a case for soft photons and gravitons having the capacity for getting rid of so much information.

"In particular, I still don't see that the conserved charges they are referring to actually encode all the information that's in the field configuration," Hossenfelder wrote in a blog post on Thursday.

So, we're kind of at the "cool story, bro" phase. The ideas are worth pursuing, so much as they can be pursued, but proper answers are a long ways off.