At the heart of every black hole sits a problem. As they sizzle away into nothingness over the eons, they take with them a small piece of the Universe. Which, quite frankly, just isn’t in the rule book.
It’s a paradox the late Stephen Hawking left us with as a part of his revolutionary work on these monstrous objects, inspiring researchers to tinker with potential solutions for the better part of half a century.
Somewhere between the two greatest theories ever to be constructed in physics there’s a tiny but significant flaw. Finding a solution would allow us to either model general relativity as a particle-like system or comprehend quantum physics against the rolling background of space and time. If not a combination of both.
One recent attempt at a new theory by physicists from the UK, the US, and Italy has certainly stirred some interest in the general media, though it will be some time before we know one way or another if it’s the solution we so desperately seek.
Mathematically it’s a clever new spin on an idea that’s been kicked around a while – one that poses black holes are kind of ‘hairy’.
To understand why a hairy black hole could be a useful one as far as paradoxes go, it’s important to know why there’s a paradox to begin with.
Black holes are masses of matter packed so tight, their gravity puckers space and time to the point that nothing can muster the velocity required to escape.
Ordinarily this wouldn’t be a huge issue. But about half a century ago, Hawking came to the realization that black holes must ‘shine’ in a rather unique way. Their warping of the Universe would change the wave-like nature of surrounding quantum fields such that a form of heat radiation was produced.
To make the mathematics all balance, this means black holes would gradually radiate energy, shrink at an accelerating rate, and eventually pop out of existence.
Ordinarily, the information that falls into a radiating object like a star would be represented in the messy spectrum of colors that shoot from its surface. Or is left behind in its cold, dense husk after it dies.
Not so for black holes. If Hawking’s radiation theory is correct, it would all just, well, go away. Which compromises the big rule in quantum physics which says the information that makes a particle a particle is conserved in the Universe from moment to moment.
A significant part of the debate over the nature of a black hole’s information bank is the extent to which its contents’ characteristics and behavior continue to affect their surroundings even after they’ve slipped over the edge.
There are solutions for black holes in general relativity that recognize their mass, angular momentum, and charge still push and pull on their local surroundings. Any remaining connections with the Universe are described as hair, with theories that presume their persistence as ‘yes-hair theorems’.
Having a bit of fuzz would give black holes a path for their quantum information to remain stuck in the Universe, even if they do happen to fade away over time.
So theorists have been busy trying to find ways to make the laws that tell space and time how to curve mesh with the laws that tell particles how to share their information.
This new solution applies quantum thinking to gravity in the form of theoretical particles called gravitons. These aren’t bona fide particles like electrons and quarks, as nobody has seen one in the flesh yet. They might not even exist at all.
That doesn’t mean we can’t figure out what they might look like if they did, or consider possible quantum states they might operate within.
Through a series of logical steps from the way gravitons could potentially behave under certain energy conditions, the team demonstrates a reasonable model for how information inside a black hole can remain connected with surrounding space across its line-of-no-return – as slight peturbances of the black hole’s gravitational field (the hairs).
As a theory, it’s an interesting one based on a solid framework. But there’s a long way to go before we can stamp ‘solved’ on this paradox.
Broadly speaking, there are two ways science progresses. One is to see something odd, and try to account for it. The other is to guess at something odd, and then try to find it.
Having a theoretical map like this is invaluable on our journey towards a solution to one of physics most perplexing paradoxes.
This research was published in Physical Review Letters.