Six years ago, a group of astrophysicists came to the startling realisation that if certain assumptions about the quantum physics of black holes were true, their horizons should be ablaze with high energy particles.
This “firewall” would pose a big problem, not just for suicidal space tourists, but for physics itself. It just shouldn’t be there.
We might be able to breathe a little easier now – but only if we imagine black holes as rather hairy places.
Three physicists from Ohio State University in the US have come up with an innovative way to sort out a problem that’s been frustrating black hole enthusiasts for the past few years by turning to string theory.
Black holes are objects of such insane density, stuff that goes in can’t ever move fast enough to get out again, and is turned into something called a quantum mixed state.
One thing that does seem to come out of black holes is paradoxes. Plenty of them.
And we’re about to run you through most of the big ones.
The paradox hit list
Arguably the most famous paradox is the contradiction between information being messed up inside a black hole and a physical law that says information can’t be created or destroyed.
The late Stephen Hawking proposed a loophole that could get around this problem. Fluctuations in the fields surrounding a black hole could result in pairs of particles that would normally cancel each other out.
If one close to the hole’s edge slipped in, and its antiparticle partner flew off, the whole ‘information can’t be destroyed’ issue might have a solution.
Hawking radiation has never been observed, but it’s still a fairly safe bet as far as physicists are concerned. Unfortunately, it comes with its own paradox.
Even separated on two sides of a black hole’s horizon, they remain entangled thanks to their shared origin. This is a quantum mechanics way of saying that if you measure a property of one particle, you immediately know something about the other.
A particle that disappears inside the black hole also entangles with all of the Hawking radiation that has sizzled away from the black hole in the past. Unfortunately this flies in the face of a yet another fundamental property, called the “monogamy of entanglement“.
Like the Universe’s most lovesick coupling, two particles that are maximally entangled with one another can’t bring a third into their relationship. A particle headed into the heart of a black hole is committing some serious adultery.
Divorce is possible in the particle world, but it’s messy. And violent.
While the general noise of quantum fluctuations will gradually erode even the most robust entanglements over time, about a decade ago physicists discovered something called Entanglement Sudden Death (ESD).
Applied to black holes, it gave the married particles an immediate way out. But the sudden breakup was theorised to release enormous amounts of energy in one hit, creating a firewall of doom on the threshold of the black hole’s event horizon.
Which, as you might guess, results in yet another paradox, this one involving the equivalence principle.
In effect, an observer crossing the event horizon in freefall should feel as if they’re floating freely in space. A tourist heading into a black hole might be slowly stretched into a human noodle by tidal forces, but that first step over the line-of-no-return should feel no different to the journey preceding it.
Being blasted with intense energy would almost certainly qualify as ‘a sudden change’, marking the event horizon in a way that physics says shouldn’t be possible.
So what’s the answer?
Now, physicist Samir Mathur doesn’t think that boundary is all it’s cracked up to be.
“What we’ve shown in this new study is a flaw in the firewall argument,” says Mathur.
Mathur’s previous work revolves around string theory, which describes the information making up particles in the form of tiny one-dimensional loops vibrating in multiple dimensions.
Together with colleagues, Mathur showed how these strings don’t suffer the same fate as the particles they comprise, ultimately preserving information within the singularity. The whole information paradox could technically be averted.
But that doesn’t necessarily mean there’s no Hawking radiation, and therefore no firewall. So Mathur has now shown those strings could turn a smoothly defined boundary into something a little more … fuzzy.
“The firewall argument had seemed like a quick way to prove that something falling through the horizon burns up,” says Mathur.
“But we now see that there cannot be any such quick argument; what happens can only be decided by detailed calculations in string theory.”
Objects that approach black holes would instead face a negligible chance of striking any higher energy particles produced by ESD, meaning once again that boundary isn’t a special place as far as our intrepid space tourist is concerned.
“Once a person falling into the black hole is tangled up in strings, there’s no easy way to decide what he will feel,” says Mathur.
String theory is still one of those nice ideas waiting for empirical support.
The good news is this could be one way to find that evidence. The bad news is any fearless scientist willing to test it would earn any accolades posthumously.
Not that offering free hugs of fuzzy space balls wouldn’t be great for the space tourism industry.
This research was published in Journal of High Energy Physics.